While sailing with Seattle friends who voyaged to New Zealand on their J-42.... we came back to the boat one night, flipped on the breaker for the lights and... pow, off instantly. The owner's an electrical engineer and did what any EE would do. He put his meter on the circuit and flipped the breaker on again... result was 100 amps and near-instant trip on this 15 amp circuit. After an hour of dividing up the loads, he found the problem. The manufacturers for this oldish boat (hull 2 or 3) had routed a pair of light fixture supply wires up and over a stainless chainplate. A wooden trim piece was squashed down hard over it, forcing the wires into a sharp turn. The trim was charred but did not ignite. We were very lucky not to return to a sunken lump of molten fiberglass.