Shore Power Fault - Above my pay grade.....

[Davidasailor26]

The local marine shop only had a 2 pole, 30A breaker in stock and they suggested just connecting Neutral and Ground together. I was surprised by this, since everything I read indicated that Neutral and Ground should not be connected on a boat

You are correct - Neutral and ground only get tied together at the power source. Unless you have an inverter or generator onboard this means the neutral and ground never get tied together on the boat. The risk of tying them together is that a bad connection on the neutral could force current onto the ground, potentially through the water back to earth. That’s bad for underwater metals and any swimmers in the area.

I installed the new breaker and left Neutral and Ground disconnected from each other. I measured 115 between Live and Neutral and Live and Ground. But with it configured like this, turning on a load such as the LED lamp, resulted in the voltage between Live and Neutral dropping from 115V to ~100V. If it was a higher load, such as a 700W heater, it wouldn't even run and the reverse polarity indicator on the panel would glow.

I then connected Neutral to Ground, and everything works as expected.

I’ll add another vote that this is caused by a bad connection somewhere in the negative line. The voltage drop indicates a bad connection, and the fact that the drop goes away when the neutral and ground are connected means the problem is on the neutral side.

 

[Jan11]

Being in a boatyard the pedestal you are connected to is a long way from the source. If properly wired the neutral and ground are only connected at that source. Along the way from the source to your pedestal there are a lot of other pedestals and likely other loads - lights, equipment. There are voltage drops in the neutral wire same as the hot wire and none in the ground wire. You have leakage currents in the cables. There may be currents flowing in the ground wire to other sources producing voltage drops. Measurements to an unloaded neutral wire and ground could also be affected by using a digital voltmeter which typically has a fractional microamp input as opposed to an analog meter with typically 50uA or more. Based on readings to four significant figures they were using a digital meter. The readings might have been better if you had a load connected at the boat making the leakage currents insignificant.

Frequently in a boatyard the reverse polarity indicator doesn't work as intended. This is also a reason to use an isolation transformer so it becomes the source allowing the neutral to be connected to the boat's ground. Virtually no one does.

Based on some or all of these possible situations your friend doesn't have an on board problem.

 

[Whitespeed]

If the bonding is done correctly, then there is zero volts potential between neutral and ground.
So, you go back to the dock outlet. If the PD between N & G is anything but zero, you can stop there.
Meters have a high input impedance. When you measure the dock voltage with no load, then the PD between N & G must be zero.
My interpretation is that the N G bonding at source isn’t a clean connection. The other possibility, (although unlikely) is a floating ground at the pedestal.

This all starts at the dock outlet, and go linearly from there.

While I didn't think to test at the pedestal, I did test N to G at the boat panel with N and G disconnected from each other. My recollection is that I did this test with the LED lamp connected and powered and I saw approximately a 20V difference between N and G. I don't recall if I checked N to G with no load. I made no measurements at the pedestal.

Just another idea for debug... Recreate the conditions where you see the 100V (I guess at the breaker on the boat) with the ground and neutral disconnected and the 10W load. Then see if you can measure those voltage with extension cord connected up starting at the pedestal. If you see the 100 VAC at the pedestal between H/N, the marina has a problem. But if that pedestal under that load is 115VAC, the problem is "down stream" and you could continue to measure the voltages in the "chain" to isolate where the problem is or possibly check for continuity.

Thanks for the suggestion. I don't know how accessible the pedestal wiring is and I'm reluctant to go poking around inside of it. But I'll keep your idea in mind.

FYI, we can assume that the marina pedestal does not have gfci or elci because you would never have been able to connect ground and neutral at the boat without the gfci tripping.

Safe assumption. From the quick look I had, there just appears to be a standard, single pole breaker.

If the boat is in fresh water. best to get this fixed (ie ground and neutral should not be directly connected at the boat) as its possibly a safety hazard to someone in the water near the boat.

My concern as well. However, from what I recall, the AC ground on the boat is not connected to the DC negative bus, so there may not be a path to the water. But still not ideal.

Nigel

 

[Whitespeed]

You are correct - Neutral and ground only get tied together at the power source. Unless you have an inverter or generator onboard this means the neutral and ground never get tied together on the boat. The risk of tying them together is that a bad connection on the neutral could force current onto the ground, potentially through the water back to earth. That’s bad for underwater metals and any swimmers in the area.

I’ll add another vote that this is caused by a bad connection somewhere in the negative line. The voltage drop indicates a bad connection, and the fact that the drop goes away when the neutral and ground are connected means the problem is on the neutral side.

There was an inverter but it was removed long ago apparently. Agreed about the risk to metals and swimmers. Interestingly though, the ground bus is not connected to the DC negative bus. So the AC ground is isolated.

Based on some or all of these possible situations your friend doesn't have an on board problem.

That's looking more and more like it's the case.

Nigel

 

[Deparlocean]

Hello all of you!
I read a bit of the thread but not all. Here are my conclusions for the differences of voltages on the ground terminal when connected to post but not to boat: The resulting increase of voltage over the lenght of cable results from induced voltage from magnetism, the principle of transformers, and increases with lentgh of wire.
As for trying to know if your cables are good, I would check independently the resistance of each circuit from end to end of cables and check the isolation from each others. Me too I think that the problem of voltage on the ground wire comes from the post.