[Editor’s note: In part one of the shore power series, we discussed the advantages of shore power over 12 VDC systems and defined some terms and practices. In this, the second and final part of the shore power story, we delve into galvanic isolators, isolation transformers, generators, inverters, GFIs and more.]
Many boatowners may have heard a marina or boatyard called a “hot marina” and wondered exactly what the term meant and if the condition it names is dangerous for them or their boat. Fortunately, for those voyagers who are regularly plugged into shore power, in the majority of cases the situation is not as sinister as it may sound. Voyagers should be aware, however, of the problems that shore power can cause.
The term “hot”, when attributed to a marina or dock, may refer to two distinctly different scenarios. One case may involve loose AC current coursing through the dockside water. This is a very definite danger to swimmers and divers. As mentioned in part one of this series (“The shore power program”, Issue 119 Jan./Feb. 2002) it takes very little current to paralyze the muscles of an unwary swimmer or diver, indirectly leading to drowning. (It should be noted that there is very little evidence to support the commonly held belief that stray AC current can cause corrosion. All the stray-current corrosion examples I have seen in my experience have, without exception, been direct-current induced.)
When most sailors refer to hot docks, they are, sometimes unknowingly, referencing the corrosion potential rather than the risk presented to swimmers. This does indeed exist, and in the majority of cases, the shore power is indeed responsible in an indirect manner. Each time you plug in your shore power cable, whether it’s 110 or 220 VAC, 30 or 50 amp, you are connecting your boat’s AC grounding and bonding system to the dock’s AC safety grounding grid. Herein lies the problem; every other vessel that is plugged into the dockside shore power system is now connected to your vessel through this grounding network.
The potential for disaster is now clear and present. You’ve dutifully ensured that your vessel’s electrical system is sound and in compliance with American Boat and Yacht Council specifications, and you replace your zinc anodes regularly. Your slip neighbor, however, hasn’t visited his boat in months. His zincs are depleted, and what’s more, he has hooked up an automotive battery charger to keep his batteries topped off so they’ll stay ahead of that pesky stuffing-box leak. Your zincs are now protecting his underwater metals because his are long gone. Furthermore, his automotive battery charger is leaking AC current into the DC system, creating an electrocution hazard.
It’s not difficult to imagine how this scenario could create all sorts of problems for your boat. Once your zincs are gone, the next least noble underwater metal will throw itself on its sword in order to protect its, as well as your neighbor’s, brethren. This may be your prop, strut, shaft or a through-hull fitting.Two solutions
Fortunately, there are two solutions at hand. The first, and least expensive, is a good galvanic isolator. The second is an isolation transformer (more on these later). A galvanic isolator is installed in-line in the shore power green wire aboard your boat, between the shore power deck inlet and the electrical panel. No ground connections are permitted between the galvanic isolator and the inlet.
Using a series of diodes (and a capacitor on better models), this device will stop galvanic DC voltage/current (the root of all corrosion), up to about 1.2 volts. After that, the diodes can no longer block the current, and the isolator becomes ineffective. However, most galvanically induced corrosion happens below this voltage, so the isolator is effective.
In addition to its voltage limitation, the other failing of the galvanic isolator is when it’s carrying AC fault current. In this case, the diodes do not block, as they shouldn’t. However, if the diodes are not blocking because they are conveying faulty AC current, destructive DC current may piggyback along for the ride, again rendering the isolator ineffective.
In spite of an isolator’s limitations, every vessel that’s plugged in, for even the briefest time, should be equipped with a galvanic isolator. It is an excellent and inexpensive defense against shore-side-induced galvanic corrosion. Ensure that the unit you choose, and its subsequent installation, complies with the latest version of the ABYC standard (version A-28) for galvanic isolators.A new source of power
If, however, you are searching for the ultimate in protection against shore-power-induced corrosion (as well as reverse polarity and some electrocution situations), then look no further than the isolation transformer. This device, when employed, becomes essentially a new source of power aboard, much like a generator. Using transformer windings, all direct connections with the shore are eliminated. No galvanic or stray current, no matter how great, can sneak past the isolation transformer sentry. Additionally, isolation transformers are immune to reverse polarity situations. This can be useful, both ashore and afloat.
