Overload NC contacts, best place to land them. 2

[bklauba]

As requested, this is a new thread, taking up the subject of: For purposes of both safety and ease of diagnosis, where is the best place to put NC contacts that deny activation of a motor contactors: 'Tween the contacts and neutral or 'tween contacts and hot feed to coil?

Below is the exchange that led to this issue, and my apologies for changing the subject on "fused neutral" thread.

jistre (Mechanical) 26 Mar 08 14:02
Heck, I only took Volts for Dolts in college, and I know enough to say "YIKES!" on this one. The first maintenance guy who assumes that the equipment is de-energized because the breaker's open is going to be very shocked to find out he's wrong.


bklauba (Industrial) 26 Mar 08 14:32
. . . . which is why putting a NC Overload contact 'tween a contactor coil and neutral sure looks like tortured logic. (Sorry if you guys think this is a subject change, but, not really.) Putting it "in front" of the coil, so the coil gets NO JUICE when the NC Overload contact is open seems just so right!

BK

 

[bklauba]

The logic in the immediately preceding thread also applies to my question:

"However, from dealing with maintenance and plant workers in general, I think you have to assume that the majority of them don't know the intricacies of breaker positioning and will always assume that a breaker at the panel is a way to de-energize the panel. If they have to walk across the room to pull the bus breaker or they can just flip the panel breaker, there are some workers who will always choose to flip the panel breaker no matter how much you've trained them that it doesn't safe the equipment. Just from a human factors viewpoint, I'd say that a single pole neutral breaker living by itself on a piece of equipment is always dangerous. To those who don't know better, there is the impression that it safes the equipment."

Likewise, a maintenance bloke is told that a motor is not running. He opens a panel (I am not going to get into an arguement as to whether he is qualified; the reality is too many of these 200 - 480 VAC MS's are LEFT OPEN), places one meter lead on ground/neutral and the other on A1 or a terminal leading to it. If he sees the control voltage, but the contactor is not engaged, what does he think? That the neutral connection to the contactor coil is open? Or (I believe more likely) that the contactor itself is bad (open coil or jammed/stuck in OFF position).

What has been the experience of the eng-tips community?

BK

 

[dpc]

bklauba,

Please confine yourself to one question per thread.

NEMA standards call for the motor overload relay contact to be on the neutral side of the starter coil for normal full-voltage starters. It can be changed by special order.

The main reason for putting the overload contact on the neutral side is to lessen the risk of the contacts being subjected to short circuit current if any of the field devices on the line side of the coil go to ground. Historically, the overload relay contacts did not have a very high fault current rating or contact make/break rating and were prone to welding shut at times. This arrangement is also a little easier to wire.

Many engineering firms routinely require that the overload contact be moved to the line side in all starters. But I believe the NEMA standard is still to put them on the neutral side.

The attached file is a (very old) copy of a page from NEMA ICS-2 that explains the logic.

 

[jraef]

Best response I've heard on that subject dpc. I have worked at two places where I had to make this argument in the face of their long established engineering practices. Unfortunately I didn't realize that NEMA ICS-2 provided such a good explanation, although I also didn't have access to it anyway at that time.

One was a European equipment manufacturer who hired me to work in a US engineering office. They ALWAYS put the OL aux on the "hot" side. The other was a major West Coast oil company I worked for who took it so far as to instruct their field technicians to rewire any NEMA starters that came in with equipment in the field!

In both cases, their argument was something like "What happens if the control circuit goes to ground on the neutral side? Nothing happens and if the motor overloads, the contactor won't drop out". I pointed out (fruitlessly I might add) that this scenario only applies to a grounding point AHEAD of the aux. contact. Both companies were recalcitrant, although I recently did a project for that same oil company and their drawings showed the standard NEMA design now, so at some point within the last 20 years someone saw the light.

 

[waross]

I believe that there is an historical precedent also.
Back before most of us were born, many starter coils operated at line voltage. The starter was factory wired from the coil to the O/L contacts to L2. In the smaller sizes the only protection on the control circuit was the motor circuit protection.
This was in the days of the ungrounded delta so grounding was not an issue and everything was hot.
With the background of ungrounded delta power, when the old plants started using control circuit transformers, they were often ungrounded. If they were fused, both lines would be fused. (Hell to trouble shoot, every terminal in the control circuit would read 55 volts to ground, either directly or by back feed)
The 1977 NEMA ICS-2 standard is interesting.
My 1970 Standard has the same diagram, but

Authorized Engineering Information, 1-5-1977,
ICS 2-321.62 LOCATION OF OVERLOAD RELAY CONTACTS IN CIRCUIT

Was not yet a part of the standard.

When the control circuit is fed from a grounded source, the grounded line is never fused.
The NEMA scheme for the O/L contacts is prewired and does not have to be touched by the installer. I am sure that this has avoided tens of thousands of mis-wired O/L contacts over the years.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Laplacian]

My experience is to connect O/L contacts on the hot side of the coil because that is where all the other interlocks are located (stop pushbutton, vibration, temperature, and associated process trips, etc.).

