SPDs - A Quick Overview

SPDs – Surge Protection Devices – A Bitesize Overview

Everyday tasks now depend on electronic technology, changing the way electrical equipment is used in households and at work.

Products like microwaves, alarms, laptops, printers, flat-screen televisions, industrial control devices like PLCs, and washing machines are all commonplace. All of these may be susceptible to transient overvoltages, which can severely shorten the equipment’s lifespan by degrading it and damaging it.

A transient overvoltage, often known as a surge, is an abrupt rise in the voltage between two or more conductors. In a nutshell, this refers to durations ranging from a few milliseconds (thousandths of a second) to a few microseconds (millionths of a second).

Surge Protection Devices (SPDs)

SPDs shield electrical and electronic devices from transients caused by lightning, transformer switching, illumination, and motor switching.

These transients can result in device downtime, premature ageing, or total damage of electronic materials and components.

Selection Criteria

Surge protection devices are classified according to their functions:

Type 1

SPDs 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

SPDs which can prevent the spread of overvoltage’s 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.


The majority of SPDs have a window that shows when they are active. When the light is green, protection is being offered. If they are red, they are “end of life” and need to be replaced. A replaceable cartridge is frequently present and can be easily removed and replaced with a fresh functional device.

Kempston Controls is committed to finding you the ideal solution for your application. Call us today on 042 9359393, email us at electrical@kempstoncontrols.ie, or alternatively contact us here, we will be happy to help.

Kempston Controls

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