Stage Management Guide - Power
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Almost everything Backstage does requires electrical power, in a vast range of amounts. It is therefore extremely important, especially when using large amount of electricity, to understand how this power should be correctly distributed.

Please remember that electricity can kill and if you're unsure about its usage, consult a trained theatre electrician. Backstage Technical Services accept no responsibility for damage to persons or equipment from the use of this information.

The basics
Electricity is made up of moving electrons, parts of atoms which orbit the nucleus. The movement of these electrons from atom to atom is what gives motors, lights and this computer, their power. There are three main attributes that an electrical circuit will possess and these will change depending on the power supply and the items used in the circuit (eg. lights). Here's a simplified view of what they can be thought of as:
• Current (amperes/amps A): An indication of the number of electrons passing long the circuit.
• Voltage (volts V): An indication of the speed or force that is applied to the electrons.
• Resistance (ohms): An indication of the resistance a material/object has against electricity being passed through it.
Voltage (V) = Current (A) x Resistance (ohms)

AC versus DC
There are two main ways in which electricity is provided to a piece of equipment:

 Direct Current (DC) The voltage provided by the power supply is constant throughout time. Examples of such DC supplies are batteries and the output end of your mobile phone charger. This is the most useful type of power, since it can be used on most pieces of equipment and is required for electronics to operate. Alternating Current (AC) The voltage provided by the power supply varies through time, usually in a sine wave form with a frequency of 50Hz (50 times a second). The electricity runs one direction round the circuit and then moves the other direction round the circuit. AC power is easiest to make, requiring a magnet, some coils of cable and a rotating shaft. It is also the easiest to efficiently change the voltage of, with the use of a transformer (robots in disguise!).
Why use AC?
The cables in the national grid, in your house, in your theatre, in your lights, all carry an AC supply. The main reason for this is that voltage of AC supplies can be changed extremely cheaply and efficiently using a transformer.

Phases
 Single phase This is the simplest form of supplying AC, and is the way in which electricity is passed round your home. The normal AC requires 3 power cables (live, neutral and earth). Three phase Let's add a couple of extra lives, with the power supplies running 0, 120 and 240 deg out of phase, all sharing the same neutral. When one of the lives is pulling electricity, the other are pushing in the opposite direction at a lower voltage. If you add all these effects together, you'll get zero: the 3 phases balance each other out with the electricity moving just along the live cables. Since there is no electricity flowing down the neutral, we can take it away! The lives are doing all the work and we only require 4 cables (live1, live2, live3 and earth). Result! But hold on there cowboy, how do we go about pulling exactly the same amount of power through each core at every point in time? The answer is of course that we can't and so the neutral is always used in 3-phase systems. In many power supplies, the neutral cable isn't rated as highly as the other cores and so you must be very careful to balance the current as much as possible.

3-phase electricity
For carrying large amounts of electricity, 3-phase is usually the most efficient way, combining AC transmission with a minimum of cable. 3-phase isn't usually used as a primary power source, it is usually broken down into its three phases and these are used separately.

Since April 2004, the UK has officially changed the colours for each of the live phases. In any new, or recent work, L1 is coloured brown, L2 coloured black, and L3 coloured grey, with the neutral coloured blue. However, most of Backstage's equipment was built before this date, and so we continue to refer to L1, L2 and L3 as red, yellow and blue.

For Backstage usage, we try and use two of the phases for powering lights and the third for sound. This is because using dimmers on the same phase as sound equipment usually results in buzzing being produced from the speakers (due to electrical interference from the dimmers). If required, intelligent lights (which don't require dimmers) can also be used on the sound phase.

It is important to try and load the phases fairly equally since each depends on the other to carry its electricity (since we got rid of the neutral). A large imbalance, for example all the lights on one phase and the others empty, can lead to the power supply tripping out.

Although each phase carries around 240V, because they are all out-of-phase, the voltage between them is much greater (415V). For example, if you grab hold of a red phase cable and a blue phase cable, you'd get 415V passing through you: almost certainly enough to kill you. Therefore equipment on different phases should be kept apart to ensure that you can't contact both at the same time.

Generators can cause a few issues if supplying 3-phase power - it is very important that you try and load them as evenly as possible so that similar current is being drawn on each phase. Whilst several generators can be used to power a single circuit, this is extremely difficult to get right: consult a properly qualified and experienced electrical engineer if you are considering such a system.

