Sunday, December 8, 2013

The reverse polarity light AC leaking musing

If white touches green, your boat can get mean. What this freshly made up maxim means is the neutral white AC wires are never supposed to be connected to the green grounding wires on your boat period. Well almost, the gods of technical details say there is a small exception to this rule, and it is the reverse polarity light.

This is not a rocket science device. It consists of a 25K ohm resistor and a light bulb. Its function is to tell you if the shore power electrical source has the black (hot) and white (neutral) wires reversed. You will not often see this reverse polarity light glowing on your panel, and if it does you should pay attention to it.

We are able to break the rules about ground/neutral connections by using a resistor in its circuit. In the little diagram above the indicator light is connected to the incoming neutral wire, and grounded to the green wire.

The current has to pass through the 25K resistor. Why a 25K resistor? It a ohm's law, and GFI outlet thing. Without dragging you through the math, the current is limited to only 4.8 milliamps. This is typically just under the current needed to trip the GFI outlets on your boat, and likely won't cause spontaneous combustion.

So how do you know if your reverse polarity light is working? The answer is you usually don't. Most marina's are properly wired, although I suspect that serious cruisers might disagree about this from time to time. So outside of bad shore power cords, and dilapidated marinas whose wiring is maintained by night shift janitors who work in a porno theaters, you shouldn't typically see the light come on.

This brings me to part two of my story. This boat is a leaker. By that I mean it has a dry bilge, but is spewing a small amount of AC current onto the green ground wire. I know this because my magic meter has alerted me to the problem.

The magic meter is in reality a Hioki clamp on amp meter. I acquired it when I became involved in a somewhat painful effort by a yacht club to become electrically perfect.

The boat owner originally believed he was having a "zinc's are being eaten to quickly stray current problem." He may have been right. Both galvanic isolators had failed, and I replaced them. No zinc problems have occurred since then, but in the process of looking at the boat for issues, I put the amp meter clamp of woe on his shore power cord, and you can see the result. One point three AC amps where they shouldn't be.

All we know at this point something is leaking to something somewhere. This almost always means, parcing my words here carefully, that AC current is riding on the green ground system. If everything is perfect the meter should say zero. Is it on the marina side from another boat or dock wiring problems? Or does it lie within the boat itself?

In an effort to determine what's up with the boat, I do the big Y shaped autopsy cut, and flay the distribution panel wide open. I free the harnesses from their cruel oppressors the tie wraps. This lets me easily poke around and see where things are connected and do some testing with out contortions.

The leak is a bit elusive. If the main AC breaker is shut off, the shore power amp leak measurement stays about the same. Turning on and off breakers don't appreciably change the readings either. 

My ohm meter says there is a connection between neutral wiring and green ground. This is not surprising because there should be a wee bit. Remember our 25K resistor? I take a gander at the polarity light wiring. There are two polarity lights, one for each 30 amp circuit. One side of the light's wire goes to the white neutral incoming feed on the breaker, and the other wire goes to the green ground. 

The two wires that attach to the green wire are connected together on the bolt that holds a gauge in place but there is no green wire. At first I think this is wonky, it can't work because there is no ground connection. The wiring diagram clearly shows this connection.

However down lower on the panel the green ground wire is attached to another bolt that holds a different gauge all by its lonesome. These bolts are studs welded onto the panel, so the whole aluminum panel is part of the ground.

I'm really not sure how I feel about this. I guess its okay, I think. In practice though it seems to be a large unprotected ground block partially connected to the to the AC neutral wiring. Now insert tons of AC wiring breakers, and gauges into it. I dunno, and I wouldn't do it this way, even if I can't quite clearly elucidate what makes me uncomfortable.

Lets flop back to my AC leak. If the black hot wire was connected just a wee bit to the white neutral wire, you would see the reverse polarity light illuminated, and it isn't. My ohm meter says I have a high ohm connection from the black hot wire to the green ground on the input side of the breaker when the breaker is off. So what's left is for me to carefully check the shore cables and their Y pig-tale. If there are no problems there, then the shore power inlets. If that's zip... I don't know, I'll figure something out.


  1. Great article Bill. I like the panel being grounded. Better to have the current go through the ground vs. through me if a wire shorts opening and closing the panel.

    On the leak - did you disconnect the power cable from the ship to see if there was still leakage? It has to be the panel or the cable if you saw the leak with breakers off. After having a new Marina Guard installed at our Marina this summer we found dozens of leakers. Coffee makers were a frequent culprit. Boating in fresh water makes this an important topic.

  2. Always a welcome learning opportunity here, Bill. And I just purchased a Mastech clamp meter to chase down some issues now I'm on the hard, like why if I put in both 30 amp shore power cords (currently running just 15 amps each to yard power) and flip my "selector" switch from "single" to "double", do the lights go out on the boat? From answering such questions are better boat maintainers made...or electrocuted, I suppose.

  3. Having the panel face tied into the ground is pretty standard on aluminum panels. Same way you would ground the metal case of any electrical appliance. Based on your description I would be checking the cord and the run of wire from the shore inlet to the panel.

  4. it looks like one of the reverse polarity light is wired to the 'hot' lead on the breaker. Shouldn't they each just have a lead going to the neutral lead on the main breaker?

  5. Thanks Michael, I probably should have labeled the the other light. It's a 110VAC power indicator. The other 30 amp power light, and reverse polarity indicators are lower on the panel

  6. Hi
    I have reverse polarity light on my boat and it light when i hit the test buton.
    I have 30 amp plug in and to test if this is my conection on fault i plug my boat on a regular 15 amp and the test are the same,the light on tester turn on again.

    Is my boat as a bad conection?

    1. Michel, It's hard to tell from the info I have. The test light verifies the bulb/led is working. Typically this works whenever you push the test button and you have power. If you have reverse polarity the light turns on. So generally if the light turns on by itself when you plug into shore power it means you have reverse polarity. If the light is off it indicates everything is okay.

  7. If you have a battery charger or inverter in the system you need to check for leakage to the ground from them. They are notorious for it.

    1. Hi Erin, and thanks for commenting. Your right about both chargers and inverters being notorious forleaking into ground systems, and when I'm chasing down stray currents these are the first boxes I take a good look at first.

      That being said if one of these devices was leaking AC current on the neutral, it could cause the polarity light to come on if it was large enough. If it was leaking AC on the ground side it would have to pass through the 25,000 ohm resistor with enough current to light the bulb with the neutral acting as a ground. I guess it's theoretically possible but to date I have never seen this.