Connection of Surge Protection Device

Connections of a surge protection device(SPD) to the loads should be as short as possible in order to reduce the value of the voltage protection level (installed Up) on the terminals of the protected equipment.

The total length of SPD connections to the network and the earth terminal block should not exceed 50 cm.

One of the essential characteristics for the protection of equipment is the maximum voltage protection level (installed Up) that the equipment can withstand at its terminals. Accordingly, a SPD should be chosen with a voltage protection level Up adapted to protection of the equipment. The total length of the connection conductors is
L = L1+L2+L3.

For high-frequency currents, the impedance per unit length of this connection is approximately 1 µH/m.

Hence, applying Lenz's law to this connection: ΔU = L di/dt

The normalized 8/20 µs current wave, with a current amplitude of 8 kA, accordingly creates a voltage rise of 1000 V per metre of cable.

ΔU =1 x 10-6 x 8 x 103 /8 x 10-6 = 1000 V

As a result the voltage across the equipment terminals, installed Up, is:

installed Up = Up + U1 + U2

If L1+L2+L3 = 50 cm, and the wave is 8/20 µs with an amplitude of 8 kÂ, the voltage across the equipment terminals will be Up + 500 V.