Surge Protective Devices (SPD)

Surge Protective Devices (SPD) are used to protect the electrical installation, which consists of the consumer unit, wiring and accessories, from electrical power surges known as transient overvoltage.

Every piece of electronic equipment found in an industrial environment is subjected to power surges generated on the utility grid are transmitted to the equipment via incoming power lines. Users are installing surge protection devices to protect their equipment from surges.

There are three different types of Surge Protective Devices:

  1. Type 1 SPD installed at the origin, e.g. main distribution board.
  2. Type 2 SPD installed at sub-distribution boards (Combined Type 1 & 2 SPDs are available and are usually installed in consumer units).
  3. Type 3 SPD installed close to the protected load. They must only be installed as a supplement to Type 2 SPD.

Where multiple devices are required to protect the installation, they must be coordinated to ensure correct operation. Items supplied by different manufacturers should be confirmed for compatibility, the installer and manufacturers of the devices are best placed to provide guidance on this.

Type 1 SPD

SPD Type 1 must be connected upstream of the system, at the point of delivery of power energy. SPD protects buildings and people from the risk of direct lightning (fire and death) and is characterized by:

Iimp 10/350 Impulse current: Iimp corresponds to the peak value of a 10/350 μs current impulse waveform.

Type 2 SPD

SPD Type 2 devices are designed to remove all the overvoltage from supply circuits that are not likely to be directly hit by lightning. SPD Type 2 is connected downstream SPD Type 1 or SPD Type 1+2, (minimum distance 1 m) and they protect machines and tools connected to the ground and reduce the risk of economic loss.

SPD Type 2 is characterized by:

In 8/20 Nominal discharge current: The peak current (and waveform shape) through the SPD under conditions prescribed by EN 62305 to represent the surge current as a consequence of a lightning strike to the electric supply line.

Imax 8/20 Maximum discharge current: Peak value of the highest current of an 8/20 μs waveform that an SPD can discharge at least once without breaking.

Type 3 SPD

SPD type 3 devices are used to protect the end user from overvoltage. They may be installed in supply networks where SDP types 1 and/or 2 already exist. They can be installed in fixed or mobile sockets and have the following characteristic parameters:

Uoc: test voltage. This is the peak value of the no-load voltage of the combined test-generator; this has a waveform of 1.2/50 μs and can supply at the same time current with waveform 8/20 μs

What is transient overvoltage?

Transient overvoltage is defined as short duration surges of electricity which occur due to the sudden release of energy previously-stored or induced by other means. Transient overvoltage can be either naturally occurring or man-made.

Sources of power surges can be external or internal.

External transient overvoltage is:

Lightning (most damaging)

Switching of capacitor banks

Switching of large electrical loads

Power line disconnection and reconnection

Transformer switching

Electrostatic discharges

Poor power transmission and distribution grid quality

Internal transient overvoltage is:

Fuse and circuit breaker (MCCB, ACB, etc.)

Electrical motors and motor starters

HVAC devices

Variable frequency drives

Household devices such as microwave oven, computer, refrigerator

Electronic ballasts

Surge protection device technologies


This can be considered as a variable resistance that at nominal voltage has a very high ohmic value. But the resistance rapidly falls to near zero as the voltage surges. In this way, the varistor applies a near short circuit that clamps the surge voltage. The varistor is however subject to progressive degradation due to the small leakage current that occurs at the nominal voltage, and with the number of interventions. With every overvoltage that occurs the leakage current rises and accelerates the end of life for the device – which is ultimately indicated by the change from green to red in the signal window.

Spark gap:

This comprises two electrodes separated by air or gas. When a surge voltage occurs electrical arc bridges the gap and a surge current flow to limit the surge voltage to a low and constant level. The arc extinguishes only when the surge current falls below about 10 amperes. The gas guarantees a constant level of breakdown voltage since the arc is struck in a protected environment; not exposed to pressure or humidity variations or impurities as would happen if it had occurred in the air. There is, however, a delay before the device arcs and the surge current is diverted, and this is dependent on the magnitude of the original voltage surge and its rate of rising. Therefore, the voltage protection level can vary, although it is guaranteed to be less than up.

Do I have to have SPDs installed?

The current edition of the IET Wiring Regulations, BS 7671:2018, states that unless a risk assessment is carried out, protection against transient overvoltage shall be provided where the consequence caused by overvoltage could:

  • Result in serious injury to, or loss of, human life;
  • Result in interruption of public services and/or damage to cultural heritage;
  • Result in interruption of commercial or industrial activity;
  • Affect a large number of co-located individuals.

This regulation applies to all types of premises which include domestic, commercial and industrial.

The decision on whether to purchase SPDs is in the hands of the customer, but they should be provided with enough information to make an informed decision on whether they wish to omit SPDs.

Surge protection could be installed in an existing consumer unit if appropriate physical space was available or, if enough space was not available, it could be installed in an external enclosure adjacent to the existing consumer unit.


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