Chapter N

Characteristics of particular sources and loads

Protection for the supply circuit of a LV/LV transformer

From Electrical Installation Guide
HomeCharacteristics of particular sources and loadsProtection of LV/LV transformersProtection for the supply circuit of a LV/LV transformer

The protective device on the supply circuit for a LV/LV transformer must avoid the possibility of incorrect operation due to the magnetizing inrush current surge, noted above.It is necessary to use therefore:

  • Selective (i.e. slighly time-delayed) circuit-breakers of the type Compact NSX with electronic trip-unit (see Fig. N34) or
Fig. N34 – Tripping characteristic of a Compact NSX with electronic trip-unit
  • Circuit-breakers having a very high magnetic-trip setting, of the types Compact NSX or Acti 9 curve D (see Fig. N35)
Fig. N35 – Tripping characteristic of a Acti 9 curve D


A 400 V 3-phase circuit is supplying a 125 kVA 400/230 V transformer (In = 180 A) for which the first inrush current peak can reach 12 In, i.e. 12 x 180 = 2,160 A.

This current peak corresponds to a rms value of 1,530 A.

A compact NSX250N circuit-breaker with Ir setting of 200 A and Im setting at 8 x Ir would therefore be a suitable protective device.

A particular case: Overload protection installed at the secondary side of the transformer

(see Fig. N36)

Fig. N36 – Example

An advantage of overload protection located on the secondary side is that the short-circuit protection on the primary side can be set at a high value, or alternatively a circuit-breaker type MA (magnetic only) can be used. The primary side short-circuit protection setting must, however, be sufficiently sensitive to ensure its operation in the event of a short-circuit occuring on the secondary side of the transformer.

Note: The primary protection is sometimes provided by fuses, type aM. This practice has two disadvantages:

  • The fuses must be largely oversized (at least 4 times the nominal full-load rated current of the transformer)
  • In order to provide isolating facilities on the primary side, either a load-break switch or a contactor must be associated with the fuses.