In this article, originally published in the IET's Wiring Matters magazine, Geoff Cronshow provides a brief overview of the surge protection aspects of the 17th Edition of the IEE Wiring Regulations:
Section 534 of the 17th Edition deals with the installation of surge protective devices (SPDs). The requirements of Section 534 are for the selection and erection of SPDs for electrical installations of buildings in order to limit transient overvoltages of atmospheric origin transmitted via the supply distribution system and against switching overvoltages.
The requirements are also intended to protect against transient overvoltages caused by direct lightning strikes or lightning strikes in the vicinity of buildings, protected by a lightning protection system. The requirements do not take into account surge protective components, which may be incorporated in the appliances connected to the installation.
Section 534 deals with the installation of SPDs where required by Section 443 of BS 7671:2008 or where otherwise specified by the designer. This Section sets out the requirements for the selection and erection of SPDs for electrical installations of buildings to obtain a limitation of transient overvoltages of atmospheric origin transmitted via the supply distribution system and against switching overvoltages - SPDs for the protection against transient overvoltages caused by direct lightning strokes or lightning strikes in the vicinity of buildings, protected by a lightning protection system.
Surge protective components incorporated into appliances are not taken into account.
Generally, any switching operation, fault initiation, interruption, etc., in an electrical installation is followed by a transient phenomenon in which overvoltages can occur. The sudden change in the system can initiate damped oscillations with high frequencies (determined by the resonant frequencies of the network), until the system is stabilised to its new steady state.
The magnitude of the switching overvoltages depends on several parameters, such as the type of circuit, the kind of switching operation (closing, opening, restriking), the loads and the protection device. In most cases, the maximum overvoltage is up to twice the amplitude of the system voltage but higher values can occur, especially when switching inductive loads (motors, transformers) or capacitive loads or even resistive loads connected very near to the terminals of a supply transformer.
Also, interruption of short-circuit currents can cause high overvoltages. If current chopping occurs, relatively high energy can be stored in inductive loads and oscillations can occur on the load side of the opening switch or protective device.
Fire and shock hazards:
As detailed within BS EN 62305 'Protection against lightning', surges present a risk of dangerous sparking or flashover leading to possible fire and electric shock hazards. Surges also present risk of disruption, degradation and damage to electrical and electronic equipment leading to costly system downtime.
A surge protective device (SPD) is a device that is intended to limit transient overvoltages and divert surge currents. SPDs shall have the necessary capability to deal with the current levels and durations involved in the surges to be expected at their point of installation. In most cases, switching overvoltages are less damaging than lightning overvoltages and SPDs which are effective for protection against lightning overvoltages are also effective against switching surges. SPDs shall comply with BS EN 61643-11 and BS EN 61643-11/A11.
Section 534 contains detailed requirements for the selection, erection and coordination of SPDs in building installations. These include the use of SPDs and the connection of SPDs. Also, protection against overcurrent and consequences of an SPD failure are dealt with together with fault protection, SPD installation in conjunction with RCDs, measurement of the insulation resistance, and the connecting conductors.