2. misc-audio
  3. 1050449527

Measuring AC phase Question

I understand I am to reverse the plug in the AC outlet and measure from a known ground, in my case, a galvanized water pipe, and the positive probe is connected to the equipment chassis. I use a cheater plug on the power cord to allow reversing of the orientation. No other cords are connected to the equipment being tested.
You are supposed to choose the position of the cord that yields the lowest ac voltage on the chassis when the unit is on.
The problem I have is that the lowest voltage with the switch on also has by far higher voltage on the chassis when the power is switched off.
badge
bigtee
803 posts
 
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.)
Post removed 
gs5556
1,241 posts
 
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.
bob_bundus
1,834 posts
 
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.
badge OP
16 posts
 
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.
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