Surge Protection Devices (SPD) are used for electric power supply networks, telephone networks, and communication and automatic control buses.

The Surge Protection Device (SPD) is a component of the electrical installation protection system.
This device is connected in parallel on the power supply circuit of the loads that it has to protect (see Fig. J17). It can also be used at all levels of the power supply network.


Principle

SPD is designed to limit transient overvoltages of atmospheric origin and divert current waves to earth, so as to limit the amplitude of this overvoltage to a value that is not hazardous for the electrical installation and electric switchgear and controlgear.


SPD eliminates overvoltages
In common mode, between phase and neutral or earth;
In differential mode, between phase and neutral.
In the event of an overvoltage exceeding the operating threshold, the SPD conducts the energy to earth, in common mode; distributes the energy to the other live conductors, in differential mode.

The three types of SPD

Type1 SPD: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.Type 1 SPD is characterized by a 10/350 µs current wave.

Type2 SPD: It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of overvoltages in the electrical installations and protects the loads.
Type 2 SPD is characterized by an 8/20 µs current wave.


Type 3 SPD
These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.
Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).

Common characteristics

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 20 times.
Why is In important?
In corresponds to a nominal discharge current that a SPD can withstand at least 20 times: a higher value of In means a longer life for the SPD, so it is strongly recommended to choose higher values than the minimum imposed value of 5 kA.

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 5 times.
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.
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.