If you are referring to JCI controllers, then they don’t like the transformers grounded. Just about everybody else recommends it.
But the NEC requires the secondary to be grounded if the primary voltage is over 150 volts to ground. Electric reheat VAV’s present this problem, as do fan powered 277V boxes.
If thats the case, then JCI recommends that the step down transformer’s secondary be grounded, then use a 24 to 24 isolating transformer wired to the controllers.

 

....JCI controllers, then they don’t like the transformers grounded.

Can you elaborate on this? I know that Johnson has isolated comms, and IO, so they don't really care whether your 24v power is good bad grounded, whatever.... but what bad things happens when you ground it?

 

Ground is ground is ground...except when it floats or is isolated. Please ground one side of every 24vac xfmr unless otherwise stated by the manufacture.
It prevents stupid problems when multi xfmrs are used and helps in troubleshooting.
But come to think of it..don't. I like those easy service calls.

 

<snip>
Please ground one side of every 24vac xfmr unless otherwise stated by the manufacture.
It prevents stupid problems when multi xfmrs are used and helps in troubleshooting.
But come to think of it..don't. I like those easy service calls.

Good day control$,

Indeed, it is always best to consult the equipment manufacturers documents, but there are times where they are not available (older equipment, or manufacturer no longer around, etc). That said assuming that it is always best to ground one leg of a transformer's 24VAC secondary can create all sorts of issues you are not considering. For example, when you ground one leg of the 24VAC you create a great 60 Hz electrical noise antenna that can all sorts of communication issues with the equipment that is powered by the 24VAC. In cases where equipment may have mixed voltages present, grounded secondary now creates a direct path to ground if an electrical fault condition occurs... which can be a safety/shock hazard.

The bottom line is that there is no rule of thumb to ground or not ground a 24VAC leg without recognizes the consequences that can result.

Cheers,

Sam

 

That's exactly what we are seeing. The JCI control panel with transformer (120 to 24) we purchased doesn't have the secondary grounded. Then we purchased a newer transformer to replace the one in our MMC (600 to 120) which controls the contactors and anything else it was powering 120V.. This transformer has one post of the 120 secondary physical connected to ground.
We're just trying to understand why one is grounded but the other one isn't and in the straight up transformer nothing is mentioned on grounding the seconding from Hammond.


THanks

 

If I’m understanding you correctly, you have a 600v to 120v transformer with the secondary grounded. That is proper and required by the NEC. Your JCI control panel has a 120v to 24v transformer which has a an ungrounded secondary. That also meets the requirements of the NEC and the manufacturer.

 

NEC

Took a look at NEC for primary voltages over 150 volts.

This makes sense, based on my two situations. I was thinking grounding the secondary was going to create a ground loop for the DDC system.


THanks

 

https://control.com/technical-artic...ormers/#:~:text=In terms of grounding, this,trip that circuit protection device.

 

That's correct. I guess moving forward anything over 150 on the primary will get grounded on the secondary and any transformer that is 120 to 24 won't.

 

Good day Integrator,

I believe that it will depend upon the application, the secondary voltage, and the power involved. For example, the (Canadian) lighting industry routinely uses 347V to 24VAC (40VA to 75VA) transformers that are isolated.

Cheers,

Sam

 

Maybe in some cases. But I usually find the exact opposite.. A typical example, a controller with a xfmr in one can, not grounded on the secondary. And a few actuators somewhere else on another xfmr..also not grounded on the secondary. The question is not why doesn't it work. The question should be why did ever work at all without a common 24vac ground in the first place. Those devices will sometimes find the ground, even if just a rusted tech screw on the panel.

Rarely do i isolated secondary's help with a normal installs. Normally I see it add to any problems. I ground everything.

 

The question is not why doesn't it work. The question should be why did ever work at all without a common 24vac ground in the first place.
I ground everything.

