Electrical Conundrum

[jadedave]

My build is nearly complete including the electrical system for my TT . Everything electrical is working as it should be except for one small issue. I have discovered that the trailer chassis has a 22 volt AC charge as measured with a volt meter from the chassis to a wire stuck into the soil next to the trailer. If I am bare foot and on damp soil I can feel the current slightly when I touch the trailer or aluminum skin . I checked the electrical components and wiring and found that if I disconnect the AC shore cord the 22 volt charge disappears , also I found that if I just disconnect the converter unit from the onboard 120v AC outlet, the chassis charge drops to 8 volts.. Im wondering if I've got my grounding wrong . As it is, Ive connected my 120v AC 30amp shore power cord to a main 30Amp onboard breaker and then to two 15 Amp branch circuits. The ground wire on the 120v shore power cord is connected to the chassis as is the 12v DC battery and fuse block. Does anyone know if the 120v shore power ground should be isolated from the trailer chassis ? There might be some other reason for this issue as well .. Thanks for any advice ..

[Socal Tom]

Something is not right that is for sure. You shouldn't be getting voltage bleeding through the ground like that. I'd try pulling beakers to see if its in the wiring somewhere. Make sure your are only send the ground to the chasis, and not the nuetral.
Tom

[Shadow Catcher]

There should be no AC grounding to the chassis. Our trailer came with both the DC and AC grounded to Chassis, it fried the converter and toasted an AGM battery. Since all the 12V has separate ground wires I got rid of all grounds to the chassis.

[H.A.]

......

[jadedave]

Many thanks for the concern and advice from the tnttt community . I double checked my AC wiring and found that the on board neutrals and grounds were separate, I also verified that I had a safety ground (green wire from shore cable to chassis ground block . Lastly I inspected my shore cable and discovered that the ground terminal (green) wire had become detached inside the plug. Once repaired, the chassis voltage to ground dropped to near zero. Problem solved.
I do plan on purchasing a three light electrical checker as it is cheap insurance in the event that a camp ground hook up is damaged or defective. Id rather buy a checker than risk toasting a laptop , converter box or worse.

[Dale M.]

Good finds.... Both ac (safety) and dc grounds can and should be bonded to frame and skin.... As in your case if you have a potential difference there is a problem and diligent trouble shooting is required and and you case it found two problems....

As like in extension cord to shore power, yes there was a problem but its was not apparent visually.... These thing need to be checked occasionally just for this reason...

Happy camping...

Dale

[kludge]

Being an electrical engineer, I have seen situations where there is measurable voltage on the "ground" of power supply circuits.

First, let's get the terminology -- the word "ground" is used casually, and in many cases it only means that it is a "common" or the return side of a voltage source like a battery or power supply (like the DC converter in your trailer). Then there is EARTH GROUND which is the electric potential of the earth, or at least the small piece of it that you are standing on.

If an electric system is not EARTH GROUNDED then it's "common" or return point is said to be "floating" (like the negative side of the tow vehicle's battery).

The "common" is just a reference point from which you measure all other voltages in a system, on a car, it's the negative side of the battery.

When power comes into your house, the neutral wire from the transformer on the telephone pole is connected to the neutral bus bar in your electrical panel. Then a big fat EARTH GROUND wire is also connected to the neutral bus bar and also to your water pipes (which are buried in the earth) or a big long copper stake to EARTH GROUND your house and all the wiring in it. It's used for safety in case of a fault occurring somewhere in your house wiring or in an appliance. Electric current will flow in the ground wire in the case of a short and the breaker (or fuse) will open and prevent shock or fire.

OK, now for some explanation...

I have seen several power supplies, especially ones with autoformers (a type of transformer), in my day that are capable of putting enough voltage on the neutral or "common" side of the output, that you can feel a tingle or a buzz if the case is touched lightly. I have been able to fix a few of these by actually grounding the transformer and case and wiring it differently (sometimes also by changing the cord to a 3-prong type). This means that the case and transformer are no longer "floating". When the "floating" neutral is connected to the "ground" or "common" of your trailer, and the DC coverter is putting voltage its return side (the negative connection), then there can definitely be a difference of potential that you can measure and feel between it and the earth.

This is not something I would recommend doing if you don't totally know what you are doing - you could easily kill yourself or a loved one if you do it wrong.

A well meaning technician almost killed me once. I asked for a 5kVA (5,000 Watt) transformer to be installed at my lab at first job out of college (designing gas ignition controls for appliances - some used in RV's -- e.g. Atwood, Suburban) to power European appliances which used 230VAC. He installed is according to US code (two hots) and I almost died when I plugged in the appliance and the whole chassis was carrying 115VAC!!!