Motor control 95/96

[kenaslan]

I have heard that points 95/96 on a motor controller may no longer open the Neutral to the motor control circuit. If this is correct does anyone have the code reference? This was mentioned in a AB seminar recently.

Thanks
Ken

 

[don_resqcapt19]

There has been no recent change in the NEC. He may have been talking about 430.74, but that is not new.

 

[Smart $]

Listed controllers aren't bound to NEC wiring requirements. Perhaps ANSI, NEMA and UL updated the standards to match the NEC...???

 

[templdl]

I have heard that points 95/96 on a motor controller may no longer open the Neutral to the motor control circuit. If this is correct does anyone have the code reference? This was mentioned in a AB seminar recently.

Thanks
Ken

It may be that the statement intended to be referring to opening a neutral by itself. I have always been of the understanding that the only time that that you are allowed to "switch" a neutral is when the line or lines are opened simultaneously which completely isolates the load from the source.

 

[don_resqcapt19]

It may be that the statement intended to be referring to opening a neutral by itself. I have always been of the understanding that the only time that that you are allowed to "switch" a neutral is when the line or lines are opened simultaneously which completely isolates the load from the source.

The overload relay in a very large majority of the motor starters opens the grounded conductor when the relay senses the motor is overloaded. None of these relays also open the ungrounded control circuit conductor.

 

[templdl]

The overload relay in a very large majority of the motor starters opens the grounded conductor when the relay senses the motor is overloaded. None of these relays also open the ungrounded control circuit conductor.

Understood, You are so correct. For some reason I was thinking of the power circuit on the contactor and not the control circuit itself.
I have draws many a control circuits and should have known better. I drew control circuits for 11 MCCs with numerous buckets from size 1 NR and R to size 5 NR and R on one job that I sold and trained one of my customers on how to draw a common control schematic that we used in order to make their order entry more expedient.
I even trained my distributor to do control circuit modifications.
And there is really no good reason to disconnect the unfused side of the CPT that is commonly the grounded conductor (which really isn't a neutral). Depending upon how the control circuit is energized if it has a CPT then there should be a door handle mech that you pull down to open the door which opens the disconnect, disconnecting power to the the CPT.
If separate power there is commonly an inlk switch on the door hndl mech that disconnects the power from the control circuit. And, there should be some sort of signage which indicated that there is separate power, the wiring may be yellow instead of red if I not mistaken. Blue is commonly uses for DC if I remember correctly.
As such one should never be able to open an enclosure without disconnecting the power first.

 

[kenaslan]

The overload relay in a very large majority of the motor starters opens the grounded conductor when the relay senses the motor is overloaded. None of these relays also open the ungrounded control circuit conductor.

This is the situation in which I refer to. However come to think about it it might have been a CEC issue for large industrial mining complex in Canada that I did.

On a personal note I think that opening the grounded conductor is a bad/unsafe practice. This will leave the field controls hot as the grounded conductor is still energized. But most large complexes no longer use CPTs in the MCC. Most control now are powered from the PLC.