Isolation transformers possess one additional attribute: They reduce the likelihood of electrocution. One rainy December several years ago, in preparation for working on a customer’s boat that was in winter lay-up, I hefted an aluminum ladder against the vessel’s aluminum toe rail. As soon as the ladder made contact with the rail, I received a healthy electrical jolt through both arms. This occurred in spite of the fact that I was wearing rubber-soled boots. Fortunately, the ladder’s rubber feet were folded up for use on gravel. This enabled the majority of the current to pass through the comparatively low resistance of the ladder, rather than me. After recovering from the initial shock, both figuratively and literally, I shut the breaker supplying the boat’s shore power cord and proceeded to investigate the cause of the fault.
It did not take long to discover that the vessel’s AC electrical system was in a state of reverse polarity. The owner had made his own shore power adapter, allowing the boat to be plugged into an ordinary 15-amp utility receptacle (which was not, unfortunately for me, a ground-fault interrupter). In doing so, however, he reversed the hot and neutral conductors. In this situation, all onboard AC equipment continued to work. It was only when the right series of events came together (rain and an aluminum ladder) that a potentially deadly situation reared its head.
The reversal meant that all bonded items, including the toe rail, became energized with hot AC current. The ladder and I completed the path to ground. If this vessel had been equipped with an isolation transformer, and it easily could have been, I would never have been unfortunate enough to experience this shocking situation firsthand. Immune to reverse polarity problems
The reasons why an isolation transformer could have prevented my getting a shock are twofold. In order to be shocked by or aboard a vessel that is equipped with an isolation transformer, you must make contact with one of the vessel’s own hot conductors (or something energized by it) and the vessel’s own neutral-grounding conductor. Electricity that emanates from an isolation transformer only wants to return to that isolation transformer, not earth. This attribute has special and obvious significance for swimmers. Additionally, isolation transformers are immune to reverse polarity faults generated ashore. As it was, the simple reversal of two wires, and the lack of an isolation transformer, nearly led to a catastrophe.
One final note on the transformer story: There exists a close relative to the isolation transformer, it’s called a polarization transformer. The polarization transformer shares a common ground with shore and thus does nothing to prevent shore-induced galvanic or stray-current corrosion. Its main strength lies in its ability to correct reverse polarity anomalies, such as the one mentioned previously.
Generators and inverters present their own special set of circumstances in relation to shore power. While these devices are not shore power, strictly speaking, they have much in common with the energy created by the utility company.
Generators, often referred to as gensets, produce electricity by utilizing rotating magnetic fields driven by an internal combustion engine. An entire article could be written about these devices. However, the important point to remember for the purpose of this discussion is that, just like the utility company, generators produce lethal current/voltage combinations.
Much like the isolation transformer, in order to be shocked by current produced by a generator, you must make contact with the generator’s own hot and neutral or grounding conductor, not earth. The neutral and grounding conductors are terminated at the same location within the generator enclosure. It would appear then, that this would violate the cardinal rule of separation of neutral and grounding buses aboard, mentioned in part one of this article. This is one of three exceptions, the other two being isolation transformers (remember, they should be thought of as energy producers) and inverters.
An added wrinkle
There is, however, an added wrinkle to this rule. As long as the generator is running and supplying power, these two conductors are just fine as a couple. However, when the genset is dormant and shore power connected, these two must be temporarily separated. It sounds complicated, but it’s not. Every properly wired generator must utilize either a multi-pole roll switch or lockout-style circuit breakers. These make it impossible to connect the neutral and grounding circuits when they are not supposed to be, i.e., when the shore power cable is connected and in use.
Inverters share some similarities with gensets, primarily in the area of lethality. Make no mistake about it, just because inverters use batteries as their energy source, they produce current and voltage that’s not dissimilar from ordinary 120 VAC shore power, perhaps with the exception of the configuration of the sine wave. Most inverters produce square, rather than sinusoidal, sine waves; however, this does not affect their inherent electrocution danger.
