Short-Circuit Strength
Short-circuit proof according to the definition of DIN EN IEC 61558-1:2019-12 is a transformer in which the temperature, even in the event of an overload or short-circuit, does not exceed the values permitted for this transformer and which continues to meet the requirements of the above standard after the overload or short-circuit has been removed*.
A distinction is made between
- not short-circuit proof transformer,
- absolutely short-circuit proof transformer and
- conditionally short-circuit proof transformer.
A non-short-circuit proof transformer is intended to be protected against overload, short-circuit and otherwise unacceptably high temperatures by a suitable protective device fitted by the user which is not part of the transformer.
With an absolutely short-circuit-proof transformer, compliance with specified limit values for currents and temperatures is ensured by the design. For example, very small transformers up to approx. 2…3VA have such high-impedance windings that these transformers are absolutely short-circuit-proof.
A conditionally short-circuit proof transformer is equipped with a protective device. This interrupts or reduces the current in the input or output circuit in the event of an overload or short circuit. If the protective device is resettable, the original function of the transformer is restored after removing the overload, cooling down the transformer, and resetting the protective device (see resettable temperature limiters).
Addition: Small transformers – especially small print transformers – in a conditionally short-circuit-proof design are often equipped with temperature fuses (“thermal fuse”) permanently installed in the transformer. After an overload or short circuit, the transformer must be replaced.
If a BREMER transformer is not explicitly defined as short-circuit proof, it is a non-short-circuit proof transformer. In this case, it is intended that the transformer is protected against overload, short circuit and other inadmissible temperatures by suitable protective devices fitted by the user.
Remarks:
* This does not mean that all types of short-circuit proof transformers are still functional. Among other things, transformers equipped with a non-resettable and non-replaceable protective device must be replaced after it has tripped (thermal fuse).