David Karol

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Jan 10, 2011
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We’ve had a few posts recently about blown up gear due to power problems. Here is a basic metering procedure that should give you a good indication of the safety of a standard North American receptacle (NEMA 5-15R or 5-20R).
Standard disclaimers: 

Performing this test can give you reasonable assurance that a receptacle is properly wired and functioning, however it does not guarantee that the receptacle is good – there are still possible fault conditions that may not be revealed with this test.
Performing this test requires sticking conductive things – multi-meter probes – into an unknown electrical supply. This has the potential to create an electric shock hazard if you touch the metal tips of your meter probes (duh), and if things are really wrong, you may cause your multi-meter (especially if it’s a cheap one) to explode.

Proceed at your own risk (though testing is a lot less risky than plugging gear into an unknown receptacle).
Anatomy of a Receptacle
An electrical...

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A floating safety ground can measure tens of volts WRT neutral or line, when using a high impedance VOM. This stray voltage is low current and generally not a shock hazard by itself, but the lack of a true safety ground connection leaves three wire gear unprotected.

JR