Motor Control Circuits

[volt102]

Need some help here.

430.74 Disconnection

(A) General Motor control circuits shall be arranged so that they will be disconnected from all sources of supply when the disconnecting means is in the open position. The disconnecting means shall be permitted to consist of two or more separate devices, one of which disconnects the motor and the controller from the source(s) of power supply for the motor, and the other(s), the motor control circuit(s) from its power supply. Where separate devices are used, they shall be located immediately adjacent to each other.

1.) I get the first sentence.
2.) Permitted to be two devices

• 
  2a.) one of which disconnects the motor and the controller from the source(s) of power supply for the motor.
  
  2b.) and the other, the motor control circuit(s) from its power supply.

3.) I get the last.

My question is what is it that these two permitted devices are accomplishing that 430.102 A + B aren't doing already, or even the main breaker for what it matters. Isn't the first sentence enough?

Am I missing something?

Jim
New Hampshire

 

[kingpb]

The control circuits could be powered from a different source, usually at a lower voltage; especially when the MCC is 480V.

 

[petersonra]

The control circuits could be powered from a different source, usually at a lower voltage; especially when the MCC is 480V.

It is not unusual for the control circuits in MCCs to come from the same source for all the buckets. In that case you typically add an auxiliary contact to the bucket disconnect to disconnect the control circuit.

I usually prefer a separate control xfmr in each bucket but a few places do not want that for various reasons.