Strange low voltage on shore power

[vetting]

I have a 50 foot 30 amp power cord and receptacle. I started having intermittent issues with getting a reverse polarity warning and low voltage on my boat. It slowly got worse over the last 2 weeks. It went from getting a reverse polarity light if I turned on anymore more than a small overhead light to getting the reverse polarity light if I just had the main breaker switch on. I checked everything on the back of the AC panel and everything checked out fine. I did some diagnostic work and did some testing with a volt meter.

With the power to the plug, the end of the plug unplugged from the boat I get the following readings.
Black to ground - 120v
Black to white - 120v
White to ground - 0v

So that tested fine. So then I plugged the cord back into the receptacle with the main breaker and all of the breakers turned off on my AC panel. I took a reading from the back of the receptacle on the actual screws that hold down the wires.

Black to ground - 120v
Black to white - 41v
White to ground - 0v

I thoroughly cleaned the contacts on the receptacle and on the cord and got the same 40 to 42 volt reading on black to white.

Could the receptacle be bad? Is there anything else that I can try?

 

[alldodge]

Sounds like you have a single 50 amp 220 service and are using a Y connector. Is this correct?

No take that back, you have a 30 amp 120V connector

Take a pic of what you have

 

[dingbat]

Test 1 = no load on wires.
Test 2 = load on wire. Voltage drop, reverse polarity lights with increased loads = wire / connection unable to carry the current causing an unbalanced condition which set off the polarity warning.

Best guess: Bad wire / connection in power cord, a short/ loose wire (ground) in the fuse box, polarity problem (neutral and hot reversed), invertor going bad.

 

[Grandad]

I suspect your neutral path has a poor connection somewhere. I'd set up the test again, confirming the 41 volt reading, plug in some load and test again. With the same load still plugged in, start testing between every white conductor and neutral terminal to find that lost 79 volts. When you find 2 points in series on the white/neutral path that shows 79 volts as you bridge them with your tester, you've found the poor connection point.

By the way, don't always trust your meter. I have a digital multimeter that recently indicated I had higher than normal voltage of 150 volts AC. When I tested again with my Simpson 260 workhorse analog multimeter, the voltage read 120. I found out that the 9 volt battery in the digital meter was dead. After replacing the battery, the voltage reading returned 120 volts. - Grandad

 

[vetting]

Yeah but Im im testing right on the back of the receptacle, so wouldnt that mean its losing 80 volts between the plug (which reads 120 fine) and the internals of the receptacle? Basically wouldnt it be the the same as having no wires going to the back of the receptacle and doing a voltage test on the back of the receptacle?

 

[vetting]

The company that makes the receptacle and cord are sending me a new receptacle. They think it may be a problem with the safety thermostat inside of the receptacle. As a final test tonight, Im going to remove all 3 wires from the back of the receptacle and do the white to black test again to see what the voltage is.

 

[gm280]

vetting, I'd be very careful until you do find the problem. This isn't a mere 12 volt circuit. Be safe.

Grandad Those Simpsons 260 (any series) are absolutely great analog meters. Nothing beats an analog meter for varying resistance and fuel sensor reading or even watching capacitors charge. Digital meters have the place and I have a few Flukes, but you have to have a good analog as well. I also have a Bruno AN/PSM-6. Another fantastic analog meter. But I like the Simpson meter better. It doesn't take any special battery type.

 

[StarTed]

Another trick if you have a "wiggy" (solenoid voltage tester) for 120 to 600 VAC is to clip the DVM to the leads and then probe the circuit. The load provided by the wiggy prevents from reading stray voltages due to the load added by the solenoid in the tester.

A person can also use a light bulb for loading. A 500 watt bulb will show up about any neutral problem.

 

[Grandad]

I'm struggling here a bit. From what you've said, I understand you have a receptacle with an internal thermostat that's got a different voltage between test points on the front of the receptacle from the voltage between test points on the back of the receptacle. Certainly does sound like you've narrowed the problem down to this device. I don't think I've seen one of these critters before. :uncomfortableness: - Grandad

 

[vetting]

It ended up being the receptacle - most likely the thermostat inside. The manufacturer sent me a replacement under warranty and works like a charm.