The Surge Protection Device (SPD)
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). It can also be used at all levels of the power supply network.
This is the most commonly used and most efficient type of overvoltage protection.
Fig– Principle of protection system in parallel
SPD connected in parallel has a high impedance. Once the transient overvoltage appears in the system, the impedance of the device decreases so surge current is driven through the SPD, bypassing the sensitive equipment.
Principle SPD is designed to limit transient over voltages 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 control gear.
SPD eliminates over voltages - 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 Type 1 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.
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.
Type 1 SPD is characterized by a 10/350 µs current wave.
Type 2 SPD 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 over voltages 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).
SPD normative definition Fig. J18 – SPD standard definition
| Direct lightning stroke | Indirect lightning stroke |
| IEC 61643-1 | Class I test | Class II test | Class III test |
| IEC 61643-11/2011 | Type 1 :  | Type 2 :  | Type 3 :  |
| EN/IEC 61643-11 | Type 1 | Type 2 | Type 3 |
| Former VDE 0675v | B | C | D |
| Type of test wave | 10/350 | 8/20 | 1.2/50 + 8/20 |
- Note 1: There exist T1+ T2 SPD (or Type 1 + 2 SPD) combining protection of loads against direct and indirect lightning strokes.
