OT....24 VAC transformer

[Skidood]

Hi all, I know this sounds like a dumb question, but.....I'm thinking out loud...and maybe I shouldn't do that....
Upcoming project involves installing a new control system on a machine that has several 24 VAC solenoid valves.
My new control (which will be fed with 120 VAC ) will include a small power transformer (120V on the primary, 24 VAC from the secondary winding w/ 50W rated output).
What's to stop me from tying one leg of the secondary winding to the 120 VAC neutral? (as opposed to having a full floating 24 VAC). FWIW the AC neutral will not be tied to earth ground.
I can't think of any reason why this would inadvisable, other than I don't think it's done too often and may raise the eyebrow of an inspector.
I also believe that if it is left floating (not tied to the neutral), the electrical inspector would want to see a fuse on both sides of the output winding.
Any thoughts on this? Thanks in advance.

 

[Bit_Bucket_07]

Why is your 120 VAC neutral not bonded to earth ground?

 

[BryanG]

Some interesting info for you:
https://control.com/technical-articles/grounding-for-control-transformers/
https://www.aptsources.com/wp-content/uploads/pdfs/Floating-Output.pdf
So your first question should be if the grounding of one leg of the transformer is set by regulation, I don't know the Canadian electrical code.
And to answer part of your question:
Generally two legs of a transformer output are fused/ptotected when the transformer is centre tapped, and the centre tap is grounded. Fuses only react to excess current, so you have to ask where excess current will come from. With a grounded system, if the 'hot' leg goes to earth then a large current will flow and the fuse will trip. With an ungrounded system if one leg goes to ground there will be no large current flow and so, there will be no indication that the circuit is now grounded due to a fault.
My reason for always grounding a transformer or DC supply output is that I will know that there is a fault. Taking that to a logical conclusion, if I were to use an ungrounded system I would want a ground fault detection system so that I would know a fault had occurred.

 

[Skidood]

Why is your 120 VAC neutral not bonded to earth ground?

In my experience, in this part of the world, tying the 120 VAC neutral to earth is not typically done inside a control box. AFAIK, Neutral is grounded at the supply, wherever that is. That's all I can offer....

 

[Levi G]

what's to stop me from tying one leg of the secondary winding to the 120 vac neutral?

why?

 

[Bit_Bucket_07]

In my experience, in this part of the world, tying the 120 VAC neutral to earth is not typically done inside a control box. AFAIK, Neutral is grounded at the supply, wherever that is. That's all I can offer....

Right. The neutral is only connected to earth ground at the service supply point, but the neutral is still at earth ground potential.

It sounded to me as if you were describing a floating neutral on your 120 VAC power.

I see no problem with grounding the neutral on your 24 VAC transformer, and in some instances, it is required by code.

 

[Skidood]

why?

I would have one neutral in the system instead of 2 separate neutrals, and one less fuse and fuseholder to buy and wire up .

Right. The neutral is only connected to earth ground at the service supply point, but the neutral is still at earth ground potential.

It sounded to me as if you were describing a floating neutral on your 120 VAC power.

I see no problem with grounding the neutral on your 24 VAC transformer, and in some instances, it is required by code.

Good. if its required by some code somewhere, you can be pretty sure it is in the code up here. Also, my background is in electronics and I just have this weird uncomfortable feeling about a fully floating 24 VAC power circuit. The machine (when I looked at it) has a lot of old leaky dripping valves and there is a lot of wetness around (in stark contrast to my love life), so this just exacerbates my uneasy feeling.

 

[NetNathan]

I assume your control system (120 and 24VAC) has its own control transformers.
If both neutrals are grounded at the SAME point, I believe it is okay.
If grounded at different points, the 24VAC system would be using ground as path, assuming you are using neutral from 120VAC system.

To solve all doubt....You can get a control transformer with a dual voltage secondary....

 

[Levi G]

I have an inspector tell me that it is required to ground the secondary neutral if the xfmr is 100VA or greater.

It seems like a bad practice to use the 120v Nuetral if the Secondary has a nuetral

 

[Bit_Bucket_07]

I have an inspector tell me that it is required to ground the secondary neutral if the xfmr is 100VA or greater.

It seems like a bad practice to use the 120v Nuetral if the Secondary has a nuetral

I agree. I'd use a separate neutral bar for the 24 VAC transformer.

 

[I_Automation]

I have an inspector tell me that it is required to ground the secondary neutral if the xfmr is 100VA or greater.

It seems like a bad practice to use the 120v Nuetral if the Secondary has a nuetral

I always ground the secondary of a transformer, and DC power supply -V, unless I have a specific reason (IE - floating center 2 hand controls with the relay coils in the center of dual redundant switches, buttons and contacts).


A single Neutral/Common is highly preferred over separate 120AC Neutral / 24AC Neutral / 0VDC Common.


I have encountered this even in panels with 48VAC mixed in.

 

[lfe]

Many years ago an inspector had me ground one side of all supplies with transformer or power supply, there can be no floating supplies.

Since then, I stopped using the neutral in the installations and started using a bare common bar isolated from the support plate and grounded at a single point.

All the 0V cables are connected to this bar, even they are AC or DC and I also saved many Weidmuller-type guide terminals.

 

[Bit_Bucket_07]

Many years ago an inspector had me ground one side of all supplies with transformer or power supply, there can be no floating supplies.

Since then, I stopped using the neutral in the installations and started using a bare common bar isolated from the support plate and grounded at a single point.

All the 0V cables are connected to this bar, even they are AC or DC and I also saved many Weidmuller-type guide terminals.

I wouldn't recommend doing that. It creates a safety hazard.

https://waypointinspection.com/neutrals-grounds-subpanels/

 

[lfe]

I wouldn't recommend doing that. It creates a safety hazard.

https://waypointinspection.com/neutrals-grounds-subpanels/

I have made dozens of installations with this system, passing different conformity tests.

I have not said to use the same bars for 0V common and ground. I use separate bare bars.

I call it 0V common since I don't use the neutral of the power supply.

In addition, with the power supply neutral, there is a risk that it will be disconnected somewhere in the installation and this can cause overvoltages.

Here, in Europe, in new installations, they usually no longer supply you with a neutral cable, only with the 3 400V live cables and the ground cable

In addition, the system minimizes the problem of grounding loops.

All bare bars are grounded with thick wire.

 

[joseph_e2]

I'm not a code expert....but...in my experience with the US NEC:



A "separately derived system" starts with a transformer. The grounded conductor (neutral) needs to be bonded to ground at one and only one point. It kind of sounds like your 120V power comes from outside the machine and the 24VAC is generated inside the machine. With that being the case, you would bond the 24VAC neutral to ground at its transformer and isolate the neutrals because they're separately derived systems. To prevent cross-connection, you would use a different color wire for the second neutral, either gray or white with a gray stripe or something.



You can probably use the machine's incoming grounding conductor to bond the 24VAC neutral to ground but not the 120VAC neutral.