Hi are we supposed to tie one output wire of this transformer to equipment ground? i don't see that done in many instances. is it relevant? thanks

 

Depends on the application, especially LV circuitry.
In 120vac control circuits where the 120v is isolated then it is permissible to re-reference earth ground by taking one of the secondary's to earth creating a grounded neutral for safety purposes
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Not usually. 24V is generally regarded as safer if neither side is earthed, so that no short circuits can be created by the other winding touching earth, known as Separated Extra Low Voltage.

 

There is usually no reason to connect either side of an isolated 24 volt transformer winding to "ground". In fact there is not an adequate reason to connect one side of any ISOLATED power source to "ground." If there is no connection to the common ground at one side of an isolated circuit then when a grounded person contacts the other side no current will flow and no shock will be received. That is a primary reason for keeping circuit isolated from ground.
There is a large religion that believes differently and it is totally hopeless to argue with them.

 

There is usually no reason to connect either side of an isolated 24 volt transformer winding to "ground". In fact there is not an adequate reason to connect one side of any ISOLATED power source to "ground." If there is no connection to the common ground at one side of an isolated circuit then when a grounded person contacts the other side no current will flow and no shock will be received. That is a primary reason for keeping circuit isolated from ground.
There is a large religion that believes differently and it is totally hopeless to argue with them.

My exception in post #2 is outlined in NFPA79 and NFPA70 (NEC) that any control circuit that is isolated can be re-referenced to Earth GND in order to resume safe practice.
In some cases LV radio reception can also befifit from earth grounding, but for a different reason.

 

The mechanism of improving safety by grounding one side of an otherwise isolated circuit has not been explained in a way that makes any sense, Sort of like, If the sky fell we will be better off inside.

 

The mechanism of improving safety by grounding one side of an otherwise isolated circuit has not been explained in a way that makes any sense, Sort of like, If the sky fell we will be better off inside.

What part don't you get?

NFPA79
Control transformer GND re-reference

 

What part don't you get?

NFPA79
Control transformer GND re-reference

Max, I see where one side of the isolated decondary is connected to some variety of "ground", but tghat does not explain why assuring that one side of the circuit is a certain shock hazard relative to common ground is safer than having niether side of the circuit being a shock hazard relative to common ground.

And I am quite aware that some organizations insist on this. I also recall at least one auto plant where every control cabinet had to have "ground detect" lights to warn if either side of the isolated power had become grounded.

 

i googled "does transformer secondary need to be grounded" and this came up. So I guess they want secondary to be ground so the circuit can trip. is that true? can't you also just put a fuse on an ungrounded secondary?

https://control.com/technical-articles/grounding-for-control-transformers/

 

i googled "does transformer secondary need to be grounded" and this came up. So I guess they want secondary to be ground so the circuit can trip. is that true? can't you also just put a fuse on an ungrounded secondary?

https://control.com/technical-articles/grounding-for-control-transformers/

The article certainly goes into detail as to how grounding is done, but the explanation as to WHY it is done is rather nebulous. It seems far more a description of how to be like everybody else, instead of THIS is why it is important.
The only part that offers any explanation is the small portion where it talks about a failure of some insulation causing a fuse to fail and remove power from the portion of the grounded system. Of course, if that system in the example were not grounded then the hazard of shock would be less, because of the isolation.

 

And I am quite aware that some organizations insist on this. I also recall at least one auto plant where every control cabinet had to have "ground detect" lights to warn if either side of the isolated power had become grounded.

And then it’s no longer isolated because it is connected to ground via a Ground-detect lamp, which, if it is a 24V filament lamp, has quite a low resistance to ground when cold.

 

And then it’s no longer isolated because it is connected to ground via a Ground-detect lamp, which, if it is a 24V filament lamp, has quite a low resistance to ground when cold.

Totally correct, but the lamps were all Allen Bradley 800T series push to test transformer types. So the 120 volts was connected through the primary of a transformer. That meant that each side was 60 volts off ground. It also meant that no single ground would pop a fuse and stop production, as the machines were all production line items. And at the line cost of several thousand dollars per minute it made sense.
Also, a current limited 60 volt circuit is not going to deliver a fatal shock to most folks. Of course, none of the shocks that I have received over the years were fatal, either.
With a 24 volt system there is no serious shock hazard for most people, and with the type of insulation used on most furnace thermostat wiring accidental connections to sharp edged ductwork could be a problem.

 

i googled "does transformer secondary need to be grounded" and this came up. So I guess they want secondary to be ground so the circuit can trip. is that true? can't you also just put a fuse on an ungrounded secondary?

A few more examples come to mind.
1/ PC tower & table top power supplies are isolated LV supplies where the common is referenced to Earth via the ground plane screw to case.
2/ Many popular models of treadmills have an isolated LV supply for the console, the common of which is taken to earth GND
3/ Much of the electronics in the DIY CNC field are from a few different suppliers and are isolated from each other, but this often causes spurious noise problems unless the PS commons are not connected and earthed.

If you are using a ungrounded supply, each 'live' leg of the supply is normally fused.

 

And I am quite aware that some organizations insist on this.

Some 'Organizations' include just about all Service Co. suppliers world wide for safety reasons.
I suggest you get hold of a copy of 'Book on Grounding' by Eustace Soares, it is used by NEC and CEC as a reference and published by the IAEI (International Assoc. of Electrical Inspectors).

 

Max, I see where one side of the isolated decondary is connected to some variety of "ground"

BTW, that is meant to indicate one side of the secondary is connected to the service entry ground reference conductor.

 

Some 'Organizations' include just about all Service Co. suppliers world wide for safety reasons.
I suggest you get hold of a copy of 'Book on Grounding' by Eustace Soares, it is used by NEC and CEC as a reference and published by the IAEI (International Assoc. of Electrical Inspectors).

OK, I see now that it is quite a bit a "C.Y.A." practice, which certainly explains things. So I will not be attacking others "religious practices."
BUT for a 24 volt system it really does not offer a safety benefit.
AND quite a few of the power supplies for some of my laptop and portable computers have only 2 pin plugs for the mains connection.

 

Portable PC's are an exception, all desk/tower PC/s in my experience have earth grounded LV DC supply MOBO's.
It is not always done for safety reasons, there are a few others.

 

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!

 

There can be capacitive coupling between the primary and secondary of the transformer. If both sides of the secondary are ungrounded then the secondary may float at about 1/2 the primary voltage, say 60 VAC.

Say your 24 volt transformer is input to a low voltage power supply for some electronics you are working on.
When you touch something in your circuit with a grounded soldering tip... The IC sees 60 VAC...

And it is exciting to see a spark between your soldering tip and the circuit.

 

It is a VERY POOR CHOICE to solder connections on an active circuit. In fact, it is usually close to stupid!!!

It would be a rather poor quality transformer that had any serious capacitive coupling between primary and secondary. Besides that, it is appropriate to tie the core of a power transformer to the "green wire" ground.
And no matter what sort of application, it is seldom any benefit to tie a 24 volt AC system to earth ground. With the transformer core connected a hazard condition is removed. And if it is part of a power supply, the AC connections may not be at the supply output comon potential, such as in a bridge rectifier or a full wave voltage double circuit.