Detailed characteristics of the external SCPD: Difference between revisions
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== Current wave withstand == | == Current wave withstand == | ||
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*For a given rating, the current wave withstand capability is better with a circuit breaker than with a fuse device. | *For a given rating, the current wave withstand capability is better with a circuit breaker than with a fuse device. | ||
{{FigureRef| | {{FigureRef|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, | *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. | :'''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, | *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, | ||
{{FigImage|DB422521_EN|svg| | {{FigImage|DB422521_EN|svg|J56|Comparison of SCPDs voltage wave withstand capabilities for Imax {{=}} 20 kA and Imax {{=}} 40 kA}} | ||
== Installed Up voltage protection level == | == Installed Up voltage protection level == | ||
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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. | 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. | ||
{{FigureRef| | {{FigureRef|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). | These two protection systems have the same 8/20 µs current wave withstand capability (27 kA and 30 kA respectively). | ||
{{FigImage|DB422522_EN|svg| | {{FigImage|DB422522_EN|svg|J57|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}} | ||
Latest revision as of 17:50, 20 December 2019
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. J56 – 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 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. J57 – 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
