Low voltage/Medium voltage selectivity

To carry out the selectivity study between two low and medium voltage circuit-breakers, the data below must first be put into a logarithm diagram (referring to a single reference voltage).

1. Transformer:

  • Connection curve (inrush);
  • Rated current;
  • Short-circuit current at the LV busbars;
  • Short-circuit withstand capacity of the transformer.

2. Distributor utility:

  • Maximum current and time limits which can be set for the protections required.

At this point, the trip curve of the main low voltage circuit-breaker must be traced so that:

  • Protection of the transformer against overload is verified (threshold I1 of protection function L close to the rated current of the transformer);
  • It is selective with the other low voltage circuit- breakers on the load side.

Once the LV protection has been defined, the curve of the medium voltage circuit-breaker voltage can be traced so that:

  • It protects the transformer against overloads (this protection is usually ensured by the low voltage circuit-breaker);
  • It lies above the inrush current curves of the transformer;
  • It lies below the representative point of the thermal withstand (this protection can be carried out by the low voltage circuit-breaker, but any short-circuit between the low voltage circuit-breaker and the terminals of the transformer remains unprotected);
  • It lies below the limits set by the distributor utility.

(*) The use of a thermometric equipment allows improving the protection of the transformer against overload.

How to get selectivity between medium voltage and low voltage devices

Before facing the problem of selectivity between the medium and low voltage circuit-breakers, it is necessary to clarify the functions of these circuit-breakers.

The MV protection on the supply side of the transformer must:

  • Protect the transformer against short-circuit;
  • Protect the transformer against faults on the supply side of the main LV circuit-breaker (if a dedicated protection is not provided);
  • Not intervene when the transformer is supplied with voltage (inrush current);
  • Be set so as to satisfy the limits imposed by the distributor utility;
  • Be set so as to be selective with the protections on the supply side (if requested).

The LV protection on the load side of the transformer must:

  • Protect the transformer against short-circuit and overload; (*)
  • Be set so as to be selective with the protections on the load side.


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