Surge Protection Devices

The inclusion of Surge Protection Devices (SPDs) in the 18th edition of the BS7671 Wiring Regulations shows how far the complexity and concern for personal safety has come.

Two forms of surge

  • LEMP - Lighting Electro Magnetic Pulse
  • SEMP - Switching Electro Magnetic Pulse

To be clear, SPDs do no protect an installation from lighting strikes, only a fitted lighting protection system to BS EN 62305n will prevent a direct strike from damaging the property. SPDs provide protection from two forms of surge:-

LEMP - Lighting Electro Magnetic Pulse

SEMP - Switching Electro Magnetic Pulse

What are surges?

Surges are transients overvoltage or surge in short durations. Increaseing in voltage measured between two or more conductors.

The increase in voltage will vary from a few volts to thousands of volts.

In this context short means anything from microseconds (millionths of a second) to a few milliseconds (thousandths of a second) in duration.

This voltage exists between two or more conductors.

For a mains power supply, these conductors would be the line, neutral and earth.

These can be created from atmospheric (lightning) or switching (Industrial) occurrences.

The consequences of a surge

Without adequate protection the greatest consequence is a risk to human life, particularly in circumstances where medical equipment is involved and could be damaged.

Secondly, there is the potential downtime in production e.g. in a workshop, retail outlet or office.

‘Transient overvoltage’, is technically and descriptively the best terminology. However, transients are also referred to as surges, spikes and glitches.

Changes to the 18th Edition BS7671 Wiring

Protection against transient overvoltage shall be provided where the consequence caused by overvoltage:

Results in serious injury to, or loss of , human life or;.

Results in interruption of public services/or damage to and cultural heritage or;.

Results in interruption of commercial or industrial activity or;.

Affects a large number of co-located individuals;.

For all other cases, a risk assessment according to Regulation 443.5;.

Types of Surge Protection Devices (SPD):

Installation as defined in the standard is simple enough, the device is connected with cables to and from the unit no longer than a metre in total, and a short path to earth will ensure the effective discharge of the surge to earth.

Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).
Type 1 SPD Type 2 SPD Type 3 SPD
The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage.It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

Type 1 SPD is characterized by a 10/350 µs current wave.
The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of overvoltage's in the electrical installations and protects the loads.

Type 2 SPD is characterized by an 8/20 µs current wave.
These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

Note: If a Type 2 SPD is installed and cable lengths are less than 10m, then there is no longer a requirement to fit Type 3 surge protected extension leads. If greater than 10m then type 3 surge protection should be fitted.

For the most part a simple two pole device for single phase TT or TNS will be enough. In TNCS systems a single pole unit may be sufficient as only the live conductor comes to the property that can bring a threat from outside influences affecting that supply.

Selection Guide

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