Control Current Transformer as required by IEC60204-1

[MarnixW]

IEC 60204-1 requires a transformer for control circuits:

9.1.1 Control circuit supply
Where control circuits are supplied from an AC source, transformers having separate windings
shall be used to separate the power supply from the control supply.

Can anyone help me with the background, the contribution to safety of such a transformer?

I know sometimes usage of a transformer has advantages. For example when there is a high short circuit current or when a machine has only 3 phase supply.
But there are also situations in my opinion it is beter whitout transformer. For example when there are long cables in control current circuits. Or when the supply system has a low short circuit current.
I know also there is an exeption, but in general it is required.

So why is it required and not just allowed?

 

[RRaghunath]

The code also allows use of directly connected control supply. See the extract as belo:
9.4.3.1.5 Method d) – Control circuits not fed by a transformer
Control circuits that are not fed by a control transformer or switch mode power supply units
fitted with transformers having separate windings in accordance with IEC 61558-2-16 are only
allowed for machines with a maximum of one motor starter and/or maximum of two control
devices, in accordance with 9.1.1.