Control transformer grounding
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  1. #1
    Join Date
    Jul 2004
    Posts
    68
    Are control transformers in HVAC units grounded only for safety reasons?

  2. #2
    Join Date
    Feb 2005
    Posts
    345
    Not always, no. Controls systems can electrically reference ground in several ways. They can call the common side of an XFM ground, and they can tie the internal (DC) gound to earth ground. Often a (internal to the circuit board) tie is made between the common side of an XFM and earth ground as well. This is also typically done on a 3rd party powered tranmitter (4-20mA device for example).

    In these cases a "hard" (earth) ground reference is required to assure there's no floating ground or ground loop. The easiest (and most foolproof) way of doing this is to tie one leg the XFM to ground and to call it ground instead of common.

    On XFM's that are not grounded you can be a little more free in how you design your controls interface, but it's bad practice. Good practice dictates that even though you're dealing with AC, think of it as DC, keep the hots together, the commons together and make sure it's safe to pull common to ground at any point in your scehematic regardless of what the equipment manf. has done.

    Good question.
    Nikko

  3. #3
    Join Date
    Feb 2006
    Location
    mesquite,Tx.
    Posts
    49
    how do you know which wire off the xfmr is ntrl or grd.

  4. #4
    Join Date
    Oct 2003
    Location
    Minnesota
    Posts
    1,369
    Originally posted by nthvac
    how do you know which wire off the xfmr is ntrl or grd.
    It's the one you sink to neutral or ground.

  5. #5
    Join Date
    Feb 2006
    Location
    mesquite,Tx.
    Posts
    49
    I know but I wanted to hear some B.S. you taking all my fun away! lol

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