Bonding & Grounding System
- Once you have electricity on board, even if it’s just an engine, battery, cabin lights and VHF radio, the possibility exists of electrolytic corrosion to metal parts of the boat. The purpose of bonding, as described by ABYC, is to: (Upgrading the Cruising Sailboat, p. 273)
- provide a low-resistance electrical path inside the hull between otherwise isolated metallic objects, especially those in common contact with sea water
- prevent the possibility of electrical potential on exposed metallic enclosures of electrical equipment
- provide a low-resistance path for excessively high voltages, such as when the boat is struck by lightning
- to minimize radio interference. Boats without permanently installed electrical systems do not need bonding.
- The heart of the bonding system is a “common bonding conductor”, which is generally a length of bronze or copper metal at least ½” wide and no less than 1/32” thick. It is laid inside the hull from stem to stern and those metallic objects that are connected are done so by using #9 AWG wire or larger. Items to be bonded include engines (use the engine negative terminal), the metallic enclosures of electrical appliances, motors, generators and pump frames, fuel tanks, fuel-deck fittings and lead-lined battery trays. It is not necessary to include in the bonding system electrically isolated thru-hull fittings, if they are all of the same metal, or if they are protected by sacrificial anodes, as is common with propeller shafts. (Upgrading the Cruising Sailboat, p. 273)
- Bonding and Electrolytic Corrosion Due to Hot Marinas – Do not bond any thru-hulls or other immersed metal that can be electrically isolated. Specifically, keep your metal keel/ballast, your metal rudder shaft, your engine/prop, and all thru-hulls electrically isolated, from each other, and from the engine. It’s worth understanding the reason. In an increasing number of marinas, there are substantial DC electric currents running through the water. If your bits of immersed metal are bonded, the electric current will take the lower resistance path offered by your boat in preference to the water near your boat, and the current will flow into one of your bits of metal, through your bonding wires, and then out another bit of metal. The anodic bit of metal or thru-hull that has the misfortune to be on the “out current” side of the current running through your bonding system will also become “out metal” and will disappear, sometimes rapidly. Your zinc is only intended to protect against the modest galvanic potentials and therefore currents that are caused by the dissimilar metals that are immersed and electrically connected together on your own boat. Your zinc is incapable of supplying enough galvanic potential to protect against substantial DC currents that may be flowing in the water. These DC currents in the water will cause electrolytic corrosion to your bonded thru-hulls or metal parts. (http://www.sailmail.com/grounds.htm)
- If underwater components are not galvanically identical, bonding them completes the circuit and causes corrosion (which must be controlled with zinc anodes). Bonding does protect underwater fittings from damage caused by onboard stray currents, but it invites damage from stray currents in the water, and “hot” marinas are today the rule rather than the exception. It is easy enough to avoid onboard stray currents with good wiring practices, but you have no control over stray currents in the water….Metal through-hulls connected to rubber hose are already isolated. Likewise, the rudder shaft and/or fittings are normally isolated, but be sure components are not grounded some other way. For example, the rudder might be connected to the boat’s central ground through steering cables and pedestal wiring. A fuel tank connected to the engine with metal fuel lines is likewise electrically connected to the DC ground. In this latter case, bonding the tank for lightning protection provides a path for stray current to enter your boat at the ground plate and pass out at the prop (or vice versa). (Sailboat Electrics Simplified, p. 166)
- Where direct attachment of a zinc is not practical – bronze through-hull fittings, for example – protection from galvanic corrosion is effected by electrically connecting the fitting with a heavy electrical cable to a zinc anode, typically some anode-protected underwater fitting. This is called bonding, and while it will indeed afford the through-hull galvanic protection, it opens the door to a much more destructive stray-current corrosion. Bonding is a complex and controversial subject, but there is little doubt that bonding underwater metal components that are otherwise electrically isolated is a bad idea. A good bronze through hull with rubber hose will resist corrosion for decades without anode protection. (This Old Boat, p. 172)
- In addition to assuring the integrity of all circuits on a boat, the most effective way to prevent stray current corrosion is to have a connection (a bonding cable) from underwater metal fittings directly back to a battery negative. If a stray current occurs, it will follow the path of least resistance back to the battery, which is the wire rather than the water. The problem with this approach is that in wiring underwater metal objects back to battery negative, all these objects are simultaneously wired together, thereby establishing the prerequisite for galvanic corrosion! This is why it is better not to bond electrically isolated bronze through-hulls – just be sure no current-carrying DC wires are lying against them. (Cruising Handbook, p. 192 – 3)
- Bonding – the original idea behind bonding was to put all underwater fittings at the same potential to stop galvanic corrosion. Unfortunately, this type of bonding invites more-destructive stray-current corrosion. Bonding is still intended to put fittings at the same potential, but today the purpose is to prevent side flashes from voltage differences in the event of a lightning strike. The rules for bonding are simple: bond all sizable metal components within 1.8m (6′) of mast or rigging to the mast ground, but do not bond any submerged metal (ground plate excepted). (Sailboat Electrics Simplified, p. 164)
- The general rule for corrosion control is to bond to a single underwater component. This eliminates any possibility of providing a circuit for stray current, and it also eliminates galvanic currents except between dissimilar metals in contact. The preferred configuration is to isolate the propeller shaft from the engine so the prop doesn’t provide a ground, then use the lightning ground plate for all ground connections. If the engine is not isolated, the necessity for ground near the base of the mast means you will have two bonded components in the water. Copper and bronze are close enough on the galvanic scale that significant galvanic corrosion is not likely, but stray current corrosion is a risk. The solution in this case is to keep the lightning grounding electrically separated from the engine ground (for the DC and AC circuits). (Sailboat Electrics Simplified, p. 167)
- …from reading Nigel Calder on this subject, the immersed ground plate IS the one place where everything is connected. By everything, I mean the DC negative main bus, and the non-current carrying main bus (including connections from the chainplates, mast, fuel tank, bonding system, zincs), and the main lightning conductor from the mast. A chart showing this is on p. 233 of Calder’s Boatowner’s Mechanical and Electrical Manual. (http://plasticclassicforum.com/forum/)
- ….stray current corrosion, where you’re near someone else’s bad wiring job and the current flows through your boat’s bonding system, destroying your underwater fittings. The best way to avoid it (according to Don) is to have a single underwater ground point for all systems (like Nigel says), and to keep all other underwater fittings isolated from this ground.And here’s something interesting about the size of the ground plate: “If your keel is encapsulated, a copper ground plate is needed. Lightning dissipates from the edge of the plate, so the perimeter of the plate should be at least 4 feet (1.2m) if you sail in salt water. If there is any chance that you might sail in freshwater, the ground plate should have at least 24 feet (7.3m) of sharp edge, usually accomplished by attaching a 12-foot (3.7m) length of 1-inch (2.5cm) copper strap fore and aft. Bronze bolts are preferred over stainless steel for bolting the plate to the hull and for cable attachment.” So if you don’t sail in freshwater and don’t mind having reduced lightning protection should you sneak up a river in your travels, you can get by with a significantly smaller underwater plate. (http://plasticclassicforum.com/forum/)
