Total newbie accident question


Old 09-09-05, 05:46 PM
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Total newbie accident question

I would like to share this experience in hopes that I can avoid similar (stupid?) mistakes in future by learning from this one:

I decided to change my light switches today. First round, as I was doing this the very first time, I proceeded very cautiously. Turned off power to the room, unscrewed the switch and connected the new one. Everything goes well.

Then, I decide to experiment. For the next round, before turning off the power supply to the other room, I took a multimeter, and with the probe in the 'com' section (is this ground?), I touched the hot wire. I did not do this on I said, I didn't know what I was doing. I was just following instructions where it say to take a voltmeter and touch across the two screws (one for hot wire, one for ground) to make sure it's not getting power. I did that, but without turning off the power supply.

So as soon as I touched the screw, there was this huge spark.

I just wanted to know what the reason was for this. My own hunch (in my very limited knowledge) is that I touched the hot wire with the wrong probe (the com one). Is this it?

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Old 09-09-05, 08:19 PM
Join Date: Sep 2003
Location: Central New York State
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What you did was you shorted the hot wire to the return wire. The spark was the electricity jumping the gap as you created the short.

Using your voltmeter, you have to set the dial to measure AC volts, at some range greater than 120 volts. You then touch one probe to the hot wire and the other probe to the return wire. You do not touch both screws with the same probe or have your voltmeter set to measure resistance (ohms). With AC it makes no difference which probe you use for the hot and which you use for the return, although most people use the red probe for the hot wire.

If I may say so, what you did was stupid. It sounds like you don't know how to use the meter you have, and that you lack a basic understanding of electricity. Please read the instructions that came with the meter and read one or two books on electricity before proceeding. Electricity can kill. You could have easily killed yourself, or started a fire. Electricity is not something to play with.
Old 09-10-05, 02:38 AM
Join Date: Oct 2004
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You left out a lot of information.
Some metes have a jack for measuring amps and a jack for measuring voltage/ohms.

If you insert the probe into the jack for amps then try to measure voltage, sparks will fly.

Note for beginners:
Beginners should never-never try to checks amps without fully-fully knowing how to use this function.
Old 09-10-05, 09:24 AM
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Thank you racraft and GWIZ for your answers.

Yes, you may say so, and I admit, it was beyond stupid of me to 'play' around with it! At least I have learnt my lesson! You're right, I will pick up a few books before I do anything else now. Thanks...

Now, I did have the probes in the "Volt/Ohm" jack and "com" jack, and the dial was set to V-AC, and finally I touched only the black probe to the hot wire (the black probe for me is the one connected to 'com'). So, it seems I did everything you said?

Anyway, once again, I only intend to learn from this discussion. I'm not gonig to go out and try it again without learning up more information. Thanks.

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