Switching neutral in 120 VAC control ckt

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Where does NEC specifically note that switching of neutral on a 120 VAC control ckt is prohibited? I have a fabricated control panel that's full of control ckts with contacts on both sides of 120 VAC relay coils.

Even with a bit of searching, I can't seem to find the specifics.

While on the subject, how can a standard OL contact be wired between the coil & neutral in 99.99% of 120VAC control circuits to begin with? I've never been fond of that arrangement, even though it's been the industry-standard almost forever.
 

charlie b

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I can't think of a reason that switching the neutral would be prohibited. Switching only the neutral, and not the ungrounded leg, would be "double plus ungood." But switching both legs at the same time causes no safety concerns. I know of an industry that requires it, in a specific circumstance, and their panels have accomodations for two pole breakers (i.e., hot and and neutral) just for that purpose.
 
I'm aware of locations other locations with switched neutrals, such as within a 4-pole ATS, and in Article 514 (which requires simultaneous switching of all current-carrying conductors). But in this case, he is proposing a 120 VAC control panel with relay coils, all of which have a number of different contacts on BOTH sides of the coils. What should be wired as white/grey? Is it only the last conductor in the rung that is actually tied to the neutral in the control panel? Safety-wise, the proposed arrangement gives me the creeps. I've never deliberately switched a neutral unless the power conductors are switched along with it. I thought (somewhere) it specifically noted so.
 

augie47

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I believe the section you are looking for is 430.74
430.74 Electrical Arrangement of Control Circuits.
Where one side of the motor control circuit is grounded, the motor control circuit shall be arranged so that an accidental ground in the control circuit remote from the motor controller will (1) not start the motor and (2) not bypass manually operated shutdown devices or automatic safety shutdown devices.

The neutral feeding the OLs is a good question but since that wiring is interanl to a listed product the NEC would have no bearing.
 
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Thanks. Your reference is the closest I can come up with.

Unfortunately, he has the control circuit laid out so that the contacts which are switching the neutral leg are actually the "stop" portion of his control circuit. Therefore, it would appear to meet the letter-of-the-law for Reference 1). I could likely make a case that a failure of the "stop" contact would violate Reference 2). In any case, it is very poor design.
 

augie47

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If you have access to the NEC ('08) Handbook it has a good illustration of what you describe and how it's a violation.
 

jap

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Switching only the neutral, and not the ungrounded leg, would be "double plus ungood."


But thats exactly the way most motor control circuits are wired on a motor starter (That I have come across).
The Neutral to the Coil is broken through the Overload Contacts, the hot to the coil in some cases remains energized.(In cases where the Ungrounded Control Power is brought directly to the coil by a maintained switch instead of (a momentary pushbutton where the aux. 2&3 would open if the starter fell out, therefore disconnecting the Hot to the coil also.)

I worked for an engineer in a plant that specifically drew the neutral to be brought directly to the coil and have the Ungrounded Control power broke throught the overloads.

This is the way I prefer to set up the Control Power.
 
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That was the point in my original comment. Other than the "it's been that way forever" response, sticking the normally-closed OL contact on the neutral side of the starter coil has irritated me for years. The situation with this particular control panel just takes the issue to the extreem. Personally, I prefer to install OL's such that it breaks the hot-leg on the other side of the coil, just like any other control contact. That said, I know my schematics irritate the poor guy wiring it since most combination starters are factory-wired with it on the neutral side of the coil. Thanks, good discussion.
 

jap

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I'm right there with you.
The one good thing is the color of the conductor from the load side of the of the Overload contact to the coil on a starter is generally not "White or Grey", leaving it open for us to do with it what we want.
 

petersonra

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I believe the section you are looking for is 430.74
430.74 Electrical Arrangement of Control Circuits.
Where one side of the motor control circuit is grounded, the motor control circuit shall be arranged so that an accidental ground in the control circuit remote from the motor controller will (1) not start the motor and (2) not bypass manually operated shutdown devices or automatic safety shutdown devices.

The neutral feeding the OLs is a good question but since that wiring is interanl to a listed product the NEC would have no bearing.

Read carefully what I highlghted in red. This allows the OL contact to be on the neutral side as long as it is not remote.
 

benaround

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I'm right there with you.
The one good thing is the color of the conductor from the load side of the of the Overload contact to the coil on a starter is generally not "White or Grey", leaving it open for us to do with it what we want.

Not so fast, the schematic diagram from the manafacturer will show the other side of the

control circuit wired thru the overloads.
 

IMM_Doctor

Senior Member
AHA! - The reason may be a reversing starter

AHA! - The reason may be a reversing starter

I agree with all posts.

I would NEVER switch a grounded conductor. It would be very easy to revise a control schematic to have the N.C. OL (Overload contact) in FRONT of the coil on the ungrounded conductor on the coil of a motor starter coil, and the other side of the coil attached solidly to the unswitched, grouded conductor....

BUT

What are you going to do when you have a reversing starter, where you have only ONE N.C. OL contact, and two seperate motor starter coils?

We think, the key reason that switching of the grounded conductor (neutral) is allowed, as long as it is only done withing the local confines the "motor starter" itself, and not done remotely.

There is no 'one size fits all" answer. In the future, We may elect to draw single motor starters with the ungrounded conductor through the overload contact, and the grounded conductor tied directly to the oppsite side of the coil. BUT, when faced with a reversing starter with two coils, we will still continue to use the standard practice of switching the neutral to BOTH coils through the overload contacts.

My $0.02
 

LarryFine

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What are you going to do when you have a reversing starter, where you have only ONE N.C. OL contact, and two seperate motor starter coils?
The only solution there would be to put the O/L's ahead of the point where FWD and REV split.
 

Cold Fusion

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... I prefer to install OL's such that it breaks the hot-leg on the other side of the coil, just like any other control contact. That said, I know my schematics irritate the poor guy wiring it since most combination starters are factory-wired with it on the neutral side of the coil. Thanks, good discussion.
It would definitely irritate me if I was called out in the dead of the night to troubleshoot and I found that someone had change the factory wiring just because that was their preference.

I might even jump to considering the rewire an "Act of Aggression" if the changes were not documented.

However:

As IMM said, there is no "one size fits all".

ATEX (I think) spec requires switching the positive side. But I rarely work to that spec.

And, unless there is a reason, why not stick with what has worked well for the last 150 years? Why is putting the overload on eht line side of the coil safer?

cf
 

don_resqcapt19

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One argument against putting the overload in the hot side is the possibility that a control circuit short circuit or ground fault may weld the overload relay contacts closed and then you lose the overload protection.
As far as the factory wiring of a motor stater, the standard is with the overload in the grounded conductor, but you can order them with the overload in the hot side of the control circuit...just a longer delivery time. I have done a couple of jobs where the design specified the overloads in the hot side.
 

kkscheid

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Control Circuit Motor OL's

Control Circuit Motor OL's

Years ago, when I worked for a certain automotive manufacturer that is now controlled by the Gov't, they directed us in Plant Engineering to design control circuits with the OL's on the "hot" (ungrounded) side of the starter coil. The manufacturer would supply our starters that way if ordered properly, but if not our electricians were to revise the wiring accordingly to the schematic.

The reasoning was that if a wire came loose on the grounded side of the starter coil and went to ground, the motor OL's would not provide protection for the motor. It was deemed a safety issue. They must have had an issue somewhere in the company that affected production downtime and this was their response.
 
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