PLEASE HELP: Understanding Stage Electrics

[Tekkie]

Hey Everybody,

 

I just started up this topic because I have had real trouble finding out about stage electrics and was wondering if you would help people like me to understand what actually goes on in all those cables.

 

If you could share some of you knowledge in a '..for dummies' style that that would be great as I want to work in lighitng and stage electrics when I leave education but cant get my head round the way volts, amps, watts, etc work. Forgive me if I seem ignorant...

 

Firstly: what happens in the theatre: the electricity comes in through 1 cable from outside, it then goes into a consumer unit, mcb, rcd :s? what happens to it then..distros, splitters...?

 

What happens with 3 phase?? an example I found...

http://people.bath.ac.uk/su2bc/infoguides/smc/power/images/distro.gif

 

I understand it being seperated into 3 1ph (how btw?) but then a 63A supply turns into 6x 16A supplys (6x16=96 NOT 63 :s) - same with the other 2, the ampage gets seperated into outputs of totally different amounts (32A being seperated into 4x 16A sockets). firstly, how does it get seperated like that, and secondly how come it can get seperated into output supplys that dont add up - surely 1 63A 1ph turning into 2x 63A 1ph cant be done without the dimmers not getting enough power - it would be 2x 31.5???????

 

The different coloured Ceeform sockets: the blue ones (240V) - they come in different ampages - 16, 32, 63, and 125A - what do those ampages mean - is it just a way of describing the size of the plug/socket, or does it mean that they will 'break?' if a device tries to 'suck' more amps than the amount given? eg. you see moving lights with 16A plugs, does that mean they 'sip?' 16As from the ring main? could you change a 13A fused plug from a desk lamp into a 16A ceeform and plug it in without the light exploding?

 

Could you make a ring main that has eg. 20 16A sockets running up to the ceiling supplying intellingent lights with power - I've seen theatres with loads of 16A sockets? - surely the electricity would get used up if it came from just, for instance, 4x 16amp outputs on a distro or wherever all this power comes from :s :s - you could only power 4 lights before all the electricity would be used up/the mcb tripped....orrr, can you have as many sockets (powering moving lights) on a ring main as you like and you could use them all even though the distro is 'apparently' only outputting a certain ampage. WHERE WOULD THE ELECTRICITY COME FROM?!!..sorry :) ..

 

Those are the main points - one last thing 'power supplys', how can something ghave 240V going into it, then output/supply a given amount of amps that can be used before the mcb trips?

 

So yeah, loads of stuff im eager to learn except I just dont understand it and most of the stuff I wrote is no-doubt phrased badly and I'm talking about electricity like it comes from a water tap. If you could share your knowledge though then I would understand and not be so clueless.. cheers

 

Alex

[ChrisD]

Wow, ok. This is a big-un...

 

Google is your friend for a start - this has been covered many times before I'm sure.

 

Also, look on the Wiki. If you honestly find no articles, I'll write one myself about the physics behind power distribution and such, but I'm pretty sure someone else has already done it.

 

A bit about Potential Difference, Current and Power first.

 

Potential Difference is the posh way of saying Voltage. It's measured in Volts. In the UK it comes out of the substation at 230v, on 3 phases. What that means is, the average difference in potential (see how we get the name?) between any Live and Earth is 230v. The average voltage between phases is about 415v. All equipment in a ring main recieves the same voltage (pretty much). So if you plug 2 things in, they don't get 115v each.

 

Current is mesured in Amps. Scientifically speaking, this is the rate of flow of charge, and this is where the water analogy ties in.

 

Power is measured in Watts, but normally referred to in kW. A Par 64 with a CP62 lamp draws 1kw.

 

If you know the wattage of a light, and you want to know how much power it draws, it's really simple to do. Power (P), Voltage (V) and Current (I) are all connected with the following simple formula, which is in about 100 different places all over this forum...

 

P=V*I

 

So, you know a fixture is 1kw, and mains voltage is 230v...

