Generator Power

[Matt]

so what should stage lighting circuits have on them, mcb's, coz the bars are eartherd?

[kalmatthew]

so what should stage lighting circuits have on them, mcb's, coz the bars are eartherd?

At school we have both RCB's and MCB's. but we were told when we had our new pacs fitted that the halls earth bond was lose .

[gareth]

so what should stage lighting circuits have on them, mcb's, coz the bars are eartherd?

It's common for individual circuits to be MDB protected, with an RCD on a block of circuits (1 per rack, say, or 1 per block of 8 in an LD90).

[Matt]

and in english...

 

(ps sent you a pm, read please)

[Tomo]

What's normally fitted to a Grid-powered venue is:

 

A very large fuse right at the start, to protect the National Grid local loop from serious faults in your system - you can't touch that and have to get a National Grid-approved electrician to replace it, especially as it can't be isolated.

This should never blow!

 

An electricity meter(!)

 

An isolator switch to allow you to alter your system safely.

 

An MCB to protect your system from serious faults.

 

An RCD to protect from earth leakage - some older venues have a seperate "Earth Trip" which you are supposed to earth everything on stage to, and it detects current in the earth line directly. It's not as safe as an RCD though.

 

A series of small MCBs or fuses and often isolator switches, one for each circuit - in lighting there's normally one per dimmer channel, otherwise there's one per ring main.

 

In some venues there is also a fire-alarm relay, which kills power to certain ring mains and supplies if the fire alarm goes off, to kill anything electrical that may have caused the fire. If you've got one the amps should be on that ring, but the cans system shouldn't.

(Of course, our venue has carefully arranged it so the amps stay on but the cans go off, so we've moved our cans base station to a daft place)

 

This isn't an exhaustive list, but it covers most venues - many venues have several supplies of course.

 

As has been mentioned an RCD checks that the incoming and outgoing current are the same, and trips if there is larger than the rated difference for longer than the rated delay and turns off in the rated time.

 

Anyway, the upshot of this is that you should have an RCD on any mobile system, as if a wire gets cut or damaged (or someone sticks their fingers in the socket) the chances are that it won't cause an overcurrent big enough to trip an MCB, but will cause enough current leakage to trip the RCD.

[Ben...]

Anyway, the upshot of this is that you should have an RCD on any mobile system, as if a wire gets cut or damaged (or someone sticks their fingers in the socket) the chances are that it won't cause an overcurrent big enough to trip an MCB, but will cause enough current leakage to trip the RCD.

And, to clarify, you don't need an earth for RCDs to work.

[themadhippy]

And, to clarify, you don't need an earth for RCDs to work.

mmm depends on the defanition of earth if nothing was earthed (I meen substations,genarators ect) we wouldnt need rcd's as theres no return path for any fault currents to flow

[Shaun Foster]

A very large fuse right at the start, to protect the National Grid local loop from serious faults in your system - you can't touch that and have to get a National Grid-approved electrician to replace it, especially as it can't be isolated.

 

Out of interest, does anyone know how you work on unisolated mains?

[Matt]

I was gonna say turn it off, but then I re-read the question...

 

erm... just be extremely careful and use insulated tools I guess?

 

anyway, you could pull it out, just not touch the connection inside the holder, otherwise you would be dead in less than the time it takes to say " I ". you just get some more of those little metal tags they use to secure it, done it plenty of times... although I shouldn't really

 

always treat electricity with respect, thats my motto.

 

anyway, the earth just goes straight into the groud (the stuff called soil) hence the name earth

[themadhippy]

Out of interest, does anyone know how you work on unisolated mains

you dont, but if your on an insulated surface(rubber mat) and olny come into contact with one conductor at a time you should be ok,just dont touch anything else.

I seem to recall from the regs that a permit to work is required for all live working.

[Bryson]

I think, as hippy says, the answer is we don't. Not people in our kind of work environment, anyway. There should always be an isolator for the stuff we do. If you do have occasion to have to work on live mains, I'd expect you to be a "real" electrician, and not need our advice.

 

I always (particularly with "heavy" mains) check twice if it's isolated. Ask someone else to look and see if they think it's isolated. Use a mains-testing screwdriver or multimeter. Then tentatively touch the live with the back of my right hand (so you can't grip, and it's nowhere near your heart.) while holding your left shoulder with you left hand (to make sure it's not touching anything that would give the mains a path across your heart). Then work on it.

 

That said, 240 probably won't actually kill you. (I'm still here.) 415 probably will. But nothing is certain. Insulated screwdrivers, scary rubber gloves, rubber mats etc are all good precautions, but nothing beats turning the bloody thing off.

[Ben...]

Out of interest, does anyone know how you work on unisolated mains?

Very, very carefully... ideally with long plastic sticks or, failing that, amusing chain-mail get ups;

 

http://www.nationalgrid.com/uk/activities/..._live_line.html

[themadhippy]

out of intrest how many of us when testing wether a circuit is dead or not actually test the meter on a knowen live voltage before and after testing the circuit?

[Andrew C]

RCDs trip as a reaction to current in the earth wire (of a certain ammount and duration)

 

No No NO!

 

Older earth leakage devices detected a voltage in the earth wire (voltage sensing) but were phased out due to the possibility of failing to trip if the earth wire is damaged. Modern RCDs detect an imbalance between the currents in the live and neutral. It doesn't matter if this current flows along the earth wire or through you to ground, the device trips out.

 

There are three ratings for an RCD.

 

1) max current rating the device should be used for. It will NOT trip if you overload. (see RCBO below)

 

2) Leakage current 30ma is "normal" for most applications at final circuits, and 10ma for a greater level of protection.

 

3) Trip time. This dictates how long the device will allow the rated trip current to flow. 30ms normal, 10ms for increased protection.

 

Further upstream you may find less sensitive devices. This is to allow for discrimination. ie If your 30ma device on stage trips out when a carpenter nails through a cable, you don't want another 30ma trip knocking out the whole building. Some devices have adjustable sensitivity.

 

RCBO are devices that do all the above, AND trip on over current faults. These typically occupy 2 modules in a breakerboard and are comparatively expensive.

 

 

(Sorry, if I'd read page two of the thread, I'd have seen that most of this had already been said )

[Bryson]

Perhaps I should have inserted the word "effectively" in there.