Measuring AC phase Question

First, are you measuring the equipment in isolation? That is: nothing else is connected to the component being tested

that question is already answered in the posted text.

If you decide to go inside the equipment instead, then annotate the mod such that it can be reversed later before any future sale - otherwise it's now modded / nonstandard equipment with diminshed resale value & may not pass the U.L. If it blows up later then this could come back to haunt you.

thanks for your suggestions so far. All tests were done with no cabling connected. Each unit came with a two prong plug with no ground. I did replace the attached cords with a IEC receptacle. My concern is the high voltage on the chassis when the unit is switched off. On one unit, it measures 10 volts.

Remove the neutral wire from the outlet and measure the voltage from the nuetral to ground. If this is 10 volts, then you have either a high resistance in the neutral, poor utility neutral connection, or the neutral is not grounded properly at the panel. Other possibilites are high neutral currents (harmonics) from computers, fluorescent lighting, and motors, and neutral and ground wiring either interchanged or in contact with each other somewhere in the house wiring.

Some manufacturers bypass the UL rating with the 2-prong outlets by selling direct (no dealers). The reason is to minimize ground loops. The safety factor is using power supply transformers with 2,000 MOhm primary resistance, which effectively limits the current from either of the mains to chassis (ground, in this case). The neutral of the 2-prong cord is isolated from the chassis, but it still can pick up the stray current from the wiring system.

A ten volt drop across the chassis is high. I would contact an electrician to check the service wiring and grounding, including testing ground to earth resistance (should be less than 25 ohms) and to install a dedicated circuit for your gear.