- Bonding is NOT grounding. To prevent galvanic corrosion due to dissimilar metals in contact with the water, they need to be connected together. Some experts believe this should not be done on woodboats. Two dissimilar metals in an electrolyte (water) form a battery. But by connecting them we neutralize this. Metal fittings are connected to bring all of them to exactly the same electrical potential,zero. Then no current flows between them and no corrosion occurs. The bonding wire is connected to the grounding point on the boat. But this is not part of the electrical system, and current should never flow in the bonding wire. If current is flowing in the bonding wire then something is wrong. Do not confuse this with grounding.S o grounding is important for safety. It protects people from shock hazard. It is important to make sure it is installed correctly. (http://newboatbuilders.com/docs/Grounding.pdf)
- From the point of view of safety it makes no difference whether or not the DC system is bonded to the hull. (http://www.smartgauge.co.uk/earthing.html)
- ..if a shore-power cord is properly and safely wired, including the ground connection back ashore and a connection from AC ground to DC ground, every time the shore-power cord is plugged in, there is a risk of corrosion of any underwater metal fittings connected to the DC ground – primarily the propeller and propeller shaft, and any bonded through-hulls. This is because the various ground connections lead from the under-water metal back ashore. When another boat plugs into shore power, its underwater metal will be similarly connected, and the two boats are effectively wired together through the AC ground wire. When different metals (the underwater hardware on the two boats) are put in an electrically conductive solution (in this case, seawater) and wired together (the ground wires), in effect, a giant battery is created. A small voltage and current results; this is of no concern. What is of concern is that the voltage and current are created by the dissolution of one of the metals. This is known as galvanic corrosion. Breaking the ground circuit back ashore by disconnecting the ground wire breaks the circuit that causes galvanic corrosion, but only at the expense of avoiding protection against potential electrocution for those onboard and in the surrounding water. However, the circuit can be safely broken by installing either an isolation transformer or a galvanic isolator in the ground wire. (A galvanic isolator is a device that blocks low-level galvanic – that is corrosion-producing – voltages and currents, but passes higher-level AC voltages and currents.) Galvanic vary significantly in quality. Whatever is used should have a continuous-current rating equal to the rating of the shore-power cord (e.g., 16, 30 or 50 amps) and should also incorporate a capacitor. An even more effective way to block galvanic corrosion is with an isolation transformer, but these are bulky, heavy and expensive, and therefore rarely fitted. However, for boats cruising overseas they can be set up to enable the boat to plug into different shoreside voltages. (Cruising Handbook, p. 190 – 191)
- If two shore-power inlets are fitted, two galvanic isolators or isolation transformers are needed. Remember that a ship-to-shore cable TV hookup or telephone connection almost always by passes and, therefore, voids the galvanic isolation provided by either of the devices. The TV and telephone need their own galvanic protection. (Cruising Handbook, p. 192)
- Corrosion brought on boat by the shore-power cord is not the only source of galvanic corrosion on boats. If the boat itself is immersed metal objects that are wired together or other wise in electrical contact (e.g., the physical contact of a bronze propeller with its stainless steel propeller shaft), any differences in the composition of the metals is likely to lead to galvanic corrosion. The best way to stop this corrosion is to keep the various metal masses electrically isolated. This is why, for example, it is generally a bad idea to bond together (i.e., connect with a wire) otherwise isolated bronze through-hulls. However, some metals cannot be isolated (e.g, the propeller and its shaft) and some are deliberately interconnected for other reasons, such as the prevention of stray current corrosion and protection against lightning strikes. (Cruising Handbook, p. 192)
- Stray current corrosion is relatively uncommon but can be quite devastating if it occurs. In worst-case scenarios, it can wipe out underwater hardware in days, even hours. It occurs when an immersed metal object becomes a conduit for DC current (AC does not cause corrosion), feeding the current into the water. An example is a through-hull against which a positive DC wire has been rubbing until the insulation breaks down, allowing battery current to flow through the fitting. Just as with AC currents, this current seeks a path back to its source – in this case, the battery. The most likely patch is through the water to the propeller, then up the propeller shaft, and through the engine bock to the negative battery cable attached to the engine. Any metal that feeds a current into the water (in this case the through-hull) will be corroded; the metal receiving the current – the propeller – will be ok. The higher the current, the greater is the rate of corrosion. (Cruising Handbook, p. 192 – 3)
- Galvanic Corrosion – Connecting the green wire to an underwater fitting completes the circuit between your boat and all other nearby boats with their own green wires grounded. With seawater as the electrolyte, every grounded fitting essentially becomes part of a big battery. If your fittings are less noble on the galvanic scale than your neighbors’…they are anodes and begin to erode. This can be bad news if you have an aluminum outdrive in the water and your neighbor’ underwater fittings are bronze and stainless. (Sailboat Electrics Simplified, p. 147)
- When the grounding wire connection is perfect, connecting the green wire to the DC ground lug still has the negative consequence of electrically connecting some or all of the underwater metal fittings on your boat to those on other boats with their own green wires grounded. With seawater as the electrolyte, every grounded fitting essentially becomes part of a big battery….A serious stray leak can fully consume underwater components in a matter of hours…Disconnected the green wire from the ground lug on the engine does avoid both galvanic and stray current corrosion caused by other boats, but it leaves you horribly exposed to the potential tragedy of electrocuting or drowning someone dear to you who’s swimming of your boat. Do not disconnect the green wire. The safer strategy for breaking the green-wire connection to other boats is to unplug. If you leave your boat plugged in mostly to power a battery charger, a small solar panel will be easier on your boat and better for your batteries. (This Old Boat, p. 309)
- If you must stay plugged in, install a galvanic isolator – essentially a pair of diodes in series connected in parallel to a second pair conducting in the opposite direction. The opposing pairs of diodes let the isolator pass current in both directions, allowing both AC and DC to flow freely…diodes need a little “push” to get them to conduct. In a silicon diode this consumes about 0.6V, so two in series will block all current flow unless the voltage exceeds 1.2 volts. All galvanic voltages between underwater metals are lower than this and so are most stray currents by the time they reach your boat through the water, so this device essentially disconnects the green wire for the currents that cause corrosion but maintains the connection for AC leakage. Since the isolator is inserted into the green wire, any failure opens the grounding circuit with hazardous consequences, so its diodes must be sufficiently hefty to carry short-circuit current -up to 3,000 amps in a 30 amp circuit – long enough for the circuit breaker to trip. A parallel capacitor that would still pass AC in the event of a diode failure is a desirable feature. (This Old Boat, p. 309)
- From the point of view of galvanic corrosion it makes no difference whether or not the DC system is bonded to the hull (but the hull must never be used as a return path in the manner of vehicle wiring). However, if the DC positive side is bonded to the hull this can have huge implications for galvanic corrosion. Remember, the most positive (voltage wise) point will be the point that erodes. The negative point will receive a plating from the most positive point. (http://www.smartgauge.co.uk/earthing.html)
Corrosion – Zincs
