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Last edited 01 Nov 2021
Voltage is an electromotive force or potential difference measured in Volts. In an electrical circuit’s power source, voltage is the force that moves electrons, as a current, through a conducting loop. It can be imagined in the same way as water pressure forcing water around a plumbing system.
The term 'potential difference' refers to the energy difference between two points in a circuit, and is a measure of how much potential energy exists to move electrons from one point to another. This quantity determines the amount of work that can be done through the circuit.
The unit ‘volt’ (V) was named after the physicist Alessandro Volta who invented the voltaic pile, an early form of household battery. The voltage between two points in a circuit can be measured using a voltmeter. One volt is the difference in electric potential between two points of a conducting loop when a current of one ampere dissipates one watt of power between those points.
Voltage can be either a direct current (DC), as in the case of the potential difference between the terminals of an electrochemical cell, or alternating current (AC), as in the case of the terminals of a common utility outlet. Direct current maintains the same polarity while in an alternating current it reverses direction periodically. The frequency, measured in hertz (Hz), is the number of cycles per second.
Life safety and firefighting power supplies, second edition, written by Tony May and published by BSRIA in 2021, gives the following definitions of supply voltage:
- Extra low voltage (ELV) - 50 V or less
- Lov voltage (LV) - 50 V to 1 kV
- Medium voltage (MV) - 1 kV to 36 kV
- High voltage (HV) - 36 kV to 150 kV
- Extra high voltage (EHV) - more than 150 kV
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