Reverse Polarity and Inverter Damage

[dlochner]

A boater at my marina is claiming his Inverter was damaged because the marina had reverse polarity at his pedestal.

It is my understanding that most if not all AC electrical devices will continue to work with reversed polarity, however, since the neutral wire is now protected by the circuit breaker if there is a fault the circuit will remain hot because of the reverse polarity.

Is the boater's claim reasonable? Sorry, these are all the details I have.

 

[Project_Mayhem]

Well technically AC reverses polarity 60 times per second so I would have to say his claim has no merit without digging further into the exact operation of his electrical system

 

[Davidasailor26]

Well technically AC reverses polarity 60 times per second so I would have to say his claim has no merit without digging further into the exact operation of his electrical system

AC reverses polarity, but it does it by bringing the hot wire above and below the potential of the neutral wire. The neutral wire remains at the same potential as ground. In a reverse polarity situation the wire that should be hot remains at ground potential, and the wire that should be at ground potential goes up and down. So it’s not quite electrically the same as you described, which is where problems can occur. Usually it doesn’t matter, but in some situations where you expect neutral to be at ground potential bad things can happen.

Having said all that, I’m skeptical too. If the inverter is purely stand-alone, just providing power to a dedicated plug from the batteries, then the shore side polarity should make no difference, since the circuits should never touch. If the inverter is installed to provide power to the boat’s AC plugs then it must have some way to cut out the shore hot and neutral connections whenever it’s in inverting mode. Once those connections are disabled then the polarity of shore power is irrelevant. Before they’re cut (like if the inverter is on standby mode) then the inverter shouldn’t be connecting ground with neutral on the boat, so again the potential difference between neutral and ground shouldn’t matter. The only thing I can think of is maybe there’s some electronics in that switch that could somehow get shorted by the hot neutral, but I don’t see intuitively how that could be.

 

[JoeWhite]

I would think that the AC is connected to a stepdown transformer, then used. Therefore the reversal of polarity will not cause damage.

 

[dlochner]

then the shore side polarity should make no difference, since the circuits should never touch.

I think this is the key. The AC wiring may be faulty. If the AC Neutral and Ground are connected on the boat before the inverter and the AC Ground and DC negative are bonded, then reverse polarity would allow AC into the DC system which might then damage the inverter.

So the real culprit is the faulty wiring on the boat aided and abetted by the reverse polarity on the pedestal.

Sound plausible?

 

[Davidasailor26]

I think this is the key. The AC wiring may be faulty. If the AC Neutral and Ground are connected on the boat before the inverter and the AC Ground and DC negative are bonded, then reverse polarity would allow AC into the DC system which might then damage the inverter.

So the real culprit is the faulty wiring on the boat aided and abetted by the reverse polarity on the pedestal.

Sound plausible?

Yeah, I guess that’s possible. I can draw a circuit that looks a little like that and results in shore power going through the inverter negative on its way through the boat’s bonding system into the water, to return to shore ground. The bonds and negatives need to be in just the right places and I don’t know how realistic that is, but I guess it could happen.

I’d also be curious as to whether the inverter is properly wired with a cutout to prevent feeding both shore AC and inverter AC to outlets at the same time. That would be a more serious error, but it would result in the inverter working fine when off shore and failing if it got turned on at the dock, regardless of polarity.