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Photo: Justin Baeder
If you grew up, like I did, swimming off docks in the Great Lakes, it can be hard to understand why you have never heard of this problem until recently. But back then even the most sophisticated boats only had a battery to start the engine and to run a VHF radio. We didn't have lights and boat lifts on the docks, or water heaters and air conditioners on the boats. As often as not, we took our batteries in to the garage to charge them. In the past couple of decades, we've brought AC electricity and fresh water together in a way we never have before. "No sane person would consider plugging a hair dryer into an AC outlet, turning it on, and stepping into the water with it. But that's essentially what we're doing with our boats," Kevin Ritz says. We're plugging shore power cords into outlets and bringing AC power aboard something that is sitting in the water, and then we're stepping aboard and living inside it or we're swimming around it.
Looking at our boats that way helps to define the danger, but understanding exactly how AC gets into the water means remembering long-ago lessons from high school physics and learning some new lessons on why boat electrical systems are so different from those ashore.
First, without getting too detailed or complicated, the physics. The most basic rule about electricity, the one upon which everything we do with electricity depends, is that electricity will always find a way back to its source. Without that basic premise, we would never have the concept of a circuit that we put to good use every day when we use the washing machine or tap on our computer keys. The second fundamental rule is that electricity will take every available path back to its source, and the third is that electricity is inherently lazy. Taken together, these three rules mean that most of the current will flow along the easiest path, which in electrical terms means the one with the least resistance. We design our circuits so that path is the one that goes through the motor that powers the washing machine or the pressure pads in the keys on the computer.
But if any other path is available, some electricity will take it. And if for some reason an alternate path becomes easier than the one we intended the current to follow, a lot of electricity will flow through it. So if a wire in our washing machine shorts out, allowing the current coming in to follow a path back to its source without passing through the motor, almost all of the electricity will return to its source via an unintended, often dangerous path. When that happens, the sudden flow of a large amount of current burns out a fuse or trips a breaker (right around the time your washing machine starts to smell like melting plastic and ozone). Ashore, there is no water to offer another path to ground, and the electrical system is wired to very specific standards to minimize the danger to people — even if water is involved. On a boat or a dock, AC that finds its way out of the correct circuit and into the water will also try to return to its source, which will be the transformer on the dock or on land.
Two Different Systems
Conscientious parents who make sure their children wear life jackets may not realize the potentially lethal hazard that comes from having 120-volt electricity on their docks.
When it comes to boats, two different electrical systems written under two different standards to two different logics for two different environments meet. Understanding something about these differences — and the challenges of combining them — goes a long way toward explaining how current ends up in the water in the first place.
A DC circuit only has two wires — positive and negative — but a grounded AC circuit has three (actually, some have four because they have two hot wires, but for our purposes we'll consider the simplest case with three wires, a basic 120-volt system). AC leaves its source on the hot black wire and it returns on the white neutral wire, and the third wire is a green safety wire (ground wire) that takes any stray or fault current back to the source. Ashore, ground is just that — a metal rod or pipe that goes into the ground to a prescribed depth — and the neutral wire, as well as the safety ground wire, are tied together near this point.
To understand why you need the safety wire, consider your washing machine without it. If nothing ever went wrong with circuits, there would be no problem. All the electricity that went into the washing machine on the hot wire would come back out on the neutral wire and return to the transformer. But many things can and do go wrong. If the hot wire shorted to the metal washer frame, the outside of the washing machine would be electrified to 120 volts. If you touched it, electricity would flow through you to return back to its source via the metal grounding rod. To prevent that, most appliances have a third prong on their plugs, which is for the safety ground wire. This connects the washing machine's metal frame to the circuit's ground and provides a much easier way for electricity to return to its source than through you.
Things get much more complicated on a boat. First, there are two electrical systems — AC and DC — and the grounds for each are tied together. If that were not the case and a short occurred between the AC and DC systems, all of the wires and equipment in the DC system could potentially be carrying 120 volts AC — along with the boat's engine and underwater fittings if these were all bonded as required by the American Boat and Yacht Council's (ABYC) E-11 Standard. As counterintuitive as it is, this would pose no danger to you on the boat — there would be no voltage gradient to be bridged. You are standing in a boat, on the water, not touching the ground, so there is no easy pathway through you back to the source, and everything around you is at the same voltage. However, the boat would now be putting dangerous amounts of electrical current into the water, and the AC circuit breaker might not trip if the current level is below the circuit breaker's trip rating.
