Why are Surge Arrestors so important?

The use of Surge Arrestors in electrical power systems

Surge Arrestors fulfill important roles in industry and even residential areas. These devices protect voltage sensitive equipment connected to power lines from voltage surges induced by lightning strikes or switching within a power line.

How a Surge Arrestor works

The surge arrestor basically acts as a voltage controlled resistor. This means it changes its resistance according to the voltage which is developed across it. The surge arrestor contains a Metal Oxide Varistor (MOV) disk. This disk is a semi-conductor.  The MOV allows the arrestor to have an extremely high resistance at low voltages. When the voltage across the device rises to dangerous levels, the resistance of the MOV disk rapidly drops. The arrestor thus acts as a short circuit when high voltages are developed across it. This allows the arrestor to divert high voltage surges to ground protecting other devices connected to the power line.

Due to the working of the device, it needs to be connected in parallel to all the equipment which needs to be protected. The device also has low power requirements when operating under normal conditions due to the high resistance of the device under these conditions. The surge arrestor will thus ensure that the voltage of equipment is kept at a set value and does not rise above this specified value. This is known as voltage clamping.

[The internals of a Surge Arrestor with the MOV disks visible]

I have a Lightning Rod, why do I need a Surge Arrestor?

A common misconception is that lightning rods can fully protect all devices connected to a power line. This is however not true. Lightning rods merely divert lightning away from power lines. If the lightning strikes the power line directly, the lightning rod is powerless to stop the developed voltage surge from damaging the line and equipment. Also a lightning rod can’t protect power lines from voltages surges produced during switching within a line.

This is where a surge arrestor is necessary. The surge arrestor will nullify any voltage surges within the power line. It is thus recommended that a surge arrestor and lighting rods are installed to ensure full protection of electrical equipment and power lines.

Surge Arrestor Selection and Installation

When selecting a surge arrestor, it is very important that one keeps in mind the rated voltage of the system which needs to be protected. This is important as different surge arrestors have different rated voltages at which they operate. This is due to the fact that surge arrestors are rated to clamp circuits at a certain voltage and can only protect circuits up until a maximum allowable voltage. Surge arrestors are also rated to divert a set amount of energy. If arrestors divert more energy than this maximum energy, they will be damaged. Before installation every surge arrestor needs to be carefully chosen to ensure complete over-voltage protection.

When you are ready to install your chosen arrestor, it is also important to keep all the connection lines to the arrestor as short as possible. This is due to the fact that during an over-voltage situation long lines leading to the device can actually amplify the voltages developed across the equipment and this lessens the efficiency of the surge arrestor.

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.