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Well, yeah, again, electric shocks from mics !!

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Re: Well, yeah, again, electric shocks from mics !!

Postby Sam Spoons » Sun May 13, 2018 7:03 pm

Is there a problem with using a non polarised three conductor plug into the UK mains via an adapter with class 1 equipment using IEC three pin connectors (kettle leads). I think there is as the line and neutral can be transposed, my mate disagrees saying the RCCD would trip either way. We are not talking about poking about inside some kit here but in normal use.
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Re: Well, yeah, again, electric shocks from mics !!

Postby MarkPAman » Sun May 13, 2018 7:46 pm

An RCD trips when the current in phase and neutral is not the same. It does not care which is which.
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Re: Well, yeah, again, electric shocks from mics !!

Postby Folderol » Sun May 13, 2018 8:26 pm

These days you can get individual combined RCD/MCB modules that directly replace the standard MCBs in domestic distribution boxes.
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Re: Well, yeah, again, electric shocks from mics !!

Postby Hugh Robjohns » Sun May 13, 2018 8:36 pm

Sam Spoons wrote:Is there a problem with using a non polarised three conductor plug into the UK mains via an adapter with class 1 equipment using IEC three pin connectors (kettle leads). I think there is as the line and neutral can be transposed, my mate disagrees saying the RCCD would trip either way.

Technically, it would not be deemed a safe way of working under all conditions, and the HSE would descend like a ton of bricks in the event of 'an incident'...

In practice, though, if -- but only if -- the circuit is definitely fed from a (double pole) RCD, it is actually fully protected and would be safe.

The potential problem is that if the line-neutral are swapped the on-off switch (and fuse) may end up not being in the line feed, and thus the unit remains powered (live) even when switched off, or if the fuse fails (through old age, say). Only the (high rated) fuse in the mains distribution feed would offer (minimal) protection against fire or electrocution.

Thankfully, in either situation, an RCD would detect the resulting imbalance of line-neutral currents and disconnect the device safely... But not all UK mains supplies feature RCDs, and so it cannot be guaranteed to be a safe way of working under all conditions.

H
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Re: Well, yeah, again, electric shocks from mics !!

Postby Sam Spoons » Sun May 13, 2018 8:39 pm

Thanks Hugh, that was what my mate reckoned...... Bugger! that means he was right again :tongue: Admittedly he is better qualified than me.

FWIW we always power the stage/mix position through RCCDs
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Re: Well, yeah, again, electric shocks from mics !!

Postby kdoerbecker » Sat Dec 08, 2018 3:09 am

It has to be a bad earth ground in the venue's power. NOT AT ALL UNCOMMON. Or maybe someone broke the ground pin off the plug of a piece of your equipment (poor man's ground lift). Venues do a lot of renovations and its not uncommon for a venue manager to go on the cheap when hiring an electrician.
Bottom line is: THIS IS DANGEROUS, 5 MA WILL HURT YOU, 20 MA CAN KILL YOU.
My strong suggestion is to buy or make a 50 ft heavy duty extension cord with outlets every 5 ft. Then, run everything off of one outlet (preferrably 20 amp). You can get one with a plug that indicates proper wiring and ground. Glows green if good, red if bad.
Additional benefit, it totally and completely resolves any hum from ground loop problems.
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Re: Well, yeah, again, electric shocks from mics !!

Postby ef37a » Sat Dec 08, 2018 8:10 am

Pretty sure 5mA will stop your heart if you cop it hand to hand?

Unless I am getting the wrong mental picture, a mains cable with outlets inserted every few feet is a recipe for hum loops! You want earth currents to sum together down a common cable, not have some chassis "sitting" on the current from another.

Yes, heavy duty cable but then star out to all the gear at its end. In fact since almost everywhere has double outlets these days make two cables.

Don't use cheapo diss strips from "Rock Bottom". The strips they use in comm' racks are a bit pricey but excellent quality. Avoid anything SAID to be "surge" and RFI proofed. They rarely are, very well and can make earth noise worse.

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Re: Well, yeah, again, electric shocks from mics !!

Postby Sam Spoons » Sat Dec 08, 2018 9:59 am

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Re: Well, yeah, again, electric shocks from mics !!

Postby Hugh Robjohns » Sat Dec 08, 2018 12:43 pm

ef37a wrote:Unless I am getting the wrong mental picture, a mains cable with outlets inserted every few feet is a recipe for hum loops! You want earth currents to sum together down a common cable, not have some chassis "sitting" on the current from another.

