Terms used in electricity
The flow of electricity through wires and cables can be compared with the flow of water through pipes from a tank. When there is no tap open, the water is motionless: but when a tap is opened, the height of the water in the tank exerts pressure on the water in the pipes and forces it through the outlet. The water pressure is comparable with the voltage (V) in electricity
The rate of the water flow - similar to the flow of electrical current - is determined partly by pressure and partly by the size of the outlet. For example, a very narrow nozzle allows less water to flow that a wide pipe. Similarly in electricity, a very thin wire restricts or resists the flow of current. Electricians measure this resistance in ohms (Ω), and the flow of current in amperes or amps (A).
The higher the resistance of a piece of electrical equipment, the lower the current that will flow. To find the strength of electricity required for any equipment, divide the voltage of the supply by the resistance of the equipment. For example, if an electric heater has a resistance of 20 ohms and the voltage is 230 volts, then the current is 11.5 amps; if an electric light has a resistance of 1000 ohms, the current is 230 divided by 1000, or approximately 1/4 amp.
The amount of electricity used at any moment by equipment is called the power and is measured in watts (W). Power is calculated by multiplying the current by the voltage. A luminaire supplied with 230 volts and 11 amps has a power consumption of 2530 watts or approximately 2.5 kilowatts (2.5kW).
The Kilo Volt Amp (kVA) is also a unit of electrical power. Power 'apparent' described in kVA is not the same as power 'true' expressed in Watts. Power in kVA is larger than the power in Watts, as it is multiplied by a scaling factor, called the power factor. Power factor lies between 0 an 1; often it has value of 0.8 to 0.9, although this depends how a system is
All electrical circuits and devices require protection against fault conditions, providing safety to personnel and preventing the device from severe damage. There are several forms of protection, each of which has its merits and particular uses.
Fuses consist of an outer cartridge containing a fine wire. The wire is designed to melt, or blow, when a fault condition causes an above average current to flow through it, preventing the flow of electricity. When a fuse has blown, it must be replaced by one of identical properties to re-complete the circuit.
The Miniature Circuit Breaker is a device designed to perform the same function as a fuse, although it is resettable. When the circuit breaker activates, or trips out, it disconnects the circuit. The fault condition must be found and rectified before the MCB can be reset.
The Moulded Case Circuit Breaker works in exactly the same way as an MCB, but is rated for circuits above 63 Amps.
Residual Current Devices are designed to protect both equipment and users from fault currents between the live and earth conductors. They should be used in conjunction with one of the above methods of protection, which cannot detect this type of fault condition.
An RCD prevents fatal electric shocks by disconnecting the supply if the detected fault current exceeds a safe limit. Typically this is betwwen 30mA and 100mA. RCDs should always be used when equipment is being used outside, when there is a danger of cutting through the power cable, or when there is water present.
A Residual current device is sometimes referred to as Residual Current Circuit Breaker (RCCB) or Earth Leakage Circuit Breaker (ELCB). In the US, they are known as Ground Fault Detectors.
It is important to realise that an RCD does not detect over-currents or short circuits, and must therefore be used in conjunction with conventional protection devices. Such devices may be included within the same physical package as an RCD giving, for example, a Residual Circuit Breaker with Over-current protection (RCBO).
The Circuit Breaker for Equipment offers circuit protection for equipment, sub-systems and components. They are designed to provide highly reliable and closely specified over-current circuit protection for equipment components and low-voltage wiring. CBEs differ from minuature circuit breakers (MCBs) because MCBs are primarily used to protect mains voltage wiring.
Compared with fuses, CBEs have the advantage of being resettable and more reliable. They have the ability to discriminate between safe switch-on surges & transients and harmful prolonged over-currents; together with the capability to match tripping characteristics to system requirements.
Laying power cables
Power cables carry large currents, causing a magnetic field to be generated around them. This field can cause hum in audio cables. Power cables should thus cross audio cables at right angles, and not lie parallel to them, as this reduces the interference picked up.
Power cables should never be left in coils while under load, as this causes inductance which results in a build-up of heat and potentially a melted cable.
Three phase systems typically comprise 3 cables - the phases - that carry 240V, one neutral cable has no voltage, and an earth. The supply voltage is 120V/60Hz in the United States, 230V/50Hz in Europe and the UK, and 240V/50Hz in Australia and New Zealand.
Phase to neutral should read 230V in the UK. This could vary by 250V to 220V depending on the distance from the supply transformer and the load on the supply. Phase to phase should read 415V in the UK. Again, this will vary with the supply. Neutral to Earth should read zero. There may be a trickle voltage of 10V maximum. The neutral and earth cores are connected together at the supply: this is called Multiple Earth Neutral (MEN). Phase to Earth should read 240V in the UK. This reading will determine if there is an Earth core. If there is no earth, the reading will be zero.
All three-phases must be loaded as evenly as possible: this gives a better power factor.
The front of house control, monitor system, and musicians equipment must be on the same phase, as a fault between an instrument and sound equipment on different phases could cause a potential difference of 415V: death to anyone in it's path. If a separate power source is used for the musicians' equipment, it must be checked to ensure that it is on the same phase as the monitors and FOH. A meter reading of 415V measured between active on instrument power and active on monitor power shows that they do not share the same phase. When they are on the same phase, the reading is zero.
Other electrical terms, such as ballast and TRS are described in the glossary.