[FUN] IS vs Explosion Proof

[Stephen Mitschke]

The reason that FF does not talk about explosion proof is that it is nodifferent, at least from a network sizing point of view than a conventional"safe area" installation.Having said that, I believe the Pepprl+Fuchs as well as Hawke systems makeExplosion proof Power Supplies and Repeaters so I suggest you check outtheir web sites. Hawke is a e-hawke.com.The other thing you need to check is how many field devices/instruments canyou get that are explosion proof for the Zone you require?If you would like me to post this to the FUN list as well, please let meknow.Ian-----Original Message-----Sent: Wednesday, November 20, 2002 8:18 AMI need to know what is the difference between using Intrinsically Safe andexplosion proof on FF?I have only read about Intrinsically Safe, why on FF don't talk aboutexplosion proof?Thank for your orientationMagda C.Central University of Venezuela

[Stephen Mitschke]

FYI: Emerson Process Management's PlantWeb University has some decentinformation on the IS fieldbus FISCO model. There was a good session on ISand fieldbus at the last FRUG in San Diego. I'm also giving this topic somecoverage in the January 2003 issue of Control Engineering.Dave HarroldReed Business-----Second Message-----I may be wrong, but Foundation Fieldbus has different specifications andlimits for intrinsically safe (IS) and non-intrinsically safe installations.Intrinsic safety limits the current flow on an H1 segment to prevent apotential spark with sufficient energy to become an igniter for explosivegas mixtures. If you use explosion-proof instruments you do not need tolimit the number of nodes in an H1 segment as much as is required for IS.This typically allows an H1 non-IS segment to power up to 31 nodes, ratherthan the typical 3-7 nodes for IS. However, I doubt that the overalleconomics are as good as this would seem since explosion-proof housings forfield instruments cost much more than the non-explosion-proof housings.Dick CaroCMC Associates-----Ian's addendum-----Dick's comments above are an expansion on what I was saying with theoriginal post that if you are using Non-IS installation you will follow the'rules' without the energy limitations imposed by IS. As Dick says though,explosion proof enclosures ARE expensive and also take a LONG time toopen/close when you have to undo ALL those bolts.

[Stephen Mitschke]

One note on the Emerson PlantWeb University information. The examples shownassume Group IIC with a current maximum of 110 mA. This allows ~ 6 deviceson a barrier. This single example was shown for conceptual simplicity. The actual current, and therefore total device count depends on gas type,Group IIC or IIB, and the supply voltage. For example, FF816-FISCO definesa range of voltages and currents, which for IIB can be 14V/380mA, 15V/354mA,16V/288mA, 17V/240mA, 17.5V/213mA.This would allow ~ 12+ devices on a barrier. Since many segments have 12 orfewer devices on the entire segment, a single barrier would work for acomplete segment. This can make IS in the Foundation fieldbus world verycost effective. Regards,Tom WallaceEmerson Process Management

[Stephen Mitschke]

I did a search on Google for Fieldbus Intrinsic Safety Concept and got agood collection of links, though I know the original work was done by PTB.Does anyone have THE link that describes FISCO well?Ian-----Original Message-----I have heard a lot about FISCO but have not been able to find some detailedinformation about it. Is there a web site or book that goes into FISCO indetails?James Powell, P.Eng.Industry ConsultantCommunication SystemsSiemens Milltronics

[Stephen Mitschke]

Ian, For Class 1 Div 2 areas we at InterlinkBT have energy limiting on our 49SCproducts that allows true connectorization in the hazardous area. We dolimit the current to 55 mA at 24 V. These products are mixture ofnon-incendive components on the bus trunk line and non-incendive circuitryfor all the spurs. The 49SC series is Factory Mutual approved. Yourconversation is about FISCO which is more for Class 1 Div 1 areas or Zone 1.We are working on a solution for that but I am not familiar with details asof today.Thanks, Joseph. Waszgis Jr. (BSEE, MSMS) Western Regional Manager InterlinkBT, LLC ------- message 2 -------Great to see this spark of interest in Intrinsic Safety! As comments byothers have pointed out, FISCO does extend the conventional capability from80mA or so to 110mA (IIC), 240mA or so in IIB. It does this by dropping theconventional designed-in safety analysis and allowing explosion tested,worst-case systems. That means you also get the restrictions - segmentlength reduced to only 1000m, spurs reduced to only 30m and you can only useFISCO-compatible stuff (terminators, devices & cables). FISCO means slightlymore current at increased cost and lower reliability. (To pick up aprevious thread, FISCO cables seem to come form Germany and are not codedblack & white).Hawke's solution is unique in that it uses conventional IS designs withCENELEC and FM approvals, delivers 350mA into the IS segment and has NOrestrictions on segments (1900m total, 120m spurs, standard FF ISInstruments, standard FF-compatible cable) other than Ohm's Law. Hawke candrive 16x 20mA IS devices at 500m in IIC (Group AB). The reliability issuerelates to the conventional IS design with a single wire-wound resistor forcurrent limiting, compared to the FISCO electronic current limiting circuit(minimum 8 components) which have then to be duplicated or triplicated(since the failure mode is unpredictable).Mike O'Neill Hawke Fieldbus------- Message 3 -------One aspect you should notice is the difference between two at the devicemaintenance time.Intrinsic safe allow you to disconnect and repair one device while otherdevices on the segment are alive. Explosion proof does not. You should poweroff the entire segment to access the device most of the cases.Yoshi Morioka, Yokogawa Electric Corporation------ Message 4 ------ James, I suggest that you look at the paper Practical Aspects of FieldbusImplementation - Michael O'Neill (Hawke UK) available on the Foundationfieldbus End User Council site http://www.iceweb.com.au/ffeuca/papers.htm. There are also some other Great FF papers posted on this site. For some other links look athttp://www.iceweb.com.au/Instrument/fieldbus.htmJim Russell, Chair Foundation fieldbus End User Council Australia

