Overload wiring question

[tjacobs]

when wiring a stop start motor control circuit do the overload contacts belong on the hot or the neutral side of the coil ?
Please explain why.

TIA

 

[jim dungar]

Is this a question on a test? What are your thoughts

There are several considerations for their placement, including:
Fault handling ability
National and industry conventions
Control circuit design involving multiple coils

 

[kwired]

General rules are that all switching be done in the ungrounded conductor(s) yet is very common to see overload contacts in the grounded conductor in control circuits. I don't know why. Overload contact is very commonly placed in the "L2" side of the circuit whether it is a grounded conductor or not though and all the normal controls are all in the "L1" side of the circuit.

 

[jim dungar]

General rules are that all switching be done in the ungrounded conductor(s) yet is very common to see overload contacts in the grounded conductor in control circuits. I don't know why. Overload contact is very commonly placed in the "L2" side of the circuit whether it is a grounded conductor or not though and all the normal controls are all in the "L1" side of the circuit.

Think about decades of traditional industry practices in North America and Europe. Has one method proven to be more problematic than the other?

 

[tjacobs]

General rules are that all switching be done in the ungrounded conductor(s) yet is very common to see overload contacts in the grounded conductor in control circuits. I don't know why. Overload contact is very commonly placed in the "L2" side of the circuit whether it is a grounded conductor or not though and all the normal controls are all in the "L1" side of the circuit.

I was told by an old timer a long time ago that it had something to do with a situation where there was a loss of power while running.
When power is restored you could have the motor automatically start. Can't remember what the scenario was.
.

 

[Jraef]

I was forced to learn this issue because after working for a steel mill where everything was done with NEMA starters, I took a job with a German control mfr (Klockner-Moeller) where everything was IEC. NEMA starters had the OL on the “neutral” side of the coil (A2 on IEC coils, X2 on NEMA coils at that time), IEC standards had it on the “hot” side (A1 or X1). When I started drawing up circuits with the OL contact on the A2 side of the coils, I was admonished for doing so because of the IEC standards. At the time, I was told it was “dangerous” because of the possibility of the A2 wire possibly going to ground, resulting in the coil not dropping out if there was an overload condition. But having done it the NEMA way for years and never seeing that happen, I questioned that philosophy. My boss, being an Englishman, was at least fair enough to tell me to research it and explain to him why Americans did it differently than everyone else in the world.

First off you have to understand that the concept of it being on the “neutral” side is ONLY valid if you are using 120V coils. In early days of motor starter controls, using a line voltage rated coil was quite common because it was cheaper and simpler. So a starter on a 480V circuit used a 480V coil and in that case, because NEITHER side was grounded and thus not “neutral”, the placement of the OL contacts were irrelevant. Most of the starters I wired at the steel mill used 480V coils and we wired 480V up to the push buttons and pilot lights on the doors. That’s just the way it was done back then.

2nd, notice that I said “contacts”, plural. In older designs (prior to the 1980s or so), NEMA motor starters had separate OL relays for each phase, with each OL relay having its own separate trip contact. So to be able to drop out the coil, they had to be wired in series. You still see vestiges of that in some wiring diagrams where they will show 3 NC contacts in series for the OL in a motor starter. There actually were 3 separate devices back in the 70s. That would have represented a lot of extra work for the installing electrician, so the starter mfrs pre-wired that side of the circuit for you and it was simpler to make that common wiring on the X2 side of the coil so that it didn’t matter if the coil was 480V, 240V or 120V, they were always wired the same way. Indirectly that resulted in the contacts being on the neutral side if it was a 120V control system, but that was a result of the convenience, not a direct purposeful decision.

Then because that wiring was actually done at the factory and part of the quality test process, the likelihood of one of those factory wires going to ground was so exceedingly low that it was a non-issue.

Even though NEMA has over the years adopted several other IEC standards (1 OL relay contact on a relay that senses all three phases, A1/A2 ,95/96 terminal designations etc.), the fact that the OL relay contact is still on the A2 side of the coil on NEMA starters made for North America is now just a “conventional” concept. There is no prohibition from doing it the other way, but doing so may cause you to get some flack from old timers.

 

[don_resqcapt19]

The only issue I see with the overload relay contact on the grounded side of the control power circuit, would be the rare case where the conductor between the overload relay and the contactor coil was extended beyond the enclosure that contains the starter. I have never seen that done, but one brand of MCCs has that conductor run out to the field wiring terminals in their MCCs.

One argument I have seen for putting it on the grounded conductor side is that where it is on the hot side and the control circuit extends to various controls, there is the possibility of a ground fault "welding" the overload relay contacts closed.

 

[texie]

I was told by an old timer a long time ago that it had something to do with a situation where there was a loss of power while running.
When power is restored you could have the motor automatically start. Can't remember what the scenario was.
.

