SPD's, an abbreviation of surge protection devices, are used in power supply networks to protect electrical equipment from damage caused by power surges or transient overvoltages. These can be caused by lightning strikes and switching sources. A surge protector works by conducting any transient overvoltage to earth as soon as it's detected, limiting the voltage to less damaging levels. They are normally installed inside an existing consumer unit, although they can be set up in a separate adjacent enclosure if there is insufficient room. In part due to the increasing prevalence of lightning strikes, the 18th Edition Wiring Regulations stipulate that surge protection must be provided where deemed necessary.