 

1000 = 230 * I

 

Rearrange a bit...

 

I = 1,000 / 230

 

And therefore...

 

I = 4.347....A

I ~ 4A

 

Basically, 1kw is APPROXIMATELY 4A.

 

Right... the biggest misconception you seem to have is that a fixed current supply (i.e. a 63A supply) has to be split EXACTLY. Well, it doesn't.

 

Most dimmers are 10A (for arguments sake). That means a unit made up of 6 at its peak can draw 10A per channel, or 60A, so most people stick a 63A single connector on it. That's fine, but say you want to plug 2 into a 63A single supply, that's also fine. Current doesn't have to be split equally in a single phase situation. You can have one dimmer max-ed out (60A) and one drawing UP TO the remaning 3A. Of course, if you draw more than 63A total, your fuse will blow/mcb will trip.

 

You might think "well, that's pointless, I want 60A per dimmer" - not always. I would guess most dimmers aren't loaded to the max. Obviously, there are some exceptions, but if you think about what "sizes" lanterns come in, in terms of current, (500w, 650w, 1k, 1.2k, 2k) I'd imagine most get loaded between about 1.2k and 2k (or roughly 5-8A). So you've got 12 ways of dimming, each loaded to an average of say 7A. 7A per channel, 6*7A per rack (42A), 2 racks, (84A max). In total, you can have 9 of the 12 ways on at any one time (63A) but you don't have to!

 

Using the water analogy, the water pipe is 63A wide. If you only put 7A of load on, then only 7A of water comes out. You keep ramping this up all the way to 63, and more and more water/current comes flowing out, up to the limit. If current isn't flowing, it's not "used up", and where you split, or distribute the power, you don't have to divide it by 2.

 

With regards to connectors, the blue cee-form ones you're talking about, they don't draw a fixed amount. It's the fixture which draws the current, the connector simply connects. The rating is an upper limit on current. You shouldn't draw more than 16A through a 16A connector, but you physically can, it just might start to melt, and is potentially very _very_ dangerous. If you stuck a 16A connector on your desk lamp, it'd work fine. 13A connectors have a built in fuse, so replacing it with a 16A connector would probably make it "unsuitable for residential use" or similar, but the light would come on, no explosions!

 

Basically, movers don't all suck 16A. For instance, a 250w fixture probably sucks about 2/3A, but it will peak MUCH higher when you strike the lamp. But you could also stick a 13A socket on a mover with no problem, providing it draws less than 13A. Infact, many fixtures have IEC connectors on them (rated to 10A or lower normally).

 

Has that cleared anything up for you, or has it just got a whole load trickier?

[matt c]

The different coloured Ceeform sockets: the blue ones (240V) - they come in different ampages - 16, 32, 63, and 125A - what do those ampages mean - is it just a way of describing the size of the plug/socket, or does it mean that they will 'break?' if a device tries to 'suck' more amps than the amount given? eg. you see moving lights with 16A plugs, does that mean they 'sip?' 16As from the ring main? could you change a 13A fused plug from a desk lamp into a 16A ceeform and plug it in without the light exploding?

 

Could you make a ring main that has eg. 20 16A sockets running up to the ceiling supplying intellingent lights with power - I've seen theatres with loads of 16A sockets? - surely the electricity would get used up if it came from just, for instance, 4x 16amp outputs on a distro or wherever all this power comes from :s :s - you could only power 4 lights before all the electricity would be used up/the mcb tripped....orrr, can you have as many sockets (powering moving lights) on a ring main as you like and you could use them all even though the distro is 'apparently' only outputting a certain ampage. WHERE WOULD THE ELECTRICITY COME FROM?!!..sorry :) ..

 

 

To cover a bit of these two paragraphs....

 

As ChrisD said, if you try and draw more than 16A through a 16A connector, it will melt (eventually). To stop this from happening, this type of socket is normally wired on a radial circuit, with an MCB/fuse and a separate cable for each socket. Since there is no fuse in a 16A plug, it would be dangerous to connect it to anything larger than a 16A fused supply, as the connectors, cable and the equipment you are plugging in will not be protected.

