If a fault occurs at any point in an electrical distribution circuit, it is essential that it does not interrupt the supply to all the workshops or offices of that network. This obvious requirement leads to the necessity of rapidly isolating the detective section without depriving the other users of electrical energy; this is in fact the principle of selective tripping.
The protecting element (circuit-breaker or fuses) which is placed immediately up-stream from the part of the circuit where the fault has occurred, and this alone element, must then operate; the other protecting elements must not trip. Conventional selectivity processes (overcurrent and time tag) fulfil these requirements to a more or less satisfactory degree. The Logic Selectivity System (Merlin Gerin patent) makes it possible to attain a total seiectivity between all stages of an electrical distribution network from high to low voltages. it also allows the fault to be eliminated very rapidly and independent of the place where the fault occurred.
This makes use of protective equipment operating instantaneously (rapid circuit-breakers or fuses). The selectivity is based on the fact that the short-circuit current decreases with increasing distance from the source. It is thus used especially for low voltages where the connecting impedances are not negiigibie.
If Ir is the current setting for instantaneous tripping and loo is the short-circuit current which is established at the point of departure B, down-stream from the protection equipment,
Selectivity is total:
if Ir of A >Icc at B.
Selectivity is partial:
it Ir of A < Icc at B.
AUTHOR: Schneider Electric experts | Roland CALVAS, Francois SAUTRIAU
|Title:||Protection of electrical distribution networks by the logic selectivity system|
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