Doubt about a derivation to ground in DC

Hello,

last week I was working in a ferry boat for 350 passengers and at one electrical panel I saw a negative derivation to ground was indicated by a light.
I was checking that the derivation was probably caused by our company electronic system because when I switched off our power breakers the derivation light was off. I checked then our electronic system and I found -3V to ground. Our electronics installation always specify that negative of the battery must be isolated from ground, at least 4 Mohm. When I measured between ground and negative I measured 1.5 kohm aprox.

These boat has 4 engines with its electronic systems and only one of them was partially grounded (failure). I started looking for the partial short and I found that when I disconnected a cable harness going to the ECU the resistence changed and went to more than 4 Mohm so I supposed that caused the derivation. Then I checked again the light and it was still on, measuring with multimeter I still got the -3V.

My question is, how is possible that I got those -3V if I tested that ground and negative was already isolated???

Thanks.

well for once, be carefull who you trust. digital multimeters are fine and dandy but with tipical 10Mohm input impedance on voltage range, they can easily be thrown off. i'm not familiar with marine circuits and reglations (at least in Canada, there is dedicated electrical code for marine vesels) but generaly floating circuits are rather not popular. (by floating i don't mean on boat ). mediteranian water is salty, ship of that size has plenty of friction (to water and air), this could be induced static charge for all i know. this is why on gas stations for example (and anywhere where chemicals are dispensed), bonding is regured to avoid ignition.

Hello Panic Mode,

I´m agree with you about that kind of measurements with digital multimeters so I guess the best way to check derivations to ground would be to use a DC bulb, easy but also practical and real indication...

I know the situation I explained is not normal but I checked many times the measurements and I also found that cable harness which could have any via to ground. Really strange!!!!

Thanks.