Motor Overload Protection

[Frank Bowick]

Overload contacts are always connected in the grounded control circuit conductor? Why aren't they in the ungrounded side like any other control?

 

[Strahan]

Overload contacts are always connected in the grounded control circuit conductor? Why aren't they in the ungrounded side like any other control?

First I hope you are talking about the auxiliary contacts on the overload block which I'm sure you are.

Next I personally prefer running my ungrounded conductors through this auxiliary. Aux contact would be after control circuit and in series with the motor starter. I'mnot sure of any codes determining which way you accomplish this but you are right most installs run the grounded conductor through the overload aux.

 

[don_resqcapt19]

One reason I have read is that the contacts are located in the grounded conductor side to minimize the possibility of damage from fault currents. These are normally closed contacts that rarely open and if fault current would weld them closed no one would know until there was an overload and the contact could not open to shut down the motor. This was in a NEMA document...NEMA ICS-2 Sept. 1978

 

[jim dungar]

It is very hard to build interlocked contactors, like reversing ones, if the overload relay contact is placed on the ungrounded side.

I know you can't put a 1-pole overcurrent device in the grounded conductor, but where does it say you can not switch it? Control devices do not have to open all of the ungrounded conductors.

From a risk stand point it depends on if the overload relay contact is an internal piece of the controller or if it is remote mounted.

 

[don_resqcapt19]

Jim,
404.2(B) is often cited to support not having the overload relay contact open the grounded side conductor. I tend to agree that a relay is a switch and this rule would apply if I built a control system from component parts, but I don't agree that it applies to the internal wiring of a listed motor controller.

If you want to run the starter coil grounded conductor to a device external the starter enclosure, then 430.74 would prohibit the overload relay contact in the grounded conductor. This would only apply if you were extending the conductor on the load side of the overload relay contact, not a common installation, but I have seen it a few times.

 

[jim dungar]

404.2(B) is often cited to support not having the overload relay contact open the grounded side conductor. I tend to agree that a relay is a switch and this rule would apply if I built a control system from component parts, but I don't agree that it applies to the internal wiring of a listed motor controller.

I have a hard time saying overload relay contacts switches are subject to Article 404 as they do not meet any of the Article 100 definitions of switches nor 404.3 or 404.15.

430.74 requires remote control circuits to be configured so that a ground does not bypass safety circuits. Most, if not all, overload relay contacts are not rated to handle the short circuit currents that may occur on the ungrounded side of a control circuit.

 

[don_resqcapt19]

Jim,

404.1 Scope.
The provisions of this article shall apply to all switches, switching devices, and circuit breakers where used as switches.

In my first post I should have used the term "switching device" and not switch. It is my opinion that a relay contact is a "switching device".
As far as 430.74, it does not use the term "remote control circuit". It says you can't install a control circuit "remote from the the controller" if a ground fault on that conductor would bypass an automatic safety shut down device. The overload relay is such a device. If you extend the conductor that is run between the overload relay contact and the contactor coil, a ground fault on that conductor would prevent the overload relay from doing its job. Like I said before, this would not be a common circuit. In most cases where you are extending the control circuit X2 (grounded conductor) you are extending it from the line side of the overload relay and not the load side. An extension from the line side is not a violation of this section.

 

[jim dungar]

In my first post I should have used the term "switching device" and not switch. It is my opinion that a relay contact is a "switching device".

Your opinion versus my opinion. I contend that an overload relay contact, integral to a starter, does not fit under any of the sections of article 404.

As far as 430.74, it does not use the term "remote control circuit". It says you can't install a control circuit "remote from the the controller" if a ground fault on that conductor would bypass an automatic safety shut down device. The overload relay is such a device. If you extend the conductor that is run between the overload relay contact and the contactor coil, a ground fault on that conductor would prevent the overload relay from doing its job. Like I said before, this would not be a common circuit.

Actually, it is not uncommon to see panels with the overload relay mounted remote from the contactor (i.e. a separately mounted relay after a VFD). I have even seen multi-section panels where all of the OLR where mounted on a single rail.

If the overload relay contact is located on the ungrounded side of the coil it needs to be protected against short circuit currents. A welded contact would not be readily noticed during normal operation.

 

[don_resqcapt19]

Your opinion versus my opinion. I contend that an overload relay contact, integral to a starter, does not fit under any of the sections of article 404.

Which is what I said in my first post.
[/quote]Actually, it is not uncommon to see panels with the overload relay mounted remote from the contactor (i.e. a separately mounted relay after a VFD). I have even seen multi-section panels where all of the OLR where mounted on a single rail.[/quote]
If the overload relay is installed remotely from the motor controller, then it can't switch the grounded conductor without being in violation of 430.74. The word remotely is subjective, but I read it as meaning that the overload relay is not in the same enclosure as the motor controller, or any case where the conductor on the load side of the overload relay contact is extended beyond the enclosure that contains the overload relay.

If the overload relay contact is located on the ungrounded side of the coil it needs to be protected against short circuit currents. A welded contact would not be readily noticed during normal operation.

Yes, and the NEMA document that I cited says this is the very reason that the overload relay contact is normally connected to the grounded conductor of the control circuit. It is very similar to the reason behind the rule in 430.74. An accidental ground on the conductor between the overload and the motor controller would not be noticed under normal operation, but would prevent the overload relay from doing its job.

 

[Smart $]

...An extension from the line side is not a violation of this section.

If both the extension and the overload relay are on the grounded side of the contactor coil, it is a violation. Doesn't matter whether it is on the line or the load side of the OLR.

Any fault to ground on the remote wiring cannot cause the coil to maintain or become energized.

Remote wiring and subsequent devices always have to be on the ungrounded side of the contactor coil, regardless of the location of the OLR in the circuit.

 

[petersonra]

Overload contacts are always connected in the grounded control circuit conductor? Why aren't they in the ungrounded side like any other control?

They can be on either side. It is one of the few (maybe the only?) places the code allows you to break on the neutral side.

 

[don_resqcapt19]

If both the extension and the overload relay are on the grounded side of the contactor coil, it is a violation. Doesn't matter whether it is on the line or the load side of the OLR.

Any fault to ground on the remote wiring cannot cause the coil to maintain or become energized.

Remote wiring and subsequent devices always have to be on the ungrounded side of the contactor coil, regardless of the location of the OLR in the circuit.

Smart,
My post is about extending the conductor that runs between the coil and the overload relay contact. The extension of that one would be a code violation, however there is no issue with running the grounded conductor from the other side of the overload relay contact. This is done any time you have a pilot light at the start stop station. A ground fault on this conductor does not bypass the overload relay.

 

[Smart $]

Smart,
My post is about extending the conductor that runs between the coil and the overload relay contact. The extension of that one would be a code violation, however there is no issue with running the grounded conductor from the other side of the overload relay contact. This is done any time you have a pilot light at the start stop station. A ground fault on this conductor does not bypass the overload relay.

That is correct. A remote control circuit with device current paths parallel to the coil (and any device in series with it on the grounded side of the coil) are not a violation.