Last edit: 07/08/2023
THE DOUBT: Most of us know that Light Blue is the colour of the neutral conductor inside an industrial control panel. The doubt is the colour of the “common conductor” on the secondary of either a Power or a Control Transformer.
In order to clarify the matter, it is important to understand when a conductor can be defined as a Neutral Conductor. IEC 60445 helps us in that:
[IEC 60445:2018] 3.8 neutral conductor: conductor electrically connected to the neutral point and capable of contributing to the distribution of electric energy.
The same definition is in IEC 60204-1 article 3.1.42.
From that definition comes the fact the neutral is only the conductor connected to the common point of a MT/BT transformer.
When a transformer is used inside a control panel, that is not for distributing electrical energy but to either power receptacles or control systems. For that reason, the colour should not be light blue but black if it is a power transformer or red in case of a control one.
CONCLUSION: The colour of the cables connected to the “common pole” of a transformer inside an industrial control panel shall not be light blue but of the same colour of the cables derived from the “positive pole”. The reason is that such a conductor cannot be defined as a Neutral.
We also remind you that the “common pole” has to be grounded as indicated in the following picture.