If the neutral side was "the" way to de-energize the coil, then why aren't these other interlocks also breaking the neutral?

 

[dpc]

Not switching the neutral side of a relay coil is generally a good rule of thumb to follow. The NEMA standard for motor starters is an exception to that practice for reasons explained in the attachment to my first response.

It's been this way for a very long time and the vast majority of motor starters in the US are wired this way.

Bill - I think you're right about some of historical reasons behind this design.

Much of this has to do with convention and it is generally good to provide what the plant electricians are used to whenever possible.

 

[oldfieldguy]

In my experience I cannot remember ever seeing the overload contacts on the 'hot' side of the coil on a 480-volt motor starter.

I do see them there frequently in the control circuits of medium voltage contactors, but generally they're actually in the circuit of a master relay rather than in the circuit of the contactor coil itself.

Dealing with equipment of various vintages and manufacturers, then throwing in the innovative ideas of various end users, you're liable to see just about anything, though.

But on factory-made low voltage gear, I expect to find the overload device on the neutral side of the contactor coil.

old field guy

 

[rbulsara]

I think, NEMA way is in the USA for whatever reason, probably more historical than technical. People not having worked outside of USA may think it is the best way but I am not fully convinced of any benefit of OL contact being on neutral side of a coil. It may have been an way to hide poor OL relay contact design in the past. Also there is no saying that SCCR of other control contacts are any better.

Personally, I prefer the IEC way of keeping all switching contacts on the hot side. As for short circuit currents, control circuits should be proprely fused to open quickly on faults. IEC also makes troubleshooting of the control circuit (by checking presence or lack of control voltage at various point)easy and minimizes inadvertent energization of the coil while troubleshooting.

Having said that I have not made any deliberate attempt to change a control circuit, as long as it is the accepted practice where it is applied.

 

[dpc]

The problem with the overload contact is that it is a normally closed contact that may go for years (decades?) without opening. Any damage to the contacts will not be noticeable until it fails to shutdown the motor when it should.

 

[jraef]

The problem with the overload contact is that it is a normally closed contact that may go for years (decades?) without opening. Any damage to the contacts will not be noticeable until it fails to shutdown the motor when it should.

Makes sense, and it is only in recent years, perhaps as an influence of IEC, that OL relays have a Test function for that very reason. I know that older NEMA OLRs did not have a Test button on them until the '80s. Some still don't.

 

[ScottyUK]

Maybe it is a good solution to a non-problem, I honestly don't know. But having been on a site for nearly ten years with many hundreds of motors of all sizes and duties I've never seen a problem of the type this idea is supposed to address.

How often do NEMA starters experience a problem with overload relays failing short circuit? We very occasionally see one fail open circuit but that is largely down to the aggressive atmospheric conditions at our site, and I wish it were confined only to overload relays where in fact almost any non-hermetic switching device seems to suffer at some point.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty;
I have some very old text books.
I think that the reasons are historical and safety. The Authorized Engineering Information Re; short circuit currents and welded contacts was added to the NEMA standards in 1977. (Not in the 1970 standards). The location probably dates back 40 or 50 years before that.
The overloads come prewired, and the wiring does not have to be disturbed to connect the control circuit. Many of the old starters had the overload wiring routed in such a way as to make changes difficult.
The circuit works equally well for reversing starters.
I have seen (and you probably have also) almost every imaginable wrong connection on motor starters. It is very rare to see an overload circuit that has been rewired wrong.
And before you EU folks congratulate yourselves on superior schemes, we had an issue with back feds through the alarm contacts of about 40 IEC O/L relays. The problem was solved by rewiring all the starters to NEMA standards.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

This is likely one of those "no-winners" discussions, such as with "Red = Unsafe / Running, Green = Safe / Stopped" for pilot lights. Not even a general consensus is going to convince those who have been doing it their way forever to change.

"Can't we all just get along?" _{Rodney King, 1995}

 

[ScottyUK]

Backfeeds through alarm contacts? Normally they're volt free across here so there might be a voltage present from an external source but I can't see how wiring mods would alter that.

jraef seems to have the likely conclusion to this discussion.

And on the red / green lights question... our American-built plant uses both standards and has from Day One. We haven't changed it because people have adapted to the madness and changing it now would bring a new set of risks.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

I agree with jraef also. It is good to know different points of view and we have done that.
Scotty, the contacts were single throw double throw. The motors ran in banks of three. I can't remember the exact circuit, but the problem came when one motor tripped on overload and then the bank shut down on the defrost timer. We got a back feed from the pilot light circuit of the tripped unit to the coil circuits of the other two motors. The coils would pull in, but as soon as they did it interrupted the back feed and they dropped out. It sounded like small machine guns and you could see the red glow inside the relays as the heaters became red hot.

Bill
--------------------
"Why not the best?"
Jimmy Carter