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I'm using a multimeter (Mastercraft 052-0052-2) and I decided to see if the frequency of an outlet would read 60hz. I switched my multimeter to the FREQ setting and plugged the RED test lead into the NEUTRAL of the outlet. Upon doing this, a loud bang occurred with a huge spark. Yet the outlet still works and the breaker did not trip. Could somebody explain to me what may of occurred?

The multimeter works fine and no fuses were blown. The BLACK test lead was just resting on the carpet floor. (It probably didn't touch the carpet though, since it the metal probe sits 2cm above the carpet.)

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    \$\begingroup\$ Did your meter come with a manual? Did you put your leads in the COM & F plug-ins on your meter? \$\endgroup\$
    – Tim Spriggs
    Mar 28, 2016 at 21:30
  • \$\begingroup\$ @Tim Spriggs Yes, here is one for reference. manualslib.com/manual/953499/Mastercraft-52-0052-2.html#manual \$\endgroup\$
    – Markus Tonsaker
    Mar 28, 2016 at 21:40
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    \$\begingroup\$ Technically this CAT II rated meter isn't rated for direct outlet measurements (CAT III would be). \$\endgroup\$
    – David
    Mar 28, 2016 at 21:52
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    \$\begingroup\$ @Markusmoo see ni.com/white-paper/5019/en - CAT II is suitable for appliance measurements (which will usually be separately fused and unable to deliver the same current as a main circuit). CAT III is suitable for outlets. To take the extreme of this argument, why wouldn't your CAT II meter be suitable for the incoming supply from your utility company? That is (typically) the same voltage - but far more dangerous. Regardless this doesn't answer your question, just a curiosity. \$\endgroup\$
    – David
    Mar 28, 2016 at 22:00
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    \$\begingroup\$ @Markusmoo: It sounds to me as though you somehow shorted the live wire to ground by simultaneously making contact with the socket live terminal and either the faceplate or some internal earthed metal. \$\endgroup\$
    – Transistor
    Mar 28, 2016 at 23:31

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If you drew sparks with only a single probe, it suggests that you accidentally shorted across neutral to hot somewhere on the outlet.

Studying and understanding the manual and the limits of the meter would likely help you to avoid such mishaps. (Or even worse such as fire or electrocution.) Any kind of measurement of mains power should use considerable caution.

Unless the meter is rated for measuring mains frequency, it would be better to avoid measuring high voltages like that. For practical purposes, the frequency of the utility power is the least likely to change or be worthy of measurement. Unless you were getting power from a portable generator or some "off-grid" source.

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  • \$\begingroup\$ Possibly shorted the probe across a hot terminal and some other internal hardware at neutral or ground potential. \$\endgroup\$
    – KalleMP
    Mar 29, 2016 at 8:53
  • \$\begingroup\$ This happened a long time ago, but recently I replaced the receptacle to a more modern one and found that the screw for the HOT terminal was not screwed in. I also found a burn mark on the adjacent metal electrical housing. I guess when I put the probe in, I contorted the receptacle enough for the screw to short to the earth attached metal housing. \$\endgroup\$
    – Markus Tonsaker
    Apr 3, 2021 at 7:29

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