Old amps and safety

[creolian]

Hello all,

I noticed Michael had mentioned the potential danger of old amplifiers  on another thread that I don't want to jack. I apologize if this is old news but I occasionally still see scared and bewildered guitar players wondering why their lips get a buzz. This really can be very dangerous but it's not hard to fix.

Working the deck on a number of festivals I've had to deal with this and it's relatively simple to avoid being shocked.  Old amplifiers were built with no safety earthing or grounding  connection. The simplest way to avoid a shock is to modify the amp with a 3 pin power cord  but understandably some don't want to modify vintage gear in any way.  The problem arises when voltage is leaked to the amp chassis through loose wires, frayed insulation or whatever.  Any connection to earth will provide a path (circuit) for that electricity to flow.  The danger happens when that voltage is given a path to earth through a guitar players lips and hands by touching the guitar and a mic stand or stage railing etc simultaneously. Here in NOLA it's not uncommon to have what look like three pin properly earthed outlets that are not actually connected to earth, so...

I've been brainwashed into using a voltmeter with all amps to check and see if there Is voltage to earth. I have a probe with a clip that I connect to the amp chassis and the other goes to a known good earthing point. All amps get powered up and checked before any instrument gets plugged in. Any voltage present between amp and earth, and that amp will be a no no until the techs fix it. I use a very inexpensive multimeter for this one purpose, the probes are connected to 25' of lamp cord. I also use an outlet checker to make sure that things like neutral and earth are not miswired but that's another story for another day.

Hope everyone is well grounded All best,

Jeff

[pete1951]

For small gigs we use an RCD plug. All amps and PA go through it.
PT
Had a similar problem at a jam once, put a jack in the amps unused channel, then into an unused channel on the PA. Nothing went bang, and the lead seemed to 'shunt' the voltage to earth. Don't try this at home!

[Michael Messer]

Hi Creolian,

Thanks for that information.

This is not a pun, but in the UK the problem is amplified because our mains is 240 volts.

Shine On
Michael

[creolian]

Sept 17, 2017 16:03:07 GMT Michael Messer said:

Hi Creolian,

Thanks for that information.

This is not a pun, but in the UK the problem is amplified because our mains is 240 volts.

Shine On
Michael

Hello Michael,

The actual danger is in the amount of current flowing and not the voltage.  A few milliamperes might give the lips a tingle but a few amperes is enough to penetrate skin. High current Electricity taking a path from lips to hand going through the body core to earth is what stops the heart, sometimes resulting in a trip to the pearly gates...

In my experience I've never seen more than a lip tingling low current potential. I think most of us have heard of more serious occurrences which are not a great probability but better safe than sorry

A tesla coil will produce 100s of thousands of volts but (hopefully) not enough current to penetrate skin.

Enough DRAMA!

To date: the worst thing I've witnessed at a festival was a woman who's pepper spray had gone off in her pocket. Not knowing what was wrong initially it was a bit worrisome to have a damsel in distress jump the barricade, grabbing her crotch and screaming... Ultimately a bit of comedy and a friend for life after the medics tended and we donated some shorts.


be Safe and All best, Jeff

[Deleted]

All good advice. If you are unsure of the equipment, always avoid contact with another musician, equipment, hardware or instrument on stage. Wear thick rubber soled shoes to minimize the potential path to ground, use only one hand when handling live equipment. Wire up a gfci cord for your amp.

[Stevie]

Or use wireless equipment.

Jeff's right; "it's volts that jolts, it's mills that kills". Around 55mA is the cut-off point for avoiding fibrilation, but who wants to push it that far?

I've had a fair few whacks through the years and ignoring the pain and discomfort, at the very least I would describe them as disorienting.

I have one of those little plugs with three tiny lamps that light up to indicate the various supply faults. I once diagnosed a live neutral reversal at a campsite in The English Lake District with it. I told the site owner and he flatly denied it, so I told him that I would temporarily rewire the VW Bus but it was down to him after I'd gone. He got in an electrician after that.

There is some justification for using capacitors in our guitars that are rated at twice the peak inverse voltage of the intended AC supply voltage, and also remembering that there isn't really a ground in our guitars- only a signal return.

Also lest we forget, (I do!) all our equipment should be "PAT" tested annually (tautology!) over here in the UK, and venue owners should insist on seeing evidence prior to allowing a performance to go ahead.

Good thread Jeff.

e&oe...

[Matt]

Sept 17, 2017 16:55:03 GMT creolian said:

Sept 17, 2017 16:03:07 GMT Michael Messer said:

Hi Creolian,

Thanks for that information.

This is not a pun, but in the UK the problem is amplified because our mains is 240 volts.

Shine On
Michael

Hello Michael,

The actual danger is in the amount of current flowing and not the voltage.  A few milliamperes might give the lips a tingle but a few amperes is enough to penetrate skin. High current Electricity taking a path from lips to hand going through the body core to earth is what stops the heart, sometimes resulting in a trip to the pearly gates...

Perhaps, but for a circuit with a the same resistance twice the voltage will mean twice the current...

[Stevie]

Only for DC. In an AC circuit, that would depend upon phase angle too.

e&oe...

[blueshome]

As a slight aside, I was in a band who rehearsed in a pub in Essex and we took all the usual precautions - the singer even had a radio mike. Despite this we got mild shocks off the mic and stands. It is only when we worked out that we were standing on a nylon carpet and building up static by moving around that we understood.