Problems on the ground
Recently, evidence has been mounting that some deaths around marinas that were classified as drownings were, in fact, electrocutions. When improper, or faulty, grounding of either onboard or dockside wiring is combined with AC leaks to ground, enough current can be fed into the water to paralyze the muscles of a swimmer, resulting in a drowning that leaves no physical evidence of the causative electrocution.
This reminds us once again, if such a reminder is needed, that the marine environment is both unfriendly where electricity is concerned and also, especially in the case of AC power, potentially lethal. To provide adequate protection from shock hazards, rigorous attention to correct installation practices is necessary, with particular emphasis on a proper grounding system.
To see why the grounding system is so important, we need to look at how AC is supplied. The electricity generated by power companies is provided to consumers via two wires, one being a "hot" wire, and the other a "neutral" or return wire. Both are full current-carrying cables, the hot supplying current to appliances, the neutral forming the return path to the power plant (without which nothing would work).
At the power plant, the electric company connects the neutral cable to a buried metal plate. Among other things, this has the effect of holding the voltage on the neutral wire at "ground potential" - zero volts - allowing it to be used as a reference point from which system voltage can be measured (because of this the neutral wire is known as the grounded wire).
When a house is wired, the two wiresandmdash;hot and neutralandmdash;are run to all fixtures and appliances. The incoming electrical box is considered to be the generating source for the house, and, as such, the neutral circuits are once again connected to a buried pipe - a driven ground rod - at this point.
In the old days that was it. But in the event that a fault developed such that a piece of equipment developed a short circuit and became "hot," anyone coming into contact with this equipment completed the circuit to ground and received a severe shock. What made the situation particularly dangerous was that in many instances the fault current could not run to ground until human or other contact completed the circuit. In such circumstances, fuses and circuit breakers provide no protection: without a path to ground, the amount of current necessary to blow the fuse or trip the breaker simply does not flow. The fault remains undetected until the damage is done.
To improve system safety, a third (green or uninsulated) grounding wire has now been added to AC circuits (note the term grounding is used to distinguish this wire from the neutral or grounded wire). The grounding wire is connected to the external cases of appliances and is in turn connected at the power source (incoming panelboard) to the neutral wire, and thus to the driven ground rod. In normal circumstances, this grounding wire carries no current, but should some sort of a fault develop that makes an appliance hot, this wire will immediately conduct the fault current safely to ground. What's more, if the leak is a serious one, such as in a short circuit, as soon as it occurs the grounding wire will allow high levels of current to flow immediately blowing a fuse or tripping a breaker.
This grounding circuit is thus a redundant path to ground, paralleling the neutral circuit. It provides an essential degree of protection against many common electrical faults. Rather like a seat belt or an air bag in a car, it doesn't do any good until a problem develops, but then it might save a life. Any break in the grounding circuit, such as cutting the ground pin off an extension cord (a common sight around boat yards!), or a badly corroded ground connection on a shorepower cord, potentially leaves us with the old-style, two-wire circuit that provides fault current with no safe path to ground.Shorepower grounding
For a boat connected to shoreside power, the dockside receptacle is, in effect, the power source. The neutral and grounding cables will be grounded to a driven metal rod somewhere near the main dockside panel (the one that contains the principal overcurrent protection device - i.e., circuit breaker). When the boat is plugged in, the neutral circuit on the boat, and the grounding circuit, are connected to their respective cables and consequently both are grounded ashore at the point where the cables connect to the driven rod.
Just as in a house, fault current will be conducted safely ashore via the grounding wire. Even in the damp marine environment we still have excellent protection against shock hazards. There is, however, a snag: that same green, or uninsulated, grounding wire that is providing such essential people protection may be contributing to galvanic corrosion. This corrosion can take place even if the AC circuits are perfectly installed and functioning faultlessly. It has, in fact, nothing to do with the AC system itself, but is simply a parasitic problem that comes aboard with a proper AC installation.
Let's say two boats are lying alongside one another. Both are connected to shorepower, and are properly wired with the AC grounding circuit connected both ashore and also to the onboard DC negative and bonding system (more on this in a moment). The underwater hardware on one boat is protected by zincs but the other boat doesn't have this protection. In effect, what we have is a giant battery: the zincs on one boat form the positive plate; bronze underwater fittings on the other boat form the negative plate; and the water in which the boats are floating is the electrolyte. As soon as both boats plug into shorepower the AC grounding wire completes the circuit between the two "battery terminals"(i.e., underwater hardware on the boats), causing galvanically-generated DC current to flow along the AC grounding wire. The least noble (galvanically most active) metal will corrode (in this case, the zincs). When the zincs are depleted the next least noble metal (some underwater fitting) will start to go. (Even if both boats are protected with zincs, some corrosion will still occur. Still, there are plenty of boats that don't have zincs or whose zincs are close to depletion.)
