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Grounding secondary on 24vac transformer

Asked Sep 20, 2007, 11:27 AM — 14 Answers
On a York air handler, I have a 60hz, 240/208 -> 24vac, 40 va transformer. That's a 2 * 120v primary and (2) 24v outputs.

On one of the 24v output terminals, there is a split connector that feeds the thermostat (RC) and strangely grounds(?) on the firewall, next to the circuit board, inside the handler's housing. This is what's there. It doesn't make sense to me and has me confused; so could someone please explain this oddity of a feed line that is also grounded? The other 24v terminal by the way, feeds a relay.


Also, Needless to say, I'm troubleshooting the system. When a transformer 'blows', would it still have a partial reading on the output? I have two transformers here, the original and a new one. The original has an output of 8.5v per terminal and the new one has an output of 4.5v per terminal. I did not wire the new one up, but only connected two 120v primary feeds to test the output.

The resistance on the old and new transformers is as follows:
Old primary: common<->220 = 89ohm; common<->208 = 75ohm; 220<->208 = 13ohm
Old secondary: 24<->24 = 1ohm

New primary: common<->220 = 78ohm; common<->208 = 62ohm; common<->120 = 21ohm; 220<->208 = 16ohm
New secondary: 24<->24 = 1ohm

Both old&new: primary-secondary is 0L.


Thank you,

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14 Answers
KISS's Avatar
KISS Posts: 12,613, Reputation: 4388
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#2

Sep 20, 2007, 06:34 PM
I've been staring at this post a few times today trying to make some sense out of it without much luck, but here goes:

The secondary is, in a sense, isolated. You can connect either side to ground with no ill effects.

On a 220 volt primary of a xformer, you don't have two 120V feeds. You have one 240 volt feed.

What voltage do you measure across the primary?
Between the 220 (Common) and ground?
Between the 220 (220 term) and ground?

You can always make a cheater cord with a 120 V plug and a 0.5 A fuse and connect it to the 220 volt primary side of the xformer and plug it into a regular 120 V wall socket. The voltage at the secondary unloaded should be at least 12 V.

Windings can short partially. They can arc. They can open. Opening is more probable.

However, the insulation can breakdown and you could end up with a voltage from either the primary or secondary to the case of the transformer. This will pop the transformer if not properly fused. This failure is probable too.
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T-Top Posts: 1,871, Reputation: 531
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#3

Sep 20, 2007, 07:01 PM
Was any new parts put on before the old transformer was blown. How old is the system? If you have a two transformer system is it a mobile home?
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travlr's Avatar
travlr Posts: 7, Reputation: 1
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#4

Sep 20, 2007, 07:10 PM
Quote:
Originally Posted by KeepItSimpleStupid
I've been staring at this post a few times today trying to make some sense out of it without much luck, but here goes:

The secondary is, in a sense, isolated. You can connect either side to ground with no ill effects.

On a 220 volt primary of a xformer, you don't have two 120V feeds. You have one 240 volt feed.

What voltage do you measure across the primary?
Between the 220 (Common) and ground?
Between the 220 (220 term) and ground?

You can always make a cheater cord with a 120 V plug and a 0.5 A fuse and connect it to the 220 volt primary side of the xformer and plug it into a regular 120 V wall socket. The voltage at the secondary unloaded should be at least 12 V.

Windings can short partially. They can arc. They can open. Opening is more probable.

However, the insulation can breakdown and you could end up with a voltage from either the primary or secondary to the case of the transformer. This will pop the transformer if not properly fused. This failure is probable too.
Hi and thank you,

One is at 116v, but the other is only 100v. It measures that way at the the feed side of the circuit breaker as well, next to the unit.

At the time this problem originated, there was a shorting that strangely enough affected both the a/c as well as at least one other general house circuit. After the power "outage", my tv was clicking (weirdly) and the a/c was clicking (also weird). I shut the thermostat switch to off and unplugged the tv. Now the general circuit seams to be ok, but, I'm going to now go check some house circuits.
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travlr Posts: 7, Reputation: 1
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#5

Sep 20, 2007, 07:11 PM
Quote:
Originally Posted by T-Top
Was any new parts put on before the old transformer was blown. How old is the system? If you have a two transformer system is it a mobile home?
Hey T-Top,

No no other parts. Actually there's only one transformer. I was just reporting the partial output voltages of both the old and the "new".
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T-Top Posts: 1,871, Reputation: 531
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#6

Sep 20, 2007, 07:25 PM
You are giving us high voltage and low voltage readings one leg only. Give us L1 and L2 on transformer.
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travlr's Avatar
travlr Posts: 7, Reputation: 1
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#7

Sep 20, 2007, 07:30 PM
Quote:
Originally Posted by T-Top
You are giving us high voltage and low voltage readings one leg only. Give us L1 and L2 on transformer.
Sorry you lost me on that direction.

Oh, and the system is 13yrs old in a single family permanent structure.
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T-Top Posts: 1,871, Reputation: 531
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#8

Sep 20, 2007, 07:52 PM
Give us the primary voltage reading from both legs of power going into transformer not from one leg to ground.
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travlr's Avatar
travlr Posts: 7, Reputation: 1
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#9

Sep 20, 2007, 08:03 PM
Quote:
Originally Posted by T-Top
give us the primary voltage reading from both legs of power going into transformer not from one leg to ground.
Yeah it's only 116 across the both. Sorry about that. Characteristics of 220 is coming back to me now. It's the cumulative that's needed; individually leads to mis-interpretations of the actual voltage supplied.

I'm going to check for defective breakers.
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T-Top's Avatar
T-Top Posts: 1,871, Reputation: 531
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#10

Sep 20, 2007, 08:16 PM
Quote:
Originally Posted by travlr
Yeah it's only 116 across the both. Sorry about that. Characteristics of 220 is coming back to me now. It's the cumulative that's needed; individually leads to mis-interpretations of the actual voltage supplied.
If you need 220=check for blown fuse,breaker that has only one leg of power(turn of and back on) etc. Good luck
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