- Zincs and Protection from Galvanic Corrosion – Use zincs to protect against the galvanic currents that are set up by dissimilar metals on your boat that are immersed and that are in electric contact with one another. The best example is your bronze propeller on a stainless shaft. The best protection is to put a zinc right on the shaft next to the propeller, or a zinc on the propeller nut. An isolated bronze thru-hull doesn’t need protection because it is not in electrical contact with another immersed dissimilar metal. If electrically isolated, high quality marine bronze, is electrochemically stable in seawater; nothing good can come from connecting wires to it.Figure 1. Conductors running from the external keel or ground plate to the mast, stays and to the metal fuel tank will protect against a lighting strike, and there will be no DC connections to the engine or to the electrical system.Stainless steel is a special case. Generally, it is a bad idea to use stainless steel underwater, because it can pit. When it pits the “nobility” of the metal changes locally, and you end up with tiny galvanic couples that are made up of different parts of the same piece of metal and the pits grow deeper. One school of thought suggests that if you must use stainless steel underwater (e.g. you need its strength), then you should connect a nearby, immersed zinc to it; this protects the stainless steel from itself, reducing the rate of pitting. The electrochemistry of this assertion is compelling enough to recommend that you protect a stainless steel rudder shaft with a zinc. This may be done by mounting a zinc on the hull near the rudder shaft, and electrically connect it (inside the hull) to the stainless rudder shaft. For the reasons described above, ensure that your metal rudder shaft is not electrically connected to anything else. Your stainless steel propeller shaft will be protected from itself, by the same shaft zinc that protects the propeller from the stainless steel shaft. In both cases the pits, if they appear, will appear where the stainless steel is not exposed to the water. Trouble areas are in the cutlass bearing, inside the rudder bearing, and just inside the top of the rudder.Keep your metal keel/ballast electrically isolated from all other bits of metal. If you have the misfortune to have an external iron or steel keel, however, mount a zinc directly on it to reduce the rate of corrosion. Leave lead keels/ballast isolated. (http://www.sailmail.com/grounds.htm)
- For those objects that are bonded, protection can be provided against galvanic corrosion by also wiring an immersed sacrificial zinc anode into the circuit. The zinc is fastened to the outside of the boat and then wired inside the boat into the bonding circuit. Zinc is galvanically more active than any typical boat building metal; that is, it corrodes before any other metal. In doing so, it protects all the other metals wired in the same circuit. However, zinc does nothing to protect against stray current corrosion; the only protection is proper wiring of all onboard circuits (to eliminate sources of stray currents), with bonding as a backup. (Cruising Handbook, p. 193)
- Without sacrificial zincs the underwater metal parts of your boat are at risk from galvanic action. To protect against this destruction, every underwater metal part should have an electrically conductive contact with a zinc anode. The most direct way to achieve this is to bolt a zinc button directly to each of the underwater parts. Replace these zinc buttons and collars every year – a matter of removing the mounting screws, discarding the old zinc, fitting the new one, and re-tightening the screws. (This Old Boat, p. 171 – 2)
- A zinc on one of the pintle fittings protects the attached gudgeon fitting because the metal-to-metal contact is electrically conductive, but does not protect the other pintle fittings. Each fitting requires a zinc. Too much zinc will bubble bottom paint, you need more or larger zincs only if the existing anodes are depleted by more than 50% a year. Never paint a zinc anode or you will prevent it from doing it’s job. (This Old Boat, p. 172)
- The shaft and the propeller are both protected by the installation and renewal of a zinc collar clamps around the shaft. If the shaft bolts directly to the output flange on the transmission, your prop is electrically connected to the shaft, the engine and every other metal item on the boat that is grounded to the engine. A bronze prop is the least noble of the submerged parts of this chain and can be ravaged by corrosion unless protected by a less noble zinc. However, when the shaft is isolated with a flexible coupling, the only electrical interaction is between the shaft and prop. The corrosion potential depends on the type of alloys used for the shaft and the prop and on their relative submerged surface areas. (This Old Boat, p. 172)
- Wiring systems on boats are two-wire systems, with the hot or positive wire carrying power to the lights and appliances, and a separate, insulated wire returning to ground, generally the negative bus bar in the distribution panel, which in turn is connected to the common ground such as the engine block. If the boat is bonded, as described earlier, the bonding strip is not to be used as ground. (Upgrading the Cruising Sailboat, p. 275)
- The electrical items on a marine engine almost always use a ground-return circuit. That means the engine, rather than an insulated wire back to the battery, serves as the ground side of the circuit. Because components are electrically connected to the ground circuit by their mounting bolts, you must consider these mounts when you suspect a poor connection. A buildup of rust between mating surfaces can open the circuit as effectively as a disconnected battery cable. And speaking of battery cables, the ground return circuit is typically completed through a cable connecting the engine block to the negative battery terminal. (Sailboat Electrics Simplified, p. 79)
- Negative Ground – The negative post of all batteries should be connected to “ground” to hold it at 0 potential – relative to Earth. This is normally accomplished with a cable between the negative battery post and a bolt on the engine block, which connects the battery to earth through the propeller shaft. We refer to the bolt on the engine as the “grounding lug.” Multiple connections to ground should be made to a bus connected to the grounding lug. this is the main grounding bus. (Sailboat Electrics Simplified, p. 38)
Grounding – Alternating Current (AC)
- Should any of the various metal cases enclosing your AC system become energized, the green wire provides a low-resistance path to the ground. but what if the leak is into the DC wiring, caused, for example, by a crossed wire or a short in a dual-voltage appliance (charger, inverter, dual voltage light fixtures)? Any AC that leaks into the DC system will seek ground, meaning it will automatically travel through the wiring to the ground connection on the engine and down the prop shaft into the water. (If you have a flexible shaft coupling, you need a shaft brush to make electrical contact with the shaft.) This is essentially the same as dropping a hot wire into the water. In freshwater, this poses a real risk of electrocution for anyone in the water nearby. The better conductivity of salt water (which tends to pass the current straight down to the ground) reduces the risk of electrocution, but the current field can be (and has been) enough to paralyze muscles and cause a swimmer to drown. (Sailboat Electrics Simplified, p. 146)
- Remember that the GFCI only senses a short to ground. If you get across the hot wire and the neutral wires of a circuit, a GFCI won’t protect you. (Sailboat Electrics Simplified, p. 146)
- To reduce the risk to swimmers, the green wire should be connected to the ground terminal on the engine. This gives AC leakage into the DC system a safe path to ground. But you must be aware that grounding the green wire is not without problems. Should the green wire have a fault somewhere between the power inlet and the buried grounding plate, ground–fault current will flow into the water – just what we are trying to prevent. So it is essential to test the ground at the dock, and to keep cords and plugs in good condition. Protecting all AC circuits with GFCI’s virtually eliminates this risk. (Sailboat Electrics Simplified, p. 147)
- …neutral and grounding wires are connected together only at the power source, never on the boat. In the case of shore-power, the power source is the breaker panel servicing the dockside outlet. (Sailboat Electrics Simplified, p. 155)
- …there is an essential difference for boat wiring….In your breaker box at home the neutral wires and grounding wires all connect to the same terminal strip or bus bar. On a boat the neutral (white) conductor and the grounding (green) conductor must never be directly connected. Instead, the AC grounding wire should connect to the DC ground, usually at the ground lug on the engine…with the grounding wire so connected, if you connect the neutral wire to the grounding wire on a boat, underwater hardware becomes a current carrying path to ground. For anyone in the water nearby, this makes your boat the equivalent of a giant bug zapper. Keep the white and green wires separated. (This Old Boat, p. 304)
