Forums

Safety barrier for RFID antenna?

Started by ElderUberGeek January 15, 2007
I am planning to use a RFID antenna (reader) in a Zone 0 explosive
classified area, and it is connected to an apparatus which is located
in a non-classified zone. My question is, what type of circuit do I
need to build inside my equipment (i.e. not a separate unit) in order
to make it intrinsically safe? I know it has something to do with
zeners but I would appreciate some guidance and examples.

ElderUberGeek wrote:
> I am planning to use a RFID antenna (reader) in a Zone 0 explosive > classified area, and it is connected to an apparatus which is located
[...]
> to make it intrinsically safe? I know it has something to do with > zeners but I would appreciate some guidance and examples.
Okaaaaaaay... You do realize, this is probably the most hair-raising post I've read in c.a.e. for quite a while? It's a bit like opening a Little Golden Book and finding "Now, kids - once you've acquired your UF6, you need to build a centrifuge cascade..."
larwe wrote:

> Okaaaaaaay... You do realize, this is probably the most hair-raising > post I've read in c.a.e. for quite a while? It's a bit like opening a > Little Golden Book and finding "Now, kids - once you've acquired your > UF6, you need to build a centrifuge cascade..."
:-) I can appreciate your apprehension.... BUT, I can assure you this is acceptable industry use which has been done many times in the past and will be done many times in the future (how do you think large companies, for example, take stock say of LPG bottles in a filling plant?). Seriously, I guess you are just not familiar with the industry. Any ideas anyone?
"ElderUberGeek" <aribloch@gmail.com> wrote in message 
news:1168905118.816139.319700@51g2000cwl.googlegroups.com...
>I am planning to use a RFID antenna (reader) in a Zone 0 explosive > classified area, and it is connected to an apparatus which is located > in a non-classified zone. My question is, what type of circuit do I > need to build inside my equipment (i.e. not a separate unit) in order > to make it intrinsically safe? I know it has something to do with > zeners but I would appreciate some guidance and examples.
Whilst I'm not designing anything intrinsically safe myself at this time, it is possible that I might have to in the future so I watch these threads with interest. I'm guessing that the zeners are to dissipate energy that might be received from a big dollop of RF hitting the antenna? I agree with Lewin that the OP is hair-raising at first glance but I do hope that someone will answer the question and that ElderUberGeek doesn't blow himself (or anyone else) up while implementing it.
ElderUberGeek wrote:
> larwe wrote: > >> Okaaaaaaay... You do realize, this is probably the most hair-raising >> post I've read in c.a.e. for quite a while? It's a bit like opening a >> Little Golden Book and finding "Now, kids - once you've acquired your >> UF6, you need to build a centrifuge cascade..." > > :-) > I can appreciate your apprehension.... BUT, I can assure you this is > acceptable industry use which has been done many times in the past and > will be done many times in the future (how do you think large > companies, for example, take stock say of LPG bottles in a filling > plant?). Seriously, I guess you are just not familiar with the > industry. > > Any ideas anyone?
I think his (well, certainly my) concern was over the bits "I am planning to use a RFID antenna (reader) in a Zone 0 explosive classified area" and "I know it has something to do with zeners but..." It's like the posts in uk.d-i-y saying "I'm in the middle of re-wriring my house - which colours are live and which are neutral? Cheers" or "I'm replacing my gas boiler - how do I turn off the gas?". They just give the impression that the poster is trying something which has significant safety issues if implemented wrongly but only has a vague grasp of the concepts and no idea of the details. Now, I've no idea who you are, what your experience is, and have no idea of your industry - it was more an explanation of why, at first reading, your post might ring some warning bells to readers. Thanks David
David Hearn wrote:
> ElderUberGeek wrote: > > larwe wrote: > > > >> Okaaaaaaay... You do realize, this is probably the most hair-raising > >> post I've read in c.a.e. for quite a while? It's a bit like opening a > >> Little Golden Book and finding "Now, kids - once you've acquired your > >> UF6, you need to build a centrifuge cascade..." > > > > :-) > > I can appreciate your apprehension.... BUT, I can assure you this is > > acceptable industry use which has been done many times in the past and > > will be done many times in the future (how do you think large > > companies, for example, take stock say of LPG bottles in a filling > > plant?). Seriously, I guess you are just not familiar with the > > industry. > > > > Any ideas anyone? > > I think his (well, certainly my) concern was over the bits "I am > planning to use a RFID antenna (reader) in a Zone 0 explosive > classified area" and "I know it has something to do with > zeners but..." > > It's like the posts in uk.d-i-y saying "I'm in the middle of re-wriring > my house - which colours are live and which are neutral? Cheers" or "I'm > replacing my gas boiler - how do I turn off the gas?". They just give > the impression that the poster is trying something which has significant > safety issues if implemented wrongly but only has a vague grasp of the > concepts and no idea of the details. > > Now, I've no idea who you are, what your experience is, and have no idea > of your industry - it was more an explanation of why, at first reading, > your post might ring some warning bells to readers. > > Thanks > > David
Apart from that, there are two critical things missing. 1. Define 'Intrinsically safe'. This is probably specified for the area classification, which brings us to: 2. What are the relevant national safety standards for such zones? Do you have a copy of said specification? If you don't know (2), then you need to find out or you probably shouldn't be attempting this. Cheers PeteS
PeteS wrote:

