• Product
  • Engineered
  • Downloads
    & Services
  • News, Exhibitions
    & Case Studies
  • Access our e-catalogue anytime, anyplace E-Catalogue for iOS and Android
    On the move? You can now view our E-Catalogue on your smartphone or tablet, anytime, anyplace
    Download Pulse for iPad Pulse for iOS and Android
    Pulse gives you the ultimate access to Hager literature on your smartphone or tablet.
  • Follow us on twitter Twitter
    Join in with the #HagerGang by following us on twitter.
    Subscribe to our channel on YouTube YouTube
    Subscribe to our channel to keep up to date with all of our videos.
Download the E-Catalogue for iOS or Android
Give us a call
Download Hager literature
Register to receive our newsletter
Follow @hageruk
Watch our videos on YouTube

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.

SPDs are strongly recommended for installations that are exposed to transients, to protect sensitive and expensive electrical equipment such as TVs, washing machines, PCs, alarms etc.


The choice of SPD depends on a number of criteria such as:

  • The risk of lightning strikes
  • The exposure of the building to transients
  • The sensitivity and value of the electrical equipment that requires protection
  • Earthing system
  • Level of protection

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.

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.

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.

Quick Selection Guide

The following may assist in choosing whether SPD’s are required and the correct type of device:

  • Does the installation contain a lightning protection system?
  • Is the installation adjacent to any tall structure, tall trees or near a hill top in a lightning prone area?
  • Does the installation contain equipment where higher reliability from overvoltages is required

If the answer is YES in the above to the first two questions, it is recommended to install a Type 1+2 device. This will provide protection against surges caused by direct lightning strikes and provide protection against transient over-voltages caused by indirect lightning strikes or by switching events.

If the answer is YES to the third question then it is recommended to install Type 2 devices to provide protection against transient over-voltages caused by indirect lightning strikes or by switching events.
Author: Paul Collins
Posted: 10th April 2018
Get in contact: technical@hager.co.uk | Twitter