Protecţie la supratensiuni

The surges that may be dangerous for appliances or people are primarily temporary. Temporary surges occur as relatively short-term changes in voltage between two stationary conditions. The reason for such surges to occur can be found in lightning strokes, electromagnetic fields generated by such lightning strokes or switch manipulations. There are various surge waves – by statistical analysis, they can be grouped into three forms:

• 10/350μs current stroke wave occurring as a result of a lightning stroke

• 8/20 μs current stroke wave

• 1,2/50 μs surge wave

Such stroke waves running in electric installation can cause a lot of damage to the network or appliances connected to it. To avoid the damage caused by electrical surges, a suitable surge protection is necessary.

In compliance with the IEC 62305 as well the IEC 61643 standards the surge protection is divided into several classes or the object is divided into zones. The place of use and the characteristics of the surge protection are therefore defined by these zones.

Zone LPZ 0A represents the space exposed to the lightning stroke that can conduct the entire current of the lightning. Zone LPZ 0B is protected from the direct stroke, but within, an unweakened magnetic field is active. With the screening between zones LPZ 0B and LPZ 1 we achieve a weakened effect of the magnetic field in the LPZ 1 zone, as well as lowered currents in the conductive parts. With additional screening or potential equalization, the effect of the magnetic field is further weakened and the currents in the conductive parts further lowered in the LPZ 2 zone.

By rule, class I surge protection is used on the transition between zones LPZ 0 and LPZ 1, class II surge protection is used between zones LPZ 1 and LPZ 2, and class III surge protection is used between zones LPZ 2 and LPZ 3.

For class I surge protection, a test with a 10/350 current wave is mandatory. This means that the surge protections of this class are capable to divert currents that occur as a consequence of direct or indirect lightning strokes. The size of the stroke current depends on the applied level of protection in accordance with IEC 62305.

For the class II and class III surge protection, a test with an 8/20 current wave is mandatory. Energetically speaking, such a wave is incomparably weaker than the 10/350 wave that is mandatory for the class I protection. Class II and III surge protections are therefore capable to divert currents that occur as a consequence of switch manipulations as well as distant indirect lightning strokes (the influence of the electromagnetic field at a stroke of lightning). Class III surge protections have a lower capacity for diverting currents, as well as a lower remaining voltage UP.

With the correct use of surge protection elements or a correctly used combination of the same, the effective surge protection is possible. In other words, following the necessary precautions, the appliances connected to the electric network will not sustain damage in case a surge occurs.

The correct use of surge protection elements depends primarily on the electro-energetic system and on the level of protection the customer aims to achieve.