Impromptu welding with DMM leads

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  #1  
Old 12-29-20, 03:56 PM
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Impromptu welding with DMM leads

Checking an outlet with my multimeter. With black probe on neutral screw, I touched the red probe to the hot screw and got a pop and flash, and probe lead is a little blackened and even has a tiny chunk missing from the side. Hot screw has a little blackening.

What happened?

Outlet wasn't grounded, which I just added, and now outlet tests fine with an outlet tester. Kinda skeered to test voltage across the outlet without understanding what happened.




 

Last edited by PJmax; 12-29-20 at 04:16 PM. Reason: resized pics/labeled last pic
  #2  
Old 12-29-20, 04:09 PM
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Mystery solved. Look at the arrow in the last picture.
You caught the probe between the terminal and the box.



 
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Old 12-29-20, 04:20 PM
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You didn't have your meter probes in the current/amperage position.... did you ?
I don't think so. Not even sure how to measure amperage with probes. I have a clamp ammeter for measuring amperage, so I never switch DMM to amperage setting.
 
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Old 12-29-20, 04:25 PM
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Upon further review of your pictures.... I changed my reply.
 
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Old 12-29-20, 04:36 PM
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Mystery solved. Look at the arrow in the last picture.
You caught the probe between the terminal and the box.
Ah, that explains why there's a tiny chunk of the probe missing a little bit down from the tip - that would be where the side of the probe contacted the edge of the box.

The pic shows outlet pulled out from the box, but I attempted to measure voltage while the outlet was still in the box, so I think you nailed it.
 
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Old 12-29-20, 04:56 PM
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Mystery solved. Look at the arrow in the last picture.
You caught the probe between the terminal and the box.
That's pretty impressive that you could spot that! My first guess was that he had the DMM set for continuity or amperage.
 
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Old 12-29-20, 06:04 PM
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So when you have a dead short like that, I guess the only reason breaker didn't trip was because the lead wasn't in contact with both hot screw and box for longer than a fraction of a second?
 
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Old 12-29-20, 06:14 PM
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You can get a certain amount of arc-age before the breaker trips.
Some breakers also trip faster than others.
 
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