1/12/2013: Original post.
7/18/2020: Figure and descriptive text added
Normally in a 120V AC circuit, AC current flows from its source to its intended load on an energized, or “hot,” current-carrying conductor and then returns to the source on the neutral current-carrying conductor. In a 240V AC circuit, current can return on the second “hot” current carrying conductor or on the neutral, or both, depending upon the combined nature of the loads attached to that circuit. A safety ground is not necessary to enable basic AC electric circuits to work, but is always added, first, as a means to keep living beings safe from shock and electrocution and second, as a means to interrupt the flow of current in the case of an overload, or “short circuit.” The neutral conductor is held at ground potential by attachment to the system’s safety ground. This connection – known as the “bonding conductor” – is located at the “derived source” of the AC electric service; i.e., the “main service disconnect panel” in the shore power distribution system. The safety ground conductor is not intended to carry any of the current that needs to return from the load to the source.
A “ground fault” is an unintentional connection between either the “hot” current-carrying conductor or the neutral current-carrying conductor and any possible conductive return path that leads back to the source of AC other than the normal neutral return path. On a boat, “fault” circumstances (described in the article on Electric Shock Drownings posted on this site) can cause AC return currents to flow through the water to a shore-based infrastructure. This can be lethal to people, pets and wildlife in the water.
In the United States, the National Electric Code (NEC) and American Boat and Yacht Council (ABYC) standards require that the green safety ground and the white neutral wires in an AC electrical system MUST be bonded together at – and only at – the “separately derived source“ of the AC power being utilized by the boat at any given time. For boat electrical systems that directly connect to the marina’s shore power electrical system, the “derived source” is the shore-side electrical infrastructure. Oversimplified, you can think of this “source” as being the marina’s disconnect panel that feeds the dockside pedestal to which the boat is connected for shore power. Note, therefore, that for boats directly connected to the marina’s infrastructure, the AC “source” is not aboard the boat.
In the case of an on-board generator, it’s easy to visualize that the “source” of AC power is at the generator itself, aboard the boat. Likewise, for an on-board inverter that is operating in “invert” mode, it’s also easy to visualize that the “source” of AC power is the inverter, aboard the boat. Isolation transformer configurations may be more difficult to visualize, since shore power cords do run from the boat to the marina-provided power pedestal. Isolation transformers are built so that the transformer primaries and secondaries – including the external metal encasement of the primary-side and the internal metal supports of the secondary-side – are in fact electrically isolated from each other. Therefore, boats that have isolation transformers are actually completely isolated electrically from the marina’s electrical infrastructure. The transformer aboard the boat is the point of isolation. For a boat fit with one or more isolation transformers, the “newly derived source” of AC power is defined to be aboard the boat. For the generator, the inverter device and isolation transformers, the AC “derived source” is located aboard the boat.
The NEC and ABYC standards require that bonding be at the “source,” and only at the “source.“ The vast majority of pleasure craft are not fit with isolation transformers. For those without an isolation transformer, when connected to a shore power pedestal, the neutral-to-ground bond must be located off the boat, in the dockside electrical system. But, the neutral-to-ground bond must be located aboard the boat for on-board AC electric “source” devices like a generator, inverter or isolation transformer. Note in the figure following, the neutral-to-ground bond located at the generator.
For boats without isolation transformers, to comply with the above requirements in the case of the on-board generator, a very specifically engineered source transfer switch is required. That switch transfers the boat’s AC electrical distribution panel between either 1) off-the-boat shore power as the source of AC power or 2) the on-board generator (genset) as the source of AC power. That switch must be of a double-pole, break-before-make design. That switch design transfers BOTH the hot and the neutral conductors of the boat’s AC electrical system. When the switch is in the “shore power” position, the on-board neutral and the safety ground are bonded together ashore, via the shore power cord, but are not, and must not be, connected together on-board the boat. When the switch is in the “generator” position, the on-board neutral and the safety ground are bonded together at the frame of the genset. Transfer of the neutral conductor by the switch is what changes the origin location of the neutral-to-ground bond.
Remember, the genset, when in operation and switched online, is the “derived source” of the boat’s AC power. Realize that in that state, the incoming shore power neutral is not connected to the boat’s operational AC electrical system, even if the shore power cords are still connected to the dockside pedestal. That is prevented by the break-before-make design of the Generator Transfer Switch.
