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Questions on NEC Sections 430.74 and 430.102 #87763
04/12/04 10:08 AM
04/12/04 10:08 AM
Joe Tedesco  Offline OP
Member
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
Here's a code question from an individual concerning Sections 430.74 and 430.102.

He said in their buckets and in their panels, when they disconnect the three phase that their auxiliary contacts are still live because they are fed from a PLC in many cases.

The PLC is located elsewhere in the plant.

He is wanting to know if he is interpreting the code correctly and seeing this as incorrect.

He said that he looked through his code notes and had asked this same question in 1996 on some board somewhere, but it was never answered.

If you can please give some feedback.


Joe Tedesco, NEC Consultant
2017 / 2014 NEC & Related Books and Study Guides
Re: Questions on NEC Sections 430.74 and 430.102 #87764
04/14/04 07:12 AM
04/14/04 07:12 AM
Y
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 840
Michigan
I feel thats covered by the exceptions in 430.74.
Couple problems with not disconnecting the control power:
Someone working on a starter with aux. contacts not turned off could possibly manually engage the coil thus engerizing the
aux. contacts , pulling in a starter elsewhere.Or they could touch the hot aux contact wires.
Truth is its done all the time.

Re: Questions on NEC Sections 430.74 and 430.102 #87765
04/14/04 08:42 AM
04/14/04 08:42 AM
M
MikeK  Offline
Junior Member
Joined: Apr 2004
Posts: 3
Richmond, Va, USA
am not sure this is addressing my original question. Let's take a step back.

What I was concerned about was 430-74, which states

"Motor control circuits be arranged so that they will be disconnected from all sources of supply when the disconnecting means is in the open position."

Let's say we have several motor starters in a single panel, few enough that we do not meet exception 1. 430-102A requires a separate disconnect for each of those starters, since 1999. We are using a PLC for our control system. The PLC is powered from a service panel not fed from this starter panel. The outputs from the PLC which control the motor starters get their power from the service panel. When I turn off the disconnect for the motor starter, it appears I need to shut off the power to the coil and possibly also to the aux contact (inputs and outputs from the PLC). How can I best accomplish this?

I have been using Bussman 15149 Dead Front Fused Disconnect Switches in the past. They do not provide for an aux contact. It appears that I will need to find a disconnect switch/fuseholder that does allow for aux contacts and use this aux contact in series with the signal from the PLC to the coil.

How about pre 1999 NEC, which has not even been adopted yet in many localities? I am not required to have an individual disconnect switch for each controller. Let's say I have the main disconnect switch for the panel for all of the controllers. Would I now, use the aux off this contact to control a relay and put a contact from the relay in each output signal from the PLC?

MCC buckets are a little easier. I can use the aux contact from the disconnect to kill the power to the coil. From some statements from the ROC on the 2005 NEC, Proposal 11-49 from Mr. Garvey, it appears the committee "does not require that status signal within a motor controller be disconnected."

Please let me know if I am interpreting this section correctly. If there are better ideas, other than using individual control transformers for each starter, please let me know. Thanks.


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