Yes, the ideal arrangement for mains distribution is a star form rather than a daisy-chain, for exactly the reason you state... but a daisy chain is a lot easier to make and use and, assuming the mains cable is pretty chunky and the earth leakage currents from any plugged in gear are small, it's not likely to cause a ground-loop hum problem.

At least everything would be running from just one if the building's power sockets which is far better (from a ground-loop point of view) than using multiple sockets on opposite sides of the stage!

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Re: Well, yeah, again, electric shocks from mics !!

Postby wireman » Sat Dec 08, 2018 1:25 pm

ef37a wrote:Pretty sure 5mA will stop your heart if you cop it hand to hand?

I hope not, otherwise ELCB specs would have lower limits. I suppose however, there might be an assumption that you are going to let go of the shock hazard at currents below 30mA, but 5mA seems small.
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Re: Well, yeah, again, electric shocks from mics !!

Postby ef37a » Sat Dec 08, 2018 2:10 pm

Hugh Robjohns wrote:
ef37a wrote:Unless I am getting the wrong mental picture, a mains cable with outlets inserted every few feet is a recipe for hum loops! You want earth currents to sum together down a common cable, not have some chassis "sitting" on the current from another.

Yes, the ideal arrangement for mains distribution is a star form rather than a daisy-chain, for exactly the reason you state... but a daisy chain is a lot easier to make and use and, assuming the mains cable is pretty chunky and the earth leakage currents from any plugged in gear are small, it's not likely to cause a ground-loop hum problem.

At least everything would be running from just one if the building's power sockets which is far better (from a ground-loop point of view) than using multiple sockets on opposite sides of the stage!

H

Yes Hugh I was going to say that although that cable's topology is not ideal, if the earth wire is hefty enough, i.e. a low impedance it will be ok in most situations.

Re the lethality of currents? I seem to recall a Mythbuster programme were they were investigating the Bond stunt of the fire in the bath? They started of with a current of 40mA iirc but later ammended that to 4mA. Agreed you would have to be unlucky to cop an earth fault across the chest but I am pretty sure the "safe" current is quite low?

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Re: Well, yeah, again, electric shocks from mics !!

Postby Sam Spoons » Sat Dec 08, 2018 3:22 pm

ef37a wrote:I was going to say that although that cable's topology is not ideal, if the earth wire is hefty enough, i.e. a low impedance it will be ok in most situations.

Dave.

That cable I linked too is pretty meaty, maybe I'll check the DC resistance of the earth wire with the PAT machine later.
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Re: Well, yeah, again, electric shocks from mics !!

Postby Hugh Robjohns » Sat Dec 08, 2018 3:54 pm

ef37a wrote:Re the lethality of currents? I seem to recall a Mythbuster programme were they were investigating the Bond stunt of the fire in the bath? They started of with a current of 40mA iirc but later ammended that to 4mA.

Those numbers don't sound right to me. I was of the impression that an average person could feel 4 or 5mA, and about 10mA can be enough to cause muscle contractions preventing the 'let go' reaction, but that it took closer to 100mA to cause serious problems -- hence the standard RCD trip of 30mA which is low enough to prevent lethal currents through the body.
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Re: Well, yeah, again, electric shocks from mics !!

Postby Brian M Rose » Sat Dec 08, 2018 4:25 pm

Yes, I know I keep going on about it...
But the BBC has some excellent guidelines here:
https://www.bbc.co.uk/safety/resources/ ... hting.html

Actually more than guidelines, when I was a contractor to the BBC they were absolute requirements. They are all quite sensible and (dare I say it) could even work for musicians. I still have my BBC Safety Passport.
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Re: Well, yeah, again, electric shocks from mics !!

Postby ef37a » Sat Dec 08, 2018 4:35 pm

Hugh Robjohns wrote:
ef37a wrote:Re the lethality of currents? I seem to recall a Mythbuster programme were they were investigating the Bond stunt of the fire in the bath? They started of with a current of 40mA iirc but later ammended that to 4mA.

Those numbers don't sound right to me. I was of the impression that an average person could feel 4 or 5mA, and about 10mA can be enough to cause muscle contractions preventing the 'let go' reaction, but that it took closer to 100mA to cause serious problems -- hence the standard RCD trip of 30mA which is low enough to prevent lethal currents through the body.
Don't know either Hugh. I could definitely "feel" 25k from a TV EHT stage but that could barely deliver 1mA. Any MDs about? Defribbulators deliver so many Joules so maybe a simple current is not correct and we need to know V&I by time?

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