Electronics > Projects, Designs, and Technical Stuff

DMM Lead Arc getting voltage from power strip

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TheMG:

--- Quote from: tooki on July 26, 2020, 12:37:08 pm ---Given that the jacks don’t do current measurement, then the most likely explanation is that you inadvertently shorted the probe tips. Either to each other, or across line and ground (since the neutral and ground are connected back at the panel).

--- End quote ---

That would be pretty difficult to do on a north american receptacle. The slots for live and neutral are very narrow, in fact the probes on most DMMs insert very snug into them, which doesn't allow for any side-to-side movement of the probes so there's no way you'd be able to short the probes to each other, at least not without bending them.

It is possible however on poorly made power strips if the probes are relatively long for the probe to poke past the back of the receptacle and through the insulation of a wire, or a grounded chassis, but this would be rather unusual as most DMM probes are not much longer than a standard plug end.

DW1961:

--- Quote from: tooki on July 26, 2020, 12:37:08 pm ---
--- Quote from: DW1961 on July 25, 2020, 08:10:23 pm ---

I was checking voltage at a power strip and had an arc that partially melted the tip of my lead (It put a little melt point on it). I may have inadvertently had it on DC voltage. Would that cause an arc, or just an out of range signal on the LED? It's a good way to ruin leads, so I don't want to repeat that if possible.

--- End quote ---
Given that the jacks don’t do current measurement, then the most likely explanation is that you inadvertently shorted the probe tips. Either to each other, or across line and ground (since the neutral and ground are connected back at the panel).

--- End quote ---

You mean by pushing the leads into the power strip receptacle and shorting it on the ground in the receptacle?

DW1961:

--- Quote from: TheMG on July 26, 2020, 04:53:43 pm ---
--- Quote from: tooki on July 26, 2020, 12:37:08 pm ---Given that the jacks don’t do current measurement, then the most likely explanation is that you inadvertently shorted the probe tips. Either to each other, or across line and ground (since the neutral and ground are connected back at the panel).

--- End quote ---

That would be pretty difficult to do on a north american receptacle. The slots for live and neutral are very narrow, in fact the probes on most DMMs insert very snug into them, which doesn't allow for any side-to-side movement of the probes so there's no way you'd be able to short the probes to each other, at least not without bending them.

It is possible however on poorly made power strips if the probes are relatively long for the probe to poke past the back of the receptacle and through the insulation of a wire, or a grounded chassis, but this would be rather unusual as most DMM probes are not much longer than a standard plug end.

--- End quote ---

The lead tip is about 3/16th longer than the plug spade. I was having a hard time getting a reading in the power strip. I do remember that and it was not like doing it in a wall receptacle, so maybe I did ground it out. It's a very good explanation.

DW1961:

--- Quote from: Gyro on July 26, 2020, 11:48:16 am ---
--- Quote from: DW1961 on July 26, 2020, 04:52:09 am ---
--- Quote from: helius on July 25, 2020, 10:24:17 pm ---The "wire with a little bead on it" is a K-type thermocouple wire. It is designed for high-temperature use and isn't going to just melt (although the heatshrink could).

If your meter only reads AC amps using the clamp, it is unlikely to have any fuse.

--- End quote ---

Just got this back from the Amazon merchant:

"I received this replay back fro the seller on Amazon, for what it's worth. He is saying it has both current and voltage fuses in it:

"Our products are equipped with professional voltage and current fuses. In case of misoperation and exceeding the maximum voltage/current range, the fuse will be blown first, and then the circuit board will be destroyed."

F-it! I can tear it apart and see if it really does?! It might be interesting. If it has fuses, I'm assuming it has a way to change them. I'll ask the merchant.

--- End quote ---

Yep, they're talking complete bollocks.  ::)

DMMs use VDRs and PTC thermistors for voltage ranges, and those only come into play when it is subjected to gross overload.

It will be using a current transformer for the current jaws (hence AC only), but it's easy to clamp the high impedance current output - actually a fuse opening on the output [Ed: of the] transformer would turn it into a very high internal voltage source!

I'd return it as defective - only do a teardown if you don't have to break any seals and hide the serial number. Try to find one that isn't manufactured by "RockSeed"  ;)


P.S. A reputable supplier would apologise for melting the end of your probe, scaring the life out of you, and ask you to return it for urgent investigation - not burble on about its mythical fuses.

--- End quote ---

It was free.

What do you think? Tear down? IF it doesn't have any fuses, then they need to be called on that.

joeqsmith:
My guess is you had it in the ohms function.  This will engage the PTC and low voltage clamp and I wouldn't be at all surprised if it caused a small spark.   You would be looking at somewhere around a 3Kohm load across the line until the PTC heats up.   The low voltage clamp is typically less than 20V and won't make much of a difference.  It shouldn't damage the meter but I don't recommend playing with the line. 

In the voltage mode, I wouldn't expect the meter to start to have any effect until you reach what ever the MOVs start to conduct (assuming the meter even has them).   

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