What are Surge Protective Devices?
Surge Protective Devices (SPD) are used to protect sensitive electronic equipment such as, computers, televisions, washing machines and safety circuits, such as, fire detection systems and emergency lighting, from transient over voltages (voltage surges).
It is now common for an SPD such as that shown in Figure 1, to be fitted within the consumer unit (distribution board) of an electrical system.
What are transient over voltages?
Transient over voltages are defined as short duration surges of electricity which occur due to the sudden release of energy previously stored or induced by other means. Transient over voltages can be either naturally occurring or man-made.
How do transient over voltages occur?
Man-made transients appear due to switching of motors and transformers, along with some types of lighting. Historically this has not been an issue within domestic installations but more recently, installations are changing with the advent of new technologies such as electric vehicle charging, air/ground source heat pumps and speed-controlled washing machines have made transients much more likely to occur within domestic installations.
Natural transient over voltages occur due to indirect lightning strikes most likely to happen due to a direct lightning strike on an adjacent overhead power or telephone line causing the transient overvoltage to travel along the lines, which can cause significant damage to the electrical installation and associated equipment.
IET Wiring Regulations (BS7671)
Whilst the IET Wiring Regulations are not retrospective (Current Edition BS 7671:2018), where work is being carried out on an existing electrical circuit within an installation which has been designed and installed to a previous edition of the IET Wiring Regulations, it is necessary to ensure the modified circuit complies with the latest edition, this will only be beneficial if SPDs are installed to protect the whole installation.
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