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Protection for the supply circuit of a LV/LV transformer

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General rules of electrical installation design
Connection to the MV utility distribution network
Connection to the LV utility distribution network
MV and LV architecture selection guide for buildings
LV Distribution
Protection against electric shocks and electrical fires
Sizing and protection of conductors
LV switchgear: functions and selection
Overvoltage protection
Energy Efficiency in electrical distribution
Power Factor Correction
Power harmonics management
Characteristics of particular sources and loads
PhotoVoltaic (PV) installation
Residential premises and other special locations
ElectroMagnetic Compatibility (EMC)


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. N34Tripping 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. N35Tripping 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. N36Example

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.