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https://prod-boatus-cd2.azurewebsites.net/expert-advice/expert-advice-archive/2017/february/understanding-the-green-ground-wire

One of the classically misunderstood and overlooked systems on board most boats is the grounding/bonding system, which is fairly easy to identify: In virtually all cases, it's insulated copper wire with a covering that's green or green with a yellow stripe. Some systems over the years have also used uninsulated single-strand, heavy-gauge copper wire, or flat copper strips run along the top of structural stringers. Part of the mystery surrounding this system is that it really doesn't make anything electrical on your boat work; instead, it generally just rides along with you waiting to get called into action. But when things do go wrong, this system serves a critical role in your boat's electrical universe. Shock Hazard, AC Fault Current If your boat is equipped with a shore-power system, one of the most important roles of the grounding system is to act as an alternate current path in the event of a short circuit at any of the AC appliances on board. In the event of a short circuit to the metallic case of an appliance, the normally current-carrying wire in the circuit known as the neutral conductor (the white insulated wire with 120-volt shore-power systems) gets bypassed. Without an alternate route for the fault current to travel back to the source of power, the case of the equipment becomes live. Here, the green grounding wire takes over and directs the short-circuit current back to its source and trips a circuit breaker, turning the power off. One of the potential problems that the above arrangement doesn't deal with effectively is a low-level AC fault within an appliance. As appliances age, the components inside the device can gradually lose their insulating properties, allowing for some leakage of fault current to the case, which ultimately enters into the green-wire grounding system. For a circuit that normally carried, say, 12 amps, the leakage current might only be 3 or 4 amps because the actual point of leakage still has a fairly high electrical resistance; it won't allow the full 12-amp fault to leak by. So now we have 3 or 4 amps of AC current flowing through the grounding system. Assuming the original circuit was engineered correctly, the wiring can certainly handle the current, but it also means that the circuit breaker, which was sized to carry the maximum load on the circuit before opening, will not trip because the fault current is far below the trip point of the breaker. In the final analysis, this means that the low-level leakage current can migrate its way around the grounding system indefinitely, and no one on board will ever be the wiser. A potentially deadly shock could be the result, both for crew in the boat and even a swimmer in the water. All of this describes the scenario that prompted newer American Boat & Yacht Council (ABYC) requirements for complete onboard ground-fault protection via either an isolation transformer or an equipment leakage circuit interrupter, or ELCI, device installed in the shore-power system (these are like the typical GFCI outlets in your house, but for the entire shore-power system). Much has been written about this topic over the last several years under the heading "ESD," or electric-shock drowning. DC Fault Current/Bonding DC fault current can manifest itself in several ways, all potentially catastrophic. Once again, the grounding system may get called into action to mitigate the potential issues. As with AC systems, we call upon the grounding system to divert fault current back to the source of power and ensure that a fuse blows or a circuit breaker trips, turning the power supply off. DC equipment-case short circuits can be quite dramatic, depending on the capacity of the battery bank supplying the system. The capacity of a battery bank connected in a parallel configuration (the most common approach) is cumulative, and it doesn't take very many batteries to end up with extremely high available amperage, typically measured in the thousands of amps. This is still relatively low voltage, but it's enough amperage to quite easily start fires on board. According to BoatUS Marine Insurance claims files, 12-volt DC circuits are responsible for nearly a third of all boat fires. When all is said and done, the importance of properly sized case ground connections for such things as battery chargers and DC-to-AC inverters cannot be overemphasized. ABYC recommends that case ground conductors for metal-cased battery chargers and inverter chargers be no less than one wire gauge size smaller than the DC positive conductor connected to the unit. The reason for this is in the event of a short circuit to the metal case from that DC conductor, we want a conductor with adequate fault current carrying capacity to divert potentially very high battery short-circuit current back to the source directly. This ensures that the comparatively small AC grounding conductor on the power supply (battery charger) or AC output side (the DC-to-AC inverter) of the unit carries little if any fault current. The timing on all of this is measured in milliseconds, just long enough to blow the DC fuse in the circuit. The bonding system that connects all of the metal components located below your boat's waterline is also connected directly to the grounding system for both your AC components and the high-current DC components, like battery chargers and inverters.