stray voltage?

[Victoria28]

Hi, 1992 Catalina #246 - I am installing a replacement Seaward 6 gal tank. 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.
Thanks

 

[Don S/V ILLusion]

Ground potential is almost never zero except in an ideal world. Assuming you were disconnected from both shore and battery power when you measured this, it is:
a. an indication of the meters internal ground potential,
b. an indication of voltage potential between two dissociated grounds (most likely), or
c. a galvanic voltage which fluctuates because of a faulty connection someplace (least likely).

In the case of "b", if you have your Ac and Dc grounds bonded, you have identified the problem.

My guess is that you have, as many people do, multiple DC ground points on your boat which is the single worst offender causing corrosion.

In any of the above, if you have underwater metals (e.g., shaft, prop, thruhulls) which exhibit no adverse effects, the course of action might be nothing more than "watchful waiting".

 

[Victoria28]

Don, thanks for this - I was in fact still plugged to shore with the main AC breaker shut off - DC system still active. This boat does go through shaft zincs about 2 every 3 years and the the last tank died of what appears to be tank corrosion - (this Seaward is an aluminum tank in a galv. box held together with SS screws). I checked the dock plug voltage and got a reading of 114.5V.

 

[Don S/V ILLusion]

If your zincs last 2-3 years, you don't have a problem