 

Ring Mains are usually only used with normal 13A square pin sockets that have a fuse in the plug. The fuse will protect the cable to the equipment, the connector, and offer a bit of protection to the equipment itself.

 

With either, generally you have as many sockets as you like (either in a ring main, or on individually fused radials) and use as many as you like, up to the maximum available current, so for example, if you had a 63A supply with 20 16A sockets, you could use 4 of them at close to capacity, but you could also use all of them with a 3A load. Don't forget that the fuses or circuit breakers will stop you from overloading anything.

 

It's the same principle with ring mains. You might have 30 13A sockets on a ring main, but (assuming it's protected by a 32A MCB) you could only draw full power from 2 of them, plus a little bit on another. But you could also plug in lots of little loads.

 

Matt.

[Tekkie]

Wow I'd just like to say thanks to both of you guys for replying so quickly and in a way that's easy to understand.

 

Chris, you've actually made things a whole lot easier, I tried google, and wikipedia, and the blue room wiki, but I just couldnt find a decent explaination so thanks :ph34r: , just a few things,

 

1)Do you get a unit that seperates the 3 phase supply from the substation in 3 seperate 1ph 63A supplies, and if so, do houses have to do that - I ask because my house just has a consumer unit thats got loads of trip switches (mcb's I think) as do most, but I dont think the 3-phases are seperated meaning 3 groups of switches recieving 1 seperated phase eg, consumer unit with 12 seperate trips (mcb's?) gets the 3ph from the substation --> something turns the 3 phases into 3x 1ph --> those phases then get distribued to 3 groups of mcb's, ie, 4 mcb's get one phase, another 4 gets another etc..

 

2) Is a distro just a unit you plug into the 63A supply (from the consumer unit/mcb) which lets you plug in lots of things to it, like an 8-way extention cord you can buy, just with different sockets ie, some 13A, some 16A, some 15A etc..

 

3) What happens at huge venues where you need thousands of amps being supplied, would you just plug the power from the substation into a very large consumer unit with loads if mcb's fused at 1000+ amps?? would it just be easier for big venues to be built with loads of different cables coming from the substation, then there would circuits running on lots of different phases making thing run smoother?

 

4) Are the mcb's in houses (or stages) fused usually at 32A (or any specific ampage) because that is all that is the maximum ampage that is considered to be needed by the appliances? - why are mcb's used in houses - all the plugs have fuses in them (the 13A ones). In technical theatre why are mcb's used to 'protect' dimmers? all the dimmers I've seen have seperate fuses for each channel - why do more (and sometimes limiting) fuses have to be used on them?. I assume the only reason why they have ratings for a circuit of (16A for instance) sockets is because there arent any fuses anywhere else in the circuit so it wouldnt be safe. (See below), if ive covered this below in my reply to Matt then thats fine but just to clarify, surely if in a circuit fused at the mcb at 32A for instance, 4 16A sockets each connected to movers sucking 3As each, if one of them goes mental and starts drawing an unsafe ampage *for some reason??*, surely it wouldn't trip because unless it was so high it drew above 32A (unlikely), it would just continue to 'flip-out' and break.

 

 

MATT...thanks for the help, could you go into slightly more detail about the differences of ring mains and radial circuits. I know a ring is where it is fed back into the consumer unit *for some reason??* and a radial circuit stops after the last socket. Just wondering how this could help to explain how a radial circuit helps stop the melting of the cables (is it as simple as: if a fixture draws over the rated ampage only 1 mcb will trip therefore only 1 fixture will turn off instead of all the fixtures on the whole ring losing power?

