Why does a DMM give suspect AC curent readings?

Question

I am using a digital DMM (Digitech QM1535) to measure the current drawn by currents at my switchboard. We have a higher thn expected power bill, so I pulled fuses and measured currents in the various circuits to see if one was unexpectedly high.

On a lighting circuit with about 1Kw of incandescent bulbs attached I get a reading of 21A. Yeah right. Further, an ammeter should pass th current through, bringing the lights on. Not so.

The meter works fine on all other ranges and neither internal fuse is blown.

I feel I am doing something fundamentally wrong. I am quite conversant with home and commercial wiring though never done this test before. I am inclined to blame the meter. Can anyone shed light on why this is happening?

Selecting different ranges manually (its a fully auto meter) I get different but very high currents. And why aren't the circuits becoming active with th meter in series? An ammeter has a very low forward resistance.

Any help would be appreciated.

And please, no lectures on mains voltage dangers - I have a background in electronics and building wiring spanning 25 years so I'm not wantin a diatribe on how ikely I am to kill myself.

Thanks everyone.

Answer 1

It is most likely the meter you are using. To measure non-sinusoidal AC waveforms accurately requires a True RMS meter. If there are any distortions of the waveforms, many cheaper meters will give erroneous results.

Check for Dimmers on the lighting circuit.

Answer 2

really sounds like the meter has a problem, perhaps a damaged current sense resistor.

Can you get a 0.1 ohm (10 watt) resistor? you can measure voltage across that and use it as a current probe.

But are you sure you are hooking the meter up correctly? This doesn't appear to be a clamp on ammeter type, so you have to be opening the circuit somewhere. I would guess you are removing the fuse and clipping the ammeter leads to the fuse terminals, which should work.

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