Burned Grounding Conductor

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PapaGreg

Member
A rural school building on co-op power has a vacuum pump sewage system in half of an addition to the school. The system operates on 460 volt 3-phase power. The control panel is fed by a 100 amp 460 volt disconnect that is powered by a 208 volt to 460 volt step up transformer. There is a 200 amp 208 volt disconnect protecting the transformer that is fed from a 200 amp fused disconnect in a MDP 200 feet away. The XO lug in the transformer is bonded to the frame and the circuit grounding conductors are attached to the bonding point and connected back to the MDP. The ground wire insulation is burned off or bubbled in every raceway of this system. One of the fuses in the MDP switch that feeds this circuit is open. There are signs of overheating on the contactors to the pump motors. There are power problems with co-op power supply and a standby generator system that was operating when we arrived as the open fuse caused the gen. to run when the co-op power came back on. Any ideas on this.
 

hurk27

Senior Member
Single core transformer should not have the primary XO bonded, just the frame of the transformer, this will cause overheating of the transformer and grounding conductors when the secondary is not balanced.

The primary (208/120) X/O should be floated.
If there is an X/O on the secondary, it must be bonded.

Not sure what caused the fused to open, but this could have lead to the imbalance that cause the high current on the X/O and grounding.
 
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Jljohnson

Senior Member
Location
Colorado
If your transformer is, as Hurk eluded to, actually a 480 delta to 120/208 wye transformer that has been reverse fed and XO is bonded, your meltdown is definitely caused by the bond at XO. I've seen this a number of times in a string of carwashes in my area that had 480 volt equipment connected in that manner. Power company loses a phase and "poof" the smoke starts coming out of equipment. One of the building steel wires at a site was sleeved up a wall in PVC conduit and the PVC literally melted into a puddle on the floor before they called me. Square D makes a transformer and I'm sure others do as well that is designed for to be fed with the 208 on the primary and will give a 277/480 volt output on the secondary.
 

wookie7471

Member
Location
North Florida
Has there been any welding performed close by?
I have had something similar happen to me before when some welding was being performed on a piece of equipment. The welder connected his ground lead to the piece of equipment and started welding, he then moved to piece of equipment located across the isle. The #10 grounding conductor carried the load for a little while. It was when the piece of sealtite containing the ground wire caught on fire the welder figured there might be a problem with where he attached his groung lead.
 

kwired

Electron manager
Location
NE Nebraska
If it is a reverse fed transformer with only H1 H2 H3 as the 480 volt leads then it needs to have the derived system be corner grounded or ungrounded with a ground fault monitoring system.
 

mtfallsmikey

Senior Member
Friday's stupid question

Friday's stupid question

Has there been any welding performed close by?
I have had something similar happen to me before when some welding was being performed on a piece of equipment. The welder connected his ground lead to the piece of equipment and started welding, he then moved to piece of equipment located across the isle. The #10 grounding conductor carried the load for a little while. It was when the piece of sealtite containing the ground