Electricity + Control January 2018

EARTHING + LIGHTNING PROTECTION

Figure 1: SPD with Class I spark gaps and Class II varistors connected in parallel with pluggable spark gaps and varistors; for the protection

of three-phase five-wire power systems.

Varistors Varistors are the most frequently used compo- nents in Class I, Class II and Class III SPDs. They are the best choice to divert man-made switch- ing overvoltages. Most varistors are designed to discharge 8/20 µs surge currents. Some varistors are capable of discharging 10/350 µs lightning cur- rents. Varistors are voltage-limiting components. During the whole conduction phase of a varistor, the residual voltage is always significantly high- er than the supply voltage of the power system. Varistors are capable of providing a sufficient protection effect for sensitive equipment, during short-duration surge impulses (e.g. 8/20 µs). Due to the relatively high level of the residual voltage, during the discharge of long-duration lightning cur- rents (e.g. 10/350 µs), varistors can cause undesir- able electrical stress for downstream equipment. Therefore varistors are not the best choice at in- stallation locations where long-duration lightning currents (e.g. 10/350 µs) can be expected. Varis- tors are prone to ageing. Due to ageing, a leakage current can flow through a varistor and can heat up a varistor slowly. Therefore varistors, for the instal- lation between energised conductors, are always equipped with a thermal disconnect device. Gas-discharge tubes Gas-discharge tubes (GDTs) are fully encapsulat- ed, non-triggered miniature spark gaps. They are used for Class I, Class II and Class III SPDs. GDTs are voltage-switching components with a high dis- charge capacity and a very low level of the residual voltage during the conduction phase. Because of the very low level of the residual voltage, GDTs can't quench power-follow currents efficiently. Therefore it is not permissible to install a GDT be- tween two energised conductors or between an energised and a grounded conductor. GDTs usual- ly get installed between neutral and PE conductor. The spark-over voltage of GDTs depends on the rate of rise of the voltage. During the discharge of fast-rising voltage impulses, the spark-over volt-

• During the conduction phase of the SPD the voltage level of the let-through voltage shall be low enough – for an efficient protection during longer-duration lightning impulses. Spark gaps Class I sparks gaps are themost powerful surge-pro- tective components. Spark gaps are voltage-switch- ing components, and they are capable of divert- ing high-energy long-duration lightning currents (10/350 µs, I imp ). As soon as a spark gap has become fully conductive, the level of the residual voltage is so low that there is no longer any electrical stress for downstream equipment. Because of the low level of the residual voltage, during the conduction phase, Class I spark gaps are by far the best choice for the first stage of protection. Some spark gaps are designed for use between neutral conductor and PE conductor (N/PE spark gaps). Other spark gaps are designed for use between line and neutral conductor (L/N spark gaps). Modern L/N spark gaps have an arc-burning voltage which is about as high as the peak voltage of the supplying power system. State-of-the-art spark gaps are encapsulated, elec- tronically triggered, fast-acting, entirely free of pow- er-follow current and their protection level and their residual voltage is low enough for the protection of sensitive electronic equipment.

Modern encapsulated and triggered L/N spark gap free of power-follow current.

Electricity + Control

JANUARY 2018

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