I couldn't agree more here. But JCI stuff is a different animal to some extent.
I was on a site where they were looking for low bidders to rip out their 4 year old Alerton stuff. I asked for a day of service to count against the proposal... I kept finding AC voltage on 0-10v signals... all kinds of mess. Grounded all the secondaries and the system was great....
They still want to get the Alerton stuff out. But now they are not looking for competitive bids. Grounding the secondary is a first step on a good install the VAST majority of the time.

 

I've heard and see this point of view before, so I guess we agree to disagree on controllers. But I do agree the whole rs-485 things a mess!

 

Good day control$,

I think you missed the point I was trying to make in that understanding the "why" is very important and to not simply doing something because of what was done before or what is "routinely" done. Secondly, there is no right or wrong way of powering controllers... it is based upon the controller design and other factors which can include various approvals and/or electrical standards that need to be met.

On my side, there has never been a need, but then again I have other things to be mindful off. In most cases all of my controllers are within panels that have mixed voltages present (i.e. class 1 and class 2). Since my low voltage/class 2 controllers are isolated there is much less chance of having a safety/shock hazard if someone mistakenly ... or a rouge wire goes awry... connects/touches a class 1 voltage to my controller or its inputs. If my controller was connected or referenced directly to ground, it would be a very bad day for both the equipment and anyone present. The other consideration is ground voltage differentials which can be present. One assumes earth ground is the same potential everywhere, and that is not always the case between floors or buildings. For the latter a local lightning strike can significantly affect each building's "ground" potential... and thus can cause very large ground differential currents which could lead to safety/shock hazards... and if not, then potentially cause electrical failures in equipment.

Cheers,

Sam

 

Read your technical instructions, some controllers want a 24v AC power supply that's bonded and some don't want it bonded.

DO NOT MIX the two.

I know from experience if you bond a power supply for a jace edge that doesn't want it you will blow up the jace and your laptop.

 

I know from experience if you bond a power supply for a jace edge that doesn't want it you will blow up the jace and your laptop.

I want pictures!

 

Sam.

It has nothing to do with routine and more to do about reliability and ability to troubleshoot. Again, there are the cases where it's correct to float the transformer.

 

Good day control$,

I appear to be unable to explain my point... it has nothing to do with reliability and troubleshooting ... nor is it more or less correct to ground or to isolated the 24VAC power supply. Can you not grasp this? Read what MaxBurn just posted, as perhaps you can better understand what he posted.

That said, do as you see fit, as I give up... I was simply trying to present reasons why one should not "always" do something without understanding the why.

Cheers,

Sam

 

Sam.

I respect your knowledge so I'm trying to understand your position. No harm done since your not working on my equipment. My secondary's are grounded.

 

Awesome as usual, Sam. I didn't know that there is often an internal 100 Ohm resistor between the RS485 interface and a controllers 24v reference.

Makes sense. The documentation I seen often shows grounding an RS485 ref terminal through a 100 Ohm resistor.... I usually use a 120 as I have them lying around as a termination resistor.

Very interesting.

 

Good day numbawunfela,

Your comment of "I didn't know that there is often an internal 100 Ohm resistor between the RS485 interface and a controllers 24v reference." is incorrect. The 100 ohm series resistor is typically placed between the RS485's "ref" terminal and the reference "ground" used by the RS485's transceiver (which you correctly stated). The RS485's transceiver "could" be the same one of the equipment's input power supply, but one cannot make this assumption. You will note that I used the term "local reference" ground in my original comment. This specific detail was on purpose as there is the potential of misunderstanding the term "ground", as "ground" means different things depending upon the context. Ground could mean... Earth Ground... or a common voltage reference voltage that other voltages are referenced to.

On a side note, this "ref" terminal series resistor is meant to limit or minimize differential currents that can exist between interconnected RS485 device. The voltage across it should be small... and if not, then you have large differential currents which could create misinterpretation of the electrical levels as seen by the RS485 transceiver.

Cheers,

Sam