Fuses, earths and circuit breakers
Probably the most important part of a power supply is the ways in which you can prevent it killing you. There are three main pieces of equipment that stop you electrocuting yourself:

Fuses
Think you know what a fuse's purpose is? Well, here's the important bit:

A fuse is not designed to stop you getting electrocuted

Its purpose is to be the most delicate part in the equipment, if a large current passes through the equipment, it is the fuse that melts first, thereby saving the equipment. This is most important in your house, where a fault in your wiring could result in large currents passing through your ring mains. These ring mains are buried in your walls and unless the power is cut (eg. through the use of a fuse) then the first you'd know about it is when the hot wires set on fire, burning down your house. If a fuse should be in a piece of equipment, make sure that it is of the correct rating and never bypass a fuse connection: tinfoil bad, correct fuse good!

Earths
Good news: the earth is meant to stop you getting electrocuted. Each mains cable should have an earth (yellow and green wire) and it should be properly connected up. Since electricity follows the path of least resistance, a faulty light should pass current through the earth cable instead of through you. That's the theory, but it relies on the earth being undamaged through all the cables, and the light having an effective connection between its earth wire and the external metal casing. You can see why it's important to electrically test each component and why you should never use something that has a dodgy earth connection. One important thing to remember is that an earth doesn't cut the power to a component, it just drains the electricity away elsewhere.

RCD (Residual current detectors)
RCBs are devices which can protect you from electrocution by cutting off the power. They monitor the differences in the current flowing through the live and neutral: if this difference is large enough then the power is cut. They can often trip when you initially switch the power on and can be reset once the problem has been fixed. A circuit breaker should be used on power tools to protect against cable damage.

When drawing large amounts of power, the best place for a circuit breaker is as far upstream as possible; on the power supply that is plugged into the wall. Any additional RCDs should be used close to the load.

Circuit breakers generally reside in power supplies but can also be inserted into other parts of the circuit (eg. just before the dimmers) to cover specific pieces of equipment.

MCB (Miniature Circuit Breaker)
An MCB is an extremely useful device which monitors the amount of current being drawn through a cable (or device). They are often used on large dimmers which can drawn large amounts of power at different times (eg. if lights are flashed). The MCB will then trip out if too much power is drawn by the dimmers, protecting the main power supplies and cabling.

In general MCBs should be used on any high-power device with an unpredictable power draw. They protect the main power supply and can be reset quickly, allowing you to get the show back up and running with the minimum of fuss.

Isolators
These are switching boxes used in installed power supplies. Unlike a standard switch, they should only be turned on or off if no power is flowing into the isolator (otherwise a large arc will occur). They don't form part of the electrical system's protection.

 A 'bare minimum' of protection for the sample power distro system. Note that the RCD protects the entire system whilst MCBs are used to prevent excess power draw through 3 phases.

Unfortunately, you can't just keep shoving stuff onto an electrical circuit and expect no problems to occur. A circuit can only be loaded up to a certain extent before too much current is drawn, then wires start heating up and fuses start blowing. These problems are especially acute when you're doing an event with lots of lights in a venue which doesn't have a large power supply. Here are some things to watch out for:
• Check what supply you've got, if people don't know then check the power distribution at the fuse box.
• Your circuit is limited by the weakest component: if you have a 32A ring main and use 13A fuses, then you can only run 13A through your circuit.
• Ring mains may or may not be able to take 13A.
• When using 3-phase, ensure that the phases are fairly evenly loaded.
• Ensure that there are adequate circuit breakers.
• Be aware that the supply into the building might not be as large as the fuse rating in the house. For very large events requiring lots of power, try to find out as much as possible in advance.
• When adding 13A items like 4-ways, remember that they have their own 13A fuse, limiting the power through them.

Terminal boxes
These (often grey) boxes are the outputs of large installed electrical power systems and can be found mounted on walls in a number of venues. They can be opened using a key and then wired into a power supply using a bare cable to whichever connector you require. Don't even think about trying to fiddle with a terminal box if you don't know exactly what you're doing: even when switched off, there are still accessible live parts.

How many lights can I use?
First find out the maximum number of amps that your circuit can take. Then find out the power of your light (eg. Par 64 = 1000W). Now:

Power = Current x Voltage

so each Par 64 will use 1000W / 240V = around 4 amps. Therefore a 13A circuit can take 13 / 4 = 3 lights.

BUT
A cold light will draw a larger current than hot lamp and therefore less lights can be used. This also applies to flashing lights meaning that our 13A circuit is more likely to only take 2 lights.

