Protection
Electrical installations almost always require protection against short-circuits wherever there is an electrical discontinuity. This most often corresponds to points where there is a change in conductor cross-section. The short-circuit current must be calculated at each level in the installation in view of determining the characteristics of the equipment required to withstand or break the fault current.
The flow chart in Figure 1 indicates the procedure for determining the various short circuit currents and the resulting parameters for the different protection devices of a low-voltage installation.
In order to correctly select and adjust the protection devices, the graphs in Figures 2, 3 and 4 are used. Two values of the short-circuit current must be evaluated:
The maximum short-circuit current, used to determine
- The breaking capacity of the circuit breakers
- The making capacity of the circuit breakers
- The electrodynamic withstand capacity of the wiring system and switchgear
The maximum short-circuit current corresponds to a short-circuit in the immediate vicinity of the downstream terminals of the protection device. It must be calculated accurately and used with a safety margin.
The minimum short-circuit current, essential
when selecting the time-current curve for circuit breakers and fuses, in particular when:
- Cables are long and/or the source impedance is relatively high (generators, UPSs)
- Protection of life depends on circuit breaker or fuse operation, essentially the case for TN and IT electrical systems
Note that the minimum short-circuit current corresponds to a short-circuit at the end of the protected line, generally phase-to-earth for LV and phase-to-phase for HV (neutral not distributed), under the least severe operating conditions (fault at the end of a feeder and not just downstream from a protection device, one transformer in service when two can be connected, etc.).
AUTHOR: Schneider Electric experts | Benoît de METZ-NOBLAT, Frédéric DUMAS, Christophe POULAIN
Title: | Calculation of short-circuit currents |
Format: | |
Size: | 0.75MB |
Pages: | 35 |
Download: | Right here | Video Courses | Membership | Download Updates |
Thanks for this kind article…this portal has been really helpful for me in understanding
Although this Article as basic knowledge., But this Article very-very useful for application Short circuits., Thank you very much to Mr Edvard.
I AM UNABLE TO OPEN AND DOWNLOAD ANY OF YOUR PDF ELECTRICAL REPORTS
PLEASE HELP!
ANY SUGGESTIONS
THANKS, KEEP UP THE GOOD WORK.
Thanks for this article…this portal has been really helpful for me in understanding various concepts
I’m very happy for you, just keep on learning and do not hasitate to ask anything!
I am beginner, its difficult for me for understanding it, do you have some tips for me?. but thank you,,this site is very useful for me. i will hard work to be better electrical engineer.