Electrical problem

[Bob & Tracey]

We have an electrical problem I would like to run by the great people on this board before I start taking things apart. When I connect a volt meter between the shell of our bus and the ground (the actual earth) I get 45 volts AC. If I switch off circuit breakers the voltage will drop a little more with each breaker. I also have 27 volts AC between the positive and negitive terminals in the 12 volt system even when energized with 12 volts DC. I was told a power converter could cause stray voltage but I do not understand why each circuit breaker would have only a small effect on the voltage.

Any thoughts would be appreciated.

Bob

[eddieboy]

This sounds very similar to a problem I encountered at a friends house a few years ago.  I was reading 90 volts ac from the concrete slab in the garage to the roller guides on the garage door.  The garage circuit had lost the neutral lead coming from the main panel in the house.  Somehow, the voltage was bleeding through the windings of the opener motor. At least that is where I think it was coming from.  As you turn off breakers, you are opening the paths to neutral or possibly to ground if something is not wired right.  I would turn off all breakers and start with your source of ac. Meter hot to neutral and hot to ground.  Then look at ground to neutral.  You should only have voltage from hot to neutral and from hot to ground.  If there is a difference in the readings, you may have an open in the neutral.  Good luck and be careful.
Ed

[Chopper Scott]

Strange things happen when a ground is not right. I think Ed mailed it in his post. That's what I would be looking for. Later

[Len Silva]

Don't waste any time fixing this.  That 45 volts could turn into a deadly 120 volts at any time, depending on what the problem turns out to be.

It looks to me like you may have two problems which may or may not be related.

First of all, you don't have an adequate ground connection. A good solid ground would dissipate that voltage and/or trip a breaker at the source.

Secondly, something is causing that voltage to get to the chassis.

So:

1. Measure from the ground pin on the plug to the chassis. It should be as close to zero ohms as possible.
The ground connection should be at a separate strip in your main panel and should also be bonded to the chassis.

Be sure that the ground connection is good at the source.

2. Measure from the ground pin on the plug to the neutral pin on the plug, it should be open. If not, the neutral is grounded somewhere.

3. Check all the neutral connections in your panel(s), make sure they are clean and tight, and not connected to ground.

I think these checks will lead you to your problem.

I wouldn't worry at this point about the stray AC riding on the DC, assuming it goes away when the converter/charger is turned off.

[ol713]

Hi;
    Your problem is related to a ground problem.
    in your circuit breaker panel,  check to see if
    the ground and neutral bars are seperated. I
    think this is a must an RV.

                            Good luck
                                     Merle.

[Sean]

Bob,

It's hard to diagnose these sorts of problems by remote control.  But from your description, I would bet money that

• Ground and Neutral are bonded together someplace in your coach.  That could be, as already suggested, in the main panel, or it could be almost anywhere else, including at the end of a branch circuit (turning the branch circuit's breaker off will not affect this).

And

• The coach ground and the shore ground are not connected.  That disconnect could be in the coach wiring, the power cord, the shore receptacle, or the shore branch circuit wiring.

This is a potentially deadly combination, and the advice to fix it right away is, ahem, dead on.  Just to add a few steps to Len's excellent diagnostic procedure:

1.  Unplug the coach from shore power.  Make sure any generator or inverters aboard are also disconnected.
2.  Connect your ohmmeter or continuity tester between the ground and neutral slots on any outlet on board the coach.  On a 15-amp receptacle, the ground is the U- or O-shaped hole, and the neutral is the longer of the two slots.
3.  Read for continuity or resistance.  You should read "open" which is no continuity or "infinite" ohms.

If it reads anything other than open, you have a ground-to-neutral connection someplace that must be dealt with.  This is a problem in and of itself, irrespective of the other issue (which makes this problem more potentially deadly).  If this is what you see, we can guide you through finding it.

Next:

4.  Connect your shore cable to the coach (if it is the detachable type) -- do not connect it to the shore receptacle.
5.  Connect an ohmmeter or continuity tester between the ground slot on any coach outlet and the ground tang of the shore cord.  The ground tang is the U- or O-shaped pin -- the other two or three will be flat "blades."
6.  Read for continuity or resistance.  You should read a dead short -- either full continuity, or a very low number of ohms, in the 1-20 ohm range.

If it reads anything other than a dead short, you have a ground continuity problem in the coach wiring or shore cable.  Again, let us know and we can help find it.

If that reads fine, let's check the shore receptacle:

7.  Turn off shore power at the breaker.
8.  Connect your ohmmeter or continuity tester between the ground and neutral slots on the shore receptacle.  The ground is the U- or O-shaped hole, and the neutral is the center slot on a 50-amp (4-blade) receptacle.  If you have a 30-amp receptacle, we'll need to show you a picture.
9.  Read for continuity or resistance.  You should read a dead short -- either full continuity, or a very low number of ohms, in the 1-20 ohm range.