- …[grounding] provides a low resistance path to ground should any AC wiring come in contact with various metal housing. But what if the metal enclosure also contains DC wiring, as it does in a battery charger, an inverter, and a dual voltage light fixture, or what if the AC and DC wiring are just in close proximity and the unexpected contact with the DC wiring? AC that leaks into the DC system will seek ground, meaning it will travel through the DC wiring to the ground connection on the engine and down the prop shaft to the water. This is essentially the same as dropped a hot wire into the water. In fresh water this poses a real and immediate danger of electrocution for anyone in the water nearby….This danger is the reason we connect the green wire directly to the grounding terminal on the engine. The green wire provides AC leakage seeking ground through the DC wiring a lower-resistance path than the engine and prop shaft, eliminating risk to swimmers – but only if the grounding wire connection is sound. If there is a fault in the grounding wire anywhere between the boat and the buried ground plate – suppose, for example, that the grounding wire connection inside the dock outlet has corroded open – the prop shaft again becomes a path to ground with its attendant risk to swimmers, but now not just from AC to DC leakage but from all ground fault current. GFCIs eliminate this risk for circuits they protect, but something as seemingly innocuous as corrosion on the ground prong of your dock cord can make circuits unprotected by a GFCI lethal. It is essential to test the ground connection at the dock and to maintain cords and plugs in good condition. (This Old Boat, p. 308 – 9)
- See Practical Sailor August 15, 1995 for a detailed treatment of the green wire. The best solution is a heavy and expensive isolation transformer. The acceptable solution (for the rest of us) is to install a light and inexpensive Galvanic Isolator in the green wire, between the shorepower cord socket on your boat, and the connection to the boat’s AC panel. Then, connect the grounding conductor (green) of the AC panel directly to the engine negative terminal or its bus. Note that this meets ABYC’s recommendations. In choosing Galvanic Isolators, make sure that you select one that has a continuous current rating that is at least 135% the current rating on the circuit breaker on your dock box. Certain Galvanic Isolators (e.g. Quicksilver) include large capacitors in parallel with the isolation diodes, which in certain situations theoretically provide better galvanic protection. Unfortunately, these units cost substantially more than conventional Galvanic Isolators. If you feel like spending real money on galvanic isolation, you might as well do it right and buy an isolation transformer. It is also a good idea to use a Ground Fault Interrupter (GFI) in your AC wiring. GFI’s will occasionally “nuisance trip” due to the humidity surrounding the wiring on boats, but the additional safety that they offer (particularly to nearby swimmers) in disconnecting power in the presence of ground currents is worth the nuisance. If your GFI starts to nuisance trip, it is probably a very good idea to track down and clean up your damp wiring in any event. (http://www.sailmail.com/grounds.htm)
- ….security against AC shorts is provided by using a ground fault circuit interrupter (GFCI), also known as a residual current circuit breaker (RCC). This is a special kind of breaker that is tripped by very small leaks to ground. Ideally, all AC outlets on a boat will be GFCI-protected; at the very least, outlets in the galley and head should be protected In the US, the trip limit is set to 5 mA (milliamps) (Cruising Handbook, p. 190)
- Faults in an AC power system on board can be lethal, and for this reason, ground fault circuits (GFCI) have become quite popular…GFCI’s that are incorporated into the circuit breakers and trip open with a leak as little as five milliamps. (Upgrading the Cruising Sailboat, p. 280)
- When a boat is plugged into shore power, the power source is the dock. The nature of electricity is such that “leaks” attempt to return to their source. This means that if for any reason AC equipment on the boat develops a dangerous electrical fault, the fault current will be looking for a conductive path back to the dock. Many electrical appliances have not just two wires (the “hot” and “neutral”, but also a third (the “ground” or “earth”) that is connected to the case to the appliance. In typical circumstances, the ground wire does nothing, but if the hot wire shorts to the appliance case (making the equipment potentially dangerous to touch); the ground wire will safely conduct the fault current back to shore. If the ground wire does not have a low resistance path to shore, any short to an appliance case will seek another path, generally from the boat to the water and then through the water. Anybody on the boat or swimming in the water can become part of the path, running the risk of being electrocuted. To prevent this, any shore power connection must have a low-resistance connection to the shore-side ground. Ideally, the AC distribution panel has a ground-continuity test light that enables the connection to be checked. If it does not, every tie the boat is plugged into a new shore-power outlet, boatowners are well advised to have it checked or learn to check it themselves. (Cruising Handbook, p. 190)
- Any time a shore-power connection has reverse polarity or lacks ground continuity, it should be disconnected until the problem is resolved (which may mean rewiring the marina). (Cruising Handbook, p. 192)
- Ground Fault Interrupt (GFCI)– Regular circuit breakers are essentially fire-protection devices and offer no protection against electrical shock. A ground-fault circuit interrupter (GFCI), on the other hand, provides a high degree of shock protection. If you accidentally touch an energized wire or component and you are grounded, the GFCI disconnects the circuit in about 1/40 of a second, too little time for the current to build to a dangerous level. To minimize nuisance tripping, it is better to protect each circuit individually. Replacing the first outlet (counting from the distribution panel) with a GFCI outlet protects the rest of the downstream outlets in the circuit. Be sure the wires marked LINE are connected to the power source and those marked LOAD feed the remainder of the circuit [See image from same location as source] (Sailboat Electrics Simplified, p. 145)
- …the grounding wire, and somewhere back upstream from your boat, it too is connected to a metal rod in the ground. At the other end the grounding wire connects to the metal housing that contain the wiring and the appliances. In the case of plug-in equipment, the grounding wire connection is made via the third socket – the one that in a 15 amp receptacle looks like a tunnel entrance. In a 30 amp receptacle it is the socket with the crook. The purpose of this wire is to always provide a path to ground with a lower resistance than through you. (This Old Boat, p. 304)
Grounding – Direct Current (DC)
- Every light or appliance should be wired with its own DC return wire. Never use the mast, engine, or other metal object as part of the return circuit. The DC load returns of all branch circuits should be tied to the negative bus of the DC distribution panel. In turn, the negative bus of the DC distribution panel should be connected to the engine negative terminal or its bus. The battery negative is also connected to the engine negative terminal or its bus. The key factor here is that the yacht’s electrical system is connected to seawater ground at one point only, via the engine negative terminal or its bus. See figure one. (http://www.sailmail.com/grounds.htm)
- There are also times when AC appliances short into DC circuits. At some point, a boat’s DC circuits almost always have a connection to the water, through either the engine block and propeller shaft, or a ground plate, or a through hull – or all three. If the DC system is not wired back to the AC ground, AC leakage current is likely to be fed into the water, with potential lethal results. For this reason, on boats with an AC shore-power connection there should always be a connection between the DC and AC ground (the exception, once again, being a boat with an isolation transformer). The connection allows AC leakage currents into DC circuits to find their way back to the AC ground, and, from there, down the shore-power cord back to where they belong. This AC-to-DC ground connection should also be checked as a matter of basic safety. (Cruising Handbook, p. 190)
Grounding – Lightning Protection
- when electricity seeks ground due to the imbalance of electrical charges between atmosphere and the ground. (All in the Same Boat, p. 111)
- The bottom line: you can’t really predict which boat lightning will strike….We are concerned with what happens if lightning does strike. And when it does, an inconceivably huge amount of energy not only hits your boat, it also has to find a way out. It doesn’t dissipate; it seeks ground. If it doesn’t have a good way out, it’s going to find a way out anyway. People inside boats have been electrocuted as the energy ricocheted, seeking ground…the energy jumped from one chainplate to the other, killing both people. (All in the Same Boat, p. 111)
- Two basic theories have been advanced as to how boaters should deal with the problem: (All in the Same Boat, p. 111)
- Avoid all ground connections so that the atmospheric change won’t seek ground through the boat.
