AC Leakage Tester

Discussion in 'General Electronics Chat' started by someonesdad, Feb 22, 2010.

  1. someonesdad

    Thread Starter Senior Member

    Jul 7, 2009
    I've had a 10 amp Variac in a particle board box and I figure it's time it got a better home (a nice sheet metal box) and some meters for current and voltage. At the same time, it would be pretty easy to add in some circuitry to measure AC leakage currents of things.

    I'm considering adding the circuitry shown in the attached bitmap. Here are some notes I made to myself about how this circuitry is intended to be used.
    Operation: the DUT must be tested with its power switch in both on and off positions. The AC power supplied must be with normal polarity and reversed polarity (both positions of S1). An AC voltmeter is connected to the indicated jack (a wideband true RMS meter is preferable) and a leakage probe is used to probe the metal cover and chassis points on the DUT. The first measurements are made with the AC voltmeter on e.g. a 300 VAC scale with switch S3 open to ensure there are no hazardous voltages present on the DUT. If safe, then S3 is closed and the voltmeter is set to the 3 V scale. Any reading over 0.75 Vrms indicates a leakage current greater than 0.5 mA at 60 Hz. This is about half of the point where humans can sense a current flowing through them.

    Note the test measurement is done while momentarily operating switch S2 that ungrounds the DUT. S2 is momentary because this is potentially a hazardous condition if the DUT has excessive leakage current.

    Before testing, the ground conductor in the DUT's power cord or jack must have its resistance measured with respect to a known good ground connection. It must be a high resistance; otherwise, the DUT is still grounded and an unsafe leakage current condition might not be detected.

    The "leakage probe" will just be a DMM probe that fits into a banana jack; I haven't decided if it will be the old style plain 4 mm jack or a shrouded jack.

    Does this circuit look appropriate for the job? The switches will be capable of taking a 15 A load and will be military surplus stuff I have in my junk box. The outlet will be a single outlet, not a duplex one. Construction will be heavy-duty and mechanically rigid (this thing will easily outlast me). It will be carefully insulated.

    Do you see any potential hazards or pitfalls that I might have missed?