NEMA versus IEC Motor Starters/Contactors

Status
Not open for further replies.
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Last week I attended a dog and pony show sponsored by Schneider Electric, (SquareD) with the advertised topic of intelligent motor control centers. Without all the boring details, it was not what I thought it was going to be. However, it did give me the opportunity to ask them about the push using IEC contactors in starters.

The following is my interpretation/paraphrase of the conversation:

Q1: It seems that IEC starters are showing up more frequently as new equipment and as replacements. Since the methods of calculating the short circuit currents is different for IEEE and IEC devices, how are you rating the IEC equipment?

A1: We agree that IEEE and IEC calculation methods are different and therefore the ratings do not mean the same thing, because the short circuit values mean different things.

Q2: Then how are IEC devices being provided in lieu of NEMA devices, or as replacements, especially since there is no cross reference between the two?

A2: IEC devices come in many more sizes then the NEMA sizes and therefore you will end up with an IEC starter that is larger, maybe two sizes, then what you would get with a NEMA starter.

Q3: Even if the starter is two sizes larger, how do you know it has the short circuit rating capable of handling the IEEE calculated current?

A3: We have performed factory tests to see if they will carry the fault current, but we do not go so far as to provide dual rating nameplates. But our technical folks can help with sizing and providing an appropriate IEC size.

Q4: So in essence there is a false sense of security being provided that it won't blow up if a fault occurs. So unless someone performs the IEC 60909 calculations you don't know what the proper rating is.

A4: If your replacing NEMA starters/contactors then you should stay with NEMA devices, and shouldn't mix them. Unfortunately, because the market is so competitive, if the specification does not preclude IEC devices, then that is what we will quote, because they cost less.

All I can say, is buyer beware. If the device you are replacing is a NEMA device, then that is what it should be replaced with, unless someone has performed the appropriate IEC 60909 calculations. Anyone your talking too, ask them if they are aware of the differences in calculation methods. If they give you a blank stare, or can't specifically tell you what or why it is different, then replace according to the method used for determining the ratings. Saving a few bucks using IEC devices instead of NEMA could unknowingly be putting someones safety at risk. I don't want that on my conscience, and I'm sure none of you do either.
 
I agree with Jim. There are millions, probably billions, of IEC contactors in service. I have not heard that they are blowing up in any numbers.

They work very well for the most common applications, especially in smaller sizes.

There are some things you want to watch out for. One that sometimes gets people is that it is more common to use class 10 overloads in an IEC starter, while most NEMA starters have class 20 overloads installed.
 
Great, you both agree, that's fantastic! Unfortunately, two wrongs don't make a right.

Devices applied to their UL listed rating................should mean that if it is an IEC device rated for 10kA, then it meets the IEC 60909 short circuit calculated currents. If you are using an IEC device in a NEMA rated MCC, and the short circuit calculations are per IEEE, then how can you possibly say you are using the device per the UL listing?

Curiosity; How many have a copy of, or have done IEC 60909 calculations, and understand the differences in the SCCR's?
 
I asked a simple question. According to the link I posted, Square D has a listing of UL approved combinations of devices.

kingpb said:
If you are using an IEC device in a NEMA rated MCC, and the short circuit calculations are per IEEE, then how can you possibly say you are using the device per the UL listing?
Click to expand...

If UL has tested the IEC device in an MCC, I am pretty confident I can say I am using per the UL listing. Are you saying that UL tests are not based on IEEE methods?

Doesn't UL test both IEC and NEMA motor control components using the same standards/procedures?
 
kingpb said:
Great, you both agree, that's fantastic! Unfortunately, two wrongs don't make a right.

Devices applied to their UL listed rating................should mean that if it is an IEC device rated for 10kA, then it meets the IEC 60909 short circuit calculated currents. If you are using an IEC device in a NEMA rated MCC, and the short circuit calculations are per IEEE, then how can you possibly say you are using the device per the UL listing?

Curiosity; How many have a copy of, or have done IEC 60909 calculations, and understand the differences in the SCCR's?
Click to expand...

First off, I would not be inclined to replace an existing NEMA motor starter in an MCC with an IEC starter.

UL lists IEC starters with certain combinations of fuses and breakers you can use. Thats what I use with them.

Here is a typical chart of UL approved combinations.

http://raise.rockwellautomation.com/RAConfig/sendsuppl.asp?CID=B34C9A7254214B75A4E5D1722A495954 &PIID=RDCPDL.$M$:\PFM\223c&tar=F$SCCR&pg=0
If it is good enough for UL, it is good enough for me.
 
I think it would be helpful to illustrate the difference in SCC calculations by use of an example. I know I have a representative one-line already modeled in ETAP that I can run using both mehtods. It will take a little clean-up but I will get to it post haste.

Bob, if i'm understanding you correctly, it sounds like you are actually using fuses as the protection ahead of the contactor and in essence doing a series rated combination. The link does not work, but is that the approach Allen bradley is taking?
 
kingpb said:
I think it would be helpful to illustrate the difference in SCC calculations by use of an example. I know I have a representative one-line already modeled in ETAP that I can run using both mehtods. It will take a little clean-up but I will get to it post haste.

Bob, if i'm understanding you correctly, it sounds like you are actually using fuses as the protection ahead of the contactor and in essence doing a series rated combination. The link does not work, but is that the approach Allen bradley is taking?
Click to expand...

