enclosure for class 1 div 1 location

[james_mcquade]

hi everyone,

i am working on a job classified as class 1 div 1 and i have specified a a nema 7,9 enclosure for the electrical controls.

as you know, this enclosure wall is thick and has to contain a fire if one should develop.

the man i am working for is a p.e. and he says that a purged nema 4 cabinet will be ok.

since i do not have the pe stamp, is there any sections of the code i can recite to help change his mind?

thanks in advance.

james

 

[rbalex]

...

since i do not have the pe stamp, is there any sections of the code i can recite to help change his mind?
...

james

Fundamentally, he?s right. See Section 500.7(D). He is specifying a ?Type X? purging system though which is fairly complex. See NFPA 496 for all the specific requirements.

 

[justdavemamm]

James, why do you want him to change his mind ?

For an enclosure more than a few cubic feet, it's way less expensive to purge.

Heck, even better to locate the controls outside of the classifed area, if you can.

I use Nema 7 if it's the only way I can do protection as it's a pita in many ways.

rbalex: An X purge is only complex the first time you use one.

 

[rbalex]

...

rbalex: An X purge is only complex the first time you use one.

I guess complex is a relative term. There are extra ?bells and whistles? for Type X and, depending on the processes involved, the operating scheme can be very inconvenient on loss of the purging medium. It?s still a pretty good way to go though for small, isolated systems.

 

[james_mcquade]

rbalex,

i looked at the code and thanks for the help.
i learned something today.

i have some more questions please.
1. purged cabinets are permitted for which it is identified.
does that mean the controls in the cabinet must be listed for that environment?

2. we have two classifications for this area, durung production - class 1 div 1, 2 hours after production - the environment no longer has explosive gases and we can get into the control panel. How do you keep people out of the enclosure during production hours when the area is class 1 div 1.?

thanks in advance
james

 

[rbalex]

1. Not necessarily. Identified is a defined term and does not automatically require listing in itself, although listing or labeling is a method of “identifying” an installation. An installation assembled/constructed under the appropriate Sections of NFPA 496 would be “identified.”
2. For the most part, an installation will always be Division 1, if it is ever Division 1. This is essentially the fall out of the “proper documentation” required by NEC Section 500.4(A). The system may be altered by a “hot-work permit” as the temporary “proper documentation.” NFPA 496 addresses "How do you keep people out of the enclosure during production hours when the area is class 1 div 1.?" (See NFPA 496. Section 5.5.2)

Edit Added NFPA 496 Section Reference

 

[james_mcquade]

thanks,

this is the first time i have had anyone ask me to design electrical controls inside the area with a nema 4 purged enclosure.

i always try to ask "what if" and see what the results are, and in this case, the answer will be a disaster.

thanks again,
james

 

[petersonra]

thanks,

this is the first time i have had anyone ask me to design electrical controls inside the area with a nema 4 purged enclosure.

i always try to ask "what if" and see what the results are, and in this case, the answer will be a disaster.

thanks again,
james

A properly designed and maintained purged/pressurized system is every bit as effective at preventing explosions and fires as an XP cabinet.

It has the side benefits of usually being less expensive to build and a whole lot easier to deal with.

However, it will use some air. Air is an expensive utility. It makes no sense to save $100 capital cost and spend 50 cents a day in air.

 

[jbower]

rbalex,

i looked at the code and thanks for the help.
i learned something today.

i have some more questions please.
1. purged cabinets are permitted for which it is identified.
does that mean the controls in the cabinet must be listed for that environment?

2. we have two classifications for this area, durung production - class 1 div 1, 2 hours after production - the environment no longer has explosive gases and we can get into the control panel. How do you keep people out of the enclosure during production hours when the area is class 1 div 1.?

thanks in advance
james

Install a pressure switch in the cabinet in series with the control voltage. When there is a pressure loss, the cabinet is shut down.

 

[petersonra]

Install a pressure switch in the cabinet in series with the control voltage. When there is a pressure loss, the cabinet is shut down.

that is necessary for a type X system (among other things).

the switch needs to be XP, and normally is on the outside of the cabinet.

you also need some way of making sure that the environment inside the cabinet is purged of any potentially hazardous gases before allowing power to be restored.

there are some other things you need as well.

it is generally easier to buy a listed system to handle all the requirements then to try and do it yourself. several companies sell them.