Protection Against Short-Circuit Effects Inside Low-Voltage Switchboards
The Std. IEC 61439-1 specifies that ASSEMBLIES (referred to hereafter as switchboards) shall be constructed so as to be capable of withstanding the thermal and dynamic stresses resulting from short-circuit currents up to the rated values.
Furthermore, switchboards shall be protected against short-circuit currents by means of circuit-breakers, fuses, or a combination of both, which may either be incorporated in the switchboard or arranged upstream. When ordering a switchboard, the user shall specify the short-circuit conditions at the point of installation.
Related Article: Appropriate Test and Sequence for Low Voltage Electrical Installations According to IEC 60364 Related Standards
This article takes into consideration the following aspects:
- The need or not to carry out the verification of the short-circuit withstand inside the assembly;
- The suitability of switchgear for a plant according to the prospective short-circuit current of the plant and of the short-circuit parameters of the switchgear;
- The suitability of the busbar system according to the short-circuit current and to the protection devices;
- The verification of the short-circuit withstand of the assembly by applying the design rules defined in the IEC 61439-1.
Verification of short-circuit withstand strength
- For switchboards having a rated short-time current (Icw) or rated conditional current (Ik) not exceeding 10 kA.
- For switchboards protected by current limiting devices having a cut-off current not exceeding 17 kA at the maximum allowable prospective short-circuit current at the terminals of the incoming circuit of the switchboard.
- For auxiliary circuits of switchboards intended to be connected to transformers whose rated power does not exceed 10 kVA for a rated secondary voltage of not less than 110 V, or 1.6 kVA for a rated secondary voltage less than 110 V, and whose short-circuit impedance is not less than 4%.
Verification Guide | Refer to IEC 61439-1 |
Short-circuit current and suitability of the switchboard for the plant
- the rated short-time current Icw
- the rated conditional short-circuit current Ik
It shall be necessary to verify that the breaking capacities of the apparatus inside the switchboard are compatible with the short-circuit values of the system.
Source: ABB |
Source: ABB |
- Electrical Installation Handbook | Protection, Control and Electrical Device | pp. 233-236
- Publisher: ABB
- Download here
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