Overvoltage overlooked


Switching events, such as starting up equipment from plant machinery or electrical network switching, generate overvoltage’s from within the building or externally. This could be from a neighbouring building using, for example, arc welders, lift motors, refrigeration equipment and air conditioning systems. Even certain types of lighting systems can generate potentially damaging overvoltage conditions.

In the UK, we have an average of 300,000 lightning ground strikes a year, the majority of them hitting open areas not affecting us. However, in some cases they hit buildings and overhead power lines. These potential harmful voltages can be inducted into the building from a direct strike or from the secondary effects of lightning, which can reach up to 6,000 volts.

An effective surge protection device (SPD) should have a low voltage let-through voltage, as this is the amount of the voltage that the equipment being protected will be subjected to. The greater the overvoltage the greater the risk of disruption, degradation and damage to the equipment connected to the electrical system. As technology moves on, components within electronic equipment continue to become smaller and, therefore, more sensitive to these types of influences.


  • Metal Oxide Varistor (MOV) – An MOV is a bipolar, non-linear resistor with a symmetrical voltage/current characteristic curve and a resistance value which decreases as the voltage increases. This is the most popular type of device, mainly due to its fast reaction times and relatively low cost of production in comparison to other technologies.
  • Gas Discharge Tube (GDT) – A sealed gas-filled spark gap consisting of two metal electrodes, which are normally spaced by a ceramic or glass insulator. Not as popular as MOV technology as they can be slightly slower in reaction time. • Silicone Avalanche Diode (SAD) – A semiconductor diode, which avalanches like a thyristor and folds back to give less dissipation. Fast in reaction times, but expensive to produce and use.

Fitting surge protection devices is very simple, providing that they are designed into the installation. This is normally achieved by allowing for a spare way in the distribution board this may be a three-phase or single-phase installation – and fitting it at the origin. The supply feeding the SPD is normally done via an over-current protection device, for example a circuit breaker. The circuit breaker is predominantly there for means of isolation purposes, to allow for commissioning, testing.

It should also be noted that the SPD should be mounted as close as possible to the incoming terminals of the circuits being protected, with cables between protective devices feeding the SPDs is kept as short as possible in order for the SPD to operate at its best performing clamping voltage.

Type 1 Class 1 are fitted at the origin of a supply where the building has an LPS (lightning protection system) fitted, or has an overhead supply. This SPD will protect against damage caused by a direct lightning strike.

Type 2 Class 2 are fitted at the origin, if no LPS system or overhead supply is present, co-ordinated downstream from a Type 1 Class 1 in sub distribution boards if required.

Type 3 Class 3 are fitted next to sensitive equipment eg fire alarm panels, CCTV, server rooms and PC work stations or co-ordinated downstream from a Type 2 Class 2 if required.

The benefits of fitting SPDs far outweigh the initial cost, so why not offer to fit SPDs on all installations?

For more information on the range, contact Omak Agencies on 028 9055 0202 or visit www.omakagencies.com