... While using my multimeter to check the existing wiring to ensure that the power was off prior to hook up I noticed that while
there was no reading between black and white, or black and green,
I did get a reading between white and green -
it quickly fluctuated back and forth from 0.1V to as high as 1.2V.
Any ideas what is going on here? is this a problem? I have decided to not
proceed with the hookup until I feel confident about this.
With boat wiring almost nothing can be taken for granted.
Is this an AC circuit? (I guessing yes but...) I suspect the jumping around could be the result of a frequency other than 60hz (meter may not like that). Some meters will measure the frequency (which can often say alot).
If it is the AC only then white is neutral and green is ground (just being clear). These circuits are not broken when breakers are opened (agian a reminder to me and being clear).
Unplug boat, measure voltage between green and white, record both voltage and frequency or record voltage when using AC voltage setting and DC voltage setting (be sure to use correct voltage setting for meter so you do not measure a 120volt circuit with 12 volt meter setting)
If voltage is zero with cord unplugged, but 0.1 to 1.2vac when plugged in then you have traced it down to just the AC circuit. Under idea conditions the voltage between these two points is zero but really they are not.
The only point at which they should be connected is back in the panel in your house (or close to the ground point for the AC system on shore) but really there are lots of other little connections (wet plugs for example) and inductive coupling.
If this was my boat and I had normal zinc use and everything worked fine, I would likely ignore it. If it was a customers boat I would start checking to be sure everything was clean and dry and the isolation system was working properly, ect ect then I'd say it was a shore problem.
Kinda hard to say it is a problem, depends on so many things, like zinc use. I expect that the DC negative is well grounded to the AC ground....and so on.
Another boat in the marina can cause this (voltage between neutral and ground) so sometimes shutting off power via breakers can give more info.
Thats all off the top of my head at this point.