You are referring to a 3 wire start/stop which will not restart automatically after a power interruption. This is unrelated to the OP question.

 

[texie]

As discussed, in the NEMA world the factory OLs are in the grounded conductor when 120 coil is used. But this side of the circuit can't be extended beyond the controller enclosure due to the possibility of a ground fault energizing the coil. This has been in the NEC for decades. See 430.74.

 

[kwired]

Think about decades of traditional industry practices in North America and Europe. Has one method proven to be more problematic than the other?

I don't see it being a problem functionality wise, just seems odd that in general we not supposed to switch grounded conductor yet we do so with these motor overload contacts quite often.

If I am wiring up my own motor controller arrangement (particularly with 120 control volts) I will hit the motor overload contact then run to the coil, and put the grounded conductor on other side of coil. If using say a NEMA starter that is already prewired with a lead to overload and lead from coil to "3" on the holding contact - I usually run grounded conductor to the overload, just because.

 

[kwired]

As discussed, in the NEMA world the factory OLs are in the grounded conductor when 120 coil is used. But this side of the circuit can't be extended beyond the controller enclosure due to the possibility of a ground fault energizing the coil. This has been in the NEC for decades. See 430.74.

A ground fault won't energize the coil, it can bypass the overload protection though.

 

[texie]

A ground fault won't energize the coil, it can bypass the overload protection though.

It could depending on the control circuit setup.

 

[kwired]

It could depending on the control circuit setup.

If standard three wire control with holding contact, it shouldn't.

If start switch or holding contact stuck closed for any reason contactor is pulling in anyway unless overload contact opens. Then any bypass to ground will still allow to pull in but you also have overload bypass situation which is bad even if no other problems with start/holding contact.

 

[texie]

If standard three wire control with holding contact, it shouldn't.

If start switch or holding contact stuck closed for any reason contactor is pulling in anyway unless overload contact opens. Then any bypass to ground will still allow to pull in but you also have overload bypass situation which is bad even if no other problems with start/holding contact.

Who said I was speaking of only 3 wire control. Why do you always state the obvious?

 

[ptonsparky]

Who said I was speaking of only 3 wire control. Why do you always state the obvious?

Because there are many that read these threads, don't post and do not have the experience or education of others here.
Obvious is subjective.

 

[kwired]

Who said I was speaking of only 3 wire control. Why do you always state the obvious?

I thought we were talking about a three wire control, maybe we weren't?

 

[garbo]

when wiring a stop start motor control circuit do the overload contacts belong on the hot or the neutral side of the coil ?
Please explain why.

If the starter coil is 120 Volts AC then the overload block is wired to the grounded conductor ( yep neutral ). I for one questioned this to my shop teacher over 50 years ago for not being the safest way. If for some reason the other end of the overload block was to be grounded you would loose critical motor overload . More then once I have seen somebody disconnected the neutral side of motor starter and run it out to limit switches.

 

[kwired]

If you had a 240 volt coil and still had the overload contact in the L2 side but all other controls in the L1 side, which is pretty common, you could still have unexpected coil pull in if there is a ground fault between coil and the overload contact (and overload is open). The coil would only be seeing 120 volts, and is subject to overheating because of the low volts but if enough to cause pull in still can start the controlled load. Larger the contactor the less likely it can pull it in as well though.

 

[Jraef]

I thought we were talking about a three wire control, maybe we weren't?

Me too. The OPs post infers 3 wire control by stating “start stop control circuit”, which I interpreted as being a start button and a stop button, the definition of a 3 wire control circuit.

 

[garbo]

If you had a 240 volt coil and still had the overload contact in the L2 side but all other controls in the L1 side, which is pretty common, you could still have unexpected coil pull in if there is a ground fault between coil and the overload contact (and overload is open). The coil would only be seeing 120 volts, and is subject to overheating because of the low volts but if enough to cause pull in still can start the controlled load. Larger the contactor the less likely it can pull it in as well though.

I believe that only having halve of the coil voltage would not produce any where near the magnetism required to pull in a starter. Years ago think it was Cadillac best of the best Allen Bradley had it in their thick product catalogue the minimum required voltage percentage to what they call to seal the starter contacts. Had to be over 85%. Have to be carefull when selecting the minimum size controll transformer when installing several motor starters especially if they are all capable of starting at the same time. Some people only used the sealed current ( maybe 13 to 18 watts for a NEMA size 1 starter ) and not the current it takes to seal contacts. Some Danfoss FC model early drives with the 3 starter bypass function had problems because of slightly undersized control transformer. They placed a 5 to 10 second time on delay timer so only the M1 start contactor that feed the drive would energize then 5 to 10 seconds later the M2 drive output contactor would pull in. Learned so much from drive tech that performed start ups. Would always have a question or two for them and learned more about drives then a few drive classes I attended.