Surge protection devices （SPDs）

Surge protection devices (SPDs) are installed in parallel to the electric line to be protected. At the mains rated voltage, they are comparable to an open circuit and have high impedance at their ends. In the presence of an overvoltage, this impedance falls to very low values, closing the circuit to earth. Once the overvoltage has ended, their impedance rises again rapidly to the initial value (very high), returning to open loop conditions.

1. Protection zones
Standards define the LPZs (Lightning Protection Zones), which indicate the different zones at risk. These are distinguished as:
(1) LPZ 0A: Area outside a building not protected by LPS (e.g. lightning rods) where a direct lightning strike is possible. In this zone, there is total exposure to induced electromagnetic fields.
(2) LPZ 0B: Area outside a building protected by LPS; therefore, a direct lightning strike is not possible. In this zone, there is total exposure to induced electromagnetic fields.
(3) LPZ 1: Area inside a building so protected against direct lightning strikes. In this zone, there is the possibility of very high overvoltage and of induced electromagnetic fields which may be attenuated depending on the degree of screening. This zone must be protected by an SPD type 1 at the boundary with zone LPZ 0A or 0B.
(4) LPZ 2: Area inside a building (e.g. in a room), in which there is the possibility of low overvoltage since they are limited by SPDs installed upstream. This zone must be protected by an SPD type 2 at the boundary with zone LPZ 1.
(5) LPZ 3: Area inside a building (e.g. the system connected to a socket in a room) characterized by very sensitive equipment, in which there is the possibility of very low overvoltage as they are limited by SPDs installed upstream.
This zone must be protected by an SPD type 3 at the boundary with zone LPZ 2.

2. Installation category
For the correct choice of the SPD, the dielectric strength of the equipment to protect needs to be considered. This level is established by IEC 60664-1 standard.
A 230/400V system specifies:
Installation category I: 1.5kV for equipment containing “particularly sensitive” electronic circuits (for example, electronic devices like PCs or TVs)
Installation category II: 2.5kV for non electronic devices (for example, household appliances or electric tools)
Installation category III: 4kV for devices being part of the fixed system (for example, distribution boards, switching devices, isolators, ducting and their accessories)
Installation category IV: 6kV for devices installed upstream of the distribution board (for example, delivery point with the distribution system).

3. Definitions and ratings
(1) Maximum continuous voltage Uc:
Maximum value of AC or DC voltage that the SPD is capable of permanently withstanding without activating or getting damaged; this is its rated voltage.
(2) Protection level voltage Up: Maximum value of the voltage between the terminals of the SPD in presence of an impulsive overvoltage. It is a fundamental parameter to correctly choose the SPD; account of it must be taken in relation to the impulse voltage of the equipment to be protected.
(3) Impulse current Iimp:
Peak value of the current that flows in the SPD with a 10/350 waveform (activation must be guaranteed for 20 times without damage).
It is used to classify SPDs in test class I.
(4) Rated discharge current In: Peak value of the current that circulates in the SPD with an 8/20 waveform (activation must be guaranteed for 20 times without damage). It is used to classify SPDs in test class II.
(5) No-load discharge voltage Uoc: Peak value of the non-load discharge voltage delivered by the test generator with 1.2/50 waveform simultaneously with a short-circuit current with 8/20 waveform, applied at the terminals of the SPD. It is used to classify SPDs in test class III.
(6) Maximum discharge current Imax:
Peak value of the current that flows in the SPD with an 8/20 waveform. An SPD is capable of withstanding it at least once.