Why am I getting mini electric shocks off the microphone?

fretfinder

I played a gig on Saturday at a pub we haven't played in for years and whenever my lips touched the SM58 mic grille I got a mini electric shock. It was so weird/uncomfortable that we had to stop and dig out a foam windshield to fit over the mic. I thought it might be because I'd ballsed up the earth wiring on the LP I was using, as I'd just fitted a new wiring loom and new pickups. But in the second set I changed to my PRS McCarty, which I've used for dozens of gigs with never a problem before, and still got the shocks off the mic. I've used Strats at previous gigs and other Les Pauls before with no problem like this, so wondered if it could be something to do with the pub wiring. Anyone got any idea why that should be happening?

IanSavage

Residual current between the earths on your amp (which you're connected to via the strings whichever guitar you use) and the PA (which the mic grille will be earthed to). Creates a potential difference between your guitar strings and your mouth when it's in contact with the grille, hence the shock as a current manifests - in very rare cases these can be quite serious / fatal, so if you ever get that again check/change what things are plugged into. You want to fit an RCD (residual current detector) to your amp, by the way, so that if the earth IS dodgy it'll shut the amp off before you get a shock.

Personally I use a wireless partly to eliminate that kind of thing, and a DRY foam cover will work in a pinch - both not really addressing the problem though. If it's the venue's PA, let them know it's happening; if it's your PA, try to run it from the same set of sockets as you amps. 

Col_Decker

I used to have that with my chinese copy sm58. Not had the problem since I got a proper 58, not saying yours is a copy but maybe there's a lose wire inside it

Jalapeno

Earth / Neutral crossed on the PA possibly.

An RCD breaker will prove it.

Richardj

Agree with the above, there is a bad earth somewhere. Definitely get an RCD, make sure all of your gear is electrically checked and safe, check the venue is up to scratch.

usedtobe

God is telling you to stop playing!

TTony

I had something similar.

IIRC, both the amp and the min-PA were plugged into the same 4-way extension plugboard when I was getting the mini-shocks.

Then I plugged the min-PA box into a different power socket and all was well.

Bogwhoppit

I've also had the odd shocking gig....

ICBM

I'm sorry to say this, but in the scenario where your own gear is fine and the fault is in the PA, an RCD will *not* save you. It may shut off your amp, but that won't help because the earth will still be connected, and that's where the current leaking from the PA is going to, via you - it makes no difference if your amp is powered up or not. Don't rely on an RCD or think that it gives you any protection - at worst, it gives you a false sense of security.

RCDs on the whole building or on the supply to the PA *as well* will do the job, but not on your own gear alone.

In general running everything from the *same* mains extension or socket is the best because that way you should have a common earth connection to all the equipment and the live and neutral should be the right way round - but if some of the gear is double-insulated rather than earthed, or if there's a broken earth in one of the cables, it's possible that you could get a build-up of charge that way (in which case the shock won't be dangerous, just uncomfortable).

Ideally, the strings on a guitar shouldn't be grounded anyway - it's a hazard and is done to reduce noise, *not* for safety. You can reduce the risk a great deal by fitting a high-voltage capacitor in line to the bridge/tailpiece ground connection, although you could still get a shock from the jack or a metal control plate or pickup cover.

fretfinder

Thanks guys. This wasn't a house PA, it's our drummer's (no jokes please, he has a fine set of PA equipment and lights and always brings the lot, there and back, so can't complain!) and has always worked fine in numerous other venues over the years. So this really was  a one-off occurrence. We've got a gig at a different venue in a couple of weeks so I'll see if it's back to normal or still shocking...

Col_Decker said:

I used to have that with my chinese copy sm58. Not had the problem since I got a proper 58, not saying yours is a copy but maybe there's a lose wire inside it

It's a proper SM58 not a copy - our drummer has several of them. I also stepped across to the other singer's SM58 and gave that a try and still got the mini shocks.

usedtobe said:

God is telling you to stop playing!

Maybe...   :-S

ThePrettyDamned

@icbm, the cap thing... What sort of cap do you mean? And do you mean you solder the ground wire to the cap, then the cap to the bridge to ground the strings?

I don't fancy getting shocked...

ICBM

You need a cap with a rating of at least 250VAC or 400VDC to be safe. The value isn't critical, but in the .01uF to .1uF range seems to work well. A 'Class Y' (mains-rated safety cap) is the very best type, but any cap will actually do since the chances of a cap failure at the same time as a mains fault are so remote they aren't worth worrying about.

Unsolder the bridge ground wire from wherever it's attached to in the control cavity (usually the back of a pot), solder the wire to one end of the cap and insulate this connection with heatshrink, and solder the other end of the cap back where the wire came from.

crunchman

I might do that at some point.  Not sure I have enough caps lying around to do all my guitars though.

Van_Hayden

Quite often it can be plain old static, if the gears in good order that's probably it. All live performance venues should be RCD protected on stage as most councils insist on it for the license. Next time try touching anything metal. (I don't mean kiss a radiator...)

Static shocks are also made worse by the fact your lips are moist!

(you weren't wearing PVC trousers and a polyester top were you?)

Danny1969

Sounds like the difference between 2 grounds to me, ie one ground floating above the other  maybe 30v or so and tha'ts the potential difference your feeling. The neutral is connected to earth at the sub station, earth is in effect another path from live to neutral so any difference between 2 earths will cause a current to flow

In a situation like that using a common ground will at least make sure everything is at the same. This is one of the reasons I've always preferred EMG's for live playing. AC will pass through a cap there's not a lot you can do to protect yourself if the guitar is earthed via the strings

fretfinder

Van_Hayden said:

(you weren't wearing PVC trousers and a polyester top were you?)

I wasn't, but I was wearing different shoes to normal. I usually wear the same pair of trainers every gig, but had a bad foot so wore somethng different - don't think the soles were polyester or pvc though...

ThePrettyDamned

I get some of the nastiest shocks at work, separating sheets and blankets. Or wheeling around a wheelchair, then pressing a lift call button... Ouch!

Thanks @icbm, I'll do that on my next build. Sounds like there is no reason not to!