Re: Safety barrier for RFID antenna?

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?

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.

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

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