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Electrical Contractor
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Discussion Starter · #1 · (Edited)
Got a call from one of my customers, who supplies some electrically operated equipment ( don't want to be too specific )
Anyways, he was questioning; if 2 phases on a 3 phase 208 volt 15 amp breaker were to momentarily short out, and trip the breaker, could the surge in turn blow out a phase in the 150 kva transformer that feeds the panel?
No breaker between the transformer and panel, just on the 600 volt side ( and that breaker didn't trip) And there is virtually no load on the transformer.
I can't see the transformer windings being weaker than a 15 amp breaker.
Inquiring minds need to know:)
 

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This transformer needs to get blown:



The transformer would see a bit of current spike prior to the OCPD tripping, the longer duration the worse. If it's in question you can disconnect the transformer and ohm out the windings, and megger it.
 

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Discussion Starter · #4 · (Edited)
One winding is definitely blown, and the short was momentary.
What happened was, they were swapping 2 phases ( reverse direction) when the phases touched, blew the breaker right away. ( stupid machinists)
From what I was told, the primary didn't trip, and since nothing else was connected to the panel, they don't know when the transformer phase was damaged
 

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One winding is definitely blown, and the short was momentary.
What happened was, they were swapping 2 phases ( reverse direction) when the phases touched, blew the breaker right away. ( stupid machinists)
From what I was told, the primary didn't trip, and since nothing else was connected to the panel, they don't know when the transformer phase was damaged
Reason #483 why not to work hot.
 

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Bilge Rat
motors and controls.........
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I don't see any possible way that a fault downstream of a 15 amp breaker could wreck a 150KVA transformer.

Had the breaker not tripped, I'm pretty sure that the transformer in question would have blown it to smithereens.

If there are fuses in series with the 120 side, it's possible that one of them could have blown, provided the length of small conductor is short.

I'd test voltage at the transformer terminals, and also test the high side. Use a solenoid type wiggy, not a DMM.
 

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Maybe there was already a weakness in the transformer ?
that had not yet finally failed,
and what happened was enough to be the final end.

The proverbial "straw the broke the camels back"
 

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Electrical Contractor
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Discussion Starter · #8 ·
I don't see any possible way that a fault downstream of a 15 amp breaker could wreck a 150KVA transformer.

Had the breaker not tripped, I'm pretty sure that the transformer in question would have blown it to smithereens.

If there are fuses in series with the 120 side, it's possible that one of them could have blown, provided the length of small conductor is short.

.
Maybe there was already a weakness in the transformer ?
that had not yet finally failed,
and what happened was enough to be the final end.

The proverbial "straw the broke the camels back"
Both of these thoughts were basically the first thing that came to mind.
The EC on the project is trying to lay blame for the failure on this short. Having not seen the installation, and ASSUMING, I was given all the information, it doesn't make sense that a momentary short would destroy such a large transformer. A small control transformer, for sure would be possible to fail.
Just thought if someone with more transformer experience had a different take on this.
 
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Sure, it's possible that fault could damage the transformer. It's also possible that I might get to bang Scarlett Johansson but it's not freakin' likely.

That's a very small through-fault and the only way I could see it opening a transformer winding is if the transformer was on the verge of failure already, in which case an across-the-line motor start might've done the exact same thing.
 

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150 KVA transformer is rated for 416 amps secondary current (208/120 VAC), a 15 amp circuit beaker trips on instantaneous at around 90-150 amps. The transformer saw a momentary load no different than a motor starting up and took it in stride.
 

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What happened was, they were swapping 2 phases ( reverse direction) when the phases touched, blew the breaker right away. ( stupid machinists)
Reason #483 why not to work hot.
I'm about as liberal as one can be about hot work. I work live just about everyday, often just due to my own laziness/convenience and never wear ppe. Even I wouldn't do this live. Why would someone swap phases on a transformer live? You might as well shut it down. The loads aren't going to have power to them anyway while you are swapping phases around. It's this type of idiot that causes the rest of us to be saddled with all of the ridiculous safety rules (I don't follow them anyway, but that's not the point). If people used basic common sense, nfpa 70e wouldn't even exist.
 

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EBFD6 said:
I'm about as liberal as one can be about hot work. I work live just about everyday, often just due to my own laziness/convenience and never wear ppe. Even I wouldn't do this live. Why would someone swap phases on a transformer live? You might as well shut it down. The loads aren't going to have power to them anyway while you are swapping phases around. It's this type of idiot that causes the rest of us to be saddled with all of the ridiculous safety rules (I don't follow them anyway, but that's not the point). If people used basic common sense, nfpa 70e wouldn't even exist.
They weren't swapping phases on the transformer itself. It was a 3 pole 15A circuit.
 

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They weren't swapping phases on the transformer itself. It was a 3 pole 15A circuit.
That's just as bad. Whatever that circuit was feeding obviously wasn't being used while the phases were being swapped, shut it off. Like I said, morons like that make all of us that do hot work look bad. A couple of idiots make a bad decision and the safety nazis will be all over everyone's case.
 

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Not likely, but if there was a defect in the windings it could happen. Id more likely think a splice or lug blew off.
 

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I'm curious about the statement that there is nothing between the 150kVA transformer and a 15A 3pole breaker or that the only protection is the transformer primary OCPD.... I find that difficult to accept. I'd be willing to bet there is a fused disconnect somewhere that the user is unaware of and one fuse is blown
 
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