A wee electric shock...

J

John B. Miller

Freshman
Sep 13, 2011
14
0
0
Waterloo, ON, Canada
www.audiocraft.ca
Today I played and sang at an elementary school where the school kids set up the SOS system that the school owns, and I operated the system (from the stage, mixer beside my piano). After the event, I began unpatching things, and when I reached around behind the mixer to pull a TRS plug out of an AUX send, I got an electric shock. It wasn’t the worst shock I’ve ever received, but certainly enough to make me want to let go of the connector.

It happened when I had the TRS plug part-way out of the jack. The other end of the cable wasn’t connected to anything. The mixer was still powered up, with universal 48v switch on, and connected to a pair of powered speakers. The school is about 10 years old, and the sound system is around 6 months old.

Now I’m no electrical genius, but I don’t think this should happen! This is the mixer:

http://www.alesis.com/multimix12r

Unfortunately, I didn’t have my circuit tester or multimeter with me, and I was in a hurry to leave, so I didn’t test anything or try to troubleshoot it. I will likely go back to the school when I have some free time and try to re-create it the scenario.

Any thoughts on what might be up here?
 
Re: A wee electric shock...

I had this happen when I was touching the disconnected RG6 cable coming out of my $5 HDTV tuner box. This can happen with some ungrounded devices with floating power supplies. Strangely, it doesn't necessarily indicate a problem other than cheap design, though there could be something else going on that's more serious.
 
Re: A wee electric shock...

No that doesn't sound right.

With a simple VOM check for near 0V between outlet grounds, a long 3 wire extension cord will simplify making the measurement.

JR
 
Re: A wee electric shock...

You didn't notice if the third prong of the power cord had been removed, did you?

Without a ground reference, the chassis will tend to float around half the line voltage, or about 60 VAC. The current should be pretty low, but you might still feel it. With everything patched together, it would have got a ground from the other equipment (assuming some or all of it had grounded plugs), but once you unplugged the last patch cord, there would be potential between metal parts that your hands are now bridging.

I really hate people that chop off ground prongs!

GTD
 
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Re: A wee electric shock...

Geoff Doane said:
You didn't notice if the third prong of the power cord had been removed, did you?

Without a ground reference, the chassis will tend to float around half the line voltage, or about 60 VAC. The current should be pretty low, but you might still feel it. With everything patched together, it would have got a ground from the other equipment (assuming some or all of it had grounded plugs), but once you unplugged the last patch cord, there would be potential between metal parts that your hands are now bridging.

I really hate people that chop off ground prongs!

GTD
Click to expand...


With that in mind - the outlet itself may not be grounded.
 
Re: A wee electric shock...

People have actually been killed by old 2 circuit wiring incorrectly wired to 3 circuit outlets, with a hot instead of neutral ground connection.

Hopefully this isn't the case, but there are simple outlet checkers and a VOM is your friend.

JR
 
Re: A wee electric shock...

John B. Miller said:
Unfortunately, I didn’t have my circuit tester or multimeter with me, and I was in a hurry to leave, so I didn’t test anything or try to troubleshoot it.
Click to expand...

General rule when running audio is that this is the FIRST tool pulled out of the bag. Verify all unknown outlets before use. It only takes a couple seconds and can save your butt when something is wired wrong. You would be surprised how often that is the case!!

Yes, I've run into many issues with wiring. Actually, most of the glaring problems I've fount (Missing neutral lines, reversed polarity, ground/neutral reversed, hot on the GROUND pin...) have been in new buildings. Mistakes do happen, and in newer buildings, the outlet you are plugging into may not have been used in conjunction with other outlets, so the problems have yet to be discovered. Outlets being used by themselves wired incorrectly may work just fine in most cases. ALWAYS test.
 
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