opening grounded conductor in motor contactor in auxillary contact?

stephena

Member
Is it legal to open the grounded conductor in a motor control circuit on the auxillary contact? I thought it wasn't but in the mills I've seen a lot of it.
 

Jraef

Moderator
Staff member
Is it legal to open the grounded conductor in a motor control circuit on the auxillary contact? I thought it wasn't but in the mills I've seen a lot of it.
Yes it is. Not only is it OK, it is common enough to be the STANDARD way of wiring up a starter OL aux contact for years, in the old "JIC" wiring standard, which eventually became NFPA 79, the standard for industrial machinery. Here is how it is worded in NFPA79, section 8.3:

"Control Circuits shall be permitted to be grounded or ungrounded. Where grounding is provided, that side of the circuit common to the coils shall be grounded at the control transformer" ...
"Exception No. 2: Overload relay contacts shall be permitted to be connected between the coil and the grounded conductor where the conductors between such contacts and coils of magnetic devices do not extend beyond the control enclosure"
The concept has been debated in here time and time again, there is no consensus as to whether it is better or worse, it just is what it is and comes down to personal preference now. Basically if you and everyone else in your facility are USED to it being that way, that's the way it should be. If you are not used to it, but everyone else is, you had probably better change.

That said, here is why I was taught it was done that way, back in the days when JIC was still the predominant law in industrial facilities. The core issue is consistency. If you have a standard FVNR starter, it really doesn't matter which side of the coil you put the OL relay aux contact on, it works either way. But what if you have a FVR (Reversing) starter, where you have two coils, but only one OL relay, and that OL relay has only one set of contacts? If your standard was to have the OL aux contact on the "hot" side of the coil, you would have to have it VERY far ahead of all of the reversing starter control circuits where it is still common with everything. In the case of a stand-alone enclosed starter with a remote push button station, that means you must run your circuit from the control power source to the OL aux, out to the control station, feed the pilot devices, then back to the starter for the Seal-In, then back out to the control station, then back to the starter for the coils. LOTs of places for an error to happen or a wire to rub up against something and short to ground. If you just put the OL aux contact on the common side of both coils, it stays INSIDE of the control enclosure with very low risk of shorting to ground, fewer wires in the field, fewer errors, fewer problems later. So for an FVR starter, and a Wye-Delta starter and an RVAT starter etc., where there are multiple coils used in the same unit, it makes sense for the OL aux contact to be on a common connection side of all of the coils. Then for consistency, do it that way on the FVNR starter as well, so no matter where you look, it's always in the same place. Hence, it became the standard for industry. But because in our 120V control systems, the "common" side happens to also be the grounded neutral side, it looked like a conflict with the NEC (see below*) because both sides of that circuit were not being switched together.

It remained that way up until IEC controls started to flood our market. IEC rules prevent there EVER being a switching device on the grounded side of a control circuit, so they NEVER did it that way, despite the issues raised earlier. So if, in the 1980s, you started buying cheaper IEC starters, you got used to seeing the OL aux contacts on the other (hot) side of the coil. Newer electricians started getting confused, older guys were never quite sure why it was done the other way in the first place, assuming it was just tradition, and the standard was lost to history.

*The debate on this issue often arises again, as it has in here, because it appears to conflict with the NEC, which like the IEC rules, says no switching of the grounded circuit (neutral) unless BOTH sides of the circuit are switched. But the wording in NFPA79 is carefully chosen, notice that it says "where the conductors between such contacts and coils of magnetic devices do not extend beyond the control enclosure". That keeps the issue squarely in the realm of NFPA79, not the NEC. If one of those wires were to LEAVE the starter enclosure, then it would indeed violate the NEC, because it would involve field wiring.
 
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don_resqcapt19

Moderator
Staff member
...
*The debate on this issue often arises again, as it has in here, because it appears to conflict with the NEC, which like the IEC rules, says no switching of the grounded circuit (neutral) unless BOTH sides of the circuit are switched. But the wording in NFPA79 is carefully chosen, notice that it says "where the conductors between such contacts and coils of magnetic devices do not extend beyond the control enclosure". That keeps the issue squarely in the realm of NFPA79, not the NEC. If one of those wires were to LEAVE the starter enclosure, then it would indeed violate the NEC, because it would involve field wiring.
I don't see how NFPA 79 has anything to do with a starter that is not a direct part of a machine. In my opinion that standard does not apply to starters that are remote from the machine control panel or that control individual motors.
That being said, it is a common circuit and while the NEC does not directly permit it, section 430.74 is often cited to permit the overload relay contact to be in the grounded conductor of the control circuit.
430.74 Electrical Arrangement of Control Circuits. Where one conductor of the motor control circuit is grounded, the motor control circuit shall be arranged so that a ground fault 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.
 
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