Surge Protection Device (SPD) is used to protect the electrical distribution systems , which consists of the consumer unit, equipment, wiring and accessories, from transient overvoltage surges.
Surge Protector is always connected in parallel on the power circuit of the loads that need to protect (see Fig. 1).
This is the most commonly used and most efficient type of overvoltage protection.
Surge Protection Device has a high impedance when connected in parallel. Once the transient overvoltage appears in the system, the impedance of the device reduces so surge current is passed through the SPD, bypassing the sensitive equipment which need to be protected.
SPD is designed to limit transient overvoltages 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 equipment.
- 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.
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 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).
Is it necessary to install Surge Protective Device (SPD)?
The current edition of the IET Wiring Regulations, BS 7671:2018, states that unless a risk assessment is carried out, protection against transient overvoltage shall be provided where the consequence caused by overvoltage could:
- Result in serious injury to, or loss of, human life; or
- Result in interruption of public services and/or damage to cultural heritage; or
- Result in interruption of commercial or industrial activity; or
- Affect a large number of co-located individuals.
This regulation applies to all types of premises which include domestic, commercial and industrial.
However, In the previous edition of the IET Wiring Regulations, BS 7671:2008+A3:2015, there was an exception for some domestic dwellings to be excluded from surge protection requirements, but this has now been removed and it is now essential requirement for all types of premises including single dwelling units.
The decision on whether to install SPDs is in the hands of the client, but they should be provided with enough information to make an informed decision on whether they wish to omit SPDs or not. A decision should be made based on safety risk factors and following a cost evaluation of SPDs, which may cost as few chunks, against the cost of the electrical installation and equipment connected to it such as computers, TVs and necessary equipment, for example, smoke detection and boiler controls.
Surge protection could be installed in an existing consumer unit if appropriate physical space was available or, if not, it could be installed in an external enclosure adjacent to the existing consumer unit.