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Detailed characteristics of the external SCPD

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electrical fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential premises and other special locations
ElectroMagnetic Compatibility (EMC)
Measurement

Contents


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 J56 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. J56Comparison 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 J57 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. J57Comparison of time/current and energy limitations curves for a circuit breaker and a fuse having the same 8/20 µs current wave withstand capability