So long as a shorepower-based AC system is properly grounded, in an unmodified AC system, plugging in to shorepower will always invite corrosion. The problem is caused by precisely those steps deemed necessary to safeguard people on board. We end up with a clear conflict between people safety and boat protection. The challenge is to find an acceptable response to this situation that accomplishes both objectives.Breaking the galvanic circuit
From time to time a recommendation is still made to cut the connection between the AC grounding and DC negative circuits. In theory, this isolates the AC grounding circuit from underwater hardware, breaking the path from shore to water for DC galvanic currents, while still maintaining the shoreside AC grounding connection for people protection. On the surface a simple, cost-effective solution to the corrosion problem that does not compromise safety.
This approach is frowned on by most experts, however. There are at least four reasons:
1. In many instances the shoreside grounding connection is itself compromised, either through faulty wiring, or else because of corrosion within one or both of the shorepower cord receptacles. Should a fault current develop, and the connection to the DC negative is still intact, the current has a relatively safe path to ground through underwater hardware. (Of course, this could endanger nearby swimmers - but is considered the lesser of the two evils.) Without the DC negative connection, any time the shoreside grounding connection is defective we are, in effect, back to the old, unsafe two-wire AC system. Put another way, the connection to the DC negative provides one more line of defense against defective AC circuits or equipment.
2. Sometimes a serious leak can occur between the AC circuits and the DC negative circuitandmdash;the most likely cause being a defective battery charger, or a short between adjacent AC and DC wiring. Without the AC grounding to DC negative connection, this fault current has no safe path back ashore.
3. Proper lightning protection demands that the AC grounding circuit and the DC negative circuit be held to the same voltage potential in order to minimize the build-up of dangerous voltages in either circuit. To do this effectively the two must be electrically inter-connected.
4. Quite often, even if the AC grounding to DC negative connection were to be cut, there would still be some other, unforeseen, path to the DC negative. This might be through a piece of AC equipment which is itself in some way grounded to the DC system (generators, air conditioners, non-marine battery chargers, some water heaters) or through onboard leaks between the two circuits. The potential for corrosion would still exist but without the boatowner being aware of it, with the result that proper preventive measures would not be taken.Galvanic isolators
Rather than cut the AC grounding to DC negative connection, the correct way to galvanically isolate the AC circuit is with a galvanic isolator. These devices consist of little more than several special diodes wired in parallel to conduct in opposite directions. It takes a certain voltage (typically around 1.5 volts) to make the diodes conductive. If an isolator is installed in the AC grounding wire, unless there is a leakage current or a stray DC current in excess of 1.5 volts the diodes simply will not conduct. In normal circumstances the isolator effectively breaks the grounding circuit. But in the event of a leakage current greater than 1.5 volts, the diodes become conductive, ensuring the continuity of the grounding circuit.
It would seem that the best place to put an isolator is between the AC grounding and DC negative circuits. In this position an isolator failure would not in any way compromise the integrity of the ship to shore grounding connection. However, as noted above, in reality it is often next-to-impossible to prevent some circuit or other by-passing the isolator, rendering it ineffective. The potential for corrosion will once again exist while the boatowner is under the illusion that the problem has been solved.
As a result, the American Boat and Yacht Council (ABYC) and other authorities recommend placing an isolator in the incoming AC grounding wire immediately downstream of the shorepower inlet. It is then a simple matter to ensure that nothing bypasses it.
The obvious problem with the ABYC's location is that should the diodes fail in the open-circuited position the grounding circuit will be broken, voiding the protective function of the grounding wire. Alternatively, should the diodes fail in the conductive (shorted) position, there will be no protection against galvanic currents.
Given that diode failure in most instances is not externally visible, it becomes essential to have a high-quality isolator with both a continuous current rating, and a short-circuit rating, at least as high as that of the main breaker on the circuit in which it is installed (in practice this should be higher, since breakers typically trip at around 1.3 times their nominal current rating). There are isolators that do not have this current-carrying capability and as a result are potentially lethalandmdash;it is a case of buyer beware!
