NEMA 4 Vs Class Rating

[james_mcquade]

Hi everyone,

I started to order an HMI terminal today but ran into a problem.
While the HMI Terminal is rated NEMA 4, Its electrical rating is general purpose, ie. no Class or div rating.

This is how i currently understand things.
The NEMA 4 rating means that the enclosure and seal of the hmi display is rated for hose directed water to obtain the maximum seal. Its also oil,dustight.

The Class / Div rating is used to rate the electrical characteristics of the HMI in regards to its use in certain areas. For example, an HMI rated for class 1 div 2, class 2 div 2 groups efg means that the HMI can be safely used in class 1 / div 2 locations and also class 2 / div 2 group efg locations when the installation methods for the HMI is properly done and as well as using correct electrical installation methods are used for the wiring.

This does not mean that the HMI terminal with the NEMA 4 rating is acceptable for use in a class 1 div 2, or class 2 div 2 environment (this is where the unit was to be placed). There will be an accumulation of dust on the HMI enclosure.

Please comment, I don't want to walk into my meeting later this week and be incorrect. This plant is trying to overcome some of the previous electrical installation practices (any thing goes). They want to do the right thing, but several of the engineers of the anything goes age are still there and may fight me on this. I want to be able to present the facts and back it up with the correct rules.

Thanks in advance,
James

 

[weressl]

Hi everyone,

I started to order an HMI terminal today but ran into a problem.
While the HMI Terminal is rated NEMA 4, Its electrical rating is general purpose, ie. no Class or div rating.

This is how i currently understand things.
The NEMA 4 rating means that the enclosure and seal of the hmi display is rated for hose directed water to obtain the maximum seal. Its also oil,dustight.

The Class / Div rating is used to rate the electrical characteristics of the HMI in regards to its use in certain areas. For example, an HMI rated for class 1 div 2, class 2 div 2 groups efg means that the HMI can be safely used in class 1 / div 2 locations and also class 2 / div 2 group efg locations when the installation methods for the HMI is properly done and as well as using correct electrical installation methods are used for the wiring.

This does not mean that the HMI terminal with the NEMA 4 rating is acceptable for use in a class 1 div 2, or class 2 div 2 environment (this is where the unit was to be placed). There will be an accumulation of dust on the HMI enclosure.

Please comment, I don't want to walk into my meeting later this week and be incorrect. This plant is trying to overcome some of the previous electrical installation practices (any thing goes). They want to do the right thing, but several of the engineers of the anything goes age are still there and may fight me on this. I want to be able to present the facts and back it up with the correct rules.

Thanks in advance,
James

I'll try. An HMI installed in a class Class Z - NFPA 496 - purged NEMA 4 panel could be suitable for a Class I and/or II, Division 2 location. There may be HMI interfaces that are suitable without any purging in a Division 2 environment, although that approach would reduce your choice of selections. Troublesome items are the keypad and the "pointing device".

Peaked roof will fix your dust accumulation issue and housekeeping is a must in dust environments.

 

[james_mcquade]

weressl,

thanks for the reply.

I knew about 500.7d / purge & pressurization, but i concerned about the HMI display itself.
while the internal electronics will be protected, i don't know about the display itself.

thanks again,
james

 

[weressl]

weressl,

thanks for the reply.

I knew about 500.7d / purge & pressurization, but i concerned about the HMI display itself.
while the internal electronics will be protected, i don't know about the display itself.

thanks again,
james

You're welcome. It is routinely done with integrated LCD displays and sealed keyboard on the same vertical surface. You can ask the HMI manufacturer about:

• the static discharge from the screen, although it is not likely be an issue in Div. 2
• if it could withstand/hold the pressurization itself,
• if it would have pockets of air where ingitable vapor could remain stagnant after purging/pressurization.

 

[petersonra]

weressl,

thanks for the reply.

I knew about 500.7d / purge & pressurization, but i concerned about the HMI display itself.
while the internal electronics will be protected, i don't know about the display itself.

thanks again,
james

I think you need to ask the manufacturer of the HMI this question directly.

I had a long chat with the product manager at AB about this very question a few years ago.

It turned out that some of their panelview terminals were suitable for being purged and others not. I don't recall off hand what was and was not suitable but it had to do with the internal pressure separating the membrane keypads apart or separating the touchscreen from the CRT.

 

[weressl]

I don't recall off hand what was and was not suitable but it had to do with the internal pressure separating the membrane keypads apart or separating the touchscreen from the CRT.

Indeed, that is a recognized problem. A lot depends on how well one is able to control the air pressure inside. An overpressure gravity 'flapper' - we're deling with very low pressures - with alarm, may staisfy AB that the overpressure will not occur and at the same time it meets the Class Z.