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  1. #1
    Join Date
    Jul 2007
    Location
    south bay california
    Posts
    202
    Post Likes

    Dc bi bo infultration

    Question is due to circumstance below,
    Can the 20vdc signal be falsely sent back to its input, or enough frequency picked up in the wire to give a false signal when the wire is in the same low voltage wire bundle the 24v start stop is in, not shielded, but is copper stranded.
    Furthermore, these wires are inside a boiler with 110v power and the wire may cross its path within an inch or so.
    I have been trying to find an intermittent momentary alarm on a raypak boiler. Tech support has told me any alarm would store in the boilers history. Every day i return to the boiler, nothing resides in the history.
    The boiler has normally open contacts that close when an alarm condition takes place. These alarms happen early morning and in the evening. I have spent a very long time working on these newley installed boilers to not find a problem. I think it is very unlikely there is a prob with the bms, as this is a very reputable and capable controls contractor, and a good product they use frequently on our jobs.
    They use a jci iom4711 bi's to monitor the boilers alarm status. This is a 20vdc signal sent to the boilers n/o contacts, that is returned in the event of alarm when they close.
    Whats even weirder is the alarm contacts show closed for just 1 second in many circumstances. This is monitored through an alarm log that emails like 10 people everytime it happens. Sometimes it happens 3 times a night, other times it happens a dozen times.
    So im just doing my dudilagence to see if what tech support suggested is even a posability.
    Anyone have ans opinion in the matter, other than 'someone is always trying to blame the controls guy!'
    Tuesday im going to throw some tattle tales on the relays that get energised to close the alarm contacts to prove it is actually the boiler, but til then... I can only wonder

  2. #2
    Join Date
    Jun 2009
    Location
    StL, MO
    Posts
    645
    Post Likes
    While most modern controllers have decent electrical isolation odd things still happen.

    I am not a fan of TattleTails as they don't catch quick, small current changes. While expensive, 1/128 A fuses don't miss much.

    An easy way to pin down boiler or controls is a relay-in-box (RIB et al). Put a RIB in the JCI panel. Power the RIB through the boiler alarm contacts. Monitor the RIB contacts with the IOM.
    UA LU 562

  3. #3
    Join Date
    Feb 2008
    Location
    Washington State
    Posts
    132
    Post Likes
    Inserting a delay on make hardwired relay (or software equivalent in the controller) can filter out transient 'ghost' alarms.

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