Page 1 of 2 12 LastLast
Results 1 to 10 of 17

Thread: Intrinsically safe wiring requirements for Class 1 Div2.

  1. #1
    Join Date
    Jul 2013
    Location
    Cocoa, FL
    Posts
    4

    Intrinsically safe wiring requirements for Class 1 Div2.

    What is the voltage limit for requiring an intrinsically safe device on control wiring and motor voltages? This is in regards to a sewage lift station. There is 24VDC normally going to the floats monitoring the wet well level and to control the pump operation. We also have a SCADA/BYPASS system (in the lift station control panel) that we can switch to in the event the PLC fails to control the pumps. This system, at present, would then use 120VAC or 24VAC (depending on the lift station) float voltage to monitor the wet well level and control the pump operation. Articles 501 & 504 is the sections I have been trying to use as a reference.

  2. #2
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    15,681
    Quote Originally Posted by KrisED View Post
    What is the voltage limit for requiring an intrinsically safe device on control wiring and motor voltages? This is in regards to a sewage lift station. There is 24VDC normally going to the floats monitoring the wet well level and to control the pump operation. We also have a SCADA/BYPASS system (in the lift station control panel) that we can switch to in the event the PLC fails to control the pumps. This system, at present, would then use 120VAC or 24VAC (depending on the lift station) float voltage to monitor the wet well level and control the pump operation. Articles 501 & 504 is the sections I have been trying to use as a reference.
    there is no voltage limit. however as a practical matter, if it is IS wiring, it can't be 120 VAC since there are no IS barriers for that voltage level.

    I am not even sure you can tap in downstream of the IS barriers with relay contacts or whatever you are using to isolate the circuit.
    Bob

  3. #3
    Join Date
    Apr 2004
    Location
    Mission Viejo, CA
    Posts
    4,916
    If you have read Article 501 carefully you will find that intrinsically safe is not mentioned anywhere in it. For Class I, Division 2 you need to refer to Section 501.10(B)(3) and the various nonincendive definitions in Sections 100.2 or 500.2 (depending on NEC Edition), which reference ANSI/ISA-12.12.01 [20xx] -, Nonincendive Electrical Equipment for Use in Class I and II, Division 2, and Class III, Divisions 1 and 2 Hazardous (Classified) Locations.

    Technically, voltage is only part of the "equation". Actually, total energy transfer the is issue as expressed by Minimum Igniting Current ratio (MIC ratio) and Minimum Ignition Energy (MIE).

    While Article 504 is proper for intrinsically safe systems Section 504.2 will ultimately reference ANSI/ISA-RP 12.06.01-[20xx], Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation - Part I: Intrinsic Safety.

    The basic rules are similar with lower acceptable MIC and MIEs.

    And, in either case, don't forget your required manufacturer's control drawing.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  4. #4
    Join Date
    Jul 2013
    Location
    Cocoa, FL
    Posts
    4

    Still confused

    Still confused. Do I need to have IS equipment for the float voltage and/ or motor voltages? There are properly installed seal-offs where the wiring leaves the wet well. Do the seal-offs satisfy the requirement for not needing IS equipment since any hazardous gases will not be able to enter the control panel?

  5. #5
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    15,681
    Quote Originally Posted by KrisED View Post
    Still confused. Do I need to have IS equipment for the float voltage and/ or motor voltages? There are properly installed seal-offs where the wiring leaves the wet well. Do the seal-offs satisfy the requirement for not needing IS equipment since any hazardous gases will not be able to enter the control panel?
    you never "need" IS equipment. it is just one means of protection. other means are available such as explosion proof equipment.

    as a practical matter, there is no way to use IS as a means of protection at typical motor voltages.

    if the float switch in the wet well is protected by another means (such as explosion proof), IS would not be required for float switch wiring. just having seals does not cut it though.
    Bob

  6. #6
    Join Date
    Jul 2013
    Location
    Cocoa, FL
    Posts
    4
    Thank you petersonra.

    This is an on going issue with the engineering firm doing our design work for our sewage lift stations. My point towards them "was why have IS equipment for float control voltages of 24vdc or 24vac, when you have a motor starter with 240 or 480vac in the same cabinet, coming from the same wet well", they're reply was "it is required due to NEC 70, section 504".

  7. #7
    Join Date
    Feb 2007
    Location
    USA
    Posts
    5,356
    Quote Originally Posted by KrisED View Post
    Thank you petersonra.

    This is an on going issue with the engineering firm doing our design work for our sewage lift stations. My point towards them "was why have IS equipment for float control voltages of 24vdc or 24vac, when you have a motor starter with 240 or 480vac in the same cabinet, coming from the same wet well", they're reply was "it is required due to NEC 70, section 504".

    I have had this same concern and issue. I have also seen a manufactured cabinet without barrier for IS.

  8. #8
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    15,681
    Quote Originally Posted by Sierrasparky View Post
    I have had this same concern and issue. I have also seen a manufactured cabinet without barrier for IS.
    If there is no barrier it is not IS.

    There are a fair number of devices that have IS barriers built in so a separate barrier is not required. Generally the terminals for the IS wiring on such devices are blue in color and other terminals on the device that are not in the IS circuit are a different color.

    It is also possible that the IS barrier is in a separate box located outside the main cabinet. I have seen this done where someone wants to provide a UL508a listing for the control panel. UL does not allow IS devices inside of a UL508a control panel.
    Bob

  9. #9
    Join Date
    Apr 2004
    Location
    Mission Viejo, CA
    Posts
    4,916
    Quote Originally Posted by KrisED View Post
    Thank you petersonra.

    This is an on going issue with the engineering firm doing our design work for our sewage lift stations. My point towards them "was why have IS equipment for float control voltages of 24vdc or 24vac, when you have a motor starter with 240 or 480vac in the same cabinet, coming from the same wet well", they're reply was "it is required due to NEC 70, section 504".
    I concur with petersonra's post #5 (you never "need" IS equipment). In fact, you never need an Intrinsically Safe System at all. Article 504 is simply one permitted protection technique among the many recognized in Section 500.7. While it has its own Article [504], it is never a requirement.

    That said, your engineering firm should have been able to answer your original question. If they couldn't, they don't understand Intrinsically Safe Systems in the first place.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  10. #10
    Join Date
    Jun 2006
    Location
    Ohio
    Posts
    21,385
    Quote Originally Posted by rbalex View Post
    ...
    That said, your engineering firm should have been able to answer your original question. If they couldn't, they don't understand Intrinsically Safe Systems in the first place.
    I will have achieved my life's goal if I die with a smile on my face.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •