Short-circuit and overload protection

Short-circuit strength

Classification of short-circuit strength conform to EN 611558 Abs.3.

Short-circuit proof transformer

Here the temperature does not exceed certain set limits, if the transformer is overloaded or has a short-circuit. After the removal of the overload or the short-circuit, all requirements of the norm are still fulfilled.

Partly short-circuit proof transformer

Short-circuit proof transformer, which has a protection that opens the input or output circuit or lowers the current in those circuits, if the transformer is overloaded or has a short-circuit. After removal of the fault and after exchanging or reseting of the protective device, all necessary requirements of the norm are redeemed again. Examples for protective devices are fuses, overload breakers, temperature fuses, thermal fuse inserts, temperature limiters, PTC and automatically reacting mechanical protection switches. In case of a circuit break due to a protective device, which can not be exchanged or reseted, this means, the transformer might not operate anymore, even though the norms are redeemed.

Absolutely short-circuit proof transformer

Short-circuit proof transformer without protective device. Due to the construction, temperature won’t exceed set limits in case of an overload or short-circuit. After removal of the fault the transformer still works and all requirements of the norm are met.

Not short-circuit proof transformer

This transformer is meant to have a protective device, which is not a component of the transformer itself. AFter removal of the fault, all requirements are still met, if an external protective device was used.

Fail-safe-transformer

Transformer, which is damaged permanently in case of wrong handling which leads to a break of the input-circuit. The device is no harm for ambient or user in case of a failure.

Partly short-circuit proof transformers and not short-circuit proof transforemrs need to be fused with a protective device!

Micro fuses

The label of the transformers shows the values of the fuses for the secondary side.

A selection of laggy micro fuses with norm values conform to DIN 41662 IEC 127-2 can be found here inrush current and primary fuse.

We also deliver transformer with temperature switches (only conform to VDE with lock), thermal fuses and PTC resistors. The protective devices are built in on the primary side and are connected in series to the primary winding. A PTC (cold), built in on the primary side, turns the transformer off, if there is a short-circuit or overload on the secondary side. Due to the PTC resistor, the transformer can just be operated again, if the short-circuit is removed and the device is started after a cool down. Thus the PTC also has the function of the lock, besides the main function as overtemperature and overcurrent protector.

Thermal fuses

Selection as example:

Thermal switch without lock

Selection as example:

P T C

Selection as example:

Higher powers than 150 VA can not be fused directly with a PTC, the PTC has to control an automatic switchoff.

All tables are examples, the values can be changed by our suppliers.