If you read anything other than a dead short, there is a grounding problem in the shore-side wiring.  If you read a dead short, next:

10. Turn the shore power back on at the breaker.
11. With a 30p-15r dogbone (and, if needed, a 50p-30r dogbone), connect a 3-light circuit tester to the shore circuit.  Alternatively, you can use a two-wire 120-volt test light if you have one.  Make certain that the 3-light tester reads "OK" or a two-light tester shows good when connected between any hot and ground AND any hot and neutral, and no power shows between ground and neutral.

If none of that turns up a problem, let us know and we can take it from there.

-Sean
http://ourodyssey.blogspot.com

[boogiethecat]

Just a quick addendum to what Sean has posted- a probable "why"...

First, yes I agree you DO have a problem, it needs to be fixed asap, and Sean and Len have outlined it quite well.
But there's still the question of why it's happening, incorrect grounding notwithstanding...

Built into virtually every electronic device and especially surge protectors, there are a set of two capacitors, very low in value- one connected from "hot" to ground, and the other connected from neutral to ground.  In some versions of surge protectors these are called the "Y" capacitors.  Their function is to provide a route to ground for any RF interference that the device has "captured". 
  A side effect of these capacitors is that they tend to create a "capacitive voltage divider" that tries to force the ground system to half of the supply voltage, ie 45-55 volts AC.   Because these capacitors are small in value, they don't have the ability to do this with a lot of force; ie the current that they cause to flow is small and though it can definitely give you a tickle, it generally won't cause a dangerous condition.  As you add more and more devices containing these capacitors to your system, the current that they cause to flow in the ground line increases.
This is probably why you saw your voltage reduce as you turned off breakers one by one.

Now the big deal is, as Sean and Len have mentioned, your ground system is not working.  Because of this you will see exactly what you've seen between your bus shell and ground, and more than likely it's because of these capacitor currents- so it's possible that it's annoying but not necessarily dangerous.  In any case it must be fixed.

But before I fix problems like this I like to find out if it's from the capacitors (which might not actually be so dangerous) or from something that actually is a serious problem.
A super easy way to figure this out is: Before you repair your ground,  hook a 7 watt lightbulb (ie a small 120 volt christmas light) between your bus shell and ground and see if it lights up.  If there is enough energy there to make the bulb light up even a tiny bit, you have a second problem in addition to your bad ground.   But if the bulb doesn't light up, that means that the 45 volts you're reading doesn't pack enough wallop to be anything to worry about and is probably due to only the capacitors.

Case in point, I had the same problem with my bus, and it was simply the capacitor thing and a bad ground connection, but it wouldn't illuminate the bulb.  So I fixed the ground, no big deal.

But long ago I rented an old (100yr) house and upon my first shower got shocked when standing in the bathtub touching the drain with my foot, and simultaneously touching the metal faucet with my hand.  Turned out that I could illuminate a 50 watt lightbulb with this same connection and still read 50 volts across it- what a mother****er when taking a shower-  turned out that the house had been retrofitted with copper plumbing, the plumber had not grounded the hot copper piping which was further isolated by individual cold and hot faucets throughout the house and dielectric couplings at the water heater, and a defective clothes dryer had it's ground hooked to the hot-water copper pipes.
Upon digging into the dryer, I found that the electric heating element was touching the frame at one spot... Lucky for the homeowner that the short was on the low side of the heating element and nobody got killed!!!

So in that case it was neat to use the lightbulb trick to ascertain that there was more of a problem than just a bad ground lurking around.....

cheers
Boogie

[Bob & Tracey]

Thanks for all the help. In checking the ground I put my ohmmeter between the ground theminal on the shore power plug and the ground terminal of an outlet inside the bus and I have an open circuit. Because of where the bus is parked right now I cannot get into the bay with the AC panel, I expect to move it on Tuesday. I will let you know what I find.

Bob

[Sean]

Bob,

Make sure you also do the ground-neutral continuity test on board the bus.  It is possible that it is merely a capacitor, as Gary suggested, but it is also possible that it is an improper bond someplace.  One's a nuisance, the other's dangerous.

BTW, I realized we never answered part of your original question:

The reason why each breaker seems to contribute only a small amount to the problem is that the potential difference (voltage) between the frame and the ground is a function of the total current running through the neutral.  As each successive device is turned off, the total neutral current drops by whatever amount that single device was contributing.

-Sean
http://ourodyssey.blogspot.com

[JohnEd]

El Boogay,

You are soooo good!  And an excellent story to boot.   Caps rarely fail "open" as I recall so they are a serious failure hazard under this "no ground" compound failure condition....me thinks.

Thank you,

John

[Bob & Tracey]

Ok here it is...

I could not get a 7W bulb to glow off the voltage passing between the bus and earth.

I did have continuity between ground and the neutral.

And yes the green wire in the shore power cord was connected directly to the neutral bus bar. Funny thing is that there are a couple of green wires in the box that all are grounded to the box/bus chassis except the the one on the shore power.

I now have a big fat 0 volts between the bus and the earth and no more tickle.

Thank you all for your help, this board is great...

Have a fun and happy Fourth,
Bob