When your plug in tester reads polarity reversal and your wiggy reads 120 volts from ground to neutral and from hot to neutral what two totally different failures can cause this.
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Tom

[This message has been edited by tdhorne (edited 12-08-2004).]

Open neutral upstream and that may be a bad contact in a GFCI receptacle (covered here several times). You are seeing a back circuit through the downstream load.

I assume you are taking measurements at the receptacle. The tester is saying you have reversed polarity, that means the neutral and live conductor are reversed. It sounds like the grounded conductor (neutral) is connected to the brass colored terminal to which the live conductor is supposed to be connected, and the live conductor is connected to the white or silver colored terminal to which the grounded conductor (neutral) is to be connected.

John if this is a simple polarity reversal you wouldn't see volts on the brass screw

If you reverse hot and neutral you would see this. If you roll all 3 lines so hot is on silver, ground is on brass, and neutral is on green, you would also see this.
/mike

It's an open neutral, nothing else. Most of the cheap plug in testers will show reverse polarity on an open neutral condition since the neutral is now at 120V potential from upstream load.

Sorry I didn't notice you said 120 from hot to neutral. It must be like mike said,simply reverse polarity. Your getting 120 from neutral to grd because your neutral is really a hot.

How I got the readings described were from a fault from the hot to the Equipment Grounding Conductor with an open in the EGC between the fault and the source.
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Tom H

Tom, did and earlier fault cause the EGC to blow apart, then become energized when the breaker was reset?
Just goes to show how trying to guess a problem without being there can make a fool out of you.

So far it appears that the separation of the armor jacket of the old type BX cable caused the fault by cutting into the insulation of the hot conductor.
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Tom H