[Stephen Mitschke]

Maybe we are missing the whole point of this discussion, but we alwaysthought the great advantage of using IS, is that instruments located inhazardous areas can be connected and disconnected when the bus is running.For explosion-proof, the bus must be stopped before a device can beconnected or disconnected, because it carries power.As regards the number of devices that can be connected to an IS segment,this depends on the power supplied to the bus. You will find examplecalculations in Liptak's IEH Volume 3, Chapter 4.14 as well as our FF andPROFIBUS guidelines. In general, however, FISCO IS devices are designed toconsume less power than their IS counterparts, so you will always get moreper segment than with non-FISCO devices.Within the context of good segment design, however, getting as many deviceson a segment as possible is of secondary importance. The real criteria arehow to optimise control and what macrocycle times are allowable for yourapplication. These dictate the number of devices you can have operating ona segment - more often than not, this is less than the maximum.Peter Berrie, Dieter SchrenkEndress+Hauser Process Solutions AG

[Stephen Mitschke]

I've read two responses now (Yoshi Morioka from Yokogawa, and now Peter &Dieter from E&H) who seem to think one has to shut down / remove power froma segment if it has any explosion proof devices attached, in order to do anyservice or attach another device.In our plants, when the instrument is in a classified area, we would simplyget a "hot work permit" before working on the device, or adding anotherdevice.If explosive concentrations of vapors are always present, then I'd have toquestion whether I'd even send a tech in there (without fresh air), letalone do any "hot work". These kinds of locations are very rare in ourplants . . . un-vented sub-grade ditches, sumps, vaults, or other confinedspaces . . . we normally design so no instruments are in such places anyhow."If you make it foolproof, they'll make a better fool" . Sometimes ISstrikes me as a technique that attempts to eliminate risk, where we've donean excellent job of managing it. I'd be curious if anyone can cite anyexplosions, fires, fatalities, environmental releases, or other accidents inthe last 20 years that were the result of mis-applied or carelessly applieddevices for hazardous areas. That is, where has a spark created by aninstrument or any powered device caused a fire or explosion? I'm not suggesting that we should be lax in the application of appropriatedevices in hazardous locations, or that we should be casual about localcodes and standards. I'm just saying we shouldn't increase cost andcomplexity to fix a problem that doesn't exist.John RezabekBP

[Stephen Mitschke]

Hello Ian,I have to disagree about the two comments below concerning lower reliabilityand higher costs when using FISCO complient devices/accessories.FISCO does not make any restriction concerning the cables to be used, exceptthe specific cable parameters (i.e. capacitance, inductance and resistanceper km). The range of these parameters is that large that eventually anycable indeed can be used. FISCO does not at all specify any coding of thewires. Only exception for IS: Cable must be either blue or clearly markes as"IS".I attached the official FISCO document from the PTB, to proof that the aboveis correct. There, all restrictions are mentioned. I also added apresentation, converted to PDF. Please feel to publish the presentation. Forthe PTB report, I am sure there is a copyright for the document so please donot distribute this to the FUN subscribers without permission of PTB.Thank you,Andreas AgostinPepperl+Fuchs Pte Ltd Singapore

[Mike ONeill]

For anyone still interested in using explosionproof FF devices and concerned about maintenance issues (big boxes, lots of bolts, depowering segments, etc), we have just had a novel solution approved for locations where Exe (increased safety) is acceptable.
The Hawke TG304/308 is an Exme device coupler in an Exe box that can be opened 'live' (all live parts are either shrouded or energy-limited) and individual devices can be disconnected or added to a live segment.
The field devices can be Exd explosionproof, wired in the usual way with armoured or toughened cable, to the Exe device coupler which is fitted with Exd/e cable glands.
This gives the user the opportunity to use Exd devices, with no new wiring techniques to learn, without the weight/cost of Exd junction boxes, and retains the unique Hawke TRUNKGUARD technology for spur short-circuit protection (folds back per fault rather than simply limiting (maintaining) that fault current as aditional the segment load).