Surge Protective Devices SPDs

Transient surges represent the main cause of electrical devices failure and loss of productivity. They are the result of lightning strikes, switching operation on the electrical network or parasitic interferences.

Nowadays, in all the sectors (residential, commercial and industrial), in the data center industry, they rely on their computer systems.

A downtime in one of these computer systems, due to transient surges, can have catastrophic consequences. Loss of operation, loss of service, loss of data and of productivity involve, in most of the cases, huge consequences which are, by far, higher than the costs of the equipments for protection against overvoltages.

SPDs, or Surge Protective Devices, are designed to safeguard electrical systems and equipment against transitory surges and impulses such as, for example, those caused by lightning and operations on the electrical grid.
Transitory overvoltage surges consist of a small voltage peak of a short duration (less than a millisecond) which can reach tens of times the standard mains voltage.
The resistance to transitory surges – known as "impulse withstand" – is of great importance in electrical and electronic equipment, and for this reason equipment is fitted with systems isolating the parts connected to the phases from earth or neutral.
This isolation can vary from a few hundred volts, for sensitive electronic equipment, to several kilovolts for an electric motor.
SPDs contain at least one non-linear component (a varistor or spark gap). Their function is to divert the discharge or impulse current and limit the overvoltage to the
downstream equipment.
Operation of an SPD:
– During normal operation (e.g. in the absence of surges), the SPD has no influence on the system it is applied to. It acts like an open circuit and maintains the isolation between the active conductors and earth.
– When a voltage surge occurs, the SPD reduces its impedance within a few nanoseconds and diverts the impulse current. The SPD behaves like a closed circuit, the overvoltage is short-circuited and limited to an acceptable value for the electrical equipment connected downstream.
– Once the impulse surge has stopped, the SPD will return to its original impedance and return to the open circuit condition.

Surge protection starts at the origin of the electrical system and finishes near the most delicate equipment. The discharge energy is reduced in various stages, first with the more robust SPDs (Class 1), then with finer protection (Class 2 devices). This protection co-ordination is represented with the LPZs, which divide up the environment on the basis of the effects of the lightning strike.

The choice of the SPD depends on a series of criteria defined in the phase of assessing the risk of lightning strikes, allowing the surge protection requirements to be identified.

When is it necessary to provide for protection?
First of all, the requirements of the standards must be considered; to the analysis of these we can add recommendations based on ABB's industrial experience.
The criteria taken into consideration in this section consist of assessing the risk of a direct lightning strike on or near a building, including the financial aspect caused by equipment which may be damaged and temporary loss of operational capacity. Even in the case that protection is not indispensable, it is as well to note that, considering that a zero risk does not exist, it is always a good idea to provide for protection.

In the case that protection against lightning strikes is recommended, it is sufficient to choose the appropriate product and install it.
The choice of the SPD is based on different elements:
– The type of lightning strike, direct or indirect
– protection level Up;
– The discharge capacity: Iimp or In (10/350 μs or 8/20 μs impulse wave);
– The network's earthing system;
– The operating voltages (Uc and UT).
– Options and accessories (end-of-life indicator, pluggable cartridges, safety reserve, remote signalling).

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