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

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 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

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?

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

"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

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 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

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!)

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