HMI Screen in Flame Proof enclosure

[rajsiyer]

Hi friends, I've a peculiar problem.. We need to Install a Touch Screen/HMI in a flame proof enclosure. How does one access the touchscreen behind a toughened glass? or is it better to mount a LCD monitor inside the enclosure and provide a pointing device outside? But then I have'nt come across a Flameproof mouse yet!. Or is a purged panel an acceptable choice. I'm thinking of doing the Flameproof enclosure with 8 buttons all wired to the PLC. I feel helpless in such matters does anyone have a clue what is most economical and hassle free? I'd really appreciate the advice. Tahnks and best wishes, Raj s. Iyer

[Ken Moore]

What's the area classification?

[rajsiyer]

Thanks a million Ken, and this is not the first time you are so prompt to help.. Now for the Area & Zone classification It will be Zone 2 GRIIA,IIB(Ordinary solvents) Regards and best wishes for your success and prosperity. Raj S. Iyer

[Ken Moore]

I'm not that familiar with the Zone designations, but I think Zone 2 is sort of like NEC Div II. If so you you have a lot of options, although a touch screen is not one of them. There are many purged panels with keyboards and track balls on the market. Here is a link to a vendor I have used in the past, it may give you some ideas. http://www.protech1.com/products.php

[paulengr]

No such thing as "flame proof". Everything burns eventually with enough heat and oxygen. Most electrical/electronic gear stops at 40-70 C. Part of the limitation has to do with PVC as an insulator. Part has to do with limitations on LCD panels. There are lots of tricks of the trade to make electrical equipment fire resistant in a steel mill/foundry/kiln situation, but there are no codes or specifications for it. There is even some wire on the market that can go all the way to 1000 C without damage at least for short periods of time. If you can provide specific examples of what you are trying to do, I can definitely make recommendations on how to protect nearly anything. Time and temperature play a big factor in what you are trying to achieve. For instance, fire brick, fire sleevs, and mill board are great insulators but you've got to provide some sort of cooling on the cold face. Silicone coated kaowool "fire sleeves" do a great job of taking a hit from slag even though they would never withstand sustained heat for any length of time. Similarly, TFE wire will give you an elevated temperature range and helps survive a "hit" but doesn't help with sustained radiant heat sources even if the ambient situation is tolerable. Finally, you can do some amazing things even with ordinary bathroom silicone caulk in the right situation. And "coolers" (compressed air or AC) are frequently not the answer either. Often two sheets of "tin" roofing panels are the most effective heat shields. In a foundry, we've tried touch screens and keep coming up with too many problems. We've gone to mounting screens behind glass and putting buttons only on the outside that are interfaced to the PLC. Forget mice/keyboards. Not even worth the effort. However, you mentioned codes relating to EXPLOSIONS. This is entirely different from dealing with heat and flames. In this case, the goal is not to survive an explosion. It's usually impractical to provide enough brute strength and/or pressure relief to deal with the pressure and shock waves created in an explosion, at least not predictably. The goal is always to simply not create or propagate an explosion. The equipment is usually destroyed in the process so there is not attempt made to preserve anything. In terms of specifications, speaking in terms of the U.S. classification system, Class I/Division I is the "worst case scenario". Class I/Division II is "second worst". The C.I/D.I stuff is designed such that in the event that an explosion occurs, it is fully contained within the enclosure...in other words, it is built very heavy. C.I/D.II stuff has to be "nonsparking". This is actually harder to achieve than C.I/D.I makes these components much more rare. So if you have to go this way, unless it's available, go all the way to C.I/D.I. The European system uses ABCDE which works in a similar but slightly different manner. It is very important to read the NEC Codes to understand the differences. If you are in the U.S. the NEC Code is specifically written so that you pick ONE system or the other. You do NOT mix systems. This is a beginner's mistake when interpreting the Codes. Equipment manufacturers put BOTH specifications on them because they are trying to appeal to both groups of end users. By the way, I will use the U.S. system if I have the choice because implementation is much easier and less complicated. There is an alternative under the NEC Code though called "Intrinsically Safe". This is a little known section that nicely gets past most of the problems. When you design with IS, all the equipment must meet special requirements which are that it draws less than 100 mA and that it is low voltage. It must also not be capable of storing any appreciable energy. Between the "hazardous" and "nonhazardous" areas but mounted in the "nonhazardous" area (inside your enclosure), you have to mount an "isolation barrier" which is a bunch of zener diodes, resistors, and fuses which gaurantee that you can't go over the low voltage/low current requirement. The advantage is that as long as your devices are "intrinsically safe", your wiring and everything else can run using the normal Code rules. No special armor plated equipment and nonsparking stuff is required. This is much less expensive, and the reason that it is hard to find Class I/Division I or Division II stuff for controls. PepperL-Fuchs, Square D, Allen Bradley, IFM Efector, and several others all sell Intrinsically safe equipment. You can buy both analog and digital devices including LED panel lights, buttons, selector switches, level sensors, proximity sensors, and many other types of devices. There's really no limit here. The only place where you definitely couldn't go IS is that for instance you have to buy ATEX motors or other equivalent "explosion proof" designs. Edited February 26, 2008 by paulengr

[rajsiyer]

Thanks Paul, Thanks for your reply. Your wide experience is evident so I know that you know certain useful things. However in this case, I'm going ahead with a Magelis XBT-GT touchscreen made by Schneider and it is not IS. But Ken's suggestion of using purged panels is indeed practical and I've located vendors who could carry out the same. The link to Protech was useful to understand the specs involve. It seems to be a profession in itelf Thanks and my sincere appreciation to all you geeks who keep this site going. Great advice great place great folks here. Raj S. Iyer

[Clay B.]

I am not sure how it is in India but one thing we have to deal with in the US is our Insurance Underwriter. The reason I mention this is that we had a machine we wanted to add a touch screen to and our system met the established code but our insurance underwriter would not allow it citing failures of the system in the field. We ended up putting in a PC with an IS rated keyboard.