Bear in mind that the power drawn through a dimmer is not proportional to the dimmer setting. As you increase the dimmer setting, more current is drawn through the power supply until at about 70%, maximum power is flowing.

On large events, the greatest restriction is sometimes the amount of power available rather than the cost of the equipment. Therefore use of more efficient, lower power lighting such as Source 4s is a useful option. When calculating the power available from 3-phase power, remember than there is 3 times the stated value available; eg. a 63A 3-phase supply has 63A on each phase, totalling 189A.

Connectors
We've got our power, we know how many items can be placed on each circuit; now we just need to connect it all together.
The most common connectors used are ceeform type, an industry standard which is used by Backstage for all circuits over 15A. There are three main classes of ceeform connector:

 110VUsed in some countries, 110V supplies aren't very widespread but are occasionally used by Backstage for outdoor work. 240V 1-phaseThis connector is available in 16A, 32A, 63A and 125A, each connector increasing in size along with its power rating. 240V 3-phaseVery similar to the 1-phase variety, the 3-phase connector is also available in 16A, 32A, 63A and 125A types.

There are a few other, notable connectors:

 5A Although small and once very common, the 5A connector has almost been completely phased out although its still around in a large number of schools. Powercon Only rarely used on racks, this connector is equivalent to a locking, unfused IEC connector. IEC Otherwise known as a kettle lead, this is a simple 3-pin connector found on low-power rack units. It can handle up to 6A. 13A You know it, you've used it, it's the 13A connector. The main disadvantages in theatre usage of the 13A connector is the fact that is contains a fuse (which is difficult to find and replace) and that you can't tell if the circuit that it is on comes from a normal ring main or a dimming circuit. 15A The most common lighting connector, the 15A has no fuse and is fairly cheap and sturdy. If your cable has a 15A socket on the end, you can usually assume that the circuit has a dimmer on the other end.By not having an inbuilt fuse, it means that all the circuit protection can be centralised at your power supply, reducing the time spent finding and fixing faults. 16A ceeform Sometimes rated for outdoor use, the 16A connector is becoming increasingly used for theatrical connection rather than the less durable 15A design.Backstage try and keep 16A cables for use with undimmed supplies (eg. for intelligent lighting), thereby avoiding confusion and mistakes. 32A ceeform Sometimes used for powering small dimmers, the 32A connection is usally to big for something normal and to small for something useful. It occasionally appears on amplifier racks. Camlock & Powerlock (3-phase)The Camlock connector is used for the connection of large 3-phase supplies and is currently fitted in the Sports Hall. Unfortunately, the original camlock socket design allowed you to reach inside with your finger, and so a newer, safer design was introduced. Very large supplies (eg. 128A 3-phase, 140A 3-phase) often utilise camlock, the five connectors being used for live1, live2, live3, neutral and earth. Lectriflex You've got hundreds of lights, all needing power but you really don't want hundreds of power cables. Therefore lectriflex (or leci) uses multiple cores to send power to 6 separate channels. It requires a leci-15A spider connector which changes from leci to 6 separate 15A sockets. Bear in mind that leci can only carry 10A down each channel and that since the connector is rated to 380V, all power must be on the same phase. Socapex A very similar idea to lectriflex, carrying 6 channels of 10A power but this time using a circular connector.

15A wiring
The most common power connector used by Backstage, the 15A connector is used for almost every light we have. There's a few important points you need to remember when you want to wire a 15A plug or socket:
 Get decent plugs, sockets and cables - proper 1.5mm˛, 3 core cable is best. Don't use anything thinner and ensure that the cable is in good condition before wiring it up. Use high-temperature silicon sheathed cable to lanterns and rubber HO7 for extension cables. Cut the outer and inner sheathing carefully - any nick will result in less efficient earthing. Always ensure that the earth core is longer than the live and neutral. This ensures that if the cable is pulled, the earth connection will come out last, providing maximum protection. The cable grip ensures that the cable can't be pulled out of the connector: always use it and ensure that the screws are done up tightly. Electrically test (PAT test) the cable after it is made to make sure its OK.

Credits:
The Backstage Power Guide was written by Colin Hodges and is © BTS 2003.
No reproduction in whole or in part in any means whatsoever is permitted without written consent.

Backstage Technical Services, University of Bath Students' Union, Claverton Down, BATH, BA2 7AY
Generously supported by the University of Bath Alumni Fund.