Multimeter incorrectly set to DC when measuring AC voltage

Other Miscellaneous
1

I was testing a Light switch today that had 120V power ON with multimeter and had the meter incorrectly set to DC Auto. Big spark and I of course immediately removed probes (total contact time probably about 1/4 second - the amount of time for reflex to pull away from spark). 15 amp circuit breaker did not trip and multimeter still works as does the light switch. I replaced the light switch as a precaution since you could see what looks like an arcing spot on metal side of switch and a slight hot spot on one of the two wire screws, both where one of the probes had contacted. The multimeter probes look a little damaged but everything still works.

Concerned about whether the above might have damaged the house circuit wiring in any way. No sign of burned insulation or any other indication of any damage other than what I mentioned earlier and as I said, circuit breaker did not trip.

Appreciate any insights into what happened with my dumb move - learned an important lesson but want to better understand the physics associated with current flow and multimeters. Thanks in advance.

2

Sounds like you had the meter in the current range not voltage. Just setting to DC volts wouldn’t have had that effect, it would have read inaccurately that’s all.

The other posibility is you shorted the 120v out with a probe, but this would have only burnt 1 probe not both of them.

I’m very surprised you didn’t blow the fuse in the meter though.

I’m fairly confident that if the house is wired correctly, and it was the correct value fuse, the house wiring will be fine. The wiring should have a higher rating than the fuse that protects it, so if the fuse didn’t blow the wiring should be fine

3

You might also want to double check you have your probes in the correct terminals on your meter.

Most meters have a common terminal where your black probe goes and then two (or sometimes three) additional terminals for the positive probe.

One of those terminals will be for current only and the other 1 (or 2) will be for volts/ohms and mA. Make sure you have black in the common terminal and the red in the volts/ohms terminal and that you don’t have the meter set to measure current.