Protection of LV/LV transformers

These transformers are generally in the range of several hundreds of VA to some hundreds of kVA and are frequently used for:

• Changing the low voltage level for:
  • Auxiliary supplies to control and indication circuits
  • Lighting circuits (230 V created when the primary system is 400 V 3-phase 3-wires)

• Changing the method of earthing for certain loads having a relatively high capacitive current to earth (computer equipment) or resistive leakage current (electric ovens, industrial-heating processes, mass-cooking installations, etc.)

LV/LV transformers are generally supplied with protective systems incorporated, and the manufacturers must be consulted for details. Overcurrent protection must, in any case, be provided on the primary side. The exploitation of these transformers requires a knowledge of their particular function, together with a number of points described below.

Note: In the particular cases of LV/LV safety isolating transformers at extra-low voltage, an earthed metal screen between the primary and secondary windings is frequently required, according to circumstances, as recommended in European Standard EN 60742.

Contents

Transformer-energizing inrush current Protection for the supply circuit of a LV/LV transformer Typical electrical characteristics of LV/LV 50 Hz transformers Protection of LV/LV transformers, using Merlin Gerin circuit-breakers