GFIs on the dock
only see half the problem. A hot boat leaking DC into the water
or injecting DC current
into the ground lead will not trip a GFI.
With years of experience I can say that running round and measuring the voltage in the water
, even with the right equipment
, will give you a mountain of data that defies analysis, even by experts.
If a particular boat is having problems thier first line of defense is an isolation transformer but since most can't afford the cost, space or weight a Galvanic Isolator
is $100 of insurance
If the AC Voltage across the isolator is greater than about 0.75 volts then the methods suggested above for isolating the culprit can be used. I've measured as much as 5 volts AC on a marina ground. While the hot wires were 10 gauge or heavier the ground wire was only 16 gauge. It only takes one miswired boat returning current
via the ground instead of the neutral to create havoc on the light gauge ground wire.
The most common cause of boats putting AC current on the ground is a combination of two things. First is a not uncommon wiring
error (especially if the boat is using a domestic house breaker box) where the neutral and ground are connected together. This is WRONG. Then if that boat has a faulty shore power
connection and loses the neutral the problem is compounded when the current flows back in the ground lead.