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Safety barrier for RFID antenna?

Started by ElderUberGeek January 15, 2007
Mark makes a great point. FM will charge you about $15,000 - $20,000
and take 6-12 months for Zone 0 Approval for a single product.

My advise, if you go down the design and approval route, is don't go
through Factory Mutual (FM), if you need FM approval have CSA do the
testing for you and they will deal with FM for the paper work exchange.

The last Hazardous project I worked on took FM 2.5 years to get
approved. We had 3 power supply variants (AC, DC, BATTERY PACK), but
they original quoted us 4 months and $90,000. We payed $100,000 and
waited 2.5 years and finally just got the approval.

If your volume is low buy an off the shelf preapproved limiter, they
are spendy $100- $500, but sometimes it is better than dealing with all
the agency approval crap!!!

Eric

Eric wrote:
> Mark makes a great point. FM will charge you about $15,000 - $20,000 > and take 6-12 months for Zone 0 Approval for a single product. > > My advise, if you go down the design and approval route, is don't go > through Factory Mutual (FM), if you need FM approval have CSA do the > testing for you and they will deal with FM for the paper work exchange. > > The last Hazardous project I worked on took FM 2.5 years to get > approved. We had 3 power supply variants (AC, DC, BATTERY PACK), but > they original quoted us 4 months and $90,000. We payed $100,000 and > waited 2.5 years and finally just got the approval. > > If your volume is low buy an off the shelf preapproved limiter, they > are spendy $100- $500, but sometimes it is better than dealing with all > the agency approval crap!!! > > Eric
Good points. Yes, the certification route is long and painful... I would be willing to consider a COTS product if it were reasonable in price (but $500 or even $100 is not reasonable for me at the moment....). Are there no IS barriers that are less?
On 15 Jan 2007 19:52:01 -0800, "larwe" <zwsdotcom@gmail.com> wrote:

> >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..."
While the regulations define how much power can be delivered into the Ex area, one should remember that at RF, any conductor, which is large compared to the wavelength, can have quite large impedance variations along the conductor. If a conductor is 5 m long (about 1/4 wavelength at the 13,56 MHz ISM frequency), the open end potential can be quite significant and may cause some flashovers. Paul
On Wed, 17 Jan 2007 11:11:14 +0200, Paul Keinanen <keinanen@sci.fi> wrote:

>On 15 Jan 2007 19:52:01 -0800, "larwe" <zwsdotcom@gmail.com> wrote: > >> >>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..." > >While the regulations define how much power can be delivered into the >Ex area, one should remember that at RF, any conductor, which is large >compared to the wavelength, can have quite large impedance variations >along the conductor. If a conductor is 5 m long (about 1/4 wavelength >at the 13,56 MHz ISM frequency), the open end potential can be quite >significant and may cause some flashovers. > >Paul
And normal IS barriers are probably not suited to RF anyway - you might just about get away with it at 125KHz but not 13MHz. As RFID readers tend to inherently have high voltages on the antenna, my approach would be to have the whole reader & antenna in the hazardous zone, completely potted to the depth required by the appropriate standard - the nice thing about RFID is you don't need any external interface other than power and data, and these could be relatively easily protected.
On 16 Jan 2007 05:01:37 -0800, "ElderUberGeek" <aribloch@gmail.com>
wrote:
>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!)
It's been a few years since I did anything intrinsically safe but a good start is to check the UL isolation rules (for the voltages we were using in the external equipment, I think 120v, half an inch was the required physical isolation between the intrinsically safe part of the circuit and the external part of the circuit where air was the insulating medium. That's hard to do inside most equipment, so you might consider an external isolation box near the boundary of the zone 0 area. Potting with nonflamable materials with high breakdown characteristics might work, depending on the cabling. After the physical issues, we used opto-isolators [Toshiba's TLP421, for example]. That would work for most data rates, but not necessarily for the antenna connection. You would probably want to hang low voltage zeners to common ground for the signal wires as well as low voltage spark gaps if there is the possibility of current beyond the zener capacity. After you get that done, getting the stuff powered can be interesting too. Then you may need to get it UL (et al.) approved for the environment (if you intend to sell it). Or have an equivalent expert insure it does meet the NEC or other regulatory body's rules. That takes a while. Good luck, --Charles

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