by kludge » Tue Jun 30, 2015 6:03 pm
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!!!