Isolation transformer

Hi! I have a question regarding the use of isolation transformers in control cabinets. I have seen many diagrams in which an isolation transformer is used to provide 230V AC control voltage although the cabinets are endowed with a neutral potential from the source (3 phases+N+PE).

I have read that this is sometimes used in order to avoid ground loops, to increase safety operation by ensuring a galvanic isolation or even to increase the availability of this supply in case of a failure of the neutral conductor (why is it assumed that it fails more often than any other phase? Because it's independent from the source or the transmission system?).

Sometimes, however, the secondary winding is earthed, thus cancelling the benefits of the galvanic isolation.

I have also read that it is used in order to reduce the effect caused by the switching of the different control devices (relays, contactors, etc.) on the rest of the consumer loads, filtering out (undesired) harmonics.

Could you please explain how this transformer filters out these artifacts? Do I have to size the winding (number of turns) for the reactance to be high enough from the desired frequency upwards (for example, from the first harmonic to be filtered)?

If it had a turns ratio of 1, would it still have a significant effect on shortcircuit current? Why? Is it because of the impedance of the transformer? Where does the main contribution to this limiting effect come from?

Thank you very much for your answers!

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Hugo Caballero Figueroa
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