From: John De Armond
Subject: Re: electrical problem
Date: Tue, 14 Nov 2000 12:24:46 -0500
Larry Daniels wrote:
> The other day I was doing a roof repair and (of course) it started to
> rain right in the middle of it. I finished the repair including
> replacing the vinyl inserts. While on my knees fastening a screw at
> the bottom of the trim, I received a slight shock. At first I just
> thought it was a BIG static electricity shock.
> Well, the next day I went out to check on the job and was on a 6'
> metal step stand (like for aircraft repair). With one hand on the
> grounded step stand (stuck in the mud) I reached out and touched the
> roof and ZAP ! a really nice jolt !!
> I realized the trailer was plugged into an regular extension cord (via
> an adapter) that was missing the ground prong (really smart, huh ?).
> Anyway, my questions (finally) are:
> Does this mean I probably have a short of some kind?
> or would this be normal if the trailer was not grounded at all but
> or might I have some water standing somewhere like a light fixture, or
> a/c or something?
> or could this have been a DC shock?
Since it bit you with just a casual touch but didn't severely shock
you, that indicates that the chassis was charged with high voltage,
low current energy. Just what one would expect to find with the
ground disconnected. There are several possible normal sources. If
you have a solid state 12 volt converter, the input line filter that
keeps switching noise off the incoming line will couple a few ma to
ground as part of its normal function. Surge suppressed outlet
strips and/or surge suppressers frequently bypass both the hot and
neutral lines to ground with small capacitors. This leads to more
ma flowing toward ground. It is not all that uncommon to find the
neutral and ground strips connected inside the breaker panel. It
should not be that way but panels typically come either
interconnected or with a kit that makes it easy to do. If this is
the case, then voltage drop through the neutral leg, such as through
a light gauge extension cord, will elevate the whole ground system
and therefore the trailer chassis above ground. Usually this
results in just a few volts of potential on the chassis and the
sensation is tingling or slight jolting and not a full scale shock.
But not always. It would not be uncommon for there to be leakage to
ground through the windings of the fan motor and even the compressor
in the roof AC. A motor can leak significant current to ground
through wet, contaminated or slightly damaged windings for years
with no external indication. From my HVAC experience, I know that
often as not if one puts a clamp-on ammeter around the ground lead
of a unit, one will see some ground current flow which is almost
always caused by motor winding leakage.
The only way to be sure is to measure. Even though I'm a
measure-holic, I probably would not bother and would just hook up
the proper 3 wire cord and be done with it. If you do want to
measure the situation, the first step would be to measure the AC
current from the floating ground pin to a known good ground. If the
source of the chassis voltage is leakage and/or bypass caps, the
current will probably be below 20 or 30 ma. Another measurement is
to measure the voltage on the chassis to a known good ground. If
the voltage is near line voltage AND the current measurement above
is low, then you can be assured that the source is normal leakage.
In any event, there is likely little you can to to get rid of this
leakage. Even if you find the source and get rid of it, it is
likely that other leakage will crop up later, either from moisture
in a motor or transformer winding or because you added some other
electrical device to your RV that leaks. The proper solution is to
ensure that you always have a good ground when your rig is plugged
in. I keep one of those little ground/polarity checkers plugged
into a 30 amp to convenience outlet adapter in my rig. I ALWAYS
check the outlet at a CG before I plug in. I've found more than one
with no ground. If someone wanted to make some bux, they'd make a
30 amp pass-thru connector with the ground checker built in so that
one could leave it in line all the time.