Intrinsic Safety and RS232

Discussion in 'General Electronics Chat' started by evilclem, Sep 30, 2013.

  1. evilclem

    Thread Starter Active Member

    Dec 20, 2011
    Hi guys,

    Doing some initial research into what is required to build our products to the Ex ia standard.

    I was hoping to make our device an intrinsically safe apparatus with a Ui of 10V. The problem is that I don't know what voltage to expect from other Intrinscially Safe RS232 devices.

    Can I just throw in a safety shunt to limit myself to 10V prior to it reaching any infallible components?

    I probably need to ask the same question about our RS485 interface too.
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    the bulk of your troubles/expense will be in the connections.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    I am rethinking just how intrinsically safe an item might be if this is how a new intrinsically safe product starts out.

    You do realize that people with families count on intrinsically safe devices to be safe. They are installed in areas with flammable and/or explosive environments and no amount of training is going to help them if your device fails.
    strantor likes this.
  4. evilclem

    Thread Starter Active Member

    Dec 20, 2011
    How is that? I am aiming for Ex ia, not Ex d.
  5. evilclem

    Thread Starter Active Member

    Dec 20, 2011
    My intention is to design the device not to fail. We are designing not only to meet the standards but also to survive abuse from misuse.

    We are designing a data logger for the purpose of connecting simple apparatus in hazardous areas.

    The connection of communications circuits could also be a benefit for long term monitoring in hazardous areas.

    My primary concern is that the RS232 and RS485 standards permit voltages higher than what I can accept in parts of my circuit. The standard way to protect from overvoltage is a safety shunt as per Section 8.7 of IEC 60079.11:2011.

    I guess my question was, am I still compatible with other devices if I limit the voltages on my RS232 lines to no more than 10Vdc?
  6. evilclem

    Thread Starter Active Member

    Dec 20, 2011
    Just thought of the stored energy of the other device being an unknown. Might see if I can use galvanic isolation instead, will make things easier.
  7. evilclem

    Thread Starter Active Member

    Dec 20, 2011
    Just saw your unedited reply in my inbox. You really don't have any clue about Intrinsic Safety do you?

    It's all very well and good to carry on about the blokes getting home safely, but actually submitting useful help regarding the achievement of that goal appears to be too great a challenge for you.

    I have seen a hell of a lot of people receive no guidance at all in this forum because you all think you are better than everyone else. The fact is, you know nothing about other peoples background, nothing about how early on in their research/design they may be and nothing about what they can achieve if they receive a simple answer to a simple question.

    There are a hell of a lot of myths regarding Intrinsic Safety (and you probably believe all of them) and I was hoping that there were people in this forum who actually knew something about hazardous area design.

    Before certification drawings are certified and accredited by a certified body, a prototype must undergo rigorous assessment to ensure compliance with the standards. Section 7.1 of IEC 60079.11-2011 states the over-ratings at which the components must meet to ensure a safety factor. If a product is too complex to simulate or calculate the stored energy, then it is loaded on a spark test apparatus to determine if it will actually create an ignition.

    Now my question was in relation to the inter-connection of Intrinsically Safe devices via RS232 and possibly RS485 and whether limiting their voltages to a lower level would affect the standard. My question was not 'how do I build an intrinsically safe device around an rs232 port'. Further thoughts on this topic lead me to the idea that the manual must specify galvanic isolation for this interface, this leaves the ball in the court of the system designer (rather than myself as the product designer).

    Now you know at least a little bit about the certification requirements, perhaps you'll see that AAC has no liability concerns regarding ANY IECEx related product design questions as the product must undergo rigorous certification. As you may have also noticed, the implementation and installation of Ex certified equipment is another ballpark altogether, in which there may be a case of liability concerns.

    Some people in this place need a serious attitude re-adjustment.
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    RS232 will more than likely require zener/isolated barriers or repeaters to achieve safety..
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    No such thing. Either you plan for a failure or you've failed to plan.

    Hint: "intrinsic safety" embodies this concept.
    PackratKing and GopherT like this.