I am trying to figure out why a spark was created when I was testing the voltage across a three way switch I have.

So to make a long story short, I was trying to move some switches because I didn't like the order they were in (i.e. 3 switches and I wanted the switch closest one to the door to control the light closest to the door).

So I took off the face place and I noticed there were 3 switches behind it, but each switch was a 3-way switch. However, each of the switches are dedicated to one device (light,fan,small light). So the electrician just had a bunch of extra 3-way switches and didn't want to go and buy just regular switches.

The common and traveler terminals of each switch are not connected to anything. I incorrectly (I think) assumed that these were power and ground terminals. So I tried to measure the AC voltage using my multimeter across these terminals. What happened is a giant spark, the breaker flipped, and one of my probe leads melted. I had the multimeter on the AC voltage setting (ACV~), and it is a pretty decent multimeter (https://smile.amazon.com/INNOVA-3320-Auto-Ranging-Digital-Multimeter/dp/B000EVYGZA?sa-no-redirect=1).

I reset the breaker and the switch still works. But what bothers me the most is I have no idea what happened. I didn't short the probe leads, and it makes no sense to me why connecting two terminals would create this giant spark. Only one of the test probes melted, and only one of the screws coming out the side of the switch got black from the spark.

Can someone please explain to me what happened? Please let me know if you have more questions or if I left something out.


  • 1
    are you certain you didn't have the meter in AMP mode instead of voltage? As to why only one probe sparked, you probably had the other probe already in good contact with a terminal so there was no spark and so no damage there.
    – DoxyLover
    Commented Feb 13, 2017 at 6:54
  • I would have to guess that you did as Doxy suggested having the meter in AMPs mode or that you allowed the metal tip of one probe to bridge across a hot and neutral connection at the same time.
    – Michael Karas
    Commented Feb 13, 2017 at 9:29
  • Is it a metal box? If so, are you sure you didn't touch the box at the same time as the terminal?
    – Tester101
    Commented Feb 13, 2017 at 11:44

1 Answer 1


[$20 Innova 3320] is a pretty decent multimeter

See http://www.eevblog.com/forum/testgear/innova-3320-multimeter-teardown/

Consider 2nd hand fluke

what happened?

Any of

  • You had the red lead in the 10A socket even though the dial was set to ACV~.
    • Unlike better multimeters, that socket is unfused in this low-cost meter because decent 10A High-Rupture-Capacity fuses are expensive.
    • Some better meters have a "jack alert" feature to warn of this problem.
  • You didn't have shrouds on the probe tips and a tip slipped and shorted two contacts, not just the one you were trying to probe
    • Trying to hold two probes and a multimeter with only two hands can be awkward. This is why other voltage test devices integrate the electronics and display into a probe.
  • It's a three-socket meter with mA~ and ACV~ measured from the same sockets. This is a common design issue in low-cost meters. It is too easy to have the dial set to mA~ instead of ACV~ or for a fault in the dial mechanism to leave the sockets shorted by the mA shunt. Four-socket designs are arguably safer.

one of my probe leads melted.

That shouldn't happen on a decent multimeter. The 10A range should be protected by a 10A HRC fuse and the leads should be rated for 10A. On the whole, it is better to blow a replaceable HRC fuse than to melt something you are directly holding while it is passing >10A at 120V.

one of the screws coming out the side of the switch got black from the spark.

It might be safest to replace the switch.


  • I agree with this answer. There should be 2 points of damage where you were measuring and the probe melted on 1 side you know about if the other point is where the meter was connected on the other side it was the meter (wrong connections faulty meter that caused the problem) if the second point is on the metal of the switch or box you bridged to the ground. 3 way switches can be used as single pole and there is no problem doing this most electricians don't use 3 ways because they are more expensive unless out of single pole then a trip to the store would cost more in time and fuel.
    – Ed Beal
    Commented Feb 13, 2017 at 14:09

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