> Apart from that, there are two critical things missing. > > 1. Define 'Intrinsically safe'. This is probably specified for the area > classification, which brings us to: > > 2. What are the relevant national safety standards for such zones? Do > you have a copy of said specification? > > If you don't know (2), then you need to find out or you probably > shouldn't be attempting this. > > Cheers > > PeteS
Friends (David et-all), interesting how this has become (deteriorated?) a discussion about safety, capabilities and understanding rather than engineering... Would everyone feel better if I had posted "I need to monitor the level of gas in a mine shaft and need a barrier for the sensor", would that have been less scary for you? :) All said, it is exactly the same question... just a different (and relevant - not strange) item at the end of the line.... To answer PeteS's question about the definition of Intrinsically Safe, well, here is a link for everyone to read at leisure (http://www.crouse-hinds.com/CrouseHinds/resources/intrinsically_safe/techref/article1.cfm) but basically IS is one of the many methods of protection (as opposed to Explosion Proof, for example) which is meant to keep electrical circuits from igniting gases etc. by way of controlling the amount of heat they dissipate or ability to spark etc. Regarding standards, yes every area (zone/class/division etc.) has various applicable standards that you have to comply with according to the degree of protection required, zone etc. And yes, a copy of this is essential reading for anyone looking to design such circuits (papers are in the mail...). So having, hopefully, satisfying everyone about the above, the question remains: can anyone share any "practical" design inputs for constructing barriers in intrinsically safe equipment.... (Phew.... i need a drink!)
ElderUberGeek wrote:
> I am planning to use a RFID antenna (reader) in a Zone 0 explosive > classified area, and it is connected to an apparatus which is located > in a non-classified zone. My question is, what type of circuit do I > need to build inside my equipment (i.e. not a separate unit) in order > to make it intrinsically safe? I know it has something to do with > zeners but I would appreciate some guidance and examples.
Factory Mutual (FM) or ATEX requires double fault protection for Zone 0. That means you'll have to limit the voltage and current to the antenna to safe levels.... Low enough level so a spark will not occur if the cable to the RFID antenna is cut. I don't know if they have a RF power level requirement yet. Factory Mutual has Voltage vs Current graphs that will let you know what you have to limit your DC levels to. The most simple solution is to use three resistors in series each with a shunt zener diode for limiting the voltage. The resistors limit the current. The reason you need 3 is because of the tripple redundacy required for double fault protection. Any two components could fail and you'll still prevent a spark. Every wire going to the RFID antenna will need this resistor and zener limiting circuit. Eric
On 15 Jan 2007 15:51:59 -0800, "ElderUberGeek" <aribloch@gmail.com>
wrote:

>I am planning to use a RFID antenna (reader) in a Zone 0 explosive >classified area, and it is connected to an apparatus which is located >in a non-classified zone. My question is, what type of circuit do I >need to build inside my equipment (i.e. not a separate unit) in order >to make it intrinsically safe? I know it has something to do with >zeners but I would appreciate some guidance and examples.
I have been working on the design of RFID readers for alomost 10 years and strongly suggest you contact the manufacturer of the RFID equipment with your questions. There are numerous pieces of information needed that only they can supply. Some of them are: 1) what happens if the antenna becomes disconnected from the feed cable inside the explosive area (is the RF power shut down?; if so, how quickly?; if not, will the result violate requirements?) 2) is there a DC voltage applied to the cable to determine if the proper antenna is attached or shorted or missing? If so, will its presence cause a violation of requirements? They, the original designers are ina much better position to work with you in ensuring their product can be safely and legally used in the environment you describe. Regards -Bill Knight R O SoftWare
"ElderUberGeek" <aribloch@gmail.com> wrote in message 
news:1168905118.816139.319700@51g2000cwl.googlegroups.com...
>I am planning to use a RFID antenna (reader) in a Zone 0 explosive > classified area, and it is connected to an apparatus which is located > in a non-classified zone. My question is, what type of circuit do I > need to build inside my equipment (i.e. not a separate unit) in order > to make it intrinsically safe? I know it has something to do with > zeners but I would appreciate some guidance and examples. >
Depending on the number of units you are planning on using, it may be cheaper to just buy an IS barrier that can handle the voltage, current, capacitance, and inductance of your antenna. I did an IS barrier a while back with lots of redundant SCR's and firing circuits instead of zeners to lower our manufacturing costs, but the quote from CSA for testing was high enough that it was cheaper in our case to buy someone elses barrier. If you have to roll your own, there are several good reference books at http://www.isa.org/ Mark Walsh www.rogue-engr.com