PLCs in hazardous locations (Class 2, Group G)

[agarb]

Background - I am more of a PLC programmer than an electrician. We are quoting a few pieces of equipment, some of which will be going into a hazardous location. It has become my responsibility to try to put together a cost estimate for the required electrical hardware, junctions boxes, etc. The engineering company has stated that the control panel and PLC will be installed in an unclassified location but that devices/instrumentation installed inside the equipment will need to be suitable for class 2, div 1, group G. Devices installed on the exterior of the equipment will need to be class 2, div 2, group G. I have done a bit of reading in NFPA70, article 502, but I still have a few questions.

1. Does the PLC and associated I/O modules (installed in the unclassified location) need to be rated for the appropriate hazardous location if interfaced with devices inside the hazardous area?

2. According to article 502-6(b), class 2 div 2 locations require only that the enclosures be dust-tight. Can components (such as a remote PLC I/O rack) not specifically marked for class 2 div 2 be installed within this dust-tight enclosure?

3. It appears that dust-ignitionproof enclosures are only required in regards to class 2, div 1 locations. Article 502-5 requires appropriate sealing between dust-ignitionproof enclosures and non-dust-ignitionproof enclosures. Are there any similar requirements for sealing dust-tight enclosures from enclosures in an unclassified area?

4. What is the role of intrinsic safety? After reading 500-2(a)(4) it appears to allow for normal installations by removing the requirements of article 502.

Thanks!

 

[czw0]

[FONT=&quot]Hi ,[/FONT]
[FONT=&quot]I can’t comment so much on the American system of hazardous area safety more than to say it seems to leave a lot to be desired. In Australia to meet the requirements of a hazardous area is very very expensive. I just had an EXd box made to house a 7” HMI screen. It’s 95 kg and was $26000[/FONT]
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[FONT=&quot]I know of no PLC’s that even come close to meeting IECEX requirements for hazardous areas. You could put a PLC in a EXd Enclosure is guess but the cost would be astronomical. All our PLC’s ( an entirely hazardous site with gas and dust) are mounted outside the area and all IO passes through intrinicly safe barriers. All Motors and lighting (240v) are either Exe or EXd rated.
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[FONT=&quot]The role of main role of intrinsic safety is to limit the ignition energy to a level where a spark cannot ignite your hazardous substance. The energy required would I imagine be defined in your gas groupings and area codes.[/FONT]
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[FONT=&quot]I realise however there is a massive difference between Australian/NZ/European and US standards for Hazardous areas. [/FONT]

 

[Yuri]

Hi,

I agree with czw0, that there are probably big differences in the rules for ATEX in different country.
But effectually the goal is the same, to prevent conditions that is needed for an explosion to happen.

- Intrinsic and non arcing prevent the spark

- D en-closers prevent spreading when it happens inside the containment

- Filled up housing to prevent the gas or dust to enter

Its almost like the fuel/oxygen/spark with fire.

 

[OZEE]

The requirements for Class 2 Div 2 are "dust tight" -- which means NEMA 4, 4X, or 12 enclosures and devices OUTSIDE of the enclosure. Equipment within the enclosure has no other requirement.

Class 2 Div 1 enclosures are either going to be rated for Class 2 Div 1, which means NEMA 9 ($$$ and heavy!) or a purged 4/4X/12 enclosure.

The NEC(2011) 502.15 requires seals between the hazardous area and unclassified area. The intent is to keep dust from the unclassified area out of the hazardous area. The four methods that are approved:

1) A permanent and effective seal
2) A horizontal raceway not less than 10' long
3) A vertical raceway not less than 5' long and extending downward from the dust-ignition proof enclosure
4) A raceway installed in a manner equivalent to (2) or (3) that extends only horizontally and downward from the dust-ignition proof enclosure.

Intrinsically safe parts in a Class 2 Div 1 location are fine - but you're going to have to make sure that all of the installation instructions are met -- and that will almost always include some barriers.

 

[agarb]

Thanks OZEE,

You have confirmed what I am thinking. It appears that I can use normal PLC components (I was considering the $$$ Flex EX) and install them within the appropriate enclosures. Of course all wiring to field devices will have to be in accordance with 502.10.

Sorry about my original post. I was referencing my old copy of the NEC at home when I wrote the post. I am quoting most of it below with the code references updated to 2011 NEC.

1. Does the PLC and associated I/O modules (installed in the unclassified location) need to be rated for the appropriate hazardous location if interfaced with devices inside the hazardous area?

2. According to article 502.115(B), class 2 div 2 locations require only that the enclosures be dust-tight. Can components (such as a remote PLC I/O rack) not specifically marked for class 2 div 2 be installed within this dust-tight enclosure?

3. It appears that dust-ignitionproof enclosures are only required in regards to class 2, div 1 locations. Article 502.15 requires appropriate sealing between dust-ignitionproof enclosures and non-dust-ignitionproof enclosures. Are there any similar requirements for sealing dust-tight enclosures from enclosures in an unclassified area?

4. Deleted. Can't find similar reference in article 500 of 2011 code.