HMI Enclosure

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session88

Member
Location
USA
We are having a contractor install an HMI touch screen in our coal fired power plant. It will be installed near some coal conveyors. My understanding is that this area is a Class II, Division 1 location because even with our dust collector system running coal dust accumulates everywhere on that deck. As far as we know an area clasification study has not bin done here. Some people here are saying it is ok to buy a NEMA 4 enclosure, cutt a hole into the cover for the HMI and seal it to the enclosure door and presurize the enclosure with plant air. This home made enclosure dosen't seen right to me. I beleve the enclosure needs to be manufactured for this area. Any thoughts or an old thread someone can direct me to that might help me end up with the proper installation?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
We are having a contractor install an HMI touch screen in our coal fired power plant. It will be installed near some coal conveyors. My understanding is that this area is a Class II, Division 1 location because even with our dust collector system running coal dust accumulates everywhere on that deck. As far as we know an area clasification study has not bin done here. Some people here are saying it is ok to buy a NEMA 4 enclosure, cutt a hole into the cover for the HMI and seal it to the enclosure door and presurize the enclosure with plant air. This home made enclosure dosen't seen right to me. I beleve the enclosure needs to be manufactured for this area. Any thoughts or an old thread someone can direct me to that might help me end up with the proper installation?

The area is either classified or it is not. If it is classified it is because someone(s) qualified to do so classified it. Such a person(s) may also have determined that the area does not require classification at all.

There is likely other equipment in the area. What kind of precautions are used on the existing electrical equipment in the area.

Pressuring the enclosure is a perfectly acceptable way of reducing the level of hazard within the enclosure to non-hazardous, even if the area it is located in is classified. It is not quite as simple as hooking up air to the enclosure, but if done correctly, it is quite safe and acceptable. It's also pretty common to pressurize the enclosure just to keep contaminants out.
 
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don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
NFPA 499 in section 5.2.2 requires a Division 1 classification if the dust layer, under normal conditions, exceeds 1/8". If this is the case, 502.150(A)(2) (2011 code) will require an enclosure identified for the purpose.
 

session88

Member
Location
USA
Most of the electrical equipment in this area are Class II, Div. 1 rated. Some systems also use Class I, Div. 1 & 2 enclosures in this area.
 

justdavemamm

Senior Member
Location
Rochester NY
Some people here are saying it is ok to buy a NEMA 4 enclosure, cutt a hole into the cover for the HMI and seal it to the enclosure door and presurize the enclosure with plant air.

Anyone that says that is suspect. Purged / pressurized enclosures don't operate at plant air pressure ( typ. 90psi). They operate at 0.25 inches of water pressure (0.00903 psi).

I believe that the front of the HMI would have to be acceptably rated for the area classification, since it is exposed to the environment.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Pressurization will not necessarily solve the problem you have described. One of the major issues with Class II is dust settling on the enclosure, not just getting inside. This is especially true with Group F (coal dust) which has a relatively low autoignition temperature and is slightly conductive to boot.

Pressurization is not a bad start though. I would still make sure the HMI face was rated for Class II.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer

Pressurization is not a bad start though. I would still make sure the HMI face was rated for Class II.


I think all the AB PV+ units are rated for Class II, div 2. Not sure about others. There does not appear to be anything really different about them, so I suspect other units might also be rated that way.

If it is a division 1 area, you might be totally out of luck, short of a very pricey solution.

[ETA}
It occurs to me that there is a fairly reasonable option. There are IS mice available. You could put the HMI under a window kit inside an appropriate enclsoure, and have the mouse on the outside.
 
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