# Default Melted Ground Wire from Transformer Secondary



## Boneshaker (Jul 31, 2009)

Is the cabinet itself grounded? Check the potential from the cabinet to ground. It seems if you had a short it should have tripped your breaker. Not sure why the ground would carry enough fault current to heat up with out tripping.


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## cdnelectrician (Mar 14, 2008)

Sounds like one of the welding electrodes shorted to ground? Since it is a welding application that transformer (I would assume) is designed to handle high loads for a short period of time. If one leg of the secondary is grounded and you had a short to ground on one of the welding leads I could totally see this happening. Is the secondary fused?

I had a similar situation at a chroming plant, one of the bus bars on the rectifier was touching the steel floor grating and the other bus bar had come loose and come into contact with the rectifier cabinet. It used the conduit feeding the rectifier (primary fused at 150 amps 600 volt) as its path. The conduit was red hot and melted the feeders. I found the disconnect door 10 feet away from the disconnect and pieces of fuse all over the place. The secondary was 3 volts DC 3000 amps. The machine operator looked like he needed to change his pants when I got there!


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## wiretekaaron (Nov 22, 2009)

Here is a link to the machine so you can get an idea of what I am talking about. 

http://www.formingsystemsinc.com/pdfs/wire_forming/CFR.pdf

The cabinet has a ground lug in it. The cabinet is grounded to this, as well as the transformer and the welding controller, this also has a lead that goes to an external ground in the electrical panel. I think the breaker did eventually trip, but seems to have happened far to late. I think it is also odd that it only destroyed the ground lead to the lug in the cabinet... all the other ground wires were fine. Did it dissipate enough once it hit the ground lug not to ruin the other wires?

Secondary is not fused, only input to primaries. When I get in on Monday, I will test for a short to ground. Don't know how it would have happened though...


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## cdnelectrician (Mar 14, 2008)

wiretekaaron said:


> Here is a link to the machine so you can get an idea of what I am talking about.
> 
> http://www.formingsystemsinc.com/pdfs/wire_forming/CFR.pdf
> 
> ...


Are you sure that from that ground lug on, that is the ONLY path the current would have had? I'm pretty sure you had a short to ground on the secondary of the transformer!

BTW, the link did not work!


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## wiretekaaron (Nov 22, 2009)

Just added the link in jpeg format. For the current path... It would have to be the current lug or the welding transformer... those are the only places that it can go. I would just like to know what caused it and what can prevent it... That wire was completely smoked! I am very glad nothing else burned up. Especially since the burnt ground line was right next to the primary wire into the transformer.


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## cdnelectrician (Mar 14, 2008)

How is the machine fed? Conduit? SO cable?


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## wiretekaaron (Nov 22, 2009)

Feed line is as follows:

120/240 High leg delta at service, 100 amp breaker, 40 foot line in 1.5" conduit with #4 wire goes to 25 KVA auto-transformer to give machine 440V 3 phase. From there, one of the phases get pulled to the welding transformer and one to the weld controller. Both of these phases run through a MCB before they see each component. When the SCR fires at the weld controller, it applies voltage to the other side of the transformer primary.


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## cdnelectrician (Mar 14, 2008)

So, if one leg of that Xformer secondary is grounded, and say you have a short to ground on the other leg that it could carry the current back through the conduit and the cabinet to the weakest link. That being the # 12 ground that burnt up?


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## wiretekaaron (Nov 22, 2009)

The ground wire coming off the secondary is the wire that burnt up and it is #12, but all the ground wire inside the machine is #12 wire. I see what you are saying and it sounds most likely for the problem. I will have to check for the short on Monday. It just makes me mad because the transformer is only around 6 months old! Shouldn't get a short like that.


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## cdnelectrician (Mar 14, 2008)

I'm sure the transformer is fine. Test it out with a megger just to be sure.

Have you checked with the manufacturer to see if they have seen this problem before?


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## cdnelectrician (Mar 14, 2008)

http://www.coleparmer.ca/techinfo/techinfo.asp?htmlfile=AEMC_insulation.htm&ID=5


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## wiretekaaron (Nov 22, 2009)

Is there a recommended way to keep the ground wire from going up like it did this time. Can I put a type of fuse or CB on the ground wire off the secondary that would be able to handle that kind of current and protect the components?


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## cdnelectrician (Mar 14, 2008)

NO it would be a bad idea to install a fuse or CB in the ground. I suggest you find out what the problem is and fix it.


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## eds (Mar 21, 2009)

What kind of welding err occurred, I saw something as a newbe that might be related. I don' recall how all of this went together but there was a large steel welding table that had a 120 volt drill sitting on the table. The drill had a 4 foot chord that was plugged into a receptacle. The welder had his peice clamped to the table and as he started to weld, I could see a curl of smoke slowly circle the drill chord. The chord equipment ground had become hot enough to melt both the equipment grounds insulation and the overall chord insulation.


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