Circuit Protection | Surge Protection Devices

Surge Protection Devices

The inclusion of Surge Protection Devices (SPDs) in the 17th edition of the BS EN 7671 Wiring Regulations amendment 1 shows how far the complexity and concern for personal safety of modern installation 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 62305 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 short-term high voltage impulses often called ‘spikes’, ‘transients’ or ‘glitches’. The duration of these surges are typically measured in micro seconds.

Surges are typically produced from internal tripping events such as a lift motor in a hotel or office, lighting being turned on and off.

When there is a high energy inrush, this generates a surge and continues on around the circuits and can damage sensitive electronic equipment.

Lightning strikes also produce huge surges, up to 2km away from a direct strike. Surges can be induced in cabling systems, even when there is no direct connection to the point of a lightning strike.

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.

Finally, there is the inconvenience of replacing valuable and sensitive equipment, insurance claims, and increased insurance premiums.

Note:- Currently, cabling for data, telephones and satellites are outside the scope of the 17th edition, but installers and specifiers should still consider providing surge protection for these systems.

Changes to the 17th Edition BS 7671 Wiring
Regulations amendment 1

Two specific sections of the regulations are pertinent to SPDs, and it is important to understand these in more detail:

Section 443 refers to why and when SPD’s should be fitted.

Is simply a risk assessment,and asks the question if not installing an SPD, represents an issue to human life, public service or industrial activity. If the answer is ‘Yes’ then fitting of the SPD is mandatory.

Where some confusion arises is the reference to risk of lightning in relation to the number of thunderstorm days per year. The UK on a whole has less than 25 thunderstorm days per year.
Therefore, it is not generally required to fit SPDs to all domestic properties.
However, where higher risk to personal injury or prevention of commercial activity, further risk assessment is required.

The main driver for installation of SPDs will be insurance companies, with the potential for reduced premiums if SPDs are fitted, or the possibility of non-payment of insurance in the event of any damage or injury if not fitted.

Section 534 refers to device selection.

Refers to how to divert the surge current to earth before this surge has a way to propagate about the wiring of the installation and cause damage, by considering which device to install.

A house or building is split into Lightning Protection Zones (LPZ), as a potential surge passes between one zone to another, this is where the appropriate SPD should be fitted, e.g. between the outside (LPZ0A) to the inside of a building (LPZ1) -SPD fitted in a consumer unit.

The following is where high risk and SPDs must be fitted:

  • Risk to human life - places where people are receiving medical treatment, safety equipment used, properties with overhead TT systems
  • Public Services - museums, IT centres, libraries
  • Commercial and industrial - factories, offices, shops

Areas where SPDs should be considered and further risk assessment required:

  • Large residential - flats
  • Domestic - depending on location, cabling system etc.

There are three types of SPD, called Type (or Class) 1, 2 and 3:

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 1 Type 2 Type 3
Used on building that has an overhead(TT) supply or a lighting protection system fitted on the roof Suitable for all other installations at the incomer or after a type 1 SPD at sub distribution level.
The modern home with an underground cable supply would typically use a type 2 unit
Used after a type 2 for localised over voltage protection in control cabinets or sensitive or valuable electronic equipment. E.g. A surge protected extension lead to a large plasma TV, is an example of a type 3 device

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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