For the case of an inverter operating in “invert” mode (versus shore power pass-thru), again, the neutral and the safety ground must be bonded together on-board, since the inverter is now the “source” for at least some of the boat’s AC circuits. This connection is accomplished by a relay inside the inverter. The relay acts in the same logical manner as the generator transfer switch acted, above, except that it is automatic. The boat owner/operator doesn’t need to know it’s there. When incoming shore power is disconnected or lost unexpectedly, the internal inverter relay is de-energized, and “drops.” In the dropped state, the onboard safety ground is connected to the on-board neutral. In the dropped state, DC power from the boat’s battery bank powers the inverter electronics to produce 60-cycle AC. When shore power is restored, the relay in the inverter becomes energized again, or “picks.” The internal inverter relay has a break-before-make contactor design, so the incoming battery and shore power circuits are never cross-connected.
The extreme perversity of ground faults is that they are random, unpredictable and unknowable. They can be of very high resistance, of very low resistance, or anywhere in between. The design of the safety ground is to stand by quietly, just waiting for an abnormal condition (a fault) to occur. If a ground fault of any magnitude were to occur when connected to shore power, the safety ground green wire is intended and designed to carry the fault current ashore and dump it safely into the shore-side infrastructure source via the bonding connection. But, what if a ground fault were present when operating from the inverter or the genset? Where would the fault current go? The simple answer is, all currents, whether normal or fault, return to their “source” of AC power. So, that fault current will flow back ashore if running on shore power source, or back to the genset if running on a generator, or back to the inverter if running on an inverter, or back to the secondary of an isolation transformer.
Note: Because of the protective nature of an intact safety ground, this kind of fault is not in and of itself an imminent safety threat to people or pets on the boat, or, in and of itself, a safety threat to people in the water. However, it is a necessary precursor to a very serious, potentially lethal, evolutionary situation, and so, the subject of the article on the topic of “Electric Shock Drowning.”
When connected to, and operating from, shore power, if a ground fault condition is present, the safety ground would carry the fault current back to the source ashore. It is extremely important to realize that current will return to it’s source via all available paths. Depending on the nature of the fault itself, some portion of the total current will return to the source via the neutral (normal path) and some portion of the total current will return to the source via the safety ground (fault path). A fault current may or may not be large enough to trip an overcurrent protection device (circuit breaker) on the supply side (the “hot” side) of the circuit; that would depend on the magnitude of the resistance of the fault circuit.
One kind of fault path occurs when the neutral becomes cross-connected to the safety ground aboard the boat. In that case, and in the absence of other faults, attached equipment would continue to work normally, and overcurrent protection devices would not detect the fault. Some return current would actually flow in the safety ground instead of the neutral. If the neutral conductor was also completely detached from it’s connection to the source, all of the return current would flow in the intact safety ground. This fault commonly results from wiring errors made out of convenience or ignorance while installing or modifying electrical installations.
The safety ground, then, has several design purposes. One is to interconnect the frames and exposed metal parts of every device and metallic component aboard. Another is to clamp the neutral to a known good ground potential of zero volts. In a normally operating AC system, current flows from the source to the load in the hot wires and returns to the source from the load in the neutral wire. The safety ground should never carry any current. But that’s why it’s called a safety ground. When an abnormal situation arises, where there is a ground fault, current will flow in the safety ground. A properly functioning safety ground acts to clamp and hold any exposed fault voltage at a fixed ground potential; zero volts. That’s so that people or pets that come into contact with a metal or other conductive surface anywhere in the environment while the fault is present don’t get a shocking experience.
If you follow ABYC guidelines and the NEC, the frame of a boat’s genset, or the chassis of a boat’s inverter, are bonded to the boat’s safety ground when they are producing AC power. Anything people or pets can touch aboard is also bonded to the safety ground. So, a ground fault would not cause a difference in voltage between people or pets and the faulty device, thus no shocks or injuries. If the fault were of low enough resistance, it would be considered a “short circuit.” Current would bypass the normal path and flow, instead, through the “short.” In that case, current can be very high, the circuit breaker of the affected branch circuit would become overloaded and disconnect the offending branch circuit. If the actual fault is in an attached device, another safety device, like a GFCI, might trip the circuit. Realize, however, that neither of these disconnect outcomes are assured or guaranteed.
Fault conditions can be hard to detect. Not all fault conditions create visible symptoms or stop equipment from working. Any suspicion of abnormal behavior should be immediately investigated by a qualified marine electrician.