Surge Protection – Choosing the Right Device

[vc_row][vc_column][vc_column_text]Electrical surges, or transient overvoltages can affect a multitude of electrical systems such as those that deal with signal, power and telecoms by overloading the internal components leading ultimately to failure. Transient overvoltage can be caused by lightning strikes, reaching tens of kilovolts and lasting less than a millisecond, or from switching large electrical loads, such as motors, which might exist within the same electrical network. Needless to say, transient voltages delivered by a lightning strike can be devastating to an unprotected electrical system.

While switching transients are certainly smaller in nature than a lightning strike, and less intense, they are obviously more frequent which leads to constant stress on electronic components. Ultimately this continued stress will result in component failure, the tripping of RCDs, unplanned production outages and associated cost.

What’s at Risk from Electrical Surges?

All manner of equipment including:

  • Computers and servers
  • Building management systems
  • PLCs
  • Uninterrupted power supplies
  • Alarm systems
  • Life-saving equipment
  • Production line machines

There is a large list of machinery and devices, and not forgetting buildings themselves that could be catastrophically damaged by the effects of transient overvoltages and let’s not forget one of the most important things that will massively benefit from an effective SPD installation – people.


Which Surge Protection Device?

Surge protection devices (SPDs) can be classified into three different types, these being – Type 1, 2 and 3 according to the specific type of transient overvoltage they are destined to deal with when correctly installed.

Surge protectionMersen Surge Trap


What are Type 1 SPDs used for?

This particular surge protection device is intended for installation where the electrical services enter a building or facility and are often referred to as an equipotential bonding SPD. Type 1 SPDs are designed to protect electrical systems against major surges from direct and indirect lightning strikes. As an example of the scope of danger offered by lightning, in the UK in July 2019, in just one night there was over 48000 recorded lightning strikes, a considerable threat for an unprotected building and the personnel and equipment within.

Without a Type 1 SPD, transient overvoltages can exceed the impulse ratings of equipment or cable insulation leading to dangerous sparking and the possibility of fire. Type 1 SPDs are designed to protect against loss of life and serious injury and are not intended for protecting sensitive electronic equipment or electrical systems from switching transients. Type 1 surge protection units should, therefore, be used in conjunction with Type 2 and Type 3 models if the intention is to also protect downstream electrical systems and devices.

Type 1 SPD Protect Against:

  • Direct lightning strikes
  • Indirect lightning strikes -resistive, capacitive and inductive coupling


  •  Discharge very high currents to earth


surge protection device

ABB Furse ESP 240 Low Current Type 1, 2 and 3

What about Type 2 and Type 3 SPDs?

When you need to protect electrical and electronic equipment from the secondary effects of indirect lightning and reduce switching transients to safe levels then you will need a Type 2 and Type 3 SPD. Type 2 SPDs have the ability to discharge high currents to earth, protecting electrical systems at their point of origin and are typically found installed in distribution boards downstream of a Type 1 SPD. Type 3 SPD, with their lower discharge capacity, is used to supplement Type 2 SPDs to provide protection for critical and sensitive equipment. Type 3 Surge protection devices are typically located downstream of a Type 2 device (more than 20 metres downstream) and are installed relatively close to the electrical equipment that is being protected.

surge protection devices

Citel Type 2 and Type 2-3 SPDs

Type 2 SPD Specifics

  • Installed downstream of a Type 1 SPD
  • Ability to discharge high overvoltages to earth
  • Typically installed in a control panel

Type 3 SPD Specifics:

  • Installed downstream of a Type 2 SPD (20 metres)
  • Designed to protect sensitive equipment
  • Discharges lower-range overvoltages to earth
  • Located close to the device it is protecting

Ideally, and to ensure all electrical systems, electrical devices and personnel are protected adequately, a typical installation would use a combination of SPDs in a cascading system. In the marketplace today there is a vast array of Type 1, 2 and 3 SPDs available, including many variants that combine Types 1, 2 and 3 into a singular cost-effective unit for ease of installation and maintenance.

Surge protection devices

ABB ESP 240 Single Phase and ESP 415 3-Phase Low-Voltage Type 1, 2, and 3 SPDs

When selecting your Type 2 and 3 SPDs ensure your choice of SPD is suitable for handling the power and specifics of your particular installation, be that single or 3-phase with important consideration given to the electrical circuits operating voltage, overvoltage withstand and disconnect voltages and the demands imposed upon it by the devices of the system.

View our Type 2 SPDs

View our Type 3 SPDs


As factories and industrial processes become smarter and more automated the need to protect these valuable installations and the people involved in there operation has become more prevalent. Another consideration is that industrial control and automation devices of today have reduced in size resulting in smaller internal components that are more susceptible to damage from power surges.

lack of surge protection damage to a PCB

Overvoltage damage to a PCB


Whatever your surge protection device needs, Kempston Controls has access to a comprehensive range of surge protection devices designed to cater for every overvoltage protection eventuality. Contact us today on +44 (0) 1933 411411 or email us at to discuss your circuit protection requirements.

The IET 18th Edition

For a comprehensive guide on overvoltage protection refer to Chapter 44 section 443 of the IET Wiring Regulations 18th Edition which deals specifically with the subject of safe surge protection installation and device selection. You can order a copy here.


View our range of surge protection devices


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Kempston Controls is committed to finding you the ideal solution for your application. Call us today on +44 (0) 1933 411 411, email us at, or alternatively contact us here, we will be happy to help.

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