Properly installed inverters must be equipped with clearly labeled disconnect switches that separate them from all DC supply power. Additionally, vessels equipped with inverters should carry a placard at the main AC panel indicating that the vessel is inverter equipped. Why is this necessary? The primary reason is that you, of course, know your own vessel is equipped with an inverter. However, someone else may not. He or she may think the vessel’s AC electrical system is safe to work on as long as the vessel is unplugged.
Additionally, many inverters go into an energy-conserving sleep mode, in which they produce no AC voltage. However, they are on call to begin generating in a fraction of a second when called upon to do so. A digital or analog voltmeter may not draw enough current to awaken this sleeping giant. The hapless technician tests the lines and, finding no voltage present, begins work, only to receive a nasty and perhaps lethal shock.
As mentioned previously, inverters, like generators, possess neutral and grounding conductors that share common termination; they are connected at their source. However, since most combination inverter/chargers are not installed using roll switches, how do they separate these conductors when the time comes? The answer is by using a magnetic contact switch. The most common inverter arrangement works as follows: As soon as the inverter senses shore power, which places it in its charge or off mode, an internal switch disconnects the neutral and grounding conductors. When shore power is removed, placing it in invert mode, the magnetic switch reverses, reconnecting these two conductors.
Ground-fault interrupters (GFI), also known as ground-fault circuit interrupters (GFCI), are devices that sense current flow through both the hot and neutral conductors. By utilizing a sensitive electromagnet, they are able to sense and react to an imbalance in the flow of electricity through these two conductors. In the event of such an imbalance, as little as five milliamps (the requirements for this device are detailed in ABYC E-8.17), the circuit opens. This all happens quickly enough so that the potential victim never knows it has happened, receiving only a very brief shock, if any.
GFIs are particularly useful, just as they are in household applications, where a user may be exposed to hot current and ground. This is especially likely on weather decks, in the galley, the head and engineering spaces. In reality, this risk is present in nearly all locations aboard any vessel where AC receptacles are located. If, for instance, you were using a vacuum cleaner with a defective electrical cord (its insulation cut by a falling hatch), and you touched the uninsulated section and a grounded object, the engine block for example, you would receive a shock. However, if the vacuum were plugged into a GFI receptacle, you would just be left wondering why the vacuum had mysteriously stopped running. You may reset the GFI and carry on with your cleaning, never discovering the cause. The moral here is, if a particular appliance repeatedly trips a GFI, check it over carefully, especially its cord and insulation.
GFIs are wonderful devices that have, no doubt, saved countless lives. However, they do not work well under certain circumstances. If, for example, you should encounter both the hot and neutral conductors simultaneously (and not ground), the GFI may not register an imbalance and consequently fail to open the circuit.
Another GFI failing involves its use for protection of multiple circuits, which have a cumulative ground fault due simply to the damp marine environment. This frequently happens where an attempt is made to operate an entire boat’s AC electrical system through a GFI receptacle or circuit breaker. GFIs simply may not work under these conditions. They are best used to protect one or more receptacle locations on a single circuit. This means one GFI receptacle may be used to offer GFI protection on other receptacles that are downstream of it, provided they are properly connected, following the GFI manufacturer’s instructions.
The final measures you can take to prevent a dangerous AC fault situation involve the installation of a reverse polarity indicator and a trip-coil main AC breaker. These devices help to warn against and prevent potential reverse polarity situations (remember the electrified boatyard ladder?), respectively.
The reverse polarity indicator simply alerts the crew, with either an indicator lamp or an audible tone, to the presence of a reverse polarity situation. Ideally, this should occur as soon as the vessel is plugged into shore power, but without requiring that the main breaker be turned on. The trip-coil breaker also senses reverse polarity situations; however, instead of warning you of such a situation, it simply trips the main AC breaker. This device is an excellent adjunct to, but not replacement for, the reverse polarity indicator.
Shore power is a useful tool that will provide everything from charged batteries to hot coffee. However, like any tool, if misused it can be dangerous. Treat it with respect, and it will remain the servant and you the master.
Steve C. D’Antonio is a marine writer and photographer and is the yard manager at Zimmerman Marine Inc. in Mathews, Va.