overload contact in circuit

[jcraft]

Just wondering if there are any negatives or positives to placing the overload contacts of a standard motor control circuit on the line side of the starter coil instead of the nuetral side. On schematics I have always seen the overload contact on the nuetral side of the coil on a 120 volt control circuit. Customer wants the contacts on the line side of the coils because they had a coil short to ground once and the contact never opened the circuit and the control fuse never blew. Just your thoughts on this. Thanks

 

[LionelHutz]

There's no reason you can't.

The overload contact is always that one contact that breaks the "never switch the neutral" rule, isn't it.

I've had customers complain about this too.

 

[Skogsgurra]

Putting a contact in the neutral is BAD PRACTICE. See a lot of that in US schematics - but it still is BAD PRACTICE.

Gunnar Englund

http://www.gke.org

 

[jraef]

As I was taught way back when, the OL contact is placed on the right side of the coil so as to never be confused in the logic circuit with anything else, and so that in reversing or 2 speed starters it is guaranteed to be in the circuit regardless of which contactor is operating even though there is only one aux. contact on the OL relay. For example, in this circuit.

Keep in mind also that up until recently, it was very common in N. Am. for control circuits to be at line voltage potential, so there was no "neutral" anyway (also indicated on that drawing).

Don’t bother trying to correct my thinking, I have already changed my habit. I know it can be done exactly the same way with the OL contact on the left side of the coil, but this is just the convention that was developed early on in N. America and adopted by the (now defunct) Joint Industries Council (JIC) as the standard method of connecting them. NEMA adopted that as the factory wiring on starters so that they were all the same, regardless of brand. The practice has lingered on in spite of the very valid arguments against it fomented by IEC users when they entered the North American marketplace. The main reason is that electricians are still taught this scheme in trouble shooting training to this day, so many engineers continue on with the practice knowing that to change means educating the field force, something that is happening slowly.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

Interesting. I knew there was a reason for that bad habit - but I didn't know what the reason was.

Having said that, I still see no reason why it shouldn't be put on the live side of the coil. Were contactors once built so that it would have been impractical?

Gunnar Englund

http://www.gke.org

 

[dpc]

The NEMA standard (ICS 2) is to place the overload contact on the neutral side. The concern is the problem of welding the OL relay contacts when the control circuit is faulted. This is **much** less likely to occur if the OL contact is on the neutral side of the coil. The welded contact is generally not detected until the overload operates and the motor keeps running - usually with a bad outcome. For reversing or other multiple contactor starters, a single OL contact must be placed either in the neutral side or at the top of the line side. When at the line side, they are exposed to nearly any type of short circuit in the control circuit.

So there is a valid engineering reason why this is done and NEMA has explained this in their standards, even if you don't totally agree with it. It is also not an NEC violation as is sometimes charged. If the contact were **remote** from the starter cubicle, then it would be an NEC violation to put it on the neutral side.

I think that there are problems with either location. In the US, you can get the OL contact on the line side of the coil if you request it. If you don't specify, it will be provided on the neutral side, per NEMA recommendations.

http://www.nema.org/stds/ics2.cfm#download

You can download a copy of ICS 2 at link above. The explanation is at the bottom of page 2-10, or thereabouts.

 

[Skogsgurra]

My reasoning is this: All contacts are important. Very often, a contact that stops motion (safety stop or some other contact) is a lot more important than the overload contact. If malfunctioning, the latter could probably cause the motor to burn while the former could cause a serious accident.

So the control contacts are - in my view - at least as important as the overload contacts. Would that mean that all contacts should be in the neutral?

Gunnar Englund

http://www.gke.org

 

[jraef]

Honestly Gunnar, I don't think so. While the curent design of NEMA starters does make it difficlt to change, I would think that form followed function.

That made me think that another possible issue would be that as a pre-assembled package, a "starter" could only have been ensured to have the OL contact correctly wired in the circuit if the manufacturer did it themselves (remember, the US is much more litigious than other countries). It had to be done on one side or the other, and could have been either side. Why someone orginally chose the left side may be lost to history or be as simplistic as what I said above. That has been done that way for so long it is doubtfull that anyone who knew is still working in the field.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[jraef]

Woops, wish I had refreshed that page prior to sending that last post! Did I just accidentally infer that dpc is old? Sorry...

Good response though.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

The control contacts in overload relays were never particularly robust. I'm sure this had something to do with the NEMA standard. Maybe newer OL relays are better.

I think NEMA went to the trouble of having a separate paragraph to explain this because they agree it is *usually* best not to be switching on the neutral side. Most remote contacts are open/closed more frequently than the OL contacts, so contact welding would be more likely to be detected. Also, in many cases, the length of control circuit wiring to field devices would greatly reduce the short circuit current.

I worked (years ago) for a large consulting firm that always specified the overload relay contacts to be on the line side, and it was never a problem to get them built this way. But that was before the time that MCCs were UL-labeled. I don't know if the MCC could be UL-labeled if the OL contact wiring was not per NEMA standards, because I suspect that is the way all the testing was done. Maybe someone else knows the answer to that.

But my main point was that this has always been somewhat controversial and that NEMA has a stated engineering reason for their standard. I suspect the wording in NEC Article 430 was carefully crafted specifically to allow this practice.