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

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

Postby Hugh Robjohns » Sat Dec 08, 2018 4:45 pm

There's lots of stuff on the interweb about this, such as:

Image

https://jmkengineering.com/electric-shock-human-body/

Although an American article, it is the current that matters, not the voltage... and not the difference between 50 and 60Hz either!

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

Postby wireman » Sat Dec 08, 2018 5:15 pm

The c1 curve in one of the figures contained in the document I linked to earlier is relevant and there was a mention of the IEC 60479 standard it came from.

Anyone with some spare cash could get the latest standard which seems to be PD IEC/TS 60479-1:2005+A1:2016 Effects of current on human beings and livestock. General aspects.
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Re: Well, yeah, again, electric shocks from mics !!

Postby Trevor Johnson » Sat Dec 08, 2018 7:53 pm

As my old dad, (and others), used to say, "its volts that jolts, but mills that kills"!
He worked on radar during WWII, both on land and sea, and was seconded to the Royal Canadian Navy because of his expertise: one of his teachers was the great Watson-Watt. He knew the dangers and potential lethality of voltages.

Anyway, I followed him and became a radio amateur, growing up with some EHT power supplies, (>2KV), to power valved linear amplifiers. Clearly you make a mistake only once at 2KV, so I was taught safety at all times. In fact I was taught to regard any voltage as potentially hazardous.

In terms of audio, the voltage and current on the output capacitors of a powered up Quad 303 are more than capable of putting you into VF, depending on the circumstances (e.g. your right hand is connected to positive and your left is grabbing onto something earthed. Golden rule: put your non-working hand firmly away in a pocket. And back to the OP; if you feel a tingling sensation from your mics don't ignore it as your mic maybe looking for a new owner sooner than expected!
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Re: Well, yeah, again, electric shocks from mics !!

Postby Sam Spoons » Sat Dec 08, 2018 10:26 pm

Trevor Johnson wrote: Golden rule: put your non-working hand firmly away in a pocket.

Something I learned early in my amateur electronics (failed) career and have adhered to ever since. Good advice, obviously, as I'm still here :thumbup:
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Re: Well, yeah, again, electric shocks from mics !!

Postby ef37a » Sun Dec 09, 2018 12:06 pm

Trevor Johnson wrote:As my old dad, (and others), used to say, "its volts that jolts, but mills that kills"!
He worked on radar during WWII, both on land and sea, and was seconded to the Royal Canadian Navy because of his expertise: one of his teachers was the great Watson-Watt. He knew the dangers and potential lethality of voltages.

Anyway, I followed him and became a radio amateur, growing up with some EHT power supplies, (>2KV), to power valved linear amplifiers. Clearly you make a mistake only once at 2KV, so I was taught safety at all times. In fact I was taught to regard any voltage as potentially hazardous.

In terms of audio, the voltage and current on the output capacitors of a powered up Quad 303 are more than capable of putting you into VF, depending on the circumstances (e.g. your right hand is connected to positive and your left is grabbing onto something earthed. Golden rule: put your non-working hand firmly away in a pocket. And back to the OP; if you feel a tingling sensation from your mics don't ignore it as your mic maybe looking for a new owner sooner than expected!

Microwave ovens similarly take no prisoners. 2kV at at least an amp. We were told at an Hitachi traing course that you NEVER fire up the oven and make voltage tests, diagnosis was to be indirect and by "cold checks".
Problem was if the magnetron was the suspect (almost always was) they were 30 quid a pop and had to be ordered in. Hanging an Avo 8 on 2.5kV range then firing up from a discrete distance proved the 'tron. If the volts went to 2kV+ and stayed there it was pulling no current and thus borked (it paid however to check the heater supply!)

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

Postby wireman » Sun Dec 09, 2018 2:29 pm

It seems that there is a whole load of health-and-safety-free MO transformer fun to be had out there video1 video2.
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Re: Well, yeah, again, electric shocks from mics !!

Postby Trevor Johnson » Sun Dec 09, 2018 8:29 pm

Problem was if the magnetron was the suspect (almost always was) they were 30 quid a pop and had to be ordered in. Hanging an Avo 8 on 2.5kV range then firing up from a discrete distance proved the 'tron. If the volts went to 2kV+ and stayed there it was pulling no current and thus borked (it paid however to check the heater supply!)

The magnetron, specifically the cavity magnetron, was critical to the development of radar during WWII.

An Avo 8 on the 2.5kV range, wow! I do all my HV testing remotely with a master switch close at hand and in the vicinity of an AED. I remember a couple of years ago a well known HiFi magazine developed a kit with 300Bs in the output stages. But they were encouraging builders to prod around the valve bases with a multimeter to check voltages. I thought how insane they were!
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Re: Well, yeah, again, electric shocks from mics !!

Postby ef37a » Sun Dec 09, 2018 9:03 pm

Trevor Johnson wrote:
Problem was if the magnetron was the suspect (almost always was) they were 30 quid a pop and had to be ordered in. Hanging an Avo 8 on 2.5kV range then firing up from a discrete distance proved the 'tron. If the volts went to 2kV+ and stayed there it was pulling no current and thus borked (it paid however to check the heater supply!)

The magnetron, specifically the cavity magnetron, was critical to the development of radar during WWII.

An Avo 8 on the 2.5kV range, wow! I do all my HV testing remotely with a master switch close at hand and in the vicinity of an AED. I remember a couple of years ago a well known HiFi magazine developed a kit with 300Bs in the output stages. But they were encouraging builders to prod around the valve bases with a multimeter to check voltages. I thought how insane they were!

No Trev' I was not "poking about"! The magnetron has -2kV or so on its cathode/heater, anode is bolted to chassis. I used to hook a crock lead from heater to chassis, oven off! Then attach the Avo, remove the eathing crock then switch on. The no load volts pushed Avo over FSD slightly but if it did not start to fall after 30 secs or so I knew the magnetron was dedded.

One thing scared me though. Do you remember the huge wide screened CRT TVs? They did not last long before Plasma came in but we had to change a tube in one. The CRT came in a monster crate with two special handling hooks. It took two of us to change it and so great I think was the capacitance twixt inside and outer dags that if you copped the residual volts it would have been curtains. Had we dropped the CRT I reckon the implosion would have shredded us and flattened the building!

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

Postby Folderol » Sun Dec 09, 2018 9:26 pm

but... but... they were 'only' about 35kV. What's a little 'k' between friends :tongue:

P.S.
We had a safety earthing strap that was always used when removing a tube... before disconnecting anything. At the 'top' end it had a 100k 10W resistor and a very thin blade that could slip under the the rubber cup. It was left in place for about 1/2 hour to ensure that the somewhat high resistance internal graphite coating was completely discharged.
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Re: Well, yeah, again, electric shocks from mics !!

Postby ef37a » Mon Dec 10, 2018 9:45 am

Just under 25kV pretty sure Will otherwise punters get dosed with X rays.

Yes, you can dump it but an hour later the dielctric stress builds it back up again and the energy stored in that huge CRT was something I did not want across me!

But also, handling something THAT big under atmospheric pressure was most unnerving.

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