It would be a rather poor quality transformer that had any serious capacitive coupling between primary and secondary.

Are you sure about that? Some smart people differ with you.

"Next, let’s examine the effect of capacitance between input and output windings of a transformer when the transformer is subjected to transient overvoltages.


Two conductors separated by an insulator constitute a capacitor. In an isolating transformer, the input and output windings are separated by an insulator, and therefore constitute a capacitor.


Similarly, there is capacitance from each winding to the core. Recall that transformers are wound with insulated wire. Each turn of a winding is fully insulated from adjacent turns.


At any particular point between two windings, a single turn is adjacent to one or more single turns in another winding. Each set of two single turns in different windings constitute the plates of one of the many capacitances that exist between the two windings." From - https://incompliancemag.com/article/transients-in-secondary-circuits/

 

Sorry to say after all this discussion I’m still inconclusive about whether or not ground is needed on the secondary side of 120 to 24vac transformer. (Ac not dc, in question here.) if it’s to enable tripping then fuse the ungrounded secondary is good for that. So do we need ground on 24vac? Thank you!

It all depends on the circuit and application!
The electrical code considers any thing below 60v as non lethal etc and leaves it open to the end designer/user as to whether the application requires it or benefits from it.
If it concerns the AC side, it is very likely that it is not required, again, the application will be the deciding issue.

 

application is hvac control. i never see 24vac secondary tied to ground in all our control equipment in my building - i'm a residential building super. i went to visit a controls company they said all their enclosures have 24vac secondary tied to ground. i was like ok i guess you know best.. google article is in favor to tie to ground.. idk what the final word is.

or maybe leave transformer secondary alone and tie downstream control-panels/circuit-boards/etc to equipment ground?

also is equipment ground tied to wall outlet ground or a just floating piece of equipment metal?

 

I don't really see why it is necessary, but if you feel you feel the need to follow the advice of the Company, just tie one side to the service earth ground, wall outlet GND should suffice. to one side of the 24vac.
Just as long as any down steam equipment does not have a rectified supply etc grounded also.
May require some further investigation of what you have exactly.!

 

What happens if there is a spike on the power line/ground in your building? The line, neutral, ground all go up to 2kv?

Your 24 volt transformer output is still sitting at 0, can it hold off 2 kV or so or will it's insulation be permanently damaged?

 

What happens if there is a spike on the power line/ground in your building? The line, neutral, ground all go up to 2kv?

Your 24 volt transformer output is still sitting at 0, can it hold off 2 kV or so or will it's insulation be permanently damaged?

If the secondary is truly isolated it will probably float upward as well.
Also, understand that the "green wire ground" is tied to building steel and water pipes grounds and will have a lot of current flowing. I have seen a copper water pipe used forma ground connection burned through and flowing out water because of the current flowing to ground. So tying a circuit to that "ground" will assure that a lot of transient current flows. Much better to let it float.

 

Also, understand that the "green wire ground" is tied to building steel and water pipes grounds and will have a lot of current flowing.

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"A lot of current flowing" during some transient spike event. This is in reference to post #25. Normally there is not much current in the green wire ground system. It is for protection from assorted faults that may happen, and also it helps a bit when lightning strikes, I guess.

 

Code calls for grounding, within certain conditions, with exceptions. I know we always grounded, but I've worked on many ungrounded as well. Our CSA inspectors wanted grounds.

 

Code calls for grounding, within certain conditions, with exceptions. I know we always grounded, but I've worked on many ungrounded as well. Our CSA inspectors wanted grounds.

Itis one thing to simply demand something, it is entirely different to understand a reason. An inspector is usually a person unable to make it in business, so instead they work for local government because they know all of the words.