- Thoroughly ground the boat so that the lightning will have a quick and easy – maybe even painless – way out….we think the [first] is seriously flawed.
- Grounding is not limited to running little green wires from through-hull to through-hull to engine, although you should do this (use heavy-gauge wires insulated in green cover) to prevent one underwater metal fitting from destroying another through electrolysis. I believe the wires also help dissipate energy when lightning strikes. We attach heavy batter cable wires to our stays, and trail them in the water whenever we suspect a storm. A cable hangs from each side mainstay, and one from the backstay. At the end of the cable is a plate (copper is probably best) that encourages quick dissipation of energy. We connect the acables to the stays by unsheathing the cables about 1 ½” back from the end and clamping the bare wire to the stays with hose clamps. (All in the Same Boat, p. 112)
- Naval architect John Letcher told me that a grounded mast in fact increases the odds of your boat sustaining a lightnining strike. Lightning doesn’t actually “strike” the way it appears; rather a path from the atmosphere to the ground must be present for the charge to pass through. Of course, an ungrounded mast also can be hit, and if it is, the damage will be much greater. In any case, it’s a sticky-wicket – ground the mast and increase the odds of being struck, or go ungrounded and pray you’re never struck. As terrifying as it was to get struck by lightning, I’d opt for the grounded system. (Upgrading the Cruising Sailboat, p. 274)
- if your boat is not grounded and you wish it to be, run #8 AWG wire from each shroud of aluminum mast to a plate in the hull…of at least one square foot. Tom Colvin recommends grounding only the mast, not the shrouds. The spar is more vertical and there’s presumable less danger to the chainplate if there’s a poor wiring connection. But on deck-stepped spars without compression posts or bulkheads directly underneath there’s no way to run the ground wire straight down to the ground plate. Propellers, large metal rudders or radio transmitter ground plates can also be used for grounds. However, I don’t like using the prop shaft or any other object that requires the lightning to make an abrupt turn – the system must be as vertical as possible. Adriana has a plate in the bilge, to which the wires are connected. For this reason, do not use the engine as ground, as the only electrical path to Earth is horizontally via the propeller shaft. With wood and fiberglass hulls, the lightning ground system can be connected to any bonding system that exists to prevent surges of voltage through metal and electrical objects inside the boat. (Upgrading the Cruising Sailboat, p. 275)
- For 1959 Lightning ABYC protection, please see Upgrading the Cruising Sailboat, p. 325
- Bonding is not done just for protection against stray-current corrosion but also for protection against lightning strikes. In the case of lightning, it is not just immersed metal, but also all major metal objects that should be wired together. If a boat gets hit by lightning, any metal mass can get charged up with a very high voltage. If the metal objects are bonded together, the bonding cable holds everything at a common voltage and provides a path for the strike to run to ground through an attached ground plate (an external keel or some other immersed metal). Without bonding, there may be a massive voltage differences between metal objects, with the potential for side flashes to leap from one surface to another. (Cruising Handbook, p. 193)
- To ground a direct lightning strike, the bonding system needs cables of a certain size (4 gauge for the primary grounding cable from the masthead to the ground plate, and 6 gauge for peripheral cables to chainplates, metal water tanks, the engine, and so on; an aluminum mast is an adequate conductor, substituting for the primary grounding cable in this part of the circuit) and a ground plate with an equivalent area (electrically speaking) of at least 1 square foot of copper. Such a ground plane can be achieved by attaching a copper plate to the bottom for the boat, but it is more commonly established by making a connection to a keel bolt on an external keel. Where there is no external keel, reliance is often on the combined surface area of an accumulation of bonded objects (e.g., through-hulls and the engine, which is connected to the propeller and its shaft) rather than installing a proper ground plate. The problem with this approach is that these are often resistive paths to ground (e.g., the oil in an engine obstructs the path to ground through the crankshaft to the propeller shaft). Resistance means heat; the more current, the great the heat. In direct lighting strikes, there are many instances of bearing melting down in engines, and through-hulls melting out of fiberglass hulls. With internal ballast, an external metal plate almost has to be attached to the hull and wired to the bonding system to achieve an effective ground. (Cruising Handbook, p. 193 – 4)
- If your keel is encapsulated, a copper ground plate is needed. Lightning dissipates from the edge of the plate, so the perimeter of the plate should be at least 1.2m (4′) if you are in salt water. If there is any chance that you might sail in freshwater, the ground plate should have at least 7.3 m (24′) of sharp edge, usually accomplished by attaching a 3.7 m (12′) length of 2.5 cm (1″) copper strap fore and aft. Bronze bolts are preferred over stainless steel for bolting the plate to the hull and for the cable attachment…Do not route the strike through the engine. The propeller might have enough edge length to do the job in salt water, but passing such high voltage through the engine can damage the bearings. Likewise, never ground the mast to a sea cock. The lack of adequate conductivity is almost certain to generate enough heat to melt the fitting right out of the hull. (Sailboat Electrics Simplified, p. 162 – 3)
- Sintered bronze plates designed for grounding radios are a poor choice for conducting lightning to ground. They are less effective than solid copper at dissipating the charge of a strike and reportedly they tend to explode when heat from the strike turns trapped water to steam. (Sailboat Electrics Simplified, p. 162)
- According to a Florida Sea Grant study, only one boat in ten struck by lightning in salt water suffered damage (excluding electronics) when the mast was grounded. In freshwater, six in ten suffered some kind of hull damage. Given the poorer conductivity of freshwater, this disparity is almost certainly due to inadequate grounding. To improve the odds, fresh-water sailors should always choose a grounding plate to maximize the edge length. A 12 ft. x 1 inch plate is six times as effective at dissipating a strike as a square plate with the same area (1 sq. ft). (Sailboat Electrics Simplified, p. 163)
- Connect the mast to the underwater ground with a #4 AWG or larger cable. Because lightning travels on the surface of the conductor, solid copper strap is an even better choice. I like 1/2″ copper tubing (water pipe), first radiused then flattened. Lightning doesn’t like to change direction, so conductors should lead as sraight as possible to the ground. If a turn is required, give it a radius of 12 inches (30 cm) or more…The electrical conncetions must be perfect. The current flow in a lightning strike ranges from around 20,000 – 400,000 amps, so a 1-ohm resistance can cause a 400,00 volt-difference (I x R) from one side of the connection to the other. The result is enough heat to vaporize metal, and the resistance may encourage dangerous side flashes. Drill attachment holes in the ends of the copper strap. Cable connections should be made with mechanically attached terminals – solder will melt. Be sure all connections are clean and tight, and use copper washers to increase the contact area- except use stainless steel washers on the mast connection to minimize corrosion. Coat the assembled connection with an anti-corrosion spray, and disassemble and clean it as lteast once a year to make sure it stays resistance free. Periodically check the resistance from the mast to the ground plate with your ohmmeter. (Sailboat Electrics Simplified, p. 163 – 4)
- Although an aluminum mast offers lower resistance than steel stays, a powerful strike may nevertheless induce current flow in stays and shrouds. To provide this current a safe path to ground, the chainplates should also be connected to the underwater ground here again, the route should be as direct as possible with only large radius changes of direction. You can use #6 AWG for these secondary grounding paths….when the mat has a potential of 30,000 volts and other metal components inside the boat are essentially at 0 volts, there is some risk of the lightning jumping to the lower potential. This is called a side flash and it is extremely dangerous to anyone in its path. To minimize this risk, give the charge a lower-resistance path by connecting all significant metal masses (e.g., tanks, stove, lifelines) within 1.8 m (6′) of the mast or rigging to the ground plate. Use #6 AWG wire (or larger), and connect each component to the ground with a dedicated wire. (Sailboat Electrics Simplified, p. 164 – 5)
- What follows is based on the recommendations for lightning protection provided by the American Boat & Yacht Council, Standard E4.The primary purpose of a lightning protection system is to provide for the physical safety of all aboard your vessel. Prudent actions that should be taken during an electrical storm are:
- If at all possible remain in the cabin of a closed boat.
