Why do MCB's sometimes trip when feeding ELV luminaire transformers?

Answer:

The reason is due to the high inrush current experienced with some luminaires that exceed the protective characteristic of the MCB.

Problem analysis

When a transformer is switched on, an inrush current occurs in the primary circuit of the transformer irrespective of the load on the secondary side. The value of this inrush current varies with the design and construction of the transformer.
BS7671 (16th Edition Wiring Regulations) requires an assessment to be made to be able to select the correct protective device to match the characteristics of the lighting load. MCB manufacturers guidance for the selection of the MBC's generally assumes that the inrush current is in order of 15X the primary current. However in some cases this inrush current can be exceeded, in which case you will need to ascertain the load characteristics from the luminaire/ transformer manufacturer.

Once you have received this information you will need to match it to the characteristic provided by the MCB manufacturer.