door interlocking in Class 1 Division 2

[megloff11x]

For enclosures in non hazardous locations, I favor one of those rotary switches (UL98) with the rod that goes to the fused switch block mounted to the back panel, or the knife switch - both interlocking the door.

Since an enclosure in a Class I Division 2 location is purged and pressurized, are these kind of switches still allowed, and allowed or not, is there a better way?

The differential pressure switch needed to sense pressurization isn't required to kill power for Division 2, just raise an alarm when pressure is lost. It can be assumed that the switch will open and any sparking will occur while the enclosure is still sealed & purged. And obviously you want a time delay after re-sealing to re-establish pressure and exchange any vapors that may have crept in, before throwing the switch again.

Is there a standard practice for setting one of these up? I could comply with each requirement of NFPA 496 several different ways, but the inspector won't fuss as much if your choice is what everybody else does, and uses components that everyone else uses.

Matt

 

[petersonra]

There is no general requirement to have a disconnect switch on a control panel at all, pressurized or not. If you choose to have one, you can use whatever type you wish.

I don't believe there is a general requirement to have a d/p switch on such an installation in a division 2 area. Only some means of indication such as a gauge.

 

[megloff11x]

NFPA 496 asks for type Z pressurizing to reduce division 2 to unclassified. 4.8.1 states "detection shall be provided to indicate failure to maintain positive pressure within a protected enclosure." Later on they want at least 0.1in of H2O greater inside than out, and four atmosphere exchanges.

The only way I can see to do this is with a listed differential pressure switch. Dwyer makes one - I used them before cooling a mondo powered UV lamp that needed a big blower to keep it from melting itself. There are probably others who make listed differential pressure switches.

I haven't eard of another way to do that.

Much of what I see for switches in hazardous locations is of the bomb proof variety, which I don't think is needed in this case. I like providing an interlocked power removal to doors with high Voltage inside. The way I read the code is this is OK, but is there an easier way, or a way everyone else does it.

I'm just looking for an answer along the lines of "most everybody does it this way." What the machine does is clever. The packaging and code compliance part should be boring and not raise questions by applying a rule in a new & cute way. Unless it is a really good idea.

Matt

 

[petersonra]

detection and indication does not require a switch.

the gauge can be used to both indicate and detect.

IMHO.

 

[rbalex]

An often over looked feature of NFPA Standards other than the NEC is that an asterisk (*) appended to a Section indicates explanatory material in an Annex.

In this case, both Main Section 4.8 and Subsection 4.8.1.1 have material that almost answers the OP directly including a few ?typical? installations.

In general, for a Type Y pressurization, an alarm will be somewhere in the system although it may not necessarily be from a pressure or flow switch immediately on the enclosure. The term ?communicated? in 4.8.1.1 carries the concept that loss of pressure will be known quickly. However, because the root location is Division 2, ignitable gasses are not likely at any given time.

If only an ?indicator? is used see 4.8.4, especially 4.8.4(5). Note this does not mandate an alarm since it only applies to applications also subject to 4.3.2; however an alarm is still a good idea if the ?indicator? is not routinely monitored ? no matter how ?convenient? it is to view it. [4.8.4(1)]

 

[megloff11x]

Alarms are often ignored. When I was a kid I was gauge happy in my car, until someone pointed out that I rarely looked at them while the idiot light might be a better attention getter. But I ignored that too - fortunately it was a sending unit gone bad. My modern car has a "check engine" thing that comes on and I always ignore it because when I plug in the widget, it's also some inane sensor glitch.

Loud sirens are an annoyance especially with false alarms. This is why I unplug my smoke detector when I play amateur chef...

I would actually prefer on a machine that has an HMI to have an alarm form an annoying pop up window to get the operator's attention, unless it's something serious.

I go back to my original question. I'm looking for a standard practice for several reasons:

1. Lazy. Why re-invent the wheel here. We spent enough brain sweat inventing what the electrical cabinet runs.

2. Everybody recognizes it. I thought it was another stupid Army thing when they made us store our gear the same way in the vehicles, but the method was, if you got re-assigned to a new vehicle, you knew where everything was. This is why the big red button with the yellow behind is always E-stop. No delay figuring it out or reading the fine print.

3. The standard way has been wrung out and proven in practice. There will be no new discoveries in six months that lead to something bad happening. What the machine does is what is creative. How you package the innards should be mundane. I doubt if Frank Llyod Wright tried to come up with an artsy electrical panel in the basement, or new ways to run romex in the walls too.

Matt