24v transformer grounding

[hvac809]

I'm trying to understand what I believe to be a basic/fundamental theory, however, I seem to be over analyzing. On many hvac units the 24v control transformer common is grounded to the unit chassis. This brings the common to ground potential and helps with troubleshooting. Wouldn't grounding the common, which it obviously doesn't, unless in the event of a short, cause unwanted voltage to continuously energize the chassis? Why is it that it is safe to do this and the common has no voltage? Is the voltage lost/consumed through resistive loads in the circuit, such as contactor coils? I assume there is a logic and basic explanation to this and I just need it dumbed down. Any info is greatly appreciated, thank you!

 

[MAC702]

The common (usually blue) for the 24V control source is the only grounded part of the source, much like the neutral wire in the 120V supply with which you may be more familiar.

The power (usually red) for the 24V is the one that is doing all the work on its way to the chassis ground somewhere.

Are you an electrician who works with 120V to understand the grounded nature of the common/neutral in that arrangement? Granted, we don't bond that one to the chassis of equipment, usually, but it is at the same potential, and a neutral-to-chassis short wouldn't do anything except trip GFCI's.

I was an electrician long before I got into HVAC so I had a bit of an edge on that part.

 

[augie47]

As far as the transformer is concerned you can act like there is no "house" power as it becomes its own voltage source. Not much different that connecting a car battery to the HVAC unit. Connecting one line will not cause any current to flow.

 

[drcampbell]

Bonding one side of the 24-volt transformer secondary to the chassis will eliminate any stray voltages from the 24-volt wiring because the chassis is also bonded to the service panel, which is grounded to the Earth.
It also eliminates the possibility of a shock hazard on the 24-volt wiring in the event of a primary-to-secondary fault.

 

[hvac809]

The common (usually blue) for the 24V control source is the only grounded part of the source, much like the neutral wire in the 120V supply with which you may be more familiar.

The power (usually red) for the 24V is the one that is doing all the work on its way to the chassis ground somewhere.

Are you an electrician who works with 120V to understand the grounded nature of the common/neutral in that arrangement? Granted, we don't bond that one to the chassis of equipment, usually, but it is at the same potential, and a neutral-to-chassis short wouldn't do anything except trip GFCI's.

I was an electrician long before I got into HVAC so I had a bit of an edge on that part.

No I'm not an electrician, I'm a few years into the hvac field and a significant amount of service calls are electrical related so I'm trying to get a better understanding. I believe I'm confusing neutral of a branch circuit, such as a 120v light bulb, with the neutral at the house panel. I can't seem to grasp the idea that the 24v common grounded to the chassis doesn't carry current, yet completes a circuit. I understand not connecting neutral and ground anywhere other than at the service to prevent parallel paths, but this concept seems to contradict that? I also understand a short to ground will blow a fuse on the control side, but how is it 24v won't flow on the common? Is it because the transformer is isolating the 24v from the high voltage? Thanks for the quick response.

 

[LarryFine]

The only way 24v current would flow even on the grounded 24v wire would be if another 24v component used ground as a conductor. Any single grounded point on a circuit will carry no current under normal conditions.

 

[augie47]

No I'm not an electrician, I'm a few years into the hvac field and a significant amount of service calls are electrical related so I'm trying to get a better understanding. I believe I'm confusing neutral of a branch circuit, such as a 120v light bulb, with the neutral at the house panel. I can't seem to grasp the idea that the 24v common grounded to the chassis doesn't carry current, yet completes a circuit. I understand not connecting neutral and ground anywhere other than at the service to prevent parallel paths, but this concept seems to contradict that? I also understand a short to ground will blow a fuse on the control side, but how is it 24v won't flow on the common? Is it because the transformer is isolating the 24v from the high voltage? Thanks for the quick response.

Thats a pretty good way of looking at it

 

[MAC702]

...I can't seem to grasp the idea that the 24v common grounded to the chassis doesn't carry current, yet completes a circuit...

I think this is the rub.

The grounded conductors are conductors, they carry current. The chassis in this case is conducting the 24V control current back to the control xformer. It's minuscule, but there. The white neutral wire in a 120V power circuit is grounded (at the main panel) and conducts current, but it itself is at ground potential. It can't conduct to ground like the "hot" ungrounded conductor, typically black.

At your thermostat, one not powered just by batteries, the red supplies the power, the blue is the grounded common at the same potential as the chassis of the machine. It completes the circuit powering the t-stat.

 

[MAC702]

...I can't seem to grasp the idea that the 24v common grounded to the chassis doesn't carry current, yet completes a circuit...

I think this is the rub.

The grounded conductors are conductors, they carry current. The chassis in this case is conducting the 24V control current back to the control xformer. It's minuscule, but there. The white neutral wire in a 120V power circuit is grounded (at the main panel) and conducts current, but it itself is at ground potential. It can't conduct to ground like the "hot" ungrounded conductor, typically black.

At your thermostat, one not powered just by batteries, the red supplies the power, the blue is the grounded common at the same potential as the chassis of the machine. It completes the circuit powering the t-stat.

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.

 

[drcampbell]

... 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.

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.

... I can't seem to grasp the idea that the 24v common grounded to the chassis doesn't carry current, yet completes a circuit. ... but how is it 24v won't flow on the common? ...

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.

 

[LarryFine]

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).

 

[MAC702]

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).

 

[hvac809]

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.

 

[hvac809]

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.

 

[StarCat]

HVAC Tech

HVAC Tech

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.

 

[MAC702]

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.

 

[rlundsrud]

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.