Negative Grounds

by Jim Charles
(Maine USA)

I recently installed a small fire alarm panel into a fibreglass hulled vessel. I’m kind of new to marine electrics so excuse me if my questions are naive. The problem is that I cant get rid of an "Earth Fault" that keeps showing on the panel. I have tried to get some information from the suppliers however they wont support this panel despite selling it to us!! They suggested the ship may have the wrong type of Earthing System. As I understand it there are two types - floating Earth and negative grounded Earth. I am assuming the negative grounded just attaches the negative wire from equipment to the chassis at an earthing point (prop shaft?), like a car, while the floating Earth is independent of the DC live and neutral. When I asked an electrician about this he told me there was no earth on the boat but I could run a wire to the anode and connect this to the enclosure chassis. I didn’t want to connect the chassis to the negative in case it carried a current so I left it until I found an answer. Given this information can you suggest what type of Earthing system this vessel would have?

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Feb 18, 2008
Fire Alarm Ground Fault
by: Joel

Jim, I have been working on fire alarms for many years. The "Earth Fault" or Ground Fault light usually indicates a problem with the power limited side of the power supply. That would be a wire in the detection circuit , horn circuit, etc not a problem with the AC power feeding the panel. The wiring in a fire alarm panel is "supervised" for continuity, shorts , and grounds. All of this wiring must be isolated from any other circuits. Do not ground any (-) negative conductors. On occasion you can get a ground fault if you have a have bad or leaking back-up battery. Also..... do not take your DC power from your house batteries. The (-) terminal on these batteries is grounded and will cause a fault on your panel. You must have a separate battery that is completely isolated from the other batteries.


Aug 28, 2007
Boat Grounding
by: John Payne

Are we talking about an AC or DC powered fire panel here?. In all probability on any GRP yachts or boats the battery DC negative is grounded to the engine and by default this polarizes the system via propulsion to water via prop shaft etc so this is your ground. This means that you ground the unit as you suggested. I am perplexed about your electrician saying there is no earth, that cannot be true. If you have no grounded system them all circuit breakers would be dual pole, and in GRP boats this is almost unheard of. You must not get AC and DC earths, grounds and neutrals all mixed up and the same goes for bonding of zincs. In any case grounding chassis to this ground will not hurt.

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