Why does the circuit breaker trip when I start up my transformer?

The circuit breaker protecting the transformer may trip, even if the transformer is not under load.

A fault in the connections may be responsible, but in most cases we identify an underdimensioned circuit breaker (RCD) responding to the transformer's inrush current as the cause.

Recommendations for protecting transformers can be found in our product information sheets and on our circuit diagrams. The primary protection is usually a Type D device with a higher current than the normal primary current of the transformer. When selecting a transformer you must always check that the recommended protection is possible in the electrical installation in question. Where this is not the case it is better to go for a low inrush current (ICR) transformer.

The recommended safety devices are dimensioned for the worst-case scenarios, for example where the transformer is connected to a high voltage substation by several metres of cable. In practice it can happen that a somewhat smaller RCD will work without problems, but this can be difficult to predict. Problems can also crop up over the course of time due to external circumstances, such as modifications to the distribution network.

For transformers with a primary current lower than 25A we can supply IRC limiters. These devices ensure that the transformer starts up gently. They then switch off automatically. If necessary, devices can be installed later if problems emerge with the inrush current in an electrical installation.