Color Guard

Posted: June 1, 2014

A neglected grounding system could be shocking experience.

By: Story and photo by Deane Hislop

The electrical systems found aboard today’s cruising vessels have complex AC systems able of providing reliable power to meet all your electrical demands. Even the smallest trawlers are equipped with shore-power capability, and many have a generator and an inverter as well. If the system is maintained in good working order, problems are unlikely.

When electrical system failures do occur, they typically result in equipment that simply will not work. If this means the blender doesn’t work, life can go on without the daiquiris. But of greater concern is an electrical problem that leads to overheated wiring and a risk of electrocution.

Before working on your boat’s AC electrical system, the shore power should be unplugged, and the inverter must be disconnected — not just off — either by separating the cabling or by using a disconnect switch, from its DC battery supply so it’s incapable of potentially producing AC power.

On a boat with AC power, the green wires that run between metallic objects are your best defense against shocks. Shore-power circuits typically have a hot wire (black), a neutral wire (white) and a ground wire (green); the last leads back to a central buss, which is a path that’s direct to ground and is there to prevent faults that can cause dangerous shocks.

AC electricity, either from shore, a generator or an inverter is common on modern cruising boats. It’s typically found in 120v and sometimes 240v configurations, with 30-, 50- or 100-amp service. You’ve no doubt noticed that most modern 120v receptacles have three holes and that most plugs have three prongs. The smaller of the two blade-shaped prongs is the “hot” leg, meaning it carries the current; the larger of the two is the neutral, or grounded, leg. The round or U-shaped prong is the grounding leg, also referred to as the safety ground. Though the latter two prongs have similar names, there’s an important difference between the grounded and grounding legs in a live circuit.

The neutral, or grounded, prong and receptacle connect to the white AC wiring throughout the boat and connect or are referenced to similar wiring ashore or at the source of power, be it a generator or an inverter. In an AC circuit, the white wire returns current to the source of power, and it’s carrying current at all times, or at least whenever a load is present.

The wire that connects to the grounding leg is green, and this wire is also connected to ground ashore or to the safety ground and the neutral on the genset or inverter. In an AC circuit, the green wire doesn’t normally carry current; its sole mission is to carry fault current safely to ground or its source in order to trip a circuit breaker or blow a fuse.

In practice, if the hot wire in a circuit becomes damaged and makes contact with a metal object such as a tank, an engine block or a metallic appliance case, the fault current, or short, will be carried to ground or the source via the green grounding circuit. Aboard a boat, this circuit connects to elements of the AC system — electrical-appliance cases, the chassis inside these appliances, and outlets — and also to such objects as tanks, engines and water heaters. The green circuit provides ground-fault protection in case a hot conductor makes contact with any one of them. If the safety ground isn’t in place, a metal object could become energized, and ultimately could electrocute a crewmember who touches the object and completes a path to ground.

Visually check your AC safety ground — those green wires — and make certain that all your grounding wires are in good condition and free of corrosion. Inspect the ends of the shore-power cord and receptacles for corrosion on all prongs; examine outlets for any sign of corrosion or overheating. Finally, purchase and leave installed a portable receptacle tester, which can alert you to many, but not all, shore-power and grounded/grounding system faults.

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