Detailed characteristics of the external SCPD

Current wave withstand

The current wave withstand tests on external SCPDs show as follows:

• For a given rating and technology (NH or cylindrical fuse), the current wave withstand capability is better with an aM type fuse (motor protection) than with a gG type fuse (general use).
• For a given rating, the current wave withstand capability is better with a circuit breaker than with a fuse device.

Figure J53 below shows the results of the voltage wave withstand tests:

• to protect a SPD defined for Imax = 20 kA, the external SCPD to be chosen is either a MCB 16 A or a Fuse aM 63 A,

Note: in this case, a Fuse gG 63 A is not suitable.

• to protect a SPD defined for Imax = 40 kA, the external SCPD to be chosen is either a MCB 40 A or a Fuse aM 125 A,

Fig. J53: Comparison of SCPDs voltage wave withstand capabilities for Imax = 20 kA and Imax = 40 kA

Installed Up voltage protection level

In general:

• The voltage drop across the terminals of a circuit breaker is higher than that across the terminals of a fuse device. This is because the impedance of the circuit-breaker components (thermal and magnetic tripping devices) is higher than that of a fuse.However:

• The difference between the voltage drops remains slight for current waves not exceeding 10 kA (95% of cases);
• The installed Up voltage protection level also takes into account the cabling impedance. This can be high in the case of a fuse technology (protection device remote from the SPD) and low in the case of a circuit-breaker technology (circuit breaker close to, and even integrated into the SPD).

Note: The installed Up voltage protection level is the sum of the voltage drops:
  - in the SPD;
  - in the external SCPD;
  - in the equipment cabling

Protection from impedant short circuits

An impedant short circuit dissipates a lot of energy and should be eliminated very quickly to prevent damage to the installation and to the SPD.
Figure J54 compares the response time and the energy limitation of a protection system by a 63 A aM fuse and a 25 A circuit breaker.
These two protection systems have the same 8/20 µs current wave withstand capability (27 kA and 30 kA respectively).

Fig. J54: Comparison of time/current and energy limitations curves for a circuit breaker and a fuse having the same 8/20 µs current wave withstand capability