Electricity + Control January 2018

EARTHING + LIGHTNING PROTECTION

Surge-Protective Devices for Low-Voltage Power Systems Holger Heckler, Phoenix Contact

Nowadays more and more sensitive electronic equipment is used. Electronic devices get smaller and smaller. Saving space is beneficial, but miniaturisation has its drawbacks.

T he smaller electronics become, the trickier it is to protect electronic devices against man- made surges and against lightning-induced surges. This article explains the differences be- tween the individual surge-protective components used in modern Surge-Protective Devices (SPDs), for the protection of low-voltage power systems. For protection against transient surge-voltage and transient surge-current impulses different components can be used: spark gaps, varistors, gas-discharge tubes and suppressor diodes. SPDs can consist of: • Voltage-switching components: Spark gaps, Gas-discharge Tubes (GDTs) • Voltage-limiting components: Metal-oxide Var- istors (MOVs), suppressor diodes • Combinations of voltage-switching and volt- age-limiting components The International Electrotechnical Committee (IEC) stipulates the use of different SPDs at certain in- stallation locations. For the protection of power systems, three classes of SPDs (Class I/ Type 1, Class II/ Type 2, Class III/ Type 3) are defined in IEC 62305-4 [1] and IEC 61643-11 [2]. The most pow- erful SPDs (first stage of protection, usually Class I SPDs) shall be placed at the service entrance or at the main distribution cabinet. Class I SPDs are capable to ‘redirect’ high-energy long-duration lightning currents to ground and to other directly or indirectly grounded conductors. A single SPD, at the origin of the installation, is not capable of protecting the whole electrical system. Addition- al SPDs are needed downstream – to deal with lower-energy surge current and surge voltage impulses which are left-over by the first stage of protection.

Class I/Type 1 SPDs • Main distribution cabinet (of facilities with ex- ternal Lightning Protection System (LPS) or fa- cilities supplied by overhead lines) • High discharge capacity (lightning current im- pulse, 10/350 µs, Iimp) • Components: spark gaps, big varistors, big GDTs Class II /Type 2 SPDs: • Main distribution cabinet (of facilities supplied by buried cables and without external LPS) • Sub-distribution cabinets • Medium discharge capacity (surge-current im- pulse, 8/20 µs, I n ) • Components: medium-sized varistors, medi- um-sized GDTs Class III /Type 3 SPDs: • Installation close to sensitive equipment • Low discharge capacity (hybrid impulse, 1,2/50 µs, 8/20 µs, 2 ohm, Uoc) • Typical components: small varistors, small GDTs, sometimes suppressor diodes Main tasks of SPDs: • Lightning currents and surge currents shall be ‘redirected’ in a way that nothing gets over- heated and that there is no unwanted sparking or arcing • Surge voltage impulses and lightning-induced voltages shall be limited in a way that down- stream equipment does not get damaged • The peak voltage (voltage protection level, Up) of the SPD shall be suitable (sufficiently low) for the protection of the downstream equipment (see IEC 62305-4 [1] and IEC 61643-12 [3]).

Take Note!

The IEC stipulates the use of different SPDs at certain installation locations. Lightning protection sys- tems should be inspected periodically, A high-voltage test de- vice is needed for the proper testing of SPDs.

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Saving space is beneficial, but miniaturisation often has drawbacks.

24 Electricity + Control

JANUARY 2018