Electricity and Control April 2020

SAFETY OF PLANT, EQUIPMENT + PEOPLE

Surge protection of low-voltage power systems

Holger Heckler, Product Marketing Surge Protection, Phoenix Contact, Blomberg, Germany

More and more sensitive electrical equipment is used in low-voltage power systems. For the efficient protection of LV power systems against voltage or current surges, so-called surge protective devices (SPDs) are often used. Inside such SPDs, spark gaps, gas-discharge tubes and metal-oxide varistors are used.This article presents a close look at the technical characteristics of surge protective components which are suitable as a ‘first stage of protection’ against powerful surges.

A s electronic devices get smaller and smaller, it becomes more and more challenging to protect them against man-made surges (such as switching overvoltages) and against lightning-induced surges. Surges can affect the proper functioning of electrical equipment, they can damage equipment and they can cause dangerous sparking. Dangerous sparking can cause, for example, short circuits in electrical systems. In many cases, damages caused by subsequent short circuits are more severe than damages caused by the initial surge. Surge protective devices (SPDs) can protect electrical equipment against short-duration man-made surges and against longer-duration lightning-induced surges. According to the IEC lightning protection standards, the most powerful surge currents have to be expected at locations where power lines enter or leave a building. IEC Type 1 SPDs shall be installed – as first stage of protection – at all locations where lightning currents can be expected. At locations where only medium-energy or low-energy surges are expected, Type 2 or Type 3 SPDs are usually sufficient. Surge protective components In SPDs for the protection of power systems, mainly the following surge protective components are used: - Spark gaps Spark gaps and gas-discharge tubes (GDTs) are voltage- switching components. They are in a high-ohmic state when no surge is present, but they can change suddenly to the low-ohmic state in case of a surge. In power systems two ‘subspecies’ of voltage-switching components are used. - Between L and N: The let-through voltage of a voltage- switching component is roughly at the same voltage - Gas-discharge tubes (GDTs) - Metal-oxide varistors (MOVs)

level as the peak voltage of the electrical system which is to be protected against surges. - Between N and PE: The let-through voltage of a voltage- switching component can be significantly lower than the peak voltage of the electrical system which is to be protected against surges. Metal-oxide varistors (MOVs) are voltage-limiting components. They are in the high-ohmic state when no surge is present, but they reduce the resistance continuously in response to the voltage applied to the terminals of the respective voltage-limiting components. The let-through voltage of a voltage-limiting component is always significantly higher than the peak voltage of the electrical system which is being protected against surges.

Metal-oxide varistors

Metal-oxide varistors for Type 1 and Type 2 SPDs.

Varistors (variable resistors) are voltage-dependent resistors. They are also called metal-oxide varistors. The resistance of varistors decreases with increasing voltage levels. Most varistors are designed for the discharge of 8/20 µs surge currents. Some varistors are capable of discharging high-energy long-duration 10/350 µs lightning currents. During the whole conduction phase of a varistor,

24 Electricity + Control APRIL 2020