Selection of Surge Protection Devices

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Technical Articles

It is very important to select right Surge Protection Device for installation/equipment based on different parameters of SPD.


Below schematic will give you an idea of selection criteria for SPDs:

Selection of Type of SPD & Maximum Discharge Current (Imax):

Selection of type of surge protector and Maximum Discharge current will depend upon SPD’s Installation - location. Please see below diagram for further clarity.


After selecting Type 1/Type or 1+2 SPD, adding Type 2 SPD will depend upon Distance of distribution point and impulse withstand voltage level of the equipment/load to be protected.

Role of lightning conductor in installation:

If a lightning Rod is present in the installation and lightning strikes to this lightning rod, it provides low resistance path to heavy lightning current and maximum amount of current safely gets discharged to the ground through earthing system.


In normal conditions, typically Earthing system and all the equipments gets connected together to be at equipotential level in any installation.


During the event of direct lightning strike on lightning rod , heavy lightning current flows through earthing system which creates high voltage drop across it, based on the earthing resistance. This phenomena creates potential difference between earthing system and equipments which are connected to it and some amount of surge current can flow through the system based on potential difference generated between them.


Also, when lightning current getting discharged from lightning rod to ground, additional high magnetic fields get induced around the lightning rod due to high surge current which in turn induce high voltage peaks in line systems.


Hence, lightning rod will serve the purpose of discharging lightning energy to the ground but to eliminate surge current entering into the system, lightning rod to be associated with the Type 1/Type 1+2 SPDs at Main Panel/Electrical entry point level.

Choosing the SPD Configuration:

Based on Single phase/Three phase Electric Network, 1P+N/3P+N SPD configuration shall be selected.

Choosing SPD Nominal Voltage(Un):

SPD nominal voltage shall be selected corresponding to the system voltage of the typical SPD installation site, e.g.,230/400V AC

Choosing Short-Circuit withstand capacity of SPD (ISCCR):

ISCCR of SPD needs to be higher than the maximum short circuit value of the electrical network to which SPD is connected.

Choosing Voltage Protection Level (Up):

The user must select a surge protector with a protection level Up based on impulse withstand voltage level of the equipment. In every case, Up must be lower than the equipment’s impulse withstand capacity.


For an Example, in a distribution system, if a machine is connected which has impulse withstand voltage of 1.5kV, voltage across the machine should limit to 1.5kV when surge strikes to the system. Hence SPD’s Up value must be lower than impulse withstand voltage (1.5kV in this case) of the machine.

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