Measuring AC phase Question

determine the best phase connection, then rewire the outlet, if necessary, to reverse phase internally. This eliminates the ground cheater adapter & the chassis will always be at ground potential, theoretically reducing stray measured leakage voltage to zero. Label the outlet prominently so that you don't accidentally forget that the hot & neutral slots are inverted. This could be a safety or liability issue in the future. Disclaimer: I am not (professionally or otherwise) advising that you invert these electrical connections. I am only "answering your question". I'm sure that you understand what I'm implying.

Purchase a cheap phase polarity meter and check it. These can be bought at Sears, Home depot, etc. They also have a ground orientation mode. Remember, AC has 1 hot leg(usually the black wire), 1 neutral(white) and a ground. The neutral is tied to the ground buss along with the ground but they are not the same thing. Reversing the polarity can create problems sometimes. The switch on the unit generally opens the black wire. If you reverse it, it now opens the white. It something unexpected happens, the chasis of your unit could remain hot(worse case scenario.)

I wouldn't rewire the outlet. If you're really intent on reversing the AC polarity, buy a power cord which you can access the plug wiring. Reverse the wires in the plug, instead. This way you won't need a cheater plug nor to shave down the oversized prong.

Keep in mind, though: correct AC polarity sends power first to the switch and then through the load. Reversed polarity sends power through the load first, so there is potential avilable with the switch off (still energized). Possibility of equipment damage and injury is more likely.

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.