- No one should be in the water or have any part of their body immersed in the water.
- Do not come into contact with any components connected to the lightning protection system of a properly protected vessel. Otherwise your body could act as a conductive bridge between any items connected to the lightning conductive system. For example, you should not be in simultaneous contact with a metal steering wheel and a metal stern pulpit.
- A good lightning protective system ensures that all large masses of metal are electrically connected. This purpose should not be confused with that of the vessel’s basic bonding system. A properly installed and isolated bonding system is there to provide a low resistance electrical path to reduce electrolytic corrosion and as a measure of personal protection if there is an electrical fault in the boat’s AC/DC electrical systems.
If your sailboat is a vessel with an aluminum mast you have the starting point of a well-grounded lightning rod. This will provide a zone of protection for a radius around its base equal to the height of the lightning rod. Due to some vessels overall length, it may be necessary to install another lightning rod to encompass any areas that do not fall within the zone of protection. Don’t forget that the mast itself must be physically bonded or connected through to the common ground – one of the keel bolts or if a encapsulated keel, to the grounding plate, in order to provide optimum protection.The apex of the rod should be a minimum of six inches above any masthead device. The end should be sharpened to a point. The base of the mast or the mast step if metal, should be connected to a keel bolt on externally ballasted vessels. The preferred wire gauge is No. 6 or even better, #4AWG stranded copper. In no case should such a connection be made to a vessel with internal ballast. The result could be a hole blown through the bottom of the hull. Boats with internal ballast should have a copper ground plate of at least one square foot in size installed externally on the hull bottom. The grounding wire should then be connected to the ground plate.
All wire conductors should be kept as straight as possible. All large metal objects above and below decks should also be electrically tied into the lightning ground conductor. This is a precaution against side flashes. Large metal objects include shrouds, chainplates, toe rails, sail tracks, winches, steering wheels, and bow and stern pulpits. These items can be tied into the ground conductor wire by a minimum #8AWG stranded copper gauge wire, or connected directly to the hull ground terminus. A thorough inspection of the lightning protection system should be conducted on an annual basis as part of normal maintenance procedure. All connections should be maintained tight and corrosion free. Any corrosion will impede the flow of electricity and promote side flashes. For that reason it is important that the lightning protection system receive the same attention as the rest of the systems aboard the vessel. This should be included as a part of the annual lay-up and maintenance procedure. For additional details regarding the lightning protection standards readers should refer to American Boat and Yacht Standard E-4Source: ABYC Recommendations on Lighting Protection in The Marine Advisor, Spring 2001 (http://www.kp44.org/LightningProtectionABYC_Standards.php)
- The lightning ground needs to be a direct DC connection to the keel or to a ground plate to handle currents due to lightning strikes. So how do we keep the keel or ground plate electrically isolated as required …? The solution is to connect the keel or ground plate directly to the mast, but make sure the mast is not electrically connected to the boats DC ground system. If your steaming light, masthead light, tricolor, Windex light etc. are wired carefully and correctly, they each will have their own DC return wire; there should be no ground connection between their wiring and the mast itself. Make sure that this is the case. This should also be true of your masthead instruments. The unintended DC connection between mast and DC ground is typically made by the masthead VHF whip, which connects the shield of the coax to the bracket connected to the mast. That shield also connects to the VHF radio which is DC grounded by its power connection. The easiest solution is to insert what is called a “inner-outer DC block” into the coax. This RF device puts a capacitor in series with the center conductor, and another capacitor in series with the shield. This device is transparent to the VHF RF signals in the center conductor and shield, but blocks any DC current in either the center conductor or shield. This device can be made by a good radio technician, or purchased from radio supply houses, pre-fitted with any kind of coax connection on both ends. The commercial units look like a coax “barrel” connector. A vendor is listed at the end of the article.Once the DC connection from the mast to the VHF is broken, check for any other connections with an ohmmeter, and straighten out any other wiring errors or unintended connections. If your metal fuel tank is also bonded to the lightning ground system (per ABYC) then make sure that it does not have DC connections either to the engine via the fuel line or to the electrical system via the fuel level sensor. A piece of approved rubber fuel hose in the fuel lines to the engine solves that connection, and a well designed fuel level sensor will not make electrical contact with the tank. When you’re done, there will be heavy conductors running from the external keel or lightning ground plate to the mast, stays, and to the metal fuel tank, but there will be no DC connections to the engine or to the yacht’s electrical system. See figure 3. (http://www.sailmail.com/grounds.htm)
- Electronics Protection – A surge protector in the supply line may provide protection for a limited ranged of lightning-induced power spikes. Twist all electronics power leads so induced currents will tned to cancel. All bonding wires should cross electrical wiring at 90 degrees to minimize the inductive effect of current flowing to ground. Grounding the chassis – the metal housing – protects internal circuits and components from directly induced currents. But despite every protective effort, if your boat is struck, your electronics have only one chance in two of not becoming toast. (Sailboat Electrics Simplified, p. 171)
- …disconnect the antennae and lines running into our SSB, its tuner, radar and similar equipment.…usually keep the VHF plugged in because we may need it, but we have a spare ready to go, with a spare emergency antenna. (All in the Same Boat, p. 113)
