Prospective short circuit current

This test is conducted to verify the suitability of the equipment to withstand a prospective short-circuit current that may develop on a fault. It may also be termed the steady slate symmetrical fault current or the short-time (withstand current) rating of the equipment. When the equipment is an interrupting device, it is referred to as its symmetrical breaking current. 

It is permissible to lest just one panel of a multi panel-assembly so long as the construction of other panels is similar and busbar arrangement and supports are the same. The value of the prospective short-circuit current may be determined from a calibrated oscillogram. The test current in any phase should not vary by more than 10% of the average in the three phases and must be applied for a predetermined time of I or. 1 seconds. Unless specified otherwise, this should be considered as to be I second. 

If there is no specific marking on the apparatus, m-apparatus connected to an external power line shall be suitable for a short-circuit current of 4 kA (this is the EN 50028 wording), or, more stringently, for a prospective short-circuit current of 4 kA according to the IEC 60079-18 wording. 

The prospective short-circuit current defines the short-circuit current which can be expected in case of replacing the short-circuit point by an (ideal) electrical connection with zero impedance. Due to a short-circuit point impedance greater than zero in practice, the prospective short-circuit current indicates the upper limit of a short-circuit current in a defined grid at a given point. 

Appendix 3 of the lET Regulations gives the time/cun ent characteristics and specific values of prospective short-circuit current for a number of protective devices. 

The maximum, that is, worst case value of prospective short-circuit current is 16 kA at the supply terminals (see Electricians Guide to the Building Regulations Part P, Chapter 3). 

There has been a new addition to the information to be provided by distributors free on request to low voltage consumers. The 1988 Regulations stipulate that they must provide a written statement of maximum prospective short circuit current maximum earth loop impedance and the type and rating of supplier s fusible cut-out or switching device. The new Regulations add to this the type of earthing system (TT, TN etc). 

The prospective short circuit currents have to be determined at all relevant locations in the installation by calculation or measurement and protective devices selected to protect all conductors against thermal and mechanical effects. For new installations the designer will first have to ascertain the loop impedances and characteristics of the excess current protection at the intake from the supply company (see Regulation 313-01-01), except where private... 

The object of measuring the phase/phase or phase/neutral loop is to check that the short circuit rating of the protective devices is adequate. It should not be necessary to carry out this test on a new installation where the short circuit rating of the selected apparatus should be suitable for the calculated values of short circuit current, but the measurement is a useful check. The measurement, however, can only indicate the value when the reading is taken. Subsequent alterations in the supply network and connected apparatus can vary the value, so there should be a substantial safety margin between this ascertained value and the short circuit rating of the protection to allow for reductions in the loop impedance and a consequent increase in the prospective short circuit current. 

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