Short-circuit and overvoltage protection

Resistance to short-circuiting

Classification of the resistance to short-circuiting conform to EN 611558 Abs. 3

Short-circuit-proof transformer
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.

Micro fuses
The label of the transformer 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 at inrush current and primary fuse.
We also deliver transformers 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 will turn the transformer off, if there is a short-circuit or an overload on the secondary side. Due to the PTC resistor the transformer will just operate 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:


Selection as example:


Higher powers than 150 VA can not be fused directly with a PTC, the PTC has to control an automatic switch-off.
All tables are examples, the values can be changed by our suppliers.

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