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I used a multimeter in its 600 V AC range to measure the voltage across a socket of a surge protector that is rated 125 V - 5A.

As soon as I inserted the test leads, there was a pop and a bright flash.

The outlet is unharmed but the black probe tip has been damaged slightly and the multimeter fuse has blown.

What could have happened?

EDIT: The multimeter is a commercial electric MMM 8301s. I set the red test lead to the 10a slot and the black on to the com slot. The multimeter was set to 600 volts AC, but the outlet seemed to be a DC one. I plugged the black lead to the top left outlet and the red one to the top right one. What happened?

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    \$\begingroup\$ I would have guessed that you had it set to measure amps. But you say differently. Another possibility, if the meter forces you to move measurement cable ends into different holes depending on what you are measuring, that might be another explanation. What's the model/make of your meter? \$\endgroup\$
    – periblepsis
    Commented Jun 29, 2023 at 0:07
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    \$\begingroup\$ As @periblepsis states, odds are the + probe was plugged into the "10 A" jack, virtualy a dead short. \$\endgroup\$
    – DrMoishe Pippik
    Commented Jun 29, 2023 at 0:10
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    \$\begingroup\$ Can you post a picture of the DMM (as it was connected), and the damage? There are usually no fuses on the voltage input. There is usually a 200 mA or so fuse for current, and sometimes a large 10A or so on the 10A input \$\endgroup\$
    – PStechPaul
    Commented Jun 29, 2023 at 0:32
  • \$\begingroup\$ Which multimeter it is? An actual multimeter capable of safely measuring 600V, or a toy with similar markings? Was the meter and probes really set to measure volts, instead of current? \$\endgroup\$
    – Justme
    Commented Jun 29, 2023 at 5:26
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    \$\begingroup\$ ”red test lead to the 10a slot” Here is your problem. Please read the manual and acquaint yourself with how a multimeter works before attempting anything mains electricity related. \$\endgroup\$
    – winny
    Commented Jun 29, 2023 at 19:05

4 Answers 4

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I set the red test lead to the 10a slot and the black on to the com slot.

There you go. The 10A jack is only for current measurement, and there is a very small resistance between 10A and COM. You created a short-circuit which lasted until the fuse blew (as it's designed to).

Unless your goal is to measure high currents (above 200mA for your unit), and the mode switch is in the 10A position, the 10A jack is the wrong one to use.

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  • \$\begingroup\$ If I connected it to the other jack(not the 10a) would it blow a fuse? \$\endgroup\$
    – Dat_Derpy_Dude81
    Commented Jun 29, 2023 at 19:41
  • \$\begingroup\$ @Dat_Derpy_Dude81 no (unless something is broken). \$\endgroup\$
    – hobbs
    Commented Jun 29, 2023 at 21:32
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The meter will have a very low value resistor between the 10 A and COM sockets. This connection will not be affected by the range switch, so you essentially connected a short circuit between the AC Live and Neutral terminals of the outlet.

When measuring voltage, the red lead MUST be in the "Volt/Ohm/mA" socket on the meter, and the black lead in the COM socket, as well as having the range switch in the correct position.

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There are only two possibilities:

  1. The multimeter wasn't on a 600 V range
  • Perhaps it was on current
  • Perhaps it is defective
  1. The probes weren't connected as you thought
  • Perhaps the voltage was way above 125 V
  • Perhaps one probe shorted something

Given that meters usually have current settings and using the current setting across a large voltage give the behaviour described, the most likely explanation is that it was inadvertantly left like that. I could have sworn the meter was on volts!

A second possibility is that one probe shorted something out, but you wouldn't expect that the blow the meter's fuse. I'm sure the probe only touched one terminal!

Many labs I've seen keep the burnt things they've blown up on the wall to remind themselves to be careful.

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Many multimeters have separate volts and amps sockets. Perhaps you switched it to volts, but plugged one of the probes into the amps socket.

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