- We also have the helmsman wear heavy-duty rubber-coated work soled shoes and remain away from the strays. The area within the strays is relatively protected, theoretically The stays form a shield of sorts, alowing energy, hopefully, to pass down them to the water and away from the space between. But this doesn’t work if you are too close to the strays and they aren’t grounded well. (All in the Same Boat, p. 113)
- If you have a steel boat, you are completely grounded and probably as safe as you can be on a boat. (All in the Same Boat, p. 113)
- Our crew was saved from injury for two reasons: We were not touching the rigging or any part of the ground system or metal objects inside, and because, in the words of the ABYC Standard E-4, “A grounded conductor, or lightning protective mast, will generall deflect it to itself direct hits which might otherwise fall within a cone-shaped space, the apex of which is the top of the conductor or lightning protective mast and the base is a circle at the surface of the water having a radius of approximately two times the height of the conductor”. The probability of protection, ABYC reports, is 99%. (Upgrading the Cruising Sailboat, p. 274)
- …no one should go on deck unless necessary. If on deck, do not hold onto any metal objects (e.g., the wheel or rigging) and be especially careful not to act as a potential conductive patch between two metal objects (e.g. by hanging onto the wheel and the backstay at the given time, or a shroud and the mast.) (Cruising Handbook, p. 194)
Grounding – Radio
- Ground connections for high-frequency radios – SSB and ham – should be made with copper foil ribbon rather than wire. Radio frequency (RF) current travels on the surface of the conductor, so the more surface, the better the ground connection. That directly translates into longer-range, clearer radio transmission. 3″ wide copper foil is a good choice for RF grounding. The thickness of the foil is not important other than for durability. (Sailboat Electrics Simplified, p. 66)
- Some electronics, notably high-frequency radios – ham and SSB – require special grounding systems. A ground plane up to 100 square feet of copper or bronze screening located somewhere in the boat may be recommended. At the very least the equipment will have to be grounded to the engine with a heavy copper strap. Hull-mounted porous bronze groundplates marketed for this purpose can prove inadequate, particularly for ham and SSB transmissions. (This Old Boat, p. 301)
- The ground plane is the base from which your signal is launched, and for good transmitter performance, it cannot be too large. Because radio frequency (RF) current travels on the surface of the conductor, the ground for your high frequency radio needs to be made with 3″ wide copper foil ribbon rather than wire. (This Old Boat, p. 301)
- The usual instructions for installing ground plates recommend bonding the RF ground connection to metal tanks, the engine, lifelines, the mast and associated steel rigging (other than the insulated backstay) etc. (http://newsgroups.derkeiler.com/Archive/Rec/rec.radio.amateur.antenna/2006-05/msg01075.html)
- The thing to be careful of with ANY RF ground advice you get, no matter who you get it from, is to be skeptical and be ready to test carefully to be sure that it really works ON YOUR BOAT. There are some accepted principles that are the basis for all successful installations: (http://www.sgcworld.com/sailboatgroundtechnote.html)
- Get as much metal into your RF ground as you can. On some boats the engine, keel, thru-hulls, and even copper plates are connected together into the RF ground.
- Keep ground straps as short as possible. Connecting to your RF ground can be tricky. Often people will use a Volt-Ohmmeter to check their ground straps and declare them good because there is little or no resistance. However, the ground strap is not for DC current. An RF ground is carrying RF energy and a DC resistance to ground will not show if there is an impedance to ground at RF frequencies.
- Be aware that RF conductivity is not the same as DC conductivity. Don’t confuse your safety ground (equipment chassis, reefer, etc) with your RF ground. The RF ground is required for the ANTENNA and is an RF circuit. Your safety ground on the DC circuits is NOT intended to handle RF. While many boats connect these together successfully, it can cause interference. RF energy carried through the DC ground may get into instrumentation or other equipment. It is normally best to have the RF and DC grounds be separate.
- Dynaplates and other external devices meant to connect your RF ground to seawater can be very effective, but they will only be so if you maintain them properly. If you connect your RF ground to Dynaplates, thru-hulls, and other fittings, then you must inspect them regularly and CLEAN them regularly. Dynaplates should not be left more than 3 months without inspection and cleaning.
- Inspect your connections regularly. A salt water environment is hard on any sort of electrical connections. Your RF ground and your antenna need to be inspected regularly because the Smartuner will hide slow changes in your antenna or ground system until it can no longer compensate for them. You may operate for a long time as your fittings corrode and then find that you can’t operate at all. It will seem sudden, but the problem grows gradually.
- A useful test of the quality of your ground is to lay out several long wires on deck connected to the RF ground connection on your Smartuner. You might also throw a wire over the side to connect to seawater as well. When you remove these temporary wires, reconnect to your boat’s grounding system. The signal should get better. If it gets worse, your RF grounding system needs work.
- Bonding a lot of metal in your boat together with short, direct copper straps can create a very suitable grounding system. The engine, fuel and water tanks, the keel, and any other piece of metal of significant size can be bonded together effectively. Copper foil or wire is usually best here.
- Some boat owners install a large area of copper foil on the inside surface or their fiberglass hull and use this as an RF ground. It capacitively couples to the seawater and makes a generally excellent grounding system.
- A leaded keel also makes an excellent RF ground. Depending on the construction of the hull, you may or may not be able to make a good connection to the keel bolts.
- Overall, there are probably as many different ways to create a good RF ground as there are people giving advice about them. What works in one boat may or may not work well in another. Be prepared to adjust your RF grounding as you test it and remember that it will degrade over time, so you also need to be ready to maintain it.
