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Let's review the safety of a custom heater

Discussion in 'Electronic Design' started by Chris Carlen, Mar 27, 2007.

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  1. Chris Carlen

    Chris Carlen Guest

    Hi:

    It was built by someone who retired. A pair of band heaters go around a
    metal tank with various plumbing attached. Water and nitrogen/air will
    flow through it. Water leakage is not expected and pressure is barely
    above atm. There is also a tube heater on one of the pipes going to the
    main tank. Heaters run on 208 single phase. 20A circuit for the bands,
    and 3A for the tube.

    The maker wrapped the tank with fiberglass braid fabric thermal
    insulation. Then layer of metal tape wraps around the whole thing. I
    have no idea what covers the studs of the band heaters. I suspect only
    a thick layer of the fiberglass fabric.

    The tank was grounded with a heavy wire tying back to the power source
    ground. The high temp leads from the band and tube heaters were run
    about 6-12" in open air into a metal junction box where they tied to
    power. The tank sits unbolted and the junction box bolted to a heavy
    metal shelf plate.

    Our electrical inspection guy recommended only one absolutely necessary
    improvement before it can pass an electrical safety inspection (must
    meet OSHA and NPFA electrical safety requirements): The open air wires
    have to be protected with conduit or some mechanical shroud.

    I plan to put BX flex-armor conduit around the tube heater wires, which
    will involve devising a custom fitting to fasten the BX to the tube heater.

    Second, I plan to run 1/2" EMT conduit from the junction box to the
    heater. The inspection guy thought it would be OK that there is no way
    to fasten the conduit to the heater tank, so it would just butt against
    the outer layer of tape.

    I think he assumed that the tank was fastened to the shelf. I
    discovered it isn't. Now I think the heater needs to be bolted to the
    shelf so that it can't move relative to the conduit and shear or tug on
    the wires.

    But what else bugs me is the unknown protection for the band heater
    electrical input studs.

    What do you think is required/recommended to protect those studs? Must
    a junction box be fastened over the band heaters' studs, and the conduit
    fastened to that box? This would entail a major rework of the
    insulating tape and fabric, plus substantial additional mechanical
    engineering.

    Or is it reasonable to just bolt the tank to the shelf and run the
    conduit to butt against the outer tape covering? I don't feel satisfied
    by this. The problem is there is no way to ensure grounding of the
    metal tape, which could go live if the band heater bolts punch through
    the fabric.

    I'm inclining toward the more thorough re-work, though that will not
    make the end user happy. But I'll have to put my name on that
    inspection form.


    Thanks for input.




    --
    Good day!

    ________________________________________
    Christopher R. Carlen
    Principal Laser&Electronics Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and
    "BOGUS" from email address to reply.
     
  2. Genome

    Genome Guest

    Last time I worked in critical stuff 'after the fact' (it was already
    broken) I, ultimately, left the place.

    In my time at the place I did a couple of fmea's and fcmea's (or whatever)
    on the basis of me not signing for them (I said so coz it was silly)......
    and that was not a problem.

    I don't fly in big planes or (fundamentalists ignore this, or buy your shit
    elsewhere) military stuff and I still don't.

    You appear to be Chris Carlen.

    Problem solved.

    DNA
     
  3. Guest

    can you help me build a rocket car.
     
  4. Guest

    you are weird it dosen't make sense
     
  5. Guest

    Earth leakage trips on all the heater circuits might not be a bad
    idea.
     
  6. Rich Grise

    Rich Grise Guest

    Can you post a pic on a.b.s.e or on a webpage somewhere? I'm kinda partial
    to the shroud; it sounds like it's to protect people from getting
    electrocuted. I'd get some perforated metal, like:
    http://www.mcmaster.com/nav/enter.asp?pagenum=3592 (scroll down)
    and bend it into some shape around the flying leads and bolt it to the
    junction box, and form the tank end so that it's close enough that fingers
    can't get between the gap, but far enough so that it doesn't chafe - maybe
    protect the tank end of the shroud with some grommet strip like this:
    http://cableorganizer.com/richco/grommet-strip.htm or similar.

    Advantages: You don't have to disconnect anything; you don't have to
    drill holes to mount BX or conduit, and what would happen at the tank
    end of the conduit? You say butting up against the metal tape - I wouldn't
    trust that. It sounds to me like by mentioning "shroud" that your
    inspector guy is doing you a favor.

    And, of course, bolt the tank down. :)

    Good Luck!
    Rich
     
  7. joseph2k

    joseph2k Guest

    By the omnivore, i hate getting into situations like this. That company is
    in trouble, their "one time" jury rig has been pressed into normal service
    and is severely non-compliant.
    For OHSA requirements, bolt the tank down properly.
    For the heater connections, they must be protected against accidental
    contact and have a well grounded protective layer in between (the NEC
    requirements have gotten a lot tougher in the past ten years)any exposed
    conductor and possible human contact. Make the connection with some form
    of (flexible) conduit with a separate equipment grounding conductor.
     
  8. jasen

    jasen Guest

    here's an idea:

    Attach a couple of strips of flat bar to the back of a junction box
    and cut a suitable hole in the back then strap it in place with one of those
    banding machines that are used for lumber etc (you can hire them, and a roll
    of strap at many places (usage is charged by weight))

    The same bands could possibly be used to keep it on the shelf.

    Don't get the bands too tight or you may crush the tank.

    Bye.
    Jasen
     
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