AFAIK, they all do.

I like class CC fuses becasue they are cheap and small. You can use class J and some others as well. You can also use some circuit breakers I think and MCPs. I will see if I can get a working link.
 
kingpb said:
I think it would be helpful to illustrate the difference in SCC calculations by use of an example.
Click to expand...

I understand there are differences in the calculations. But isn't it possible for a device to have two sets of SC ratings? Doesn't a UL listing mean the device has been tested using the IEEE method? And as far as series-combination ratings go, even NEMA devices require them for fault currents above 5kA for sizes 00-3.
 
The issue I see with the UL tests is that someone (manufacturers) in their infinite wisdom have basically thrown out the standards, and contrived testing such that if the assembly can pass a test, then it is ok for some limited test current as long as you are using specifc type of fuses. (Breakers, MCPs, and some types of fuses won't work). Then throw in the fact; is it Type 1 (damaged beyond repair after a fault) or Type 2.

Conversely, if you performed the proper IEC calculations then selected the proper device, you could use breakers, MCP's, and multiple fuse types. Whereas doing the IEEE calcs, selecting the proper NEMA device also enables you to use breakers, MCP's and fuses, and provides you with same in service capabilities as Type 2.

I would be curious to know that for the "billion's" of IEC starters/contactors installed, how many of them are using the series rated combo approved by the UL gods/goddess' (try'in to be PC). No breakers, no MCP's and all the fuses are selected correctly. Better yet, how many TCC curves exist showing proper protection of the contactor, motor, and cable exist. From my humble experiences, I have seen very, very few protection curves done at the MCC level. Hence, my concern that plant electricians and field techs are being lead to believe that all is well, when in fact it is not.

IMO, projects are either IEEE/ANSI/NEMA/NEC etc. or they are International standards, and never the two shall meet.
 
kingpb said:
The issue I see with the UL tests is that someone (manufacturers) in their infinite wisdom have basically thrown out the standards, and contrived testing such that if the assembly can pass a test...
Click to expand...

This is an interesting comment. I was under the impression that UL designed their own test procedures (with a consensus from manufacturers and industry standards). This way all devices can be tested identically. So if you have a UL508E device it has under gone the same testing regardless if it is a NEMA or an IEC design.
 
I don't personally care if the starter is damaged after a short circuit, as long as the fault is safely cleared. That is one of the differences between using NEMA and IEC starters. The kind of fault that would damage an IEC starter is pretty rare.

If the starter surviving a ground fault is one of your criteria, then use NEMA starters. But keep in mind that the majority of motor starters are used on motors with smaller gauge wire. It is not all that easy to get enough current with smaller wires like that to make an IEC starter fail, especially if the fault is at the motor end.

#14 has about .25 Ohms/100 feet. If the motor is 100 feet away on a 480V system, you can only get about 2000A of fault current. #12 is about 0.16 Ohms/100 feet. You still only get 3000A of fault current.
 
jim dungar said:
This is an interesting comment. I was under the impression that UL designed their own test procedures (with a consensus from manufacturers and industry standards). This way all devices can be tested identically. So if you have a UL508E device it has under gone the same testing regardless if it is a NEMA or an IEC design.
Click to expand...

This is correct. UL is a "third party" testing authority. The tests done on components are done per UL's testing specifications, not the manufacturers or IEC. SCCR ratings are in specific combinations, but generally if a starter is made by a manufacturer who makes breakers, they list the starters with their breakers, not with a competitors. UL would require a destructive test with EVERY specific brand and type of breaker to do that.


Here's my take on IEC vs NEMA.
IEC designs came to be in the EU world after WWII when engineering was a trade, not a profession. So there were engineers at every street corner looking for work, and government programs that guaranteed them such. That meant that every single little decision on a day-to-day basis could be made by an engineer, including "rationalization" of every contactor sizing decision for every motor in every application. When a piece of machinery was changed or moved, they had an engineer (or 2) assigned to think through all the possible ways to cut costs of materials, which in post WWII were very expensive.

NEMA on the other hand, was developed at the behest of the US automotive manufacturers. They had to change out production lines every few years, but the component cost to do so was astronomical because they had to build a new line while the old line was still operating. That meant that they had to essentially double-up on control equipment. To save money, they wanted to be able to easily re-use control equipment the next time a line changed. They demanded a spec wherein a starter could be selected and used by an electrician based on very simple rules, not an engineer in an ivory tower. Those rules were designed to accommodate the worst possible thing any electrician might connect it to, essentially a punch press that used plug reversing to stop the flywheel. If they used that same starter for an equal HP motor on a pump it was overkill, but so be it. Parts and electricians were cheaper than engineering time to try to shave a few pennies off of a project cost.

In today's economy, component costs are insignificant compared to labor cost, but unfortunately our capitol expenditure rules make it so that the cheaper something is at initial installation, the better. Long range replacement costs come from a different budget. So using IEC control components is a way of reducing the initial costs of a system. the long range cost of that decision is that if someone some day wants to re-use a starter, it is no longer a simple matter of looking at the NEMA size and slapping it in. More thought needs to go in to it. The cost of THAT is deferred to when it happens, i.e. someone else's problem.
 
Status
Not open for further replies.
Top