Last night I had a call from an RV owner who said he was feeling a shock from his RV chassis while laying underneath it to change an oil filter. He had stuck a screwdriver in the dirt and measured the voltage between it and the RV's bumper, chassis, lugnuts, etc... He didn't realize that virtually everything metal on an RV is bonded to the chassis, so he didn't have to measure everything individually. He measured this voltage with the generator on, the generator off, the inverter on, the inverter off, and a few other iterations I can't remember. But what was interesting was that his measurements were showing only few hundred millivolts at most difference between the frame of the RV and the screwdriver in the dirt, yet he still said he could feel a shock when he stood on damp concrete and touched the frame of the RV. Of course, it would be impossible to feel 3/10 of a volt shock, but when I asked him what kind of meter he was using I discovered that he had it set to the DC voltage scale, not the AC scale as would be needed to measure AC hot-skin voltage. He said he selected the DC scale because he thought the shock was coming from the 12-volt battery in the engine compartment of the RV. Of course, if he has a failed EGC safety ground on his shore power connection, it's going to be AC voltage as a hot-skin. And if you set a meter to DC and try to read AC voltage it will show up as a few millivolts when reading 120-volts AC. I wonder just how many sound techs do the same thing? My beginning college students will do this in at the start of my audio electronics class, until I beat the difference between AC and DC voltage into them. And don't get me started on students who set the meter to the ohms scale, then plug the probes into 120-volts AC. I have at least one student a semester do this no matter how much I tell them not to. Ugh...