 

Once again thanks, you guys are great and I know things now that I'd probably have had to do a course somewhere to learn ;)

[Maddison]

4) Are the mcb's in houses (or stages) fused usually at 32A (or any specific ampage) because that is all that is the maximum ampage that is considered to be needed by the appliances? - why are mcb's used in houses - all the plugs have fuses in them (the 13A ones). In technical theatre why are mcb's used to 'protect' dimmers? all the dimmers I've seen have seperate fuses for each channel - why do more (and sometimes limiting) fuses have to be used on them?. I assume the only reason why they have ratings for a circuit of (16A for instance) sockets is because there arent any fuses anywhere else in the circuit so it wouldnt be safe.

 

Just answer one for now; it's a school night!

 

The purpose of fuses, and indeed any other overcurrent protection is to protect, predominantly, the wiring of the circuit which could be damaged by excessive current being drawn. As I am sure you know, when you draw a lot of power down underspecified cable, it will get very hot; hot enough to start a fire. They are rated at whatever it is that the upstream wire is rated to safely take, which will get progressively lower, as the circuit branches out. Does that make sense?

 

As for fuses on dimmers. I assume that it is because it is preferable to lose one lantern/channel of dimming rather than six.

 

HTH

[tokm]

I know things now that I'd probably have had to do a course somewhere to learn

Yes and no.. there are courses that cover things this 'basic', but there's a lot more to it all than this which any half decent sparks course (entertainment industry related or not) should cover.. No mention of Ohms law or how 3phase power works (i.e. each phase @ 120degree's apart) yet. Guess their things you can look up for yourself another time :ph34r:

 

surely if in a circuit fused at the mcb at 32A for instance, 4 16A sockets each connected to movers sucking 3As each, if one of them goes mental and starts drawing an unsafe ampage *for some reason??*, surely it wouldn't trip because unless it was so high it drew above 32A (unlikely), it would just continue to 'flip-out' and break.

In that situation, i.e. where there's an electrical fault.. The RCD on the distro would cut out, which in that sort of size distro would likely to be an overall one i.e. in the chain of things, goes between the in between the 32a inlet and before the 4x 16a MCB's that protect the 4x 16a outlets.

 

This Wiki page covers MCB's/RCD's in more detail - http://www.blue-room.org.uk/wiki/RCD

 

The rest I'll leave to someone else to explain.. I'm too tired!

 

HTH, Tom

[mac.calder]

MCB's and the like are not for the protection of the equipment. What the MCB is for, is protecting the wiring down stream from it.

 

So your Q1: Three phase - separating it - when electricity is generated, it is basically a big magnet rotating inside a number of wire coils (or vice-versa).

 

Here is an image that I find helps (look at the top part) http://www.physclips.unsw.edu.au/jw/images/power_files/3phase.gif

 

V1, V2 and V3 are separate "Phases". What that means is that in an overly simplified generator, we have three separate coils - each coil is 1/3rd of a turn ahead of the next coil. So what it means when a building has three-phase power is that the power company has taken all three outputs from that generator and are running it into the building. So there are three lines from the power company. You do not need to "separate" into phases, because they are never actually joined. Three-phase is advantageous for many industrial uses and has become the 'standard' way large volume supplies are delivered.

 

2) A distro is, in its most basic form exactly what you said HOWEVER a true distro has a number of safety features like MCB's - so (for example) 63A 3p comes in - then there are a whole bunch of 16A 1p sockets, however there may be circuit breakers on each output (or every two outputs) and various other bits and bobs.

 

3) Large venues in our industry tend to have 200A 3p outlets- my last venue for example had 4 200A supplies, 3 120A supplies, and there are 5 separate connectors, one per phase, one neutral and an earth. You connect all 5 to your distribution board and bob's your uncle. If you are in a venue with those sorts of supplies, chances are you have someone (like an electrician) to assist will all of that, including load balancing and connecting etc.

 

4) Each MCB protects everything down stream from it. Generally, each time you step down - ie from 63A to 16A, you would put in an MCB limited to the new conductor size which means that you cannot draw more power than the cabling and connectors down stream are rated for.

