Understanding Need of SPDs

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Technical Articles

Understanding Need of SPDs

Surge is an unavoidable phenomenon in the electrical system. Surge can arise due to internal or external factors and depending upon its severity, can be detrimental for the system.

What is surge?

A voltage of at least two times the system’s RMS voltage with a duration measured from 1micro second to several hundred micro seconds is called Surge.


External factors causing surge include direct and/or indirect lightning strike whereas grid switching, load switching, CB interruption are some of the internal factors.

Need for an SPD

Need for an SPD

Every equipment has rated impulse withstand voltage which indicates that equipment is capable of withstanding that much impulse voltage without failure

Impluse withstand voltage-table

What does an SPD do?

A device which diverts or limits surge current is called Surge protection Device (SPD).


SPD momentarily ‘switches’ from open circuit/high resistance mode into a low impedance mode as it sees high voltages beyond a threshold.


As shown in picture, SPDs should be connected between source and equipment, in parallel connection to the equipment to divert high voltage through it.

Divert High Voltage

SPD is an integral part of complete protection for final distribution system. MCBs provide protection against overload and short-circuit, RCDs protect against electric shock and earth leakage whereas SPDs provide protection against direct and indirect surges, as shown below.

Overload Short Circuit MCB

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