415V Switchgear: Control Supply for Motor Feeders Etc.

The most common I've seen is individual control transformers per module, with 120vac secondary (sometimes 24vac in coastal or marine locations). Standard practices for this will vary from place to place, or by customer preference.

If the control power is tapped from the main bus ( I presume phase and neutral from TPN bus system) without using the control transformer, the control circuit will be subjected to high fault level which is not desirable

Almost every MCC panel I've come across has used 110V from a common bus.

The only time I've had problems has been with large panels with a section switch on the line. The switching of the control supplies has to be carefully thought out.

I have just installed 3 motor control panels for a processing line.

The primary power feed to these were all delta 380V, no neutral.

When the panel feed size is 150mm2 the cost saving of having one less conductor to install is significant. Ditto for the motor cabling (not so large but lots of them) which was all delta.

The motor control panels also had single phase control loads in them. The neutral for these was derived through a single phase step down transformer (380/220), inside the panel enclosures, with one leg of the secondary decked to provide a single phase 220V LN bus.

Our normal practice would be to run 3C cables to MCC's. There's no point running a neutral, as you say Wal "what's the point, it's just a waste of money."

4C actually. 3P + Earth. If we ran a neutral then there would be 5.

20% conductor saving without N.

The most common practise we have followed or observed is to consider a Single phase Control transformer (415 or 380/240V AC) connected to each Bus Section. It is preferred to have dual control transformer with AUTO/Manual Changeover scheme.

However for the Incomer Control Supply, a separate control transformer is used which is connected before the Incomer. Do not consider control supply for Motor feeders from this control transformer as this may create a technical problem in the event of incomer ACB/MCCB tripping

Generally neutral Bus is not provided in MCC panels. Please remember the difference between neutral and earth connections. Never provide control supply using one phase and earth connection.

If neutral bus is available in your MCC, you can use any one phase and neutral for control supply and save the cost of control transformer.

Better to use 415/440 to 220/240 transformer, common for all motor feeders in a bus section.

A common power supply taken from one transformer fed off the the buss with an individual properly sized circuit breaker or fused isolation point for each breaker/contactor control within the MCC/Substation/Switchgear is acceptable and safe.

Keep in mind that control power for each piece of peripherial equipment being controlled by each individual switchgear cubicle shall be removed/isolated from the equipment when the contactor/breaker is opened and "racked out" for personnel protection.