Benefits of NEMA rated control relays versus IEC 1

[SparkyJunior]

My company specifies NEMA rated control relays for our projects. I presume this is because they are more durable. I do relaize they take up more space and are more costly, but is there any other benefit in specifying NEMA rated relays versus IEC?

 

[foxhoundme]

Typically the argument is that NEMA is easier to select because you do not need to know much about the application (due to robustness of the design), IEC is more catered to having to know the specifics of the application in order to select the appropriate relay. Another argument has been replacement parts for NEMA versus IEC. IEC has been considered a throw-away product if it breaks while NEMA is considered a repairable product. Today, a lot of the NEMA and IEC standard conventions have migrated from product to product and now the true arguments rest with the consumer as to which convention is more appropriate for thier application. I hope this helps!

 

[jraef]

If you are really speaking of control relays, and not motor control contactors, then there really is no difference at all in the capacity ratings. The only theoretical difference is that typically IEC relays have fixed contact sets, to which you can add more contact block assemblies to, but you cannot change the normal status of any individual contact. Many NEMA designs have contacts that are changeable, meaning that you can remove one, flip it over, and a NO contact becomes a NC contact, and each one is individually changeable. This however is not specifically in NEMA design standards, just in practice with major manufacturers. If you are using NEMA relays with fixed contacts, they are in fact no different than IEC relays.

Motor control contactors are an entirely different situation.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Laplacian]

We find that some IEC control relays don't hold up quite as well to voltage sags as their NEMA counterparts. We assumed they were the same also, until it bit us a few times on automatic motor restart circuits. Now we pay close attention to a relay's drop-out voltage where we didn't before.

 

[afterhrs]

Here some practical factors to consider when you are determining which spec. to use.

1)Life cycle of the equipment.
a)IEC = shorter cycles, 5yrs or less.
b)NEMA = longer than 10 years

2)Enviroment.
a)IEC = packaging operation.
b)NEMA = steel mill/foundry.

3)duty cycle.
a) IEC = lower (10 cycles per day)
b) NEMA = Higher (100 cycles per day)

Jreaf makes an excellent point about motor starter units. I find the key is to "upsize" by one model when selecting IEC units, or another way to put it, only use 60% of the rated capacity.

All of the above statements are based on my personal observations only, others may differ from mine. At the end of the day, that is all we have until an industrial or commercial " Consumer Reports " is actually published.


Afterhrs