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Author Topic:   Testing my boat's Ground
David Pendleton posted 04-14-2007 10:53 AM ET (US)   Profile for David Pendleton   Send Email to David Pendleton  
Is there a way to test or verify the quality of my boat's electrical system ground?

Thanks all.

jimh posted 04-14-2007 02:11 PM ET (US)     Profile for jimh  Send Email to jimh     
Dave--I am not sure if you are asking about the vessel's 12-volt negative bus or about a 115-Vac shore power system.

On most boats the 12-volt negative bus is the common point or zero voltage reference for any measurement. If you want to check on this distribution, I suggest the following:

--use a high-quality digital voltmeter (DVM)
--connect the negative lead of the voltmeter to the negative terminal of the vessel's main 12-volt battery.
--set the DVM to measure DC voltage and set the scale to its most sensitive
--make a temporary test lead which is long enough to reach from the battery to other areas of the boat where there is electrical distribution
--connect the test lead to the battery negative and observe the voltage; the measured value should be very low, around 0.01 volts
--using the long test lead, connect the test lead to the 12-volt negative terminal at various other places in the boat's primary power distribution system, and do this while all electrical loads are operating. For example, turn on all lights, pumps, radios, and other electrical loads
--check the voltage reading on the DVM at the various other negative terminals; the measured voltage should be very low, in the range of 0.1 volts.

If you find any places where the negative distribution wiring has appreciable voltage drop between it and the battery negative terminal, inspect those branch circuits for corrosion, bad connections, or damaged conductors.

The usual practice in a vessel is wire the 12-volt negative branch of the primary power distribution as a common with the engine block. The engine block is usually in electrical contact with the sea via the propeller shaft.

If there are any stray or parasitic currents in your boat which are driving current into the engine block, they can cause corrosion problems.

David Pendleton posted 04-14-2007 08:32 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Thanks, Jim!

That's just what I was looking for (12v system).

jimh posted 04-15-2007 09:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Some DVM's have a peak-and-hold function. That is a good way to capture the voltage drop between the battery and the engine when the starter motor is cranking.
David Pendleton posted 04-15-2007 10:16 AM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Mine does. I'll have to give it a try.

Thanks again.

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