- Mount your automatic tuner as close to the backstay as possible, preferably just under the after deck. Run copper ground tape from the tuner to the stern pulpit/lifelines, to the engine, and to a keel bolt. It is good practice to include the HF/SSB radio itself in this network of ground tapes. If the builder of your yacht had the foresight to bond into the hull a length of copper tape or an area of copper mesh, be sure to run a copper ground tape to this as well, and say a blessing for builders such as these. Sintered bronze ground plates (e.g. Dynaplates) can be used as radio grounds in situations where the ballast or engine is unavailable or awkward to connect. If the ballast, engine, and lifelines are available, however, they generally make a high performance ground. (http://www.sailmail.com/grounds.htm)
- The RF ground needs to be a ground for RF signals only. It does not need to conduct DC…you do not want to connect another DC ground to your engine and to your keel etc.The solution is to find a dry secure place along each of the copper RF ground tapes that are running to your engine and keel. Fasten the tape securely to an insulating piece of phenolic or to a terminal strip, cut a 1/10-inch gap across the tape, and solder several 0.15uF ceramic capacitors across the gap. These capacitors will be transparent to the RF, which will be happily grounded by the ground tape system, but they will block any DC currents from running through the RF ground system, and will avoid any resulting susceptibility to hot marina electrolytic corrosion. It is worth selecting the capacitors carefully, because they may carry a significant amount of RF current. An acceptable choice of capacitors and vendor are listed at the end of this article….To avoid making another DC ground to the engine via the HF/SSB radio copper ground strip, fasten the copper tape securely to an insulating piece of phenolic or to a terminal strip, cut a 1/10″ gap across the tape, and solder several 0.15 uF ceramic capacitors across the gap. (http://www.sailmail.com/grounds.htm)
- SB and ham radios need a good ground to provide the necessary counterpoise for transmission – like planting your feet to jump or throw. A good radio ground is a large mess of metal very close to but not necessarily touching sea water. This is usually accommodated by grounding the radio to the engine, other large metal components, and to the water through a ground plate. The lightning ground plate services well. (Sailboat Electrics Simplified, p. 169)
- Ribbon, Not Wire – Radio grounds should be made with copper foil ribbon, not wire, because the current we want the conductor to carry is RF (radio frequency), not DC. RF current travels on the surface of the conductor (lightning is also an RF event), so the more surface, the less the conductor impedes the RF current. This essentially means less of your radio’s power is wasted in the ground system, so more is radiated from your antenna. That translates into longer range and clearer signals. Use 3-inch-wide (7.6cm) foil ribbon for the best RF ground connection. Fold the ends into a point for terminal attachment. Bend the foil around corners and obstacles as shown. (Sailboat Electrics Simplified, p. 169)
- Copper Screen – Today’s automatic antenna tuners compensate for a less-than-perfect counterpoise, so few boats bother with “building” a counterpoise relying instead on the metal masses already in the boat combined with a good “connection” to the ocean. Still, for the best radio installation, about 100 sq. ft. (9.3 m^2) of copper screening inside the hull can avoid a lot of transmission problems. Hardware stores sell copper screening for windows inexpensively, and it is easy to install in below-the-waterline lockers, covered with a layer of lightweight fiberglass cloth. However, because the wire in the screening is just woven together, corrosion may eventually degrade their electrical contact. Soldering two edges before installation avoids this problem. Join the various screen panels with 3″ foil tape, also soldered. (Sailboat Electrics Simplified, p. 169)
- Stopping Direct Current – Not only does the RF ground system not need to carry DC but we don’t want it to because that allows the flow of destructive stray current. This is easily prevented by cutting the foil ribbon leading to the ground plate and installing the ends of a double bus circuit block, leaving a gap of about 1/16″. Now bridge the gap with a 0.15uf ceramic capacitor…you can solder the capacitor to the ribbon, or if the leads are long enough, simply capture them under an opposing pair of terminal screws The capacitor passes RF current but blocks DC….If other metal components bonded for lightning protection are also connected to the RF ground system, disconnect RF connections from the components to the radio and make them to the ground plate. Configured this way, a single capacitor will separate the DC ground from the RF ground. If your foil is thick enough – at least 9 mills (0.009 inches) – you can substitute the ribbon for the wire; otherwise connect the ribbon parallel to the #6 AW bonding wire.
- By convention we think of electrical current as flowing from positive to negative, but the flow of electrons is actually from the negative or ground terminal to the battery’s positive terminal. Either way, reversing connections to a circuit reverses the direction of the current flow. While lighting and heating appliances generally operate the same when polarity is reversed, most 12V motors run backwards, and electronics will, at best simply fail to operate and may be damaged or destroyed. For 12V components to operate as designed, the lead marked with a “+” must always be connected to the positive side of the circuit. (Sailboat Electrics Simplified, p. 12)
- Most DC appliances will have one wire labeled with a plus sign (+) and perhaps the other with a minus sign. The plus wire is often – but sadly not always – red. It is essential that the positive wire from the appliance ultimately connect to the positive post of the battery….if you inadvertently reverse the battery connections, you reverse the polarity to every electrical item on the boat…will damage alternator and/or regulator, and any electronics you turn on while the polarity is reversed will also suffer damage. (This Old Boat, p. 265)
- If you are installing a new 12-Volt DC system from scratch…be sure it is installed with a negative polarity grounded, which is now accepted practice and recommended by ABYC. Positive-ground electronic gear is difficult to come by. (Upgrading the Cruising Sailboat, p. 276)
- Double pole breakers, which open both sides of the circuit when tripped, fully protect the circuit even if polarity is reversed. Perhaps you will never forget to check polarity before plugging in, but I’m not that confident. (Sailboat Electrics Simplified, p. 143)
- Your AC system must have a hard-wired reverse polarity indicator if the circuits are protected with single pole breakers, but if you follow the more prudent course of using only double-pole breakers, do not omit this exra safeguard. Reversed polarity still puts ON-OFF switches on the wrong side of the circuit, leaving OFF appliances fully energized. You must know about reversed polarity, and you must correct it. A light or buzzer connected across the white and green wires will not operate as long as the white wire is at 0 potential, but reversing the polarity puts the white wire at 120V, illuminating the light or sounding the buzzer. However, since the white and green wires on a boat must never be directly connected, a momentary switch is a required part of the circuit. (Sailboat Electrics Simplified, p. 144)
- An alternative method of maintaining the required separation between white and green is to use a high-resistance indicator like a neon lamp. The circuit shown allows the indicators to operate without manual intervention, and with a resistance exceeding, 25,000 ohms, the potential current is less than 5 mA, too little to cause any mischief. This type of polarity indicator is required if your AC circuits are protected by single-pole breakers. (Sailboat Electrics Simplified, p. 144)
- We never think about polarity ashore, but its importance in the wet environment cannot be overemphasized. Not only must you verify the polarity of every dock connection, but if the polarity of any outlet is reversed, the shock risk of any appliance plugged into that outlet increases. Check all outlets for correct polarity, even those you didn’t wire. Check polarity with your voltmeter by measuring the voltage between the receptacle’s long socket (neutral) and the roundish green socket. The meter should read 0. Checked across the short socket (hot) and the grounding socket, the voltage should be normal – around 120 volts. It is a good practice to confirm the neutral connection by also measuring the voltage between the short and long sockets. [For other tests regarding polarity, more info can be found from pages 150 – 153] (Sailboat Electrics Simplified, p. 150)
- Since alternating current by definition flows in one direction then the other, reversing the connection has no effect on an AC appliance. But correct polarity is still an essential requirement in AC circuits. Why? When an overload trips the breaker, it disconnects the load from the power. But suppose connections to the dockside receptacle are reversed. That puts the breaker in the neutral side of the circuit, so the circuit is essentially unprotected. The same short now continues uninterrupted until the circuit burns open. If you are lucky, the breaker at the marina office will trip before flames break out, but don’t count on it. Shock risk is also increased. Turning off a breaker appears to remove power from the circuit because it turns off all appliances connected to that circuit. But with reversed polarity you have disconnected the appliance from ground, not from power. The circuit is still alive. (Sailboat Electrics Simplified, p. 138)
- Polarity does not matter to some items, such as lights, but you still want to connect the wire marked + to the positive side of the circuit to put the switch on the hot side. For DC motors – fans, pumps, etc. – and all electronics, correct polarity is essential. (This Old Boat, p. 282)
- You need a reverse polarity indicator even with double-pole breakers, because reversed polarity still puts switches in the neutral side, leaving “off” appliance dangerously energized. (This Old Boat, p. 307)
- The AC distribution panel should include a permanently connected polarity indicator that lights up in case of a reverse polarity (the exception is a boat with an isolation transformer). (Cruising Handbook, p. 192)
Links and Resources
- Lightning and Sailboats – http://www.marinelightning.com/ECE/SGEB17.html
- science and technology of yacht lightning protection – http://www.marinelightning.com/science.htm#Cruising%20sailboats
- Lightening Protection – http://www.bayacht.com/aaa/nl-artic/lightng/lightart.htm