This article takes a closer look at the proposed new Section 534 (Surge Protection Devices) which it is expected may be included within a future amendment to BS 7671:2008.
Section 534 contains requirements for the installation of surge protective devices (SPDs) to limit transient overvoltages where required by Section 443 of BS 7671:2008 or where otherwise specified by the designer. A risk assessment to BS EN 62305, Protection against Lightning also determines the need for SPDs.
Surge protective components incorporated into appliances are not taken into account in 534.
Both lightning strikes and electrical switching can inject what are called transient overvoltages into installations. Transient voltages are usually only a few micro seconds in duration. However their peak value can reach 6 kV. Normal electronic equipment cannot withstand this level of voltage.
Atmospheric events
Lightning is the visible discharge of static electricity. The current contained within a lightning strike varies considerably with the atmospheric conditions. Associated with this sudden discharge of current is a magnetic field that surrounds the lightning perpendicular to the direction of travel.
Lightning can impress a voltage onto a low voltage power network (or any metallic service) in a number of different ways: resistively, inductively or capacitively
Switching events
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. 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.
Surge Protection Devices
A surge protective device (SPD) is a device that is intended to limit transient over voltages and divert damaging surge current away from sensitive equipment. SPDs must have the necessary capability to deal with the current levels and durations involved in the surges to be expected at their point of installation...
To view the full article from the IET's Wiring Matters Summer 2009, click on the attachment below...