 

As for circuit designs - radial is basically a central point, which splits out into separate circuits, these circuits can then split out into smaller circuits - a bit like when you hit glass and it spiders out - and you use MCB's in the way mentioned above, each time you step down, you protect the wiring below it. A ring main is pretty much what it sounds like - a big ring. http://www.diydoctor.org.uk/project_images/ringmain/ring_main.jpg

[LeeStoddart]

1)Do you get a unit that seperates the 3 phase supply from the substation in 3 seperate 1ph 63A supplies, and if so, do houses have to do that - I ask because my house just has a consumer unit thats got loads of trip switches (mcb's I think) as do most, but I dont think the 3-phases are seperated meaning 3 groups of switches recieving 1 seperated phase eg, consumer unit with 12 seperate trips (mcb's?) gets the 3ph from the substation --> something turns the 3 phases into 3x 1ph --> those phases then get distribued to 3 groups of mcb's, ie, 4 mcb's get one phase, another 4 gets another etc..

 

To address the house part of this question... in my experience houses don't get three phase supply. So when you look at the d0m3stic consumer unit it only has a single live wire coming into it (plus a neutral and an earth connection) In my house the board installed by the electric company (or house builder) which has the consumer the company fuses, electric meter and consumer unit there is a blue dot which I guess means that my house is on blue phase - I haven't checked with my neighbours to see if they are on red and yellow. (Our houses were built before the new colours)

[lewiseason]

Hey,

 

And as for larger venues - yes they normally just have a larger current supply in the first place.

 

For example, in the electric cupboard in my school, there is a 7ft high distro unit with 1000A supply cables coming in from above, split into various zones about the school at various currents (470, 630, 63, 50 etc) I assume they are stepped down elsewhere also...

 

HTH

Lewis

[Davethsparky]

surely if in a circuit fused at the mcb at 32A for instance, 4 16A sockets each connected to movers sucking 3As each, if one of them goes mental and starts drawing an unsafe ampage *for some reason??*, surely it wouldn't trip because unless it was so high it drew above 32A (unlikely), it would just continue to 'flip-out' and break.

In that situation, i.e. where there's an electrical fault.. The RCD on the distro would cut out, which in that sort of size distro would likely to be an overall one i.e. in the chain of things, goes between the in between the 32a inlet and before the 4x 16a MCB's that protect the 4x 16a outlets.

 

 

The situation described above would not necessarily cause an rcd to operate (trip). It is described by the regulations as an overcurrent situation, i.e. a current over and above that which would normally be expected to flow is flowing. This type of fault should cause a fuse or circuit breaker to operate (disconnect the supply). The regulations refer to fuses and circuit breakers as ''overcurrent protective devices'.

An RCD (Residual Current Device) however will only operate if the current flowing through the live wire is not equal to the current flowing through the neutral wire. So if the fault current described is flowing from live to neutral an RCD will not operate, however if the fault current finds an alternative return path e.g. to earth, the RCD will operate.

This is only a very basic description of what an RCD does, there is really a lot more to it.

 

Also the moving light in question would normally have an internal fuse of some description which is designed to protect the appliance itself.

 

Wherever I have mentioned the regulations I am refering to the 17th edition IEE wiring regulations (BS7671:2008)

 

As for the supply's to houses,

an average house would normally only have a single phase supply. On a street of six houses the electricity distribution company would normally run a three-phase four-wire supply down the street (3 lives + 1 neutral). and then tap off from this to supply each house, normally bringing Live 1 and Neutral into the first and fourth house, Live 2 and Neutral into the second and fifth house, and Live three and Neutral into the third and sixth house.

This is done for two reasons, 1. This roughly balances the loads on the three phases. 2. The three phase cable will be smaller (therefore cheaper) than a single phase equivalent.

 

Also each house should be provided with an earth by one of three possible methods.

 

There is a lot more detail which could be added to this but there is no point trying to learn that untill you are happy with the basics.

 

hope this has helped.

 

p.s. Its current not amperage,