I was testing the transformer in my furnace across the COM and 120V terminals when it popped, sparked, and left a burn mark on the COM terminal. Could it be because this nucklehead had the positive lead on the 10A port?

  • Maybe a year ago I was looking for DC current drains in a car with the 10 A socket and the range on DC current--no problem. Turned off the meter and set it aside for a while without changing the lead to the V-Ohm socket. Then of course picked it up, turned the function switch to DC Volts and touched it across the 12 V battery. Spark and blown fuse costing $20+. Apr 13 at 1:59
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    Jim, that's exactly what I did - forgot to switch it over. It cost me a furnace transformer and I noticed a nick on one of my probes which I will replace. Thanks for your comment. It makes me feel more human than stupid. Apr 14 at 22:39

1 Answer 1


The resistance of a typical meter on the 10A scale is less than 0.1 ohms. This is effectively a short. When you attached the meter leads across the COM and 120V terminals, you shorted the mains voltage which is why you got a spark and a burn mark. You are lucky nothing worse happened. You probably also blew the fuse in the meter (if it has one, not all meters fuse their high current ranges). I don't know the potential knucklehead but this type of thing is all too common when the operator is not aware of what he is doing or is careless.

  • Thank you for delving into cause and effect- I am a perfect example of what can happen when one is not paying attention and I should NEVER have had my probe plugged into the 10A port. And of course you're correct that I am lucky something worse didn't happen. I knew that immediately afterwards. Thanks so much for your reply! Apr 12 at 23:30

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