Discussion in 'General Electronics Discussion' started by ceramicbrad, Dec 12, 2015.

4
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Dec 12, 2015
Hi. I am trying to learn more about transformers. I have a couple of exit signs I wish to install. There are three wires(input) labelled as follows: 120V(black) 120 V(neutral) and 277V(red). Since I only have 120V I connected up the black and white and wire nut the red... Noted that after a helper installed the unit the transformer was very hot to touch and upon examination(removal of electrical power and removing unit), noted that the red wire was shorted to the metal outlet box(the wire nut fell off).

So my question is, on a three input primary like this, what happens if the 277V becomes shorted while the 120v black and neutral are connected and powered to there respective feed wires....?(explain in electrical terms please).

2. Martaine2005

3,596
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May 12, 2015
I would think that the 227v primary that wasn't connected to AC, would get hot because of acting like a large resistor.
The input (primary) side has different resistances across each terminal due to the windings.
If you disconnect all the primaries, you can check all resistances with a multimeter.
Just a guess though.

Martin

4
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Dec 12, 2015
Martin:
Makes sense. I will measure the primary windings.... It seems that if it is grounded, then it would have lower resistance than the other windings, and thus more current would flow through it to ground assuming current could flow through it. So how would the current get to it if the windings are separate(or are they)? I assume I need to measure resistance across the red and the black windings also....?

4. Martaine2005

3,596
979
May 12, 2015
The windings on a transformer can be separate windings or tapped off at certain points.
So I can't really say.
Only an ohms test would show for sure. The white neutral and black, and neutral and red are the ones to measure.
I am sure a more experienced members can put me right and answer your question in greater detail.

Martin

5. duke37

5,364
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Jan 9, 2011
I would think that the 227V primary should be connected to the two phases of the supply.
The shorting of the transformer will cause enormous currents to flow giving heat and the likelyhood of ruining the transformer.
It would seem that the installation is faulty and dangerous.
1. Any live connection to a metal box should cause the earth leakage trip to activate. Perhaps the metal box was not earthed.
2. Excess current should blow a fuse or cause an overcurrent trip to activate.
3. Connection nuts have not been used in the UK for at least 50 years since they are unreliable.

6. Tha fios agaibh

2,188
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Aug 11, 2014
On a multi tap transformer only the line voltage your using should be hooked up to the corresponding tap. All the other tap leads will have induced voltage on them when powered up. This is why they need to be insulated.
When the 277 wire nut fell off and touched the fixture case, it shorted that portion of the primary winding out.

So yes, just hook up the 120v tap, neutral or common, and insulate the 277v wire.

dorke likes this.
7. Minder

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Apr 24, 2015
So if the 277 was grounded then it would depend on how the L & N were connected (in or out of phase with the 277) as to how much current flowed to ground in the 277 overwind, either way it would be excessive and if left as it were would most likely have burnt the primary out.
It probabally was within the limit of the breaker as they are time delay devices, and not that precise.
M.

8. Tha fios agaibh

2,188
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Aug 11, 2014

(Just a visual to see exposed windings)
The thing that saved the day was that these are tiny transformers.
277v minus the 120v portion leaves 157v divide 250va (tiny xformer) into 157 would be 1.6 amps to ground.

Last edited: Dec 12, 2015
9. Minder

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Apr 24, 2015
But it depends on if they are in phase with the applied 120v, the connection could well have amounted to anti-parallel!
Even in phase, one will oppose the other to a lesser degree.
M.

10. Minder

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Apr 24, 2015
If you want to learn what is going on, connect the L & N leave the 277 unconnected and measure the AC from N to 277 , you should see 277vac, if not reverse the N & L1 and re-measure.
M.

11. dorke

2,342
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Jun 20, 2015
While feeding the 120V connection there is voltage induced on the 227V wire(auto transformer effect) as well as on the secondary.

What you describe sounds like the drawing below.
If indeed so ,this suggest a very dangerous situation:
You either don't have an RCD(Residual-current device)
or if you have one it is faulty
or no earth connection to the device the transformer is in.
That situation should be fixed.

12. Minder

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Apr 24, 2015
As you can see from the diagram in #11, if the 277 touches ground it is connected in anti parallel, Opposing the main winding, when the L & N is connected as shown.
M.

13. Tha fios agaibh

2,188
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Aug 11, 2014
Good illustration.
But likely nothing wrong. The fault current was not simply not enough to trip the branch circuit breaker. Usually 20amps. This type of breaker can trip on 16amps, but will take hours.

14. Tha fios agaibh

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Aug 11, 2014
I think we all agree that is what happened.
But I don't think it would actually be 277v.
I'm not clear on what anti parallel is, but I'm guessing you mean out of phase?
Regardless, we are talking about a small portion of a small control transformer incapable of tripping the overcurrent protection device.

15. Minder

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Apr 24, 2015
I don't see any mention of Va size anywhere?
M.

16. Tha fios agaibh

2,188
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Aug 11, 2014
It was a guess. It might be even smaller like a 50va.

17. Tha fios agaibh

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Aug 11, 2014
As far as the tap voltage I would expect it to be 277v minus the 120v oposing phase leaving roughly 157v to ground.

If the xformer was 50va it would only pull a little over 300ma.

18. Minder

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Apr 24, 2015
If Very hot to the touch after switch on, the resultant current was probably limited to the winding resistance, i.e. Saturation.
M.

19. Tha fios agaibh

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Aug 11, 2014
Yes. Of course, It's kind of a wast of time to speculate on what type of transformer it might be.

20. duke37

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Jan 9, 2011
The VA rating of the transformer is a bit irrelevant, this is an indication of the output which the transformer will give continuously. Here the transformer was shorted and the current was limited by the wire resistance as Minder said. It will be several times that on normal full load.

The transformer will not be saturated, the flux will be lower than in normal operation due to the voltage lost in the primary.

A device such as this in the UK will normally be fed by a 5A fuse but some people will use a 1A fuse.

Do you have 30mA earth leakage trips in the US?