First we should know the common characteristics of SPD so as to select right type of SPD

  • Uc: Maximum continuous operating voltage: This is the A.C. or D.C. voltage above which the SPD becomes active. This value is chosen according to the rated voltage and the system earthing arrangement.
  • Up: Voltage protection level (at In): This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to In. The voltage protection level chosen must be below the overvoltage withstand capability of the loads. In the event of lightning strokes, the voltage across the terminals of the SPD generally remains less than Up.
  • In: Nominal discharge current: This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging minimum 19 times[2].

Why is In important?
In corresponds to a nominal discharge current that a SPD can withstand at least 19 times[2]: a higher value of In means a longer life for the SPD, so it is strongly recommended to chose higher values than the minimum imposed value of 5 kA.

When to Choose Type 1 SPD

  • Iimp: Impulse current

This is the peak value of a current of 10/350 µs waveform that the SPD is capable of discharging of discharging at least one time[3].

Why is Iimp important?
IEC 62305 standard requires a maximum impulse current value of 25 kA per pole for three-phase system. This means that for a 3P+N network the SPD should be able to withstand a total maximum impulse current of 100kA coming from the earth bonding.

  • Ifi: Autoextinguish follow current

Applicable only to the spark gap technology. This is the current (50 Hz) that the SPD is capable of interrupting by itself after flashover. This current must always be greater than the prospective short-circuit current at the point of installation.

The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage.

It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

 

When to Choose Type 2 SPD: 

  • Imax: Maximum discharge current

This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging once.

Why is Imax important?
If you compare 2 SPDs with the same In, but with different Imax: the SPD with higher Imax value has a higher "safety margin" and can withstand higher surge current without being damaged.

Type 2 surge protection constitutes the second stage in the protection concept and is installed after the type 1 SPD. To protect the system against damage, e.g., caused by switching operations, the type 2 surge protective device further limits the remaining surge voltage. Type 2 SPDs are used in the subdistribution or machine control cabinets, where no direct lightning currents occur.

  • Note 1: There exist Symbol-T1.gif + Symbol-T2.gif SPD (or Type 1 + 2 SPD) combining protection of loads against direct and indirect lightning strokes.

 

When to Choose Type 3 SPD

  • Uoc: Open-circuit voltage applied during class III (Type 3) tests.

Main applications

  • Low Voltage SPD

Very different devices, from both a technological and usage viewpoint, are designated by this term. Low voltage SPDs are modular to be easily installed inside LV switchboards.

There are also SPDs adaptable to power sockets, but these devices have a low discharge capacity.

  • SPD for communication networks

These devices protect telephone networks, switched networks and automatic control networks (bus) against overvoltages coming from outside (lightning) and those internal to the power supply network (polluting equipment, switchgear operation, etc.).

Such SPDs are also installed in RJ11, RJ45, ... connectors or integrated into loads.