Bad Transformer issue??

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jbishop

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I don't have alot of expeirience with transformers so i'm sure you guys can help me out here.
Here is the issue, we are upgrading some lighting in a factory and when working in one of the panels I noticed that the cabinet was 120v to neutral. With further investigation I found that there was a scraped # 12 THHN in an LB that was cuasing the cabinet to be hot. I also checked from the main service cabinet to the neutral in the panel I found the issue in and had the same 120v. Its a 480 3 phase service the panel with the issue was fed from a transformer that dropped the voltage down to 120/240 single phase. They have 2 other similar transformers in the building neither one of these have the same issue. We replaced the transformer and everything is fine now. My question is how can we send 120v to ground without tripping a 20 amp breaker? And how do you test for this problem? I put a megger from the transformer enclosure to each winding and the lowest reading I got was 10 G-Ohms.
Thanks
 
jbishop said:
I don't have alot of expeirience with transformers so i'm sure you guys can help me out here.
Here is the issue, we are upgrading some lighting in a factory and when working in one of the panels I noticed that the cabinet was 120v to neutral. With further investigation I found that there was a scraped # 12 THHN in an LB that was cuasing the cabinet to be hot. I also checked from the main service cabinet to the neutral in the panel I found the issue in and had the same 120v. Its a 480 3 phase service the panel with the issue was fed from a transformer that dropped the voltage down to 120/240 single phase. They have 2 other similar transformers in the building neither one of these have the same issue. We replaced the transformer and everything is fine now. My question is how can we send 120v to ground without tripping a 20 amp breaker? And how do you test for this problem? I put a megger from the transformer enclosure to each winding and the lowest reading I got was 10 G-Ohms.
Thanks
Click to expand...
Sounds like the first trans did not have a system bonding jumper.
 
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Agree.
The problem often occurs when the system bonding jumper is omitted. You need to assure the install complies with 250.30 in all respects including the system bonding jumper and connection to a grounding electrode system.
Unless there was a internal short in the transformer (extremely rare) changing the transformer should not have fixed the problem.Possibly coincidence unless the bonding jumper was installed with the new transformer in which case an overcurrent device should have opened.
It is extremely important that you address this issue as it could be a life safety situation.
As to your "how" question: In this case the transformer is a separately derived system. In order for the breaker to trip the current needs a path back to the transformer. The bonding jumper establishes that path.
 
I'll bet there was nothing wrong with the first transformer. It is scary how many transformer installs I see that are not safe or code compliant. To the OP I suggest that you draw this out and study a bit and you will see the issue. Trust me, it's worth the trouble and it makes you a better electrician. We all learn something everyday, especially here on the forum.
 
Thanks for the info.
The transformer was sitting about 2 feet above the switchgear it was powered from and the panel it fed was about 15' away.
Everything was hard piped with emt and all connectors had bond bushings on them, also there was a bond lug added to the transformer enclosure.
In the transformer the bond wire went from bond bushing to enclosure lug and then to the other bond bushing.
There were 2 legs of 480 coming in on to the H1 and H4
On the low volt side one hot was on X1 the other hot on X4 and the neutral was coming off of X2 and X3 which are tied together.
We hooked the new transformer up the same way and it worked fine, tripped the breaker as soon as it was shorted.
The other existing transformers are wired the same way as this one.
So should I run a bond jumper from the enclosure to X2/X3 where the neutral is hooked up?
Or would that cause the bond wire to carry current and be like an undersized parallel to the neutral?
Thanks again for the help!!
 
The short was from hot to ground, We took a peice of 12 THHN and put it on a 20 amp breaker, and took the other end and stripped it out and touched it to the panel tub (with no tripping), touched it to the main service tub (with no tripping), and we drove a peice of 3/4" emt about a foot in the ground and touched it to that (with no tripping).
The first panel from the transformer was fed with a 2" EMT bond bushing at transformer end and panel end with the panel end bonding to the neutral bus.
I think it was odd that we could send 120 volts to the ground at the main service and not get it to trip.
Unless I'm not remembering something right, but I'm pretty sure that's how it was.
 
Well, the resistance of the ground electrode system, if it does not include a CEE or water pipe, could have unlimited resistance, but easily over 25 ohms. Add 5 ohms at the utility ground and you have a current of only 4A at 120V. Not at all surprising that a 20A breaker would not trip.
And another practical example that the earth is often not a good fault clearing path.
 
jbishop said:
We hooked the new transformer up the same way and it worked fine, tripped the breaker as soon as it was shorted....
Click to expand...

jbishop said:
The short was from hot to ground, We took a peice of 12 THHN and put it on a 20 amp breaker, and took the other end and stripped it out and touched it to the panel tub (with no tripping), touched it to the main service tub (with no tripping), and we drove a peice of 3/4" emt about a foot in the ground and touched it to that (with no tripping).....
Click to expand...
Huh?
 
As noted in the previous posts, you must have a X2-X3 system bonding jumper between the grounded and grounding points to provide a path for fault current to flow back to your transformer.
If your branch circuit breaker tripped on hot to ground fault then you have the required path, if it does not then you do not have the necessary current path.
That would be the basic difference between the two scenarios.
 
Fifteen minutes into this youtube video Mike explains the effective ground fault current path with some pretty good graphics. Hopefully this can clarify for the OP what may be a lack of grasping the basic difference between Grounding and Bonding; and also essentially what is required to clear a fault. If you continue to maintain a grounded system without an Art 100 Effective Grounf Fault Current Path, people are going to be in extreme risk of electrocution.

http://m.youtube.com/watch?v=3vvvv5QVZoA
 
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