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 


The verification of the short-circuit withstand strength is dealt with in the Standard IEC 61439-1, where, in particular, the cases requiring this verification and the different types of verification are specified. 


The verification of the short-circuit withstand strength is not required if the following conditions are fulfilled: 

  • 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%. 
Therefore, from an engineering point of view, the need to verify the short-circuit withstand strength may be viewed as follows;

Verification Guide | Refer to IEC 61439-1




Short-circuit current and suitability of the switchboard for the plant 


The verification of the short-circuit withstand strength is based on two values stated by the manufacturer in alternative to each other: 
  • the rated short-time current Icw 
  • the rated conditional short-circuit current Ik

Based on one of these two values, it is possible to determine whether the switchboard is suitable to be installed in a particular point of the system.

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.

Rated short-time withstands current Icw is the r.m.s. value of the current relating to the short-circuit test for 1 s without openings of the protections, declared by the assembly manufacturer, that can be carried by the assembly without damage under specified conditions, defined in terms of a current and time. Different Icw values for different times (e.g. 0.2 s; 3 s) may be assigned to an assembly. The switchboard shall be able to withstand the thermal and electrodynamical stresses without damages or deformations which could compromise the operation of the system. From this test (if passed) it is possible to obtain the specific let-through energy (I2 t) which can be carried by the switchboard:


The test shall be carried out at a power factor value specified below in the Table 4 of the Std. IEC 61439-1. A factor “n” corresponding at this cosϕ value allows to determine the peak value of the short-circuit current withstood by the switchboard through the following formula:





The conditional short-circuit current is a predetermined r.m.s. value of test current to which a defined peak value corresponds and which can be withstand by the switchboard during the operating time of a specified protective device. This devices is usually the main circuit-breaker of the switchboard. 

By comparing the two values Icw and Ip with the prospective short-circuit current of the plant, it is possible to establish whether the switchboard is suitable to be installed at a specified point of the system. The following diagrams show the method to determine the compatibility of the switchboard with the plant.

Source: ABB






Source: ABB





Source:
  • Electrical Installation Handbook | Protection, Control and Electrical Device | pp. 233-236
  • Publisher: ABB
  • Download here

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