MPCB for Star Delta Starters

This has nothing whatsoever to do with electronic controls.

An IEC style MPCB (Motor Protection Circuit Breaker) which incorporates the disconnect, short circuit and adjustable thermal overload protection in one unit, is intended to feed the motor starter as the first device from the line source. So the motor thermal overload setting must be for the motor nameplate Full Load Amps.

But when you use Star Delta starting where you take advantage of using 6 leads to the motor, you typically place the thermal overload protection in only one part of the windings going through just 3 of the leads, set to 58% of the nameplate FLA of the motor, because that current flows to it the same whether it is in Star or Delta mode. This way, the overload protection sees and reacts to both the true starting current and the running current on a cumulative basis.

If you use an MPCB ahead of the entire stating/running contactor set, the starting current is not being correctly tracked, it is only going to see starting current (Star) as a fraction of the total current, yet the trip settings will be based only on the full current (Delta). The true cumulative heating effects of the starting current then are not properly included in the trip curve of the overload protection.

So for example, if the motor was turned off when it was over loaded (but not yet tripped), when you restart it, the added thermal stress of starting is not properly tracked and may allow the motor to be restarted when it should have tripped, resulting in it being damaged, which is exactly what the thermal overload is supposed to prevent.