Hi Ken & Neil - Some comments: The correct level of protection may not be intrinsic safety - it depends upon the classification of the hazardous area that the device is to be used in. If it is Class 1 Zone or Div 2, for instance, you may want to consider a non-incendive (assumes 1 fault) method, which is generally much easier than achieving intrinsic safety (assumes two simultaneous faults, and further, unless required spacings are maintained, unlimited simultaneous faults).
The max voltage of the battery would have little impact on its safety relative to the intrinsically safe standards. Per the IEC based standards, the spacings inside the battery cell most likely are subject to fault, as is the built-in PTC current limiter found on most Li-Ion cells - so, in effect, you will need to consider a fault condition where batt + is shorted to batt -, and any built-in PTC device is defeated. Surface temperatures under such conditions can get very high, and will ignite most flammable atmospheres. Also, the current output of a typical Li-Ion or NiMH AA size cell can well exceed 50 A under this fault condition, and will be way over the resistive energy curves, and would also fail the spark test. Batteries, believe it or not, especially rechargeable batteries, are actually considered a very dangerous item in hazardous locations. The easiest solution, of course, is to limit the fault current in a custom battery pack with series resistance, but this brings in a whole new set of functionality issues, like poor battery life, inaccurate charging algorithms, and shortened battery life, that must be dealt with. Alkaline cells, on the other hand, have a significant amount of internal impedance that may be considered to limit energy delivery, and are generally much easier to incorporate safely into designs than rechargables - of course, how happy is your customer likely to be if they have to change batteries every few hours? To answer Ken's original question, the standards in question present a set of ' energy limit' curves, by which there are methods to compare your circuit to the curves; if the available energy is below the curve by a sufficient margin, no spark test is required. The tough part is distilling a complex modern circuit down into a representative circuit consisting of an inductor, a capacitor, and a resistor, which can then be compared to the curves. For instance, let's take the example of a small battery powered digital do-dad. It is, of course, all small-outline SMT IC's and very small discrete components. From EN50020 for I-safe products, we find that none of the parts meet the spacings requirements for non-faultable parts, so we can assume that each pin on each IC can be shorted to its neighbor, and not counted as a fault - these just aren't counted. Now, I've taken the 6-page schematic down to a half-page of caps, inductors, and resistors. These are also not sized such that they meet the no-fault spacing requirements given by the standard, so - I'm now down to placing a short from batt + to batt(-) - and remember, the spacings inside the cell didn't meet the requirements either - and running the spark test. BOOM! This is a very simplistic view of the process and standards, I know, but hopefully it gets the point across: safety standards for hazardous locations products are quite different than those for normal use locations, so the old assumptions go right out the window! Point #2 - your resulting finished product will likely incorporate many trade-offs, like reduced battery life in order to achieve product safety compliance. Deal with it, and get your management to understand this, so that their expectations for the product are not unreasonable. My best advice: Work with your marketing department to develop a realistic hazardous location classification for the product. If the product will not be used in a Zone 1 area, don't shoot for intrinsic safety. If the product will be used in Zone 1 areas only 5% of the time, then forget that revenue and don't shoot for intrinsic safety. Educate yourself on hazardous locations classifications, and the various protection methods available. Find an expert consultant, if your design group does not have the bandwidth or expertise required. The agencies that Neil mentioned all have significant expertise, and can answer specific questions, but none can provide detailed design assistance, due to the constraints under their NRTL / NCB requirements. All of the above comments come from the bruises I got while being the product safety engineer on the design team for a intrinsically safe mobile computer (yes, Li-Ion battery powered!) rated: Class I,II,III Div 1 & 2 Groups C,D,E,F,G T4 and AEx ia IIb T4 for North America ATEX rated II 2 G D EEx ia IIb T4. I still don't consider myself an expert by any stretch. Good luck- Doug Massey Product Safety Engineer Advanced Compliance Solutions From: [email protected] [mailto:[email protected]] On Behalf Of Barker, Neil Sent: Tuesday, January 20, 2004 8:17 AM To: 'Ken Javor'; [email protected] Subject: RE: explosive atmosphere qualification Ken, The concept that you are describing is that of intrinsic safety, i.e. there is not sufficient voltage/current/energy/power available to cause ignition of a flammable atmosphere. If the intention is to deploy this device in a potentially explosive atmosphere, even if that may only occur very infrequently, then you will need to certify the apparatus as intrinsically safe. Intrinsic safety tends not to require many tests, but rather detailed examination of the circuits by a certification body. The design usually has to employ a number of techniques to limit the current/power/energy in each part of the circuit under either single or double fault conditions depending on the certification that you require. Relevant standards that you might certify to are UL913 for the US, or EN50014 + EN50020 for Europe. EN50020 contains many detailed parameters and is a useful design manual as well as a conformance standard. You cannot self-certify for intrinsic safety, and these requirements are imposed by the NEC in the US and by the ATEX Directive 94/9/EC in the EU. Some useful links that you might visit for further information are: - http://www.siraservices.com http://www.fmglobal.com http://www.ul.com http://www.csa-international.org Best regards, Neil R. Barker C.Eng. MIEE MIEEE MSEE Manager Compliance Engineering e2v technologies ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: +44 (0)1245 453616 Fax: +44 (0)1245 453410 e-mail: [email protected] Web: http://www.e2vtechnologies.com From: Ken Javor [mailto:[email protected]] Sent: 19 January 2004 16:28 To: [email protected] Subject: explosive atmosphere qualification List members, HERF question. I am looking at a small, low power, non-rf battery-operated device. Are there any guidelines under which one could say that the device is/is not possibly an ignition source? Max battery potential is 7.2 Volts. What I am trying to find out is, is an explosive atmosphere test a requirement, or are there conditions under which it is not necessary? Thank you. Ken Javor This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