Beyond this, isolator design gets controversial. The problem is that in the damp marine environment, there is often a low-level AC leakage into the grounding circuit. This may be high enough to make the diodes conductive (above 1.5 volts) but low enough to remain undetected. In such cases, the isolator is doing nothing to block galvanic currents.
One school of thought holds that a capacitorandmdash;a device that conducts AC but not DC - should be wired around the diodes. The capacitor will allow alternating currents of several amps and volts to be shorted to ground without the diodes conducting. In other words, the capacitor will maintain galvanic isolation up to higher leakage levels without compromising grounding safety. Once the carrying-capacity of the capacitor is exceeded, the diodes will become conductive as with any other isolator.
The alternative viewpoint is that the boatowner needs to be aware of, and take steps to cure, any leakage current, either AC or DC, in excess of 1.5 volts. According to this view, the correct method of isolator installation is not to include a capacitor, but to combine the isolator with a meter or warning device that will alert the boatowner to any leakage voltage high enough to make the diodes conductive. Such devices are readily available, but rarely used.
Whichever view is adopted, the reality is that the majority of isolators neither contain a capacitor, nor are fitted with a warning device. There are many which, because of improper installation, or voltage leaks in excess of 1.5 volts, are not providing galvanic isolation.Isolation transformers
Isolators are a low-cost, but only partially effective, response to the people-versus-boat-protection conundrum. Ultimately the only way to provide full people protection without courting the risk of galvanic corrosion is to install an isolation transformer on the incoming shorepower line.
The concept of an isolation transformer is straightforward. Shorepower is fed into one winding (the primary) and transferred magnetically to another (the secondary). The primary winding has a andquot;shieldandquot; which is grounded ashore. The secondary winding may or may not be grounded on board. Either way, there is no direct electrical connection between the shoreside supply and the onboard AC circuit. This gives us two advantages: 1) since the boat's power source is now the secondary side of the transformer, the only path for onboard leakage current is back to the transformer, not the dockside supply - leaks will not find a path to ground through the water; and 2) since the shoreside grounding wire is not connected to the boat's grounding circuit, there is no ship to shore path for galvanic currents.
Although the onboard circuit is sometimes ungrounded (in which case its is said to be "floating" - unfortunately we don't have space to delve into the full implications of such a system), the ABYC recommends that one side of the secondary winding be grounded on the secondary side of the transformer, with a grounding circuit tied in at this point, and the two then connected to the boat's DC negative. This has the effect of producing a "polarized" circuit on board in which the grounded side of the transformer is the neutral. In the event of either a short in a piece of AC equipment, or a leak into the DC circuits, the fault current has a direct path back to the transformer.
At first sight, the connection to the DC negative appears to bring Earth's ground back into the picture, resurrecting the potential for shock hazards to swimmers. But on closer inspection, it can be seen that regardless of this connection, the only path for fault current is back to its source, which is the transformer. Earth ground has no part to play in this circuit. The "fault current" circuit is completely contained within the boat and its wiringandmdash;swimmers will be safe.
It is important to distinguish an isolation transformer from a polarization transformer. The primary and secondary windings in the latter function as in an isolation transformer, creating a floating AC circuit on board, one side of which is once again made neutral by tying it to the boat's grounding circuit. What this does is provide a constant polarity on board, regardless of the polarity at the shoreside receptacle. But on the polarization transformer, the shoreside grounding connection is fastened to both sides of the transformer, and ultimately to the boat's DC negative: the transformer does nothing to provide galvanic isolation. To get galvanic isolation with such a transformer, it is necessary to once again fit an isolator in the grounding circuit. It is worth repeating that so long as the grounding wire is carried on board and connected to the boat's AC grounding and DC negative circuits there will be no galvanic isolation.Safety without corrosion
Given the potentially devastating consequences to both people and boats of improperly wired AC circuits we owe it to ourselves, our boats, and others around us to ensure that our installations are to the highest standards. The kind of jury-rigged circuits that one so often sees around boat yards and on board are simply not acceptable in this day and age. As we have seen, it is possible to have a high level of shock protection, without adding to corrosion problems, but this is not something that happens without close attention to details. Above all else, the grounding system must be both electrically continuous, yet galvanically isolated.
Contributing editor Nigel Calder is the author of The Boatowner's Mechanical and Electrical Manual, published by International Marine Publishing.