Surge Protection Device

Surge Protection Devices – Complete Overview

Surge protection devices play a vital role in voltage fluctuations or variations.  These fluctuations are the most common behaviours observed in any electrical system. Voltage change can vary from small voltage fluctuations for a short duration to a complete blackout for an extended period. When the voltage drops below the standard rate of tolerance for the short term, it is known as sag. When the voltage increases above the standard rated tolerance for the short span, it is known as swell. There are many fluctuations with shorter durations than swell and sag. These very short fluctuations are called transients, impulse, spikes or a notch. An increase in at least 10 per cent of voltage or current lasting for less than a few microseconds is known as a Surge. The amount of electrical energy produced during a surge, for a very short period would be significantly high as it can rise to a few thousand volts.

A surge protection device is a device that protects electrical devices from voltage spikes in alternating current (AC) circuits. Surge protection devices are used to protect the electrical installation made up of consumer unit, wiring and accessories, from electrical power surges that are generally known as transient overvoltages. The surges occurring from transient overvoltages can either result in instant failure or longer damage to the equipment. Different SPDs are used to protect the electrical installation from incoming services such as telephone lines and cable TV within the consumer unit.

Transient voltages are short duration surges of electricity that occur due to the sudden release of energy that is previously stored or induced by other means. These overvoltages can be occurring either naturally or can be man-made.

Man-made transient overvoltage occurs during the switching of motors and transformers. In recent times, new technologies such as electric vehicles, air/ ground source heat pumps and speed-controlled washing machines are showing more transients even in domestic installations. Natural transient overvoltage occurs due to indirect lightning strikes. The electrical installation and equipment associated with it can be damaged due to a direct lightning strike on an adjacent overhead power transmission line or a telephone line leading to the transient overvoltage to travel along the transmission lines 

The function of a surge protective device is to protect electrical equipment from the damaging effects of electrical surges. The rating of SPD decides the amount of protection provided by the SPD. An SPD protects an electric circuit by limiting the voltage applied to the protected circuit during the occurrence of a surge. The SPD channels the excess energy obtained from the surge away from the protected circuit into the ground. A metal oxide varistor (MOV) is a device that is used in an SPD that provides a path or excess energy. There are two main characteristics of a MOV which make them most desirable for surge protection 

  • the resistance of a MOV decreases with an increased voltage 
  • MOVs are fast-acting varistors which can respond to surge in a few nanoseconds which helps in suppressing the surge before it damages the equipment. 

The voltage limiting capability of a MOV can be measured by clamping voltage. When a surge occurs, the MOV limits the excess voltage to the clamping voltage level by diverting the surplus energy to the ground. The peak current rating or impulse current rating is the maximum current dissipated from one surge without failing the operation of an SPD.

A MOV provides a high resistance path under normal conditions. The resistance path limits the currents flowing through the MOV and allows the current to flow through. The load in all electrical devices is connected to power sources in all residential applications. The clamping voltage rating of a MOV is greater than the standard supply voltage. When a surge happens, the clamping voltage exceeds, the MOV switches from a high resistance path to a low resistance path. The excess energy generated passes through the MOV to the ground bypassing the load which is connected. Along with MOVs, SPDs also have inductors or other filter elements to reduce the effects of the surge and incorporate other protective devices to ensure the safe operation of SPDs.

Surge Voltage

Fig: Operation of MOV (Source: siemens)


There are three different categories of Surge Protection Devices:

Type 1 SPD installed at the main distribution board i.e. origin. In this type of arrangement, an SPD is mounted between the service transformer’s secondary terminal and the service transformer's main overcurrent protection device is called a surge arrester.

Type 1 SPD is characterized by a 10/350 µs current wave. 


Type 2 SPD installed at sub-distribution boards. In this type of arrangement, the SPD is mounted at the load side of the service’s main overcurrent protection device that is known as a Transient Voltage Surge Suppressor (TVSS)

Type 2 SPD is characterized by an 8/20 µs current wave. 


Type 3 SPD installed close to the protected load. This type of arrangement is a point of Use SPD which includes MOVs as well as assemblies containing MOVs.

Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs). 

A combination of Type 1 and Type 2 SPDs are also used and usually installed in consumer units. Type 3 SPDs must be used only as a supplement to Type 2 SPD.


The Requirements & Testing of any Surge protection device is as per IEC 6163-1 is to protect the equipment from transient overvoltage provided until the risk assessment is carried out. There are many consequences by the transient overvoltage that include

  • Serious injury or loss of human life.
  • Interruption of public services and damage to cultural heritage.
  • Interruption of industrial or commercial activity.
  • A large number of co-located individuals will be affected. 

Before purchasing a surge protection device, the safety risk factors must be analyzed and the cost evaluation of it must be taken into consideration as they are expensive than the cost of electrical installation and equipment connected to the SPDs. It can be installed, only if there is appropriate physical space available or if space is not enough, it can be installed in an external enclosure adjacent to the existing unit.

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