The Importance of Surge Protection

Surge Protection Devices (SPDs) are used in electrical installation systems as protection devices. They are designed to prevent excess voltage appearing at the terminals of sensitive equipment.

Should an overvoltage event occur, the surge protection device diverts the resulting excess current flow to the earth and therefore protects the electrical appliance.

Any electronic equipment can be vulnerable to transient over-voltages, which can be caused by lightning activity or a switching event. This creates a voltage spike increasing the wave’s magnitude to potentially several thousand volts. This could cause instant damage or significantly reduce a piece of equipment’s lifespan.

The need for surge protection devices can depend on numerous factors. These can include the exposure of a building to lightning induced voltage transients, the sensitivity and value of the equipment, the type of equipment used within the installation and whether there is equipment within the installation that could generate voltage transients.


Based on the type of application, different types of surge protection devices are used. Type 1 surge protection devices are used in conjunction with a lightning protection system.

They do not in themselves offer effective protection against the failure of sensitive equipment. To afford this protection they will need to be used and co-ordinated with one or more Type 2 devices.

These Type 2 devices will usually be installed in distribution boards.

Type 3 devices, which are only required to be installed as a supplement to Type 2, have a low discharge capacity and as such are installed if required near the equipment to be protected.

When installing surge protection devices, you should refer of course to BS 7671.


Making sure a connection is correct is vital when using surge protection devices. To obtain the maximum protection from the SPD, the connecting conductors should be kept as short as possible.

This is in order to minimise any additive voltages from the connecting cables, ideally this should not exceed 0.5m

The cross-sectional area of connecting conductors is also an important consideration. A Type 1 SPD shall have a cross sectional area of not less than 16mm² and for Type 2, the conductor size should be not less than 4mm² copper.


Surge protection devices can offer a visual indication to help with the inspection and testing process. Usually colour coded, if a green light is displayed then this means the device is operational.

If a red light is displayed however, it means the device has reached its end of life and needs replacing. In some cases, the SPD will have cartridges that can be simply replaced.

It’s also important to remember, when carrying out an insulation resistance test at the common value of 500V dc with a surge protection device connected as you will likely get incorrect readings. This is because the SPD will start to conduct at this value of voltage.

The SPD will be required to be disconnected before carrying out this test. Alternatively, the test voltage value can be reduced to 250V dc.