Classification of the resistance to short-circuiting conform to EN 611558 Abs. 3
Here the temperature will not exceed certain set limits, if the transformer has an overload or a short-circuit. After removing the overload or the short-circuit all requirements to the norm are still fulfilled.
Conditionally short-circuit-proof transformer
Short-circuit-proof transformer which has a protection that will open the input or output circuit or will lower the current in those circuits, if the transformer has an overload or a short-circuit. After removing the fault and after exchanging the resetting of the protective device, all necessary requirements of the norm are met again. Examples for protective devices are fuses, overload breakers, temperature fuses, thermal fuse inserts, temperature limiters, PTC and automatically reacting mechanic protection switches. In case of a circuit breaker due to a protective device, which can not be exchanged or reset. This means the transformer might not operate anymore, even though the norms are met.
By all means 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 removing the fault the transformer still works and all requirements to the norms 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 removing the fault all requirements would still be meet, if an external protective device was used.
Transformer which is damaged permanently in case of wrong handling that leads to a break of the input circuit. The device is no harm for ambient or user in case of failure.
Conditionally short-circuit-proof transformers and not short-circuit-proof transformers need to be fused with a protective device.