# grounding secondary side control transformer



## dronai

Working on a pcb board for a furnace. The control transformer should be grounded on the secondary side. How to determine which wire to ground. I'm assuming its not grounded already.


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## dronai

What I'm asking is the transformer is not already grounded on the secondary side. So just chose one side of the secondary to bond ?


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## dronai

*secondary


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## don_resqcapt19

If this is a factory installed control power transformer, why do you want to ground it?


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## John

dronai said:


> Working on a pcb board for a furnace. The control transformer should be grounded on the secondary side. How to determine which wire to ground. I'm assuming its not grounded already.


It most likely grounded through the circuitry on the pcb.

Are you having problems?


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## dronai

This is my ac guys job, and he was worrying me, by telling me to take care of that. I didn't want to toast the new board. ! Not my expertise as you can tell. Turns out the problem was bad ac condensor relay control wires. They were shorting, but not enough to see with a continuity tester. By- passed and it works. Short burned up the pcb board, and control transformer. The new board showed the secondary transformer grounded.


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## dronai

don_resqcapt19 said:


> If this is a factory installed control power transformer, why do you want to ground it?


We replaced the transformer, and because it is on the schematic. I didn't know if one of the windings off the secondary were already grounded.


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## don_resqcapt19

dronai said:


> We replaced the transformer, and because it is on the schematic. I didn't know if one of the windings off the secondary were already grounded.


Those transformers are typically ungrounded unless the primary side is 480.


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## dronai

John said:


> It most likely grounded through the circuitry on the pcb.
> 
> Are you having problems?


Thanks, and yes always  That's what keeps it interesting. I'm always looking into new areas.


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## John Valdes

Dorian. You have the drawing and you have a meter. Now ask the question one more time.


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## dronai

John Valdes said:


> Dorian. You have the drawing and you have a meter. Now ask the question one more time.


John we both tested the secondarys to ground. With my meter I got 1 V on both. My buddy with a true RMS meter got 7V on one leg, and 9V on the other ?? 27V between the two.


I do see in the drawing that the xfmer windings don't show they are grounded coming out.

If you take one of those secondary windings,and bond it at the board, like shown in the drawing, I thought I could have possibly damaged it. Originally,there was a double lug at the com terminal, with one green wire going to the pilot ignitor assembly housing. So I hooked it up the same, and the ok pilot light was not light anymore! That's when I posted here in a panic.
The new board was over $200. !! so just being careful.


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## Big John

Use a low impedance meter. Worst case, shut it off and test continuity. That you got any voltage at all seems to suggest it's floating, but I can't tell without seeing the whole schematic.

Honestly, if you reconnected it the same way you disconnected it, I wouldn't worry.

-John


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## dronai

Big John said:


> Use a low impedance meter. Worst case, shut it off and test continuity. That you got any voltage at all seems to suggest it's floating, but I can't tell without seeing the whole schematic.
> 
> Honestly, if you reconnected it the same way you disconnected it, I wouldn't worry.
> 
> -John


I don't know if you can read this drawing ? What to test with this meter ? For voltage ? What will a low impedance meter show me ? Do I need a True RMS meter ? 

I defintely learned what an inline amp reading on a meter is good for in this LV control troubleshooting. We used to see why one of the wires was sparking more than the others when separated. It showed the relay coil was using 6A !
After we eliminated the short, we got -1A


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## John Valdes

I would be using the ohm scale and just verifying the terminal marked ground is indeed attached/connected to the grounding screw on the board.
Continuity to ground or not. 
I have warned you more than once about using ground as the reference. You now can see why.


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## dronai

John Valdes said:


> I would be using the ohm scale and just verifying the terminal marked ground is indeed attached/connected to the grounding screw on the board.
> Continuity to ground or not.
> *I have warned you more than once about using ground as the reference. You now can see why*.


Yes you have :laughing: and I see what you mean.

I left out an important detail here. The replacement board was a "Universal" So it was not the same as the original. I also had to purchase a "flame ignitor sensor upgrade kit", because of the date of the furnace. This added to the confusion when the instructions and drawing had sockets that didn't match ours :blink: and we couldn't get the burners to come on. The draft inducer was not working either !! Mean time it was about 100 degrees, with the customers pacing back and forth !


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## Big John

dronai said:


> I don't know if you can read this drawing?


 Look at the right hand side of that drawing. It looks like a blowup of the LV side of the circuit, and there's a transformer at the top with a 24V secondary that is grounded.


> What to test with this meter ? For voltage? What will a low impedance meter show me? Do I need a True RMS meter ?


True RMS shouldn't make a difference; there shouldn't be anything to distort the sine-wave. If one side is grounded you should have 0V to ground on that leg, and the other side should read 27V to ground (i.e. the same voltage you get between legs.) 

Or you can just measure continuity from ground to transformer. If it's grounded, it'll show.


> I defintely learned what an inline amp reading on a meter is good for in this LV control troubleshooting. We used to see why one of the wires was sparking more than the others when separated. It showed the relay coil was using 6A !
> After we eliminated the short, we got -1A


 No reason that the appropriate ammeter shouldn't have been able to see LV current. Either the ammeter can't read accurately for very low currents (not uncommon) or there's something else wrong with how it's being tested.

-John


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## dronai

This was a new replacement xfmr so that drawing may not apply. 

I've noticed, the neutral conductor is never color coded on the secondary side of small transformers. 
After the step down thru induction of the windings, the new secondary windings (non grounded) have a reading to ground from each side of 27V yes ? Then if you bond one leg to the chassis of the furnace, whats to stop it from shorting out ?

Also, shouldn't the secondary be fused to prevent what happened in the first place ?


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## toklover

Could be a floating supply


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## John Valdes

dronai said:


> This was a new replacement xfmr so that drawing may not apply.
> 
> I've noticed, the neutral conductor is never color coded on the secondary side of small transformers.
> After the step down thru induction of the windings, the new secondary windings (non grounded) have a reading to ground from each side of 27V yes ? Then if you bond one leg to the chassis of the furnace, whats to stop it from shorting out ?
> 
> Also, shouldn't the secondary be fused to prevent what happened in the first place ?


That's because it's not a neutral until you ground it. Then it only becomes the grounded conductor. A CCC.
The primary winding and the secondary winding of a transformer have no polarity. This is AC.
It does not matter which lead you ground on the secondary.
That is why many control guys do not use white as the grounded conductor on the secondary of the control system. 
I for one like to use the same color for all control voltage conductors and use numbering for identification.
They usually mark it (the grounded conductor) #2 and use the same color as the rest. The potential is exactly the same on both secondary conductors. Each wire is one end of a coil.

Fuse either the line or the secondary. It is the same either way.


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## dronai

Thanks John :thumbsup:


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