230V control circuit

Chemenger said:

he control voltage is all 230V. Our maintenance engineer says that the control voltage, according to EIC 60204, should be stepped down to 24V. Is this mandatory?

No, Clause 9.1.2 of BS EN 60204-1 :2018 (the current Designated standard) and BS EN 60204-1:2018+A1:2025 (likely to take over as the Designated standard by 30 April 2028) states:

9.1.2 Control circuit voltages
The nominal value of the control voltage shall be consistent with the correct operation of the control circuit.
The nominal voltage of AC control circuits should preferably not exceed
– 230 V for circuits with 50 Hz nominal frequency,
– 277 V for circuits with 60 Hz nominal frequency.
The nominal voltage of DC control circuits should preferably not exceed 220 V.

I think that the confusion comes because where PELV is used (but it doesn't have to be, just if it is) there are different voltage limits to PELV in IEC 60204-1, i.e. 25 V AC or 60 V DC in BS EN 60204-1; 50 V AC or 120 V DC in BS 7671 ... unless, of course, there are lower limits in a relevant Section of Part 7). 

There are specific wiring considerations in BS EN 60204-1 for panels that contain LV controls, and specific colour-coding where LV controls remain live when the panel isolator is in the "OFF" position to open the panel (e.g. so that electronic controllers can still relay status to other panels etc.).

Hi Graham,

Thanks for your comments on this, it's very helpful.

There is still confusion in our camp where Clause 9.1.1 says, "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". This doesn't specify to what voltage the control voltage should be, but what would be the point of adding a transformer to go from a 230V supply to 230V (the maximum under Clause 9.1.2) control?? 

Essentially, I'm still being told on one hand that the supply should (although not in law) be stepped down...(to 24V).

Hi Graham,

Thanks for your comments on this, it's very helpful.

There is still confusion in our camp where Clause 9.1.1 says, "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". This doesn't specify to what voltage the control voltage should be, but what would be the point of adding a transformer to go from a 230V supply to 230V (the maximum under Clause 9.1.2) control?? 

Essentially, I'm still being told on one hand that the supply should (although not in law) be stepped down...(to 24V).

Chemenger said:

This doesn't specify to what voltage the control voltage should be, but what would be the point of adding a transformer to go from a 230V supply to 230V (the maximum under Clause 9.1.2) control?? 

This is questioning the provisions of the standard ... and I can only provide an opinion not an 'interpretation'.

This is so that electrical faults in the power circuit isn't present on the control circuit, for safety. It also permits segregation for fault-finding.

And perhaps also a safety concern, a transformer, even a 1:1 ratio unit, ensures a far lower PSSC on the control side of things, as presumably the transformer will be a low wattage unit, only just large enough to operate the contactor primary coils and any indicators with a modest safety margin,
If there are no contactors and the switches are in series with the load itself, like a DOL starter, that part of the wiring  is not a control circuit, but a power one.
Mike