Electric shock

[stingrayPete1977]

[quote name='JapanAxe' timestamp='1355530457' post='1899777']
Excellent explanation.

[/quote]
Thanks, I have made one useful post out of nearly 5000

[flyfisher]

Good points about the risks of three phase. I've very limited experience of such things but my understanding is that there are regulations requiring 'spaces' to all be wired to a single phase. However, in large venues, 3 phases might be installed thoughout a building (one on each floor perhaps?) and in such circumstances the use of long extension leads can present some real potential risks because you can,t be sure that everything is running from a single phase. As I understand things faults within a single phase supply can expose you to 240 volts but a fault within a 3-phase supply can expose you to 415 volts.

I was once told that a healthy person would be unlucky to be killed by 240v but would be lucky to survive 415v. Not a definitive statement, of course, but an indication of the relative severity.

But I've only ever worked with a 3-phase installation once and the above is mainly anecdotal from what I was told by the on-site electrical contractors at the time.

[ezbass]

I've had this off of mics in the past, especially in rehearsal rooms. It's very unpleasant for sure and I never knew why it was happening. It was always very low current, almost like licking a 9v battery. My layman's cure was to put a handkerchief over the mic and obviously one of those foam pop shields would've done the job too. I've often wondered if someone had accidentally fired up the phantom power by mistake and that was cause, but as I haven't experienced recently I haven't had a chance to check this theory and I'm not going to turn it on and try it out for poops and giggles!

[BILL POSTERS]

[quote name='JapanAxe' timestamp='1355530457' post='1899777']
Excellent explanation.

I always plug into an RCD on gigs, and carry a ring-main tester to use if I have any concerns about the venue mains supply. Biggest worries are extension leads at outdoor/marquee jobs. Most common faults are no earth (which equals no protection from lethal faults), and live/neutral reversed (which may not be as bad as it sounds if all the band are on the same supply, but is not recommended!)
[/quote]

Outdoor venues are often dodgy, I've come across situations where the sockets had no earth more than once, and we were standing on grass at one of them. luckily an RCD plug will trip if theres no earth,so it was spotted.

I think it was Stone the Crows guitarist, Les Harvey, Alex's brother who was killed

[stingrayPete1977]

Three phase can be present between two sockets fairly close to each other, often they will be installed to an engineers spec to try and acheive an even load across all three phases, so a single story venue with a stage may well have a stage ring main on one phase and dance floor area sockets on another, throw in a cleaners socket on the third phase and you could be using all three between band, lights and dj! Certainly even in primary school we wire dado sockets for computers on one phase with auxiliary sockets within an arms reach of them on another.

[flyfisher]

Interesting. My only experience with 3-phase (well, apart from unknown ones - based on that above description) was during the installation of equipment cabinets in a computer room. I wasn't involved in the electrical installation but the equipment we were loading into the cabinets was so power hungry that our cabinets couldn't all be powered from a single phase.

In the end the electrical contractors worked out some scheme to spread the load across multiple phases, but [u]because of the close proximity[/u] of cabinets on different phases, everything had to be clearly labelled with 415v warning signs.

This was in the late 80s so the regulations may have changed since then, but I can't recall ever seeing 415v warning signs in public spaces, so I've always thought that everything within (easy) reach should be on the same phase.

[stingrayPete1977]

If you think of places like kitchens then its unavoidable, some appliances are even 3 phase that unskilled persons or even the public hiring the venue are using, dishwashers, mixers etc. The thing you touch the most is a lighting bank of switches, these often have 415 present unlabeled. I would certainly not mix phases in one cabinet though and a warning notice between more than one is a sensible addition. Truth is its all around you!

[flyfisher]

Fair point about commercial kitchens using 3-phase appliances, though I was thinking more about public spaces rather than commercial/industrial ones. But, having said that, you're right that band venues are very often commercial premises anyway.

Let's be careful out there!

[stingrayPete1977]

That's it, you can hire our village hall unsupervised which includes the bar and kitchen all three phase. Rcd and plug testers all the way

[Grangur]

When designing electrics for buildings you can't avoid having 3 phases close to each other. You need to assume any 2 sockets used could easily have 415V between them. It wouldn't be such a bad idea IMHO to have it as a band rule that everyone uses an RCD and if you can, get an earth lead out from your own gear and clamp it to the earth (mic-stand?) of any other gear about so much the better.

Remember it takes no more than 30mA through your body to kill you, whatever the voltage.

[SteveO]

[quote name='JapanAxe' timestamp='1355530457' post='1899777']
...and live/neutral reversed...
[/quote]

ah, here's something that i've wondered about for a while. is it really dangerous to have live/neutral reversed? I ask because over here the plugs are symetrical so it's easy to reverse the wires by simply unplugging and inserting the plug the other way up.

[BOD2]

[quote name='SteveO' timestamp='1355672046' post='1901156']
ah, here's something that i've wondered about for a while. is it really dangerous to have live/neutral reversed? I ask because over here the plugs are symetrical so it's easy to reverse the wires by simply unplugging and inserting the plug the other way up.
[/quote]

In the UK, the mains plug has a fuse fitted. This fuse should be on the LIVE wire so that any current must pass through the fuse FIRST before reaching any other part of the circuit.

If the live and neutral are reversed then this could potentially put the mains fuse on the neutral wire where it offers less protection.

If countries where the mains plug is NOT fused then this particular issue is not relevant, although reversing the live and neutral wires may reduce the effectiveness of other safety devices in the circuit.

[SteveO]

ah, that makes sense, but isn't it AC, which would mean that every 50th of a second the live and neutral wires are reversed anyway? at least from the point of view of the electrons?

[Musky]

True, but neutral is tied to ground and the phase has a constantly varying voltage. In the UK on a single phase that's 230V to -230V.

[rmorris]

[quote name='Musky' timestamp='1355679942' post='1901295']
True, but neutral is tied to ground and the phase has a constantly varying voltage. In the UK on a single phase that's 230V to -230V.
[/quote]

+1

Essentially problems arise where 'earths' are not at reasonably close potentials due to missing or poor connections.
This can be due to connections not being made or becoming disconnected or simply through conductors which have too much impedance - usually as simple as the wire not being thick enough.
; or connections can corrode over time due to rust / oxidiseation or chemical reaction between different metals.

It's also worth mentioning that 240 (ish) Vac (RMS) means that the peak voltage to 0V is in the region of 340V.

On a slightly different note it's worth recognising that the potential problems reduce if you're not 'earthing' yourself by contact with the strings / bridge etc. of your bass/guitar. this seems generally to be okay where you use Lo-Z pickups ( emg etc ) and / or galvanic isolation - If I recall correctly WAL basses were fitted with an isolating audio transformer ?

[BOD2]

Using a radio transmitter rather than a cable between the bass and the amp will also give some level of protection (since there is no physical path to earth from the bass) but this should NOT be relied upon as a safety device. Under the right conditions an earth path will exist through your body to the floor.