The Offshore Voyaging Reference Site

Galvanic Isolator Rectifying AC to DC? Potential Death to Aluminum?

As usual immediately after launch, I checked our J/109 for stray currents using a silver chloride reference cell.

All was good until I plugged in the shore power lead and got a jump in voltage from -1.093 (natural voltage of the aluminum anode I have on the sail drive) to -1.25.

Note that all of these measurements were made with the single pole breaker off on the wharf and that turning it on made no difference.

If you decide to emulate any of the following readings, make sure that the breaker on the wharf is off, the polarity correct, and you clearly understand what you are doing. Making a mistake could get you killed.

At first I assumed that I had made some sort of wiring error, or something had changed, such as the neutral and ground connecting together on the boat—a no-no on boats without an isolation transformer. Nope.

Next I figured the galvanic isolator had failed to short circuit. But no, it passed the standard test using the diode setting on my high quality fluke meter.

What could it be?

Next I checked the ground on the wharf against the immersed reference cell and got -0.22 volts AC and -0.552 DC. Not ideal, but not terrible, or unusual, particularly when considering that the wharf shore power is over 200 meters from the transformer where the negative and ground are tied together and earthed to a stake in the ground, as required by code.

And half a volt DC is well below the 1.4 volts that the galvanic isolator is designed to block. What the heck?

Next I disconnected the galvanic isolator input and connected the meter in series measuring amps: 0 amps DC (good) but 0.28 AC (not good).

Clearly the galvanic isolator was passing AC. Again, not ideal, but then they are designed to stop low-voltage DC, not AC, and given that there is a big capacitor in there maybe this is intended behaviour.

I then reconnected the shore side of the isolator and disconnected the boat side and measured the voltage across the two and got 0.15 DC, just the amount the meter jumps when connected between the reference cell and boat’s bond system when the shore power is plugged in.

My Theory

My conclusion: the ProSafe galvanic isolator (from ProMariner) is rectifying AC ripple on the ground line and outputting DC.

If I’m right, this is a potential1 concern to anyone with an aluminum boat, rudder shaft, and/or saildrive, since any voltage over -1.1 is verging on over-protecting aluminum and it’s conceivable that if the AC ripple on a ground line was significantly higher than -.22 volts (reading on the shore power I was testing), the resulting DC output by the galvanic isolator could, when added to the voltage provided by the anode(s), exceed -1.5 volts, a potential that can, at best, blow the paint off metals and, at worst, eat aluminum in weeks2.

ProMariner Response

I have already talked to ProMariner, who were, shall we say, noncommittal, but finally admitted that maybe some rectification was going on, but then stated it was not a problem.

More To Come

I have a plan to get to the bottom of this but, in the meantime, two requests and a tip:


  1. Have any of you out there noticed a jump in hull and/or bonding voltage measured against a silver chloride reference cell when plugging in a shore power cable?
    • If so, which galvanic isolator do you have and did you check the voltage of the ground on the wharf in the same way as above?
  2. Does anybody out there, particularly you engineers, have a better theory than mine?

Anyone with a boat with any immersed aluminum and a galvanic isolator, rather than an isolation transformer, should make the above check every time we plug into a new wharf.

Further Reading

29 More tips on keeping aluminum safe from galvanic and stray current corrosion.

Thank You

Big thanks to the good people over at who worked through all of the above with me. No one I have ever talked to knows more about this subject than Bob and Tyler.

  1. Bad pun alert. ↩︎
  2. Source: Conversation with Bob at ↩︎
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Henrik Lundin

I have to admit my knowledge of AC electrics and rectifiers is not deep enough to follow a 100% your post above. But I have a question that is somewhat related to the topic.

Do you know why so called “fail safe” ABYC Galvanic Isolators are much more expensive than the Galvanic Isolators that are still allowed here in Europe?