Surge Protection

The whole nature of how electrical equipment is used in homes and at work has evolved; with everyday activities relying on electronic equipment.

Products such as computers, printers, flat screen televisions, industrial control equipment such as PLC’s, alarms, microwaves and washing machines are common place. These can all be vulnerable to transient overvoltages, which can significantly reduce the equipment’s lifespan through degradation and damage.

A transient overvoltage or surge is a short duration increase in voltage measured between two or more conductors. In short this means anything from microseconds (millionths of a second) to a few milliseconds (thousandths of a second) in duration.


Surge Protection Devices (SPD's)

SPD’s protect electrical and electronic equipment against transients, originating from lightning, switching of transformers, lighting and motors.

These transients can cause premature ageing of equipment, downtime, or complete destruction of electronic components and materials.


18th Edition Requirements

The 18th edition BS 7671 now stipulates

Protection against transient overvoltages shall be provided where the consequences caused by an overvoltage could

(i) result in serious injury to, or loss of, human life, or
(ii) result in interruption of public services and/or damage to cultural heritage, or
(iii) result in interruption of commercial or industrial activity, or
(iv) affect a large number of co-located individuals.

For all other cases, a risk assessment according to Regulation 443.5 shall be performed in order to determine if protection against transient overvoltages is required. If the risk assessment is not performed, the electrical installation shall be provided with protection against transient overvoltages.

For a single dwelling unit it will be a decision for the house owner to make whether they consider the small additional cost of the surge protection device justified to protect their installation and equipment against these damaging overvoltages


Selection Criteria

Surge protection devices are classified according to their functions:

Type 1
SPD which can discharge partial lightning current with a typical waveform 10/350 μs. Usually employs spark gap technology.This, if required, will be installed in the primary distribution board at the origin of the electrical installation. A Type 1 SPD does not in itself offer the required protection level and must be used in conjunction with coordinated type 2 devices. An installation with a lightning protection system will require a Type 1 SPD.

Type 2
SPD which can prevent the spread of overvoltages in the electrical installations and protects equipment connected to it. It usually employs metal oxide varistor (MOV) technology and is characterised by an 8/20 μs current wave.This device would normally be in sub-distribution boards and in the primary distribution board if there was no requirement for a type 1 device

Type 3
These SPDs have a low discharge capacity. They must therefore only be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads. Type 3 SPD’s are characterised by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).



Iimp – Impulse current of 10/350 μs waveform associated with Type 1 SPD’s
In – Surge current of 8/20 μs waveform associated with Type 2 SPD’s
Up - The residual voltage that is measured across the terminal of the SPD when In is applied
Uc - The maximum voltage which may be continuously applied to the SPD without it conducting.



Most SPDs have an indication window that they are operational. If the indicator is green they are providing protection. If they are red then they have reached ‘end of life’ and will need replacing. Often there is a replaceable cartridge which can simply be withdrawn and replaced with a new operational device.



More from the 18th Edition

Arc Fault Detection Devices

Arc fault protection devices (AFDD) use microprocessors to identify characteristic current flow and voltage curves that indicate an arc fault and automatically trip the affected circuit.

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Selection of RCD Types

RCD's exist in various different forms and react differently depending on the presence of DC components or different frequencies.

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Overload Protection

Devices such as switches, RCCBs etc. in distribution boards and consumer units may have historically had their rated current determined after having taken diversity into account but without having considered overload protection of the devices.

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BEAMA Guides

A collection of the latest guides relating to 18th Edition, Hager is proud to be a technical contributor to these guides which provide technical advice on a range of 18th Edition related topics.

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Related products

Check out our product solutions for surge protection below

Pre-fitted Surge Protection Consumer Units

Commercial Surge Protection

Residential Surge Protection