Intrinsically Safe Barriers 3

[whycliffrussell]

I am looking for a low impedance (less than 20 ohms) dual channel intrinsically safe zener barrier. The barrier has to be rated for CSA class I, zone I OR class I, div I, hazardous rating classification. The zener barrier is to be supplied by a 7.5VDC 1.5A power supply, and must be capable of limiting the current and voltage to 7.5VDC and 5A respectively. Anyone know of such a product? I’ve already checked with Pepperl+Fuchs and MTL and they don’t have such a product.

 

[itsmoked]

Are you serious?
The point of the barriers is to limit the current. At 7.5V you are limited to something like 200mA. Not 5A! A 5A arc at 7.5V is a 37W spark. I would have no trouble believing that could ignite class I problem.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[whycliffrussell]

Yes I am serious! How you can be so sure that a 5A 7.5VDC source can ignite a gas I have no idea!...considering that you don't even know what gas group I'm dealing with...please enlighten me! The ignition curve for the gas group that I'm dealing with...group D or (IIA) will allow for 5A of short circuit current @ 7.5VDC.

If you still have trouble believing this let me present the quesion another way. I am looking for a low impedance barrier than will limit current from a 7.5VDC source to LESS THAN 5A...I want the loop impedance of the barrier (sending and return leads) to be less then 50 ohm.

 

[jraef]

R. Stahl Intrinspaks.

http://www.rstahl.com/index.asp

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

In general, a circuit can only be made intrinsically safe if it requires less than 1 Watt of power.

Is from the site referenced by jraef above. Under the heading "INTRINSIC SAFETY 101". I am surprised to hear of a an ignition curve allowing that. Heck, I have tried for a year to find any Volt-Amps ignition curves!!

I honestly think you are in Explosion Proof Land not Intrinsically Safe Land..

I know - sorry I can't direct you to your desired solution.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[spas]

I Don't think 5Amps is IS some how.
Itsmoked is correct, you are in explosion proof land.

Cheers

Mat.

 

[whycliffrussell]

First what is the source of this quote "In general, a circuit can only be made intrinsically safe if it requires less than 1 Watt of power"...CSA, ANSI, NFPA,UL, NEC?

Depending on your location, canada, USA or Europe there are strict guidelines that dictate the requirements of intrinsic safety. And namely (based on the minimum ignition energy required by the gas present...which is in turn determined by it's concentration) this is the current and voltage seen at the boundary limits of the intrinsic safe barrier.

There are also limits on the cable, and device's capacitance and inductance, b/c in addition to what the 'source' supplies (mentioned above) the apparatus in the intrinsically safe are have limites on how much voltage, current, and therefore power that they are allowed to GENERATE...i think that this quote may be somehow refering to this...but a general quote like this does not make any sense - intrinsic safety depends on the appliataion, these is not one set rule (in CSA anyway) that makes a generalization like this.

If there is anyone familiar with CSA on this thread please drop me a line. I appreciate your comments and help but i'm not looking for a tutorial on intrinsic safey, I'm looking for a product (see beginning of thread).

 

[jsummerfield]

I will pull something out of the ISA standards. For now just forget about anything over 100 mA.

John

 

[jraef]

D'oh! I missed the 5A issue! That is a lot of current. Intrinsic safety means less than 1 watt total power to the circuit, so at 7.5VDC that is 133ma, but jsummerfield is correct, most manufacturers are going to be at 100mA or less as a margin of safety.

Besides, why would you need 5A capability when your power supply is only capable of 1.5A anyway? I would check to see if this involves a typo or miscalculation somewhere. Maybe they dropped the "m" out of 5mA or someone read 50mA and had no clue about numeration so they thought it was the same as 5A.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[JLSeagull]

Excerpts
ISA-RP12.06.01-2003
Formerly ANSI/ISA-RP12.06.01-1995 (R2002)
3.21 simple apparatus:
Simple Apparatus. An electrical component or combination of components of simple construction with well-defined electrical parameters which does not generate more than
1.5 volt, 100 milliamps and 25 milliwatts, or a passive component which does not dissipate more than 1.3 watts and which is compatible with the intrinsic safety of the
circuit in which it is used.

A.5.1.2 The values of Vmax or Ui and Imax or Ii are selected by the manufacturer of the intrinsically safe apparatus to allow connection of the intrinsically safe apparatus with as wide a variety of associated apparatus as possible. Vmax or Ui and Imax or Ii represent worst-case, associated-apparatus fault conditions … The Vmax or Ui and Imax or Ii values specified for a given intrinsically safe apparatus, taken together and compared to the ignition curves (ref. ANSI/UL 913), probably will fall in the ignition-capable area of the curve. This does not represent a problem, however, since any NRTL-approved associated apparatus must have a Voc or Uo and Isc or Io combination that is not ignition-capable. For example, an intrinsically safe apparatus with low Ci and Li values and properly rated components could realistically have a Vmax or Ui of 45 volts and an Imax or Ii of 350 mA. 350 mA is well into the ignition-capable area of the ignition curve at 45 volts. However, based on the ignition curve for Groups A and B, …

 

[Fieldbus]

You can download the FM standard 3610 or 3611 which both have Ignition curves as Appendices.
5A would be safe at 7.5V in all Gas Groups but so severely limited as far as connected cable inductance is concerned that empirical testing is the only practical way forward unless everything was closecoupled/self contained like a torch. Hence, MTL and P+F don't make barriers for those parameters.
What is the actual application?

 

[itsmoked]

Thanks for the doc tip Fieldbus.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[peebee]

Stars for itsmoked, JLS and Fieldbus.

Whycliff, you should be thanking these guys rather than throwing out snappy retorts.