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Thread: 24v transformer grounding

  1. #11
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    Quote Originally Posted by MAC702 View Post
    The 24V control is much like automotive electric (even though AC instead of DC). Bring a positive hot to where you need it, then connect the negative to the chassis ground. The frame of a car is conducting current back to the battery's negative post.
    I believe it would be a more-accurate analogy if you were to say that it's like a car where everything has its own negative wire back to the battery, and there is only a single wire connecting the battery negative terminal to the body/chassis. Nothing would actually use the body/chassis as a circuit conductor, much like the grounded conductors of our electrical services (starting at the disconnect MBJ).
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #12
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    Quote Originally Posted by drcampbell View Post
    Um, no. That's not at all how 24-volt AC circuits are configured. They have a hot wire and a return wire, same as 120-volt circuits....
    Usually yes, but I was using the example of when they don't (because sometimes they don't).

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    Quote Originally Posted by LarryFine View Post
    I believe it would be a more-accurate analogy if you were to say that it's like a car where everything has its own negative wire back to the battery, and there is only a single wire connecting the battery negative terminal to the body/chassis. Nothing would actually use the body/chassis as a circuit conductor, much like the grounded conductors of our electrical services (starting at the disconnect MBJ).
    I believe this is where the confusion is for me and I apologise if it seems as though I'm repeating myself. The common of the transformer is grounded which puts it at ground potential(0v). This makes it easy to troubleshoot and adds over current protection to the low voltage side, not sure whether an inline fuse would accomplish the same task if it weren't grounded. Anyways, as mentioned the 24v becomes it's own power source with essentially it's own neutral/ground? Instead of the transformer being center tapped like our homes we're just referencing one end of the secondary to the other? Also LarryFine added, the voltage wants to return on a single wire to the transformer that is connected to the chassis? Why is it that the chassis isn't energized? Is this just the physics/nature of electricity to go back to it's source/least resistance? Or as MAC702 said the chassis is carrying current back to the transformer but it is a negligible amount? Would that be because the loads/resistance through the low voltage circuit consume the current? I realize the neutral and ground of a main panel are only bonded at one point, but perhaps seeing them together inside the unit is what is throwing me off. If it seems like I have a misunderstanding of the basics please don't hesitate to correct me. Thanks again, appreciate the help.

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    Quote Originally Posted by hvac809 View Post
    I believe this is where the confusion is for me and I apologise if it seems as though I'm repeating myself. The common of the transformer is grounded which puts it at ground potential(0v). This makes it easy to troubleshoot and adds over current protection to the low voltage side, not sure whether an inline fuse would accomplish the same task if it weren't grounded. Anyways, as mentioned the 24v becomes it's own power source with essentially it's own neutral/ground? Instead of the transformer being center tapped like our homes we're just referencing one end of the secondary to the other? Also LarryFine added, the voltage wants to return on a single wire to the transformer that is connected to the chassis? Why is it that the chassis isn't energized? Is this just the physics/nature of electricity to go back to it's source/least resistance? Or as MAC702 said the chassis is carrying current back to the transformer but it is a negligible amount? Would that be because the loads/resistance through the low voltage circuit consume the current? I realize the neutral and ground of a main panel are only bonded at one point, but perhaps seeing them together inside the unit is what is throwing me off. If it seems like I have a misunderstanding of the basics please don't hesitate to correct me. Thanks again, appreciate the help.
    Also to add, the word common and neutral are thrown around interchangeably in the hvac field and it must drive electricians crazy because it certainly creates confusion which may be part of my problem. The neutral at the panel, grounded to earth, is not the same as say the "neutral" or "return" from a 120v light bulb.

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    HVAC Tech

    Quote Originally Posted by drcampbell View Post
    Um, no. That's not at all how 24-volt AC circuits are configured. They have a hot wire and a return wire, same as 120-volt circuits.


    The wire grounding the 24-volt common conductor completes a circuit and carries current only during a fault. Either by a 24-volt hot conductor coming in contact with the chassis, or by the 120-volt hot conductor coming in contact with the 24-volt circuit via a fault in the transformer that connects the primary to the secondary.

    Current from the 24-volt transformer will flow on the 24-volt common. An equal & opposite amount of current as is flowing in the 24-volt hot conductor.
    This is correct. Many HVACR Machines have one side of the secondary bonded to ground chassis, but all the 24 VAC loads are " wired " and do not use the chassis for a return path. Yes it can make troubleshooting easier when one is working all around and in the unit with one meter lead where power is supposed to be and one to the case.
    Microwave Radiation Dangers should be openly discussed

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    Quote Originally Posted by hvac809 View Post
    I believe this is where the confusion is for me and I apologise if it seems as though I'm repeating myself. The common of the transformer is grounded which puts it at ground potential(0v). This makes it easy to troubleshoot and adds over current protection to the low voltage side, not sure whether an inline fuse would accomplish the same task if it weren't grounded. Anyways, as mentioned the 24v becomes it's own power source with essentially it's own neutral/ground? Instead of the transformer being center tapped like our homes we're just referencing one end of the secondary to the other? Also LarryFine added, the voltage wants to return on a single wire to the transformer that is connected to the chassis? Why is it that the chassis isn't energized? Is this just the physics/nature of electricity to go back to it's source/least resistance? Or as MAC702 said the chassis is carrying current back to the transformer but it is a negligible amount? Would that be because the loads/resistance through the low voltage circuit consume the current? I realize the neutral and ground of a main panel are only bonded at one point, but perhaps seeing them together inside the unit is what is throwing me off. If it seems like I have a misunderstanding of the basics please don't hesitate to correct me. Thanks again, appreciate the help.
    Current is never "consumed;" it all makes it back to the source, usually on a wire. I work on really old machines sometimes, and have seen many that use the chassis as the return on some of the controls. I can't think of any late-model ones that do, but I did think of those old ones in my explanation. Also, the chassis is sometimes bonded to this control "common" at several places sometimes, and then will parallel the current also. Regardless, the "common" of the control 24V is tied to ground potential, like the common/neutral of a 120V source. You will get shocked if you touch 120V black to ground, but not if you touch the neutral/white to ground. You will likewise have nominal 24V (usually 28 actual) from red to blue, or red to chassis, but not blue to chassis. If you short red to chassis, you hope you only blow a fuse (and usually will on a modern machine) but can fry a transformer on an old machine with no protection. I had to crawl into many attics to diagnose and replace stuff after DIYers installed their own thermostat and didn't keep the red wire isolated from the others.

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    Bonding the 24 volt secondary is also important if there is communication wiring between units. If the secondary isn't at the same potential, it can cause issues with communication as the relative 0 volt reference isn't the same. Since the comm signals can be as small as 3 volts peak to peak, it doesn't take much to interfere with communications.
    The world is round, you will get there no matter what path you take.

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