I am going to specify instrument junction boxes for analog and digital signals in area with class 1, division 1 classification.

Explosion proof or Increased safety, which one is the best choise?

 

Class 1, Division I (not 1) indicates a US style area classification, "increased safety" is something used in the European explosion protection scheme so would not be applicable at all.

Best to get in touch with someone more familiar with the requirements, and how to meet them.

 

If you use Exd, you can't open it up until the plant shuts down. If its a permanent installation and you don't expect the need to open it up then its ok. Except its a bit heavy and normally cost more than a GRP box. Unless this is an American jobbie than GRP would be better. Lighter boxes means less transport charges and are cheaper.
The problem with GRP boxes is that they are certified for life. They get it certified only after they drilled the holes for cables. If you need to make additional cables in, specify the additional holes to be plugged. If you drill an already certified GRP box it invalidates the the cert.

 

Both are suitable for the specified area classification.You have to specify the gas group too.

Ex-proof is expensive compare to Increased safety and also to my knowledge ex-proof SS junction boxes are not available in the market.Hence,be careful while specifying the MOC.

Regards,
GB

 

In a message dated 4/23/2003 6:03:05 AM Eastern Standard Time, [email protected] writes:

> Class 1, Division I (not 1) indicates a US style area classification, "increased safety" is something used in the European explosion protection scheme so would not be applicable at
> all.

Oops. I should have said:

"Class I (not 1), Division 1"

Must remember to read what I write before hitting send key.

 

we prefer to use I.S. circuits so the need for an increased safety junction box is removed (that show I understand the rules).

Obviously you know that if you you an Exd enclosure you need the glands to go with it and most likely you will noeed armoured cable and the appropriate glands for it.

There is more the the setup of a hazardous area circuit than just a junction box, which is why it has becaome mandatory tht all designs be done by
persons qualified for hazardous areas.

Mick

 

Please refer to the National Electric Code, Sec. 500 for rules concerning Hazardous Environments. Keep in mind that UL or Factory Mutual(if a self-insured manufacturer) will or should have specific requirements as to where Sec.500 applies.

 

Check with the unit's maintenance folks AND see what is already "preferred". 8 zillion bolts to loosen at 3 a.m. is NO fun for techs and adds to downtime. If the DCS is not IS already(i.e. MTL modules/IS barriers), it could add a LOT of expense to IS it. A third option(if there are arcing devices or other kaboom-makers) is to install Z-purged FRP boxes(depends on instrument air cost/availability). A real good review of alternatives is here: http://www.okbebco.com/PDFs/EPSD/R20.pdf)

 

If your area is defined as ClassI Division 1, NEC article 501.4(A)(3) states all boxes shall be approved for ClassI Division 1, that is clear.
Your instrument junction box must meet this requirement.

The term "increased safety" refers to equipment that can be used in areas with a ClassI, Zone1 classification.

NEC Article 505.7(A) states that "Classification of areas and selection of equipment and wiring methods shall be done under the supervision of a qualified Registered Professional Engineer."

So unless your area has been reclassified to ClassI Zone 1 according to NEC Article 505.7(A), you need a box that is approved for ClassI Division1.

 

I would take a staged approach to solve this application:
1. Consider using Intrinsically Safe (IS) barriersout in the Non explosive area. This will make your digital and analogue signals completely for use in the explosive atmosphere. The choice of enclosure can then be any type. That is, the box will not need to have any type of explosion rating at all. However the wiring system will need too be installed in accordance with wiring code practice for IS wiring systems.(all components readily available (Australia)
2. Consider using Increased Safety, this type of protection is frequently used where all components within the enclosure are passive. That is, the form of protection does not allow for contactors which generate an ignition hazard by their very use. A motor junction box is a good example of an application for Increased Safety. Your application is also a good example of an application for Increased Safety. This form of protection uses increased creepage and clearance distances, lock washers, etc to ensure that an ignition spark is ever generated (Not so readily available Australia)
3. Use readily available Flameproof equipment all the way

DJW

 

My respects to the forum

I recently completed an assignment as an engineer supporting construction and commissioning (putting all the design faults right) on a petrochem plant in the middle east. I was astonished to discover the plethora of different mix and match JBs. Exd boxes with the wrong glands IS circuits mixed with 120vdc mcc circuits, you name it they did it. Made a complete moquery of increased safety. It was not part of my scope to comment on this but it left me profoundly disturbed. Only a matter of time before gas seeps into one of these Exd sheet steel boxes and a loose terminal provides a spark source. The whole area is definitely zone 1, not just by definition by by the physical plant behaviour. Needless to say the design was from across the pond where little interest is shown or knowledge retained on europeean standards and anyway they cant be sued from the middle east.

As an I&C des eng of more than 30 years I am sadly disillussioned at the very poor deal the customer gets when he places a "turnkey " contract with a construction contractor who then has design responsibility. The main independent design houses dissappeared 20 years ago when Oil majors felt they could get a cheaper quicker deal rather than employing their own design house to do the design and procurement specs (detail construction design can be left to the concrete cowboys)

Incidentally I believe the only authoritative modern book I could find (and bought) was Design for hazardous area environments

(Electrical Installations in Hazardous Areas
by Alan McMillan (Hardcover - September 1998)
Butterworth Heinemann
$160

# Hardcover: 649 pages ; Dimensions (in inches): 1.65 x 9.49 x 6.54
# Publisher: Butterworth-Heinemann; (September 1998)
# ISBN: 0750637684
# Amazon.com Sales Rank: 843,266

gives very good background for the professional engineer (as I found) though not so good on NEC systems (but then the crouse hindes catalog is pretty foolproof)

(have you ever seen the NEC handbook, it shows around 70+ different styles of domestic plug and socket types - for use depending on what state you are resident amongst others - does that tell you something?)