Neutral to Ground Short

I've never had a problem plugging into a 20 amp GFI outlet using a dogbone adapter until now. I last plugged into a GFI a month ago without problems.
Now it trips the GFI on the sticks & bricks outlet instantly. All circuits appear to work normally if powered by the Genny. However it doesn't have GFI protection.
This coach has a 30 amp system.

Just prior to plugging in I power washed the Genny and basement compartments. There is an outlet in two of the basement compartments that could have gotten some overspray. The Genny got soaked.

Checking at the Shore Power plug I get 0.6 ohms - virtually shorted between the neutral and the ground pins. Thinking the wet Genny was likely the culprit I completely unhooked it from the coach with no change.

Next I went to the breaker panel. I started pulling the 8 neutral wires one at a time. Five of the neutrals (which includes the feed neutral) had infinite resistance to the ground bus. One measured 3.72 Megohms and another 1.9 Megohms. However one was at 1.6 ohms - shorted.

Leaving this neutral disconnected and the corresponding breaker off I was able to connect to shore power without tripping the GFI.

This circuit powers six outlets: two in the bedroom slide, two in the dining/couch slide, and two in the bedroom. None of these outlets have anything plugged into them. I don't think water is a factor. There's been no change a day and a half after washing. And I don't think water could have gotten to a junction or outlet anyway.

I haven't been able to locate any of the junction boxes. There are no signs of burning or other outlet problems.

Any thoughts on troubleshooting this problem?

Thanks, Tom

Dry out the outlets. 8)

The outlets on the affected circuit did not get wet. The only two outlets that may have gotten moisture are on a circuit that works fine.

A neutral to ground short won't overheat, so there won't be any sign of burning.

Since the affected outlets are in the slides, I'd start troubleshooting there.  Locate the wiring that goes from the main unit into the slide.  Usually it's underneath the slide with either a coiled cord wrapped around a support, or maybe just a loop of Romex that flexes every time the slide goes in and out.  There's usually a junction box at each end of the flexible wiring.

If the short clears, the problem is in that box or slide.  If it doesn't, the short is elsewhere.

Common locations for neutral/ground shorts.

Patio Outlet (Water filled)

Junction boxes UNDER a slide out (Likewise)

and if you have recently installed or worked on an inverter.. THe bonding system in the inverter may have been done IMPROPERLY.

Here?s what I?ve learned today. The power distribution center(under the lavatory) feeds a junction box located under the utility cabinet. There are two cables leaving the box. One is a Rome?s cable feeding the 4 bedroom outlets. The second is a SO cable that feeds the two outlets in the port slide. Unhooking the three neutrals and testing continuity to ground revealed the short to be in the feed circuit to the port slide outlets.

This SO wire runs through the bowels of the floor and into the approx 3?  square tube that supports the slide-out. I don?t see any way to access or inspect this wire  The wire exits the support tube and enters the slide-out by the outside wall. There should be a junction box under the couch but I?ll have to remove the couch to find it.

I?m not sure if I should continue going after this or just unhook the ground lead to this circuit and forget about it. Goes against my nature to do this but it would be major surgery if it turns out to be the wire itself.

If you haven't already, try unhooking the ground to the slideout, then measure it's neutral to the other grounds.  The SO cable may be shorting against the grounded trailer frame so unhooking the cable's ground may not clear the problem.

If you have to replace a shorted cable going to the SO, you may be able to attach a new cable to the old cable and just pull the new cable through the tubes, etc. 

The Junction box if any, should be under the slideout. WIth the slide extended you should see it.

With the SO cable disconnected at the supply end there is no continuity to the other grounds (chassis). The short circuit is between the white and green of the SO cable.

This SO cable does not have a junction until it?s in the slide. I can see it exit the slide support arm and enter up through the slide floor. Accessing this junction will require removing the couch and some carpet. Arg!

Guess I should first pull the two affected outlets to see if there is a problem with them.

If I am reading correctly, the neutral is not shorted directly to chassis ground.  The short must be in the wiring in a box.  If you check the junction box, and it is good, then you will check the outlets.  Take the easy way first and check the outlets first.  Hopefully, there will be no need to get into the junction box.

Another day and no resolution.

I checked the outlets and they are fine.

I followed the SO cable from the junction box under the washer/dryer, then under the shower best I could with an endoscope type camera attached to a stiff wire. I couldn't track it all the way to it going through the floor. I was looking for things such as mouse damage or a screw / staple in the wire. All looked ok.

I did manage to get to the junction box under the couch where the SO cable comes into the slide. Now I have access to both ends of a single SO cable. Testing showed that there is a short in the cable itself. This is getting ugly.

Reminds me of when someone runs a screw thru a cable

catblaster said:

Reminds me of when someone runs a screw thru a cable

Click to expand...

Nah, nobody's ever done that  8)

My old boss had a conversion van.  During a carpet install someone ran a screw into the top of the fuel tank.  It was never a problem for 20 years, until he pulled the tank to replace a sending unit.  Didn't discover the hole until the tank was re-installed and filled up.

I had a staple in a polybutyl water line for 38 years before the staple finally rusted out and it began to leak.

Can you pull a new ground wire and cut or disconnect the shorted one at some convenient point?  Splice the new wire in where the old one is disconnected?    The ground in SO is an insulated wire, right?  So no problem if it is live (like the neutral) as long as it is capped off  at its ends and not connected to chassis ground.

Gary RV_Wizard said:

Can you pull a new ground wire and cut or disconnect the shorted one at some convenient point?  Splice the new wire in where the old one is disconnected?    The ground in SO is an insulated wire, right?  So no problem if it is live (like the neutral) as long as it is capped off  at its ends and not connected to chassis ground.

Click to expand...

Normally, in SO cable, the ground wire is NOT bare, like in a NM or Romex cable.  It will have.......Green insulation in it

Whoops....I misread your post Gary.  I thought you said that the ground was uninsulated in SO type wiring.

Right, and since it is an insulated wire, do not need to worry if it has voltage or current present from the neutral. Just need to keep the bared ends from touching chassis ground or any other wire.

I pulled an access panel that would allow servicing the freshwater tanks. This exposed the front slide support and the point where the cables enter the support. Looks like the only way to replace the wires is to remove the entire slide out. Not going to happen!

I?ve decided to clip the ground wire and live with two ungrounded outlets. I hate to give up but...

Thanks to all for the suggestions and support.

Tom

If you?re going to run without grounds, may I suggest you install gfci?s at both the outside outlets.
The gfci will see if current out doesnt match current back and will trip.
In other words, if you?re getting current through you to the ground or grounded object, the gfci will still protect you.