To prevent that from happening, the ABYC E-11 Standard calls for the AC and DC systems to be tied together, specifically for the AC grounding bus to be connected to the DC negative bus or to the engine negative. Now in the case of a short circuit between the AC and the DC system, most of the electricity will try to return to its source through the low-resistance AC ground, tripping the circuit breaker and shutting off electricity to the boat.
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Other Repercussions
But tying the AC ground to the DC negative has other repercussions.
Consider a hot water heater on a boat. If the AC hot wire shorts out to the metal jacket on the water heater, the jacket would be electrified to 120 volts. As with the washing machine, the safety ground wire provides a low-resistance path back to the source so that the metal jacket does not create a shock hazard. A large amount of current flowing through the ground wire should trip a breaker and turn off the electricity to the water heater. Leakages of small amounts of current will return along the ground wire without tripping the breaker but should still reduce the voltage in the metal jacket to non-lethal levels.
To function properly, the safety ground wire needs to offer near zero resistance so that most of the current from the short will return along the ground, tripping the circuit breaker. Unfortunately, that's extremely difficult on a boat (or on a dock). Motion loosens wires, corrosion attacks connections, chafe allows current leakage — all of which makes it harder for the stray electricity to return through the grounding system. A serious problem in the grounding system on the dock combined with an electrical fault on the boat means that the current will be looking for an easier path to its source. At least some of that current will pass through the AC ground to the DC negative where it will energize any bonded metal fittings. From there, it will find its way back to its source ashore the only way it can — through the water.
Courtesy David Rifkin
Courtesy David Rifkin
The differences between electrical systems ashore and on a boat can lead to current leakage in other ways. Here are just a few examples:
- Not using certified marine electricians. Some well-qualified and properly-certified land electricians will tie neutral to ground on the boat as they do ashore. Because the AC system is tied into the DC system on the boat, doing so will put current into the water through the DC negative bus or the engine. On a boat, neutral and ground must remain completely separate — except on inverters, generators, and the secondary side of transformers installed on the boat (see below).
- Not removing the neutral-ground connection on appliances. Some household appliances such as dryers and ovens come with a bonding strap that connects neutral to ground. To install these on a boat, this connection must be disabled. Otherwise, any fault in the grounding system will allow stray current to find its way from neutral into the DC system and out through the underwater fittings into the water. But when it comes to generators, inverters, and onboard transformers you DO want the neutral to ground connection to remain intact to make sure that the circuit breaker will trip if there is a fault. In this case, the source of your electricity is now on the boat, so any electricity trying to return to its source will stay on the boat and not enter the water.
- Mixing DC and AC systems. If DC and AC systems share the same breaker panel, or the wires from the two systems are in contact with one another, the potential exists for the AC current to find its way from a hot AC wire directly into the DC system through some sort of a short, which could energize the entire boat if a ground fault exists. This is the exact situation that killed Kevin Ritz's son, Lucas in 1999. For this reason, ABYC E-11 Standard requires the hot side of the AC and DC systems to be kept completely separated, which means if they are run together in one conduit, one of the wires must be contained in a sheath. But older boats will not have been built to that standard, and will likely have had quite a bit of wiring done on them by people who do not understand the danger. Note that in the AC system the black wire is hot, while in the DC system the black wire is the negative. Tying one into the other thinking they are part of the same system will energize every metal fitting on the boat.
ESD is so hard for boaters to accept because it makes two things most of us equate with boating — swimming off our docks and fixing our boats — potentially lethal. Yet some people who would never consider wiring their own bathroom think nothing of installing a battery charger on their boat, or a boat lift on their dock. The complexities described above should convince you that this is one area where hiring a professional makes sense, and not just any professional, but one who really understands the intricacies of boat and dock wiring. That means hiring an ABYC-certified technician to do work on the boat's electrical system, and a qualified journeyman or master electrician to work on your dock.
"If all of the wiring is perfect, there is no danger," Ritz says. Minute amounts of leaking alternating current in fresh water can kill. AC leaks can be prevented or minimized ONLY if the electrical system has been set up correctly with the proper failsafes.