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.


Switchboard Manufacturers Test Assembly to SANS 61439-1&2

Due to increased inquiries and requests from customers in the electrical industry, it has become advantageous to transition from the current SANS 60439-1 to the new SANS 61439-1&2.

Switchboard Manufacturers KZN have tested their upgraded assembly system at the South African Bureau Of Standards (SABS) NETFA Laboratory in Bronkhorstspruit. 

Shane O’Reilly and Andrew MCarthy lead the team that designed and developed the assembly that passed the tests with ease.
The tests were completed under the auspices of the Short-Circuit Laboratory Manager, Seth Mnisi.
The following tests were completed:
1) Strength of Materials-10.2
  • Resistance to corrosion.
  • Thermal stability and resistance to abnormal heat and fire of insulting materials.
  • resistance to ultra-violet (UV) radiation.
  • resistance to mechanical impact.
  • durability of marking.
  • lifting and transport.

2) Degree of Protection of Enclosures- 10.3 

Validate protection against direct contact with live parts, as well as protection against ingress of solid foreign objects and liquids, in accordance with IEC 60529.

3) Clearances and Creepage Distances – 10.4 

Verify that the clearance and creepage distance enable the assembly to withstand the following:

  • Exceptional, transient overvoltage (lighting, HV operations),
  • Operating voltage and temporary overvoltage.

4) Protection against electrical shock and integrity of protective circuits- 10.5

Verify that:

  • The effective continuity between the exposed conductive parts of the assembly and the protective circuit.
  • The short-circuit withstand strength of protective circuit.

5) Incorporation of switching devices and components-10.6-

Ensure the compliance of equipment implementation in accordance with the rules of manufacture and EMC regulations, if applicable.

6) Internal electrical circuits and connections- 10.7

Verify the conformity of implementation and dimensioning of internal circuits and connections. The following should be carefully checked:

  • Short-circuit withstand strength.
  • Temperature-rise withstand.
  • The section of the neutral conductor.
  • Identification of conductors.

7) Terminals for external conductors-10.8

Verify the compliance of implementation and dimensioning of terminals for external conductors.

8) Dielectric properties-10.9

Test each type of circuit in the assembly to ensure:

  • Power-frequency withstand voltage.
  • Impulse withstand voltage.

9) Verification of temperature rise -10.10


  • Thermal stability of the loaded assembly,
  • That the temperatures are controlled on accessible parts, connections and equipment devices.

10) Short -circuit withstand strength-10.11

In comparison to a tested reference design or by testing, verify the level of withstand assigned to the reported short circuit current (unless excluded)

11) Electromagnetic compatibility -10.12

Verify EMC requirements via tests, Except if:

  • The incorporated devices and components comply with ECM requirements for the environment that has been specified;
  • Their installation and cabling comply with the specifications of the manufacturers.

12) Mechanical operation – 10.13

Verify via tests the mechanical operation of removable parts (including any insertion locking). Enclosures, partitions and fastenings should be able to withstand the wear-and-tear of normal use under short circuit condition.

All of the above test were completed and comply with SANS/IEC 61439-2012/2011 Edition 2- part 1 & 2