[PSUBS-MAILIST] Earth Fault

[James Frankland via Personal_Submersibles]

Hi All

I have a very peculiar issue.  I have an earth fault on the boat.

My test consists of holding one probe of the meter on the positive battery
terminal and the other to the hull somewhere.  I was showing a reading of
24v.  So obviously a negative connection somewhere to the hull.

I went around everything taking things off and have tracked the fault down
to the lights.

The lights are the trustfire ones and the negative connection is grounded
to the chassis of the light.  However, when I fitted the lights, I was
aware of this and so insulated the mounting bracket from the  light casing
itself with a piece of rubber.  So theoretically, there is no physical
connection from the case to the hull.  Only the internal wire.

Anyway, if I disconnect the lights and leave them dangling on their wires,
there is no earth fault.  The lights all work, and the hull is clean of
current.

So the lights must be leaking back through the connection somehow, but I
cant see how.  The case is insulated from the mounting bracket with rubber
and the brackets are connected to the fibreglass faring, so it shouldn't
leak back?

Anyway, ive fixed it by insulating the mounting bolts with delrin washers,
but I cant see how the earth could return through a piece of rubber and
then glass fibre.  A mystery unless anyone can see something obvious?

Regards
James
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