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  1. #1

    where did it come from

    Got sent out for a no cool. System may or may not have been heating for first four months as it is in an auditourium with two other rtu feeding the space. Was told that terminal board in new carrier rooftop was bad and changed out board to keep customer happy as control company already told them that was the problem. All was good with the change out till duct smoke interlock was tied in. Original problem of chattering contacters returned with the addition of duct smoke shutdown wires to board no issues when jumper accross terminals. Found 16v present on wires to ground. Traced back could find no shorts. Removed from fire system voltage remained. Cut wiring at a 20' interval from closest j box and with both ends hanging in mid air still had voltage. Ran new shielded conductors measured to ground still had 6v. Inspected wiring pulled out strupped off outer casing no obvious cause found. Ground shield wire of new pull to j box voltage reading disappears but contactors still chatter. Added an isolation relay to the loop system run and duct smoke shutdown passes test.what gives.

  2. #2
    Join Date
    Dec 2009
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    B.C.
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    26
    Is the 24 volt transformer grounded? I've had a similar problem and it turned out that there was two transformers connected in, one from the unit and one from a dehumidifier. The transformers have their two secondary winding connections, one of which, needs a reference to ground.

  3. #3
    Join Date
    Jun 2013
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    N. Canada
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    402
    Quote Originally Posted by SurreyStevo View Post
    Is the 24 volt transformer grounded? I've had a similar problem and it turned out that there was two transformers connected in, one from the unit and one from a dehumidifier. The transformers have their two secondary winding connections, one of which, needs a reference to ground.
    Interesting... wonder if this could also have been fixed by flipping secondary of one xmer to sync the 24v supplies?

  4. #4
    Sytem only had one transformer in it brand new carrier. Wish I had model number for ya's. Maybe has something to do with other units tying to fire alarm panel? What I do know is when interlock wire was dangling both ends in air with voltage present I got confused

  5. #5
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    Quote Originally Posted by northeastnewbie View Post
    Sytem only had one transformer in it brand new carrier. Wish I had model number for ya's. Maybe has something to do with other units tying to fire alarm panel? What I do know is when interlock wire was dangling both ends in air with voltage present I got confused
    No, my xmer phasing was aimed toward SURREY. Sorry for the confusion.

    You must be working in a hi-radiation area, that 16ft open wire is acting as a dipole antennae. I could tell you its pickup frequency but don't make me go back there! Are both ends of the shield grnded? I would have been tempted to throw a DC type 5mf/50v capacitor on alarm lead to grnd.
    That wire if running parallel to hi current ac wire runs, is acting as a xmer secondary.
    I don't know if your vom is or isn't succeptable to hi-freq input.

    model # etc won't help me, never worked on anything like that, just visualizing as you describe.

    edit: NE, I'm confused and will be confusing you! The fire alarm contacts.. they're "dry" (no v.) contacts, right? Well, that ever-present voltage coming from the board, isn't that normal?

    So, ignore everything that I've said until that question is answered.

  6. #6
    Yes the contacts are dry. Unhooked the wires and tested to be sure. The conduit runs across the ceiling of an art room in a college prep school. Do you think the wire could pick up something from the fluorescents below it. T stat and duct smoke interlock of another rtu in same conduit but not expieriencing any trouble. Didn't try grounding both ends of the shielded cable since signal disappeared with one ground made just didn't solve the problem that way

  7. #7
    And yes there is voltage supplied by the board constantly but this other present voltage was interfering with the compressor contactors

  8. #8
    Join Date
    Sep 2002
    Posts
    12,562
    Are the control wires run with any high voltage wires? If so it may be induced voltage. To check just put a load across the voltage: a little relay coil or something. If it then drops to zero - it's induced voltage and not a short/cross connected circuit.

    PHM
    -------





    Quote Originally Posted by northeastnewbie View Post
    Got sent out for a no cool. System may or may not have been heating for first four months as it is in an auditourium with two other rtu feeding the space. Was told that terminal board in new carrier rooftop was bad and changed out board to keep customer happy as control company already told them that was the problem. All was good with the change out till duct smoke interlock was tied in. Original problem of chattering contacters returned with the addition of duct smoke shutdown wires to board no issues when jumper accross terminals. Found 16v present on wires to ground. Traced back could find no shorts. Removed from fire system voltage remained. Cut wiring at a 20' interval from closest j box and with both ends hanging in mid air still had voltage. Ran new shielded conductors measured to ground still had 6v. Inspected wiring pulled out strupped off outer casing no obvious cause found. Ground shield wire of new pull to j box voltage reading disappears but contactors still chatter. Added an isolation relay to the loop system run and duct smoke shutdown passes test.what gives.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  9. #9
    Join Date
    Jun 2013
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    Quote Originally Posted by northeastnewbie View Post
    And yes there is voltage supplied by the board constantly but this other present voltage was interfering with the compressor contactors
    It's your turn to be patient with me!
    Are you saying both ends of run were disconnected when you read the stray voltage? If so, my previous post does apply-inductive coupling.

    Old, ungrounded ballast.
    (Shut off various ac ckts)
    Ungrounded conduit.
    Grnding both ends may help.

    Re-read your post.
    note to all techs: dehydration makes you stupid!

    Yes, you are picking up induced voltage! Carry on with the search.
    Last edited by IRBH; 07-01-2013 at 12:24 AM. Reason: re read your post

  10. #10
    I've been re-reading my post and other than a couple spelling errors and a few examples of bad grammar I'm missing what you have me looking for. You also asked me to be patient and believe me I am there's no rush or expectations on my end. I'm gonna look up induced voltage because I understand what it is but don't know how to negate it to my own satisfaction. Thanks to everyone for their input always welcome ideas and am happy to listen to someone who knows more than me.

  11. #11
    Join Date
    Jun 2013
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    N. Canada
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    402
    Quote Originally Posted by northeastnewbie View Post
    I've been re-reading my post and other than a couple spelling errors and a few ...........
    Oops... you've fallen victim to the Canadian habit of dropping the subject in a sentence! Sorry!

    My post should literally read:
    " (I) re-red your post....." meaning that I garnered more info from your post!

    "Induced voltage" as it applies to your case means an ac carrying wire parallel to your signal wire "induces" (treats as the secondary of a xmer) an ac voltage.

    This is not my field but I would start by grnding both shield ends. Make sure the enclosing conduit is grnded. Start flipping local breakers. In theory, if your signal wires were twisted-pair, the induced voltage may cancel out.

    What's the "terminal board" ?
    Last edited by IRBH; 07-02-2013 at 01:08 AM. Reason: terminal board

  12. #12
    Join Date
    Jun 2013
    Location
    N. Canada
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    402
    You said that it ran ok with terminals jumpered, right?
    If yes, then I assume smoke contacts are NC, right?
    If yes, just have that 20ft piece of wires hooked to RTU & short the far end. Does that chatter? What I'm alluding to is that stray voltage may NOT be your problem, but poor continuity in the duct alarm. Often stray voltages only show up on super-sensitive meters, and mean nothing in regular ac ckts.

  13. #13
    Join Date
    Sep 2002
    Posts
    12,562
    Voltage doesn't come from nowhere. You either have the ends of the wires connected to a power source, or some place in the length shorted to a power source, or you have non-twisted-pair or non-shielded wires running parallel to higher voltage wires which are creating/inducing power in the wires.

    I would apply a small load to the power you are seeing. A micro-relay or something. If the power then goes to zero it is induced.

    I have seen it before but only one time was it ever a problem. A cooling relay would 'stick' in - the induced voltage wasn't enough to Pull the relay in - but sometimes it would Hold the relay in. My 'solution' was to install a 75 amp contactor in place of the cooling relay. <g> The coil voltage didn't drop so I left it.

    It turned out that Everybody in the company had been there before me and given up - and they were 'breaking the new guy's balls' by sending me. It took me a few days to figure it out but my boss told me later that I was the only mechanic who never called in and said: "I don't know what to do." I told them that they had to run a new control cable in shielded or twisted-pair but so far as I know they never did. Hell; you could hear the contactor slam in from fifty feet away. <g>
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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