"Hot" chassis - AC voltage present in chassis when on shore power

[DRMousseau]

Twice over the last 6mos, I have found stray AC voltage present in the chassis. This has been "shockingly" discovered both times, when I've been layin on the ground workin on something, anything, on the chassis. Noticed as a burning tickle on tender inner arm areas as you lightly contact the frame or chassis component,.... a familiar feeling for some of us, not so much for others who might be rather "inexperienced". Poking a probe from a multi-meter capable of measuring AC voltage into the damp ground, and touchin various frame contacts with the other prope, will confirm that this stray voltage is present,... and hopefully, not in a high reading!!! I was lucky in that only about 30VAC-60VAC was present each time.


The first time worried me, A LOT!!! I thought I had a VERY dangerous situation in my AC circuits!!! The Voltage was only present when on shore power,.... not on generator power, nor on battery/inverter power. After considerable circuit testing and checking, it was found to be a FAULTY EXTENSION CORD between the outlet and my shoreline!!! It was a repaired extension I had borrowed from a friend. Replaced with a new extention, and everything was fine. WHEW!!!


The second incident, was immediately recognized and I replaced the extension my son-in-law lent me, with a new one I had handy. Again,.... while appearing in good condition, and used by him often, it was still a faulty extention!!! This incident has caused me to add a new routine to my casual inspections. I often have a multi-meter in my hands for many various reasons. It takes only a few seconds to probe the ground and chassis for stray voltage while I attend to whatever I had the meter out for. It has also caused me to consider other safety devices such as ground-fault/surge protection.


AND,.... I now also check for stray chassis voltage EVERY TIME that I connect shore power to my home,... with, or without extension cordsets!!!
You should too!

[jeno]

thankyou i have the same problem had no ideia were to even start to look, running outside right now to change cord

[Rickf1985]

It always pays to have a meter handy. If you find that a new cord does not solve the problem or you want to be sure it is the cord and not the outlet you are plugged into you can check to be sure the ground terminal in the outlet is not open circuit. If it is then the problem starts there, fix that before spending money on a cord. OR, You can buy simple testers that plug into the outlet and have lights on the tester to let you know if the source is good or bad, they are cheap and very handy to have. I carry one in every tool pouch at work.

[jeno]

what kind of things can happen from that?

[DaveVA78Chieftain]

what kind of things can happen from that?

Electrical shock?  One common problem is the ground and neutral lead get swapped somewhere.

[jeno]

i guess what i ment was can it do any ele damage to the motorhome?

[DaveVA78Chieftain]

A neutral to ground short or a swap should not damage components however that can lead to electrical shock to humans and pets.  Some sensitive electronics may not operate properly.

Hot to neutral or hot to ground shorts can cause damage or at least cause circuit breakers to open.
Hot and neutral or hot and ground swaps are very dangerous.   Electronics can be fried, source power (shore, generator, or stick house) breakers will open and the high current draw can burn up wiring.

Some reference info related to ground faults:
120VAC Electricity In Your RV
When A Portable Generator Won’t Power Your RV
neutral to ground switching in rv & marine applications

Get a circuit checker / ground fault tester and use it

[1990HR]

A neutral to ground short

What is a neutral to ground short? Both neutral and ground wires are connected to the same bus is panels.
Are to talking about an open ground fault?

[TerryH]

"Get a circuit checker / ground fault tester and use it"

As Dave said, these are great as well as very inexpensive.
As per Rick, I have a few - one in every toolbox and a couple in appropriate cupboards inside my unit.
Very cheap and quick way to test for "cross wiring", whether inside your coach or outside in an extension cord. And a test for your cord itself.
Also a limited test for the Post wiring when you plug into a site power source.

[Rickf1985]

What is a neutral to ground short? Both neutral and ground wires are connected to the same bus is panels.
Are to talking about an open ground fault?

In most newer installations the ground and neutral buss are separate. That is the newer code. Some older installations have the same buss but not all.

[legomybago]

Most older homes don't even have a ground circuit, just a neutral. The service panels are grounded, but the wiring in the house itself is just a hot and a neutral. When you install a 3-port receptacle, it defeats the purpose of even having that third port for your electric devise you plug in. But it works fine.

[M & J]

It's mainly for what Dave says - personal safety. Kentucky requires a 1 time inspection of all campers before registering and its all safety related checks. Of which 1 is the presence of the electrical ground with the AC side of the unit. They connect to the ground pin of your shore power cord then verify chassis and outlets in the coach for continuity. They also check your 3 detectors, propane system for leaks, fire extinguisher and so on.

[DaveVA78Chieftain]

I am probably opening up a can of works here but here goes,
First we are discussing AC ground systems not DC.  2 entirely separate discussions and objectives.
This is a subject one has to understand the basics first.  If you don't then you will suffer the consequences.
First, electrical code states that the neutral and ground leads SHALL NOT be connected at a electrical subpanel.  An RV power center is considered a subpanel. 

Note: The neutral ground bar in the RV panel is also isolated from ground

When on shore power, the only place the ground and neutral leads are connected together is at the Main house (campsite) breaker panel (where the power from the power pole connects).
When using a generator,  the only place the ground and neutral leads should be connected together is at the generator. 
Note:  Some generators do connect the 2 leads together, many do not (they expect the connection to be connected elsewhere [e.g. the main house breaker panel).

The ground lead is for safety to allow a low resistance path back to the single point ground connection in case of a hot to ground or neutral to ground short.  Otherwise there should never be any current flow on the ground lead.   If, the ground and neutral lead are bonded together in the RV subpanel (or any where else for that matter), then AC current will flow back to the main breaker panel (eg house main panel) over both the neutral and ground leads (a no no).  Given that by code  the ground lead is to be attached to the case of appliances and metal encased electrical equipment (fridge, microwave, converter, and the RV chassis) a neutral to ground short or bonding allows AC voltage to be present on these metal devices.  If you are outside in the rain and you touch the metal body of the RV when this type of short is present, your body can act as the direct path to ground for the AC current rather than back to the main breaker panel.
A GFCI detector compares the current flow over the hot and neutral wires which should be equal.  A GFCI detector will open if:
1. Hot to ground short:  Current flow is higher on hot lead than neutral lead due to the current flow to ground resulting in the GFCI breaker opening due to the hot to neutral current imbalance (main breaker may also open in this case).
2. Neutral to ground short: Current flow is higher on hot lead than neutral lead because the return path current is split between the neutral and ground leads.

These type of GFCI failure detections do not have to be a direct short.  The break down of old insulation, moisture, etc. can cause current leaks.  A GFCI breaker works at the milli-amp level so it can open long before there is a major source for electrical shock.

When you do not have a GFCI based system, you use a circuit tester and multimeter to check your system.  If you have a problem, you need to isolate most everything and systematically work through all circuits in order to isolate the problem.

Rather than me trying to re-event the wheel, here are links to a series of articles by Mike Sokol at www.noshockzone.org about RV Electrical Systems.
RV Electrical Safety: Part I â€" Volts
RV Electrical Safety: Part II â€" Meters
RV Electrical Safety: Part III â€" Outlets
RV Electrical Safety: Part IV â€" Hot Skin
RV Electrical Safety: Part V â€" Amperage
RV Electrical Safety: Part VI â€" Voltage Drop
RV Electrical Safety: Part VII â€" Wattage
RV Electrical Safety: Part VIII â€" GFCI Theory
RV Electrical Safety Part IX â€" In Review
RV Electrical Safety: Part X â€" GFCI Testing
RV Electrical Safety: Part XI â€" Extension Cords
RV Electrical Safety Part XII â€" Providing Emergency Care

RV Electrical Safety: Surge Protectors vs. EMS
Mis-wiring a 120-volt RV outlet with 240-volts
Generator Ground-Neutral Bonding

[M & J]

Intersting Factoid: hes feeding the furry kids.

And yup. Can of worms big enough we all can go fishing.

[Rickf1985]

Worms, worms all over the place. I was thinking that when I posted about the isolated buss's.

Good illustration as usual Dave.

[DRMousseau]

YES!!! Thank-you Dave!!!


A tester, like the one Dave illustrated, is excellent for most folks with minimal experience or knowledge of electrical systems. It should be part of your standard equipment to quickly test the safety of your AC System each and every time you connect your shore line to an outside source. Dave also mentioned the many dangers and hazards of NOT doing so!!!


Oh and Thanks for those EXCELLENT illustrations and your comments. Knowledge that EVERYONE can benefit from!!!


I was recently asked by someone to troubleshoot an extremely "hot", emergency situation that unexpectedly appeared. It was discovered by a small child who couldn't really explain thru her tears. The parents "shockingly' found out in the same way. Turned out,.... that a tiny light bulb on their string of decorative mini-lights had been unknowingly crushed in the joint of the canopy support!!!


Summertime is filled with excitement and fun,... often in bare feet, and minimal protective clothing. But an RV becomes "an electrical appliance" when we plug it into "the post". And CAUTION should be exercised when using ANY electrical appliance or tools outdoors.

[DaveVA78Chieftain]

But an RV becomes "an electrical appliance" when we plug it into "the post". And CAUTION should be exercised when using ANY electrical appliance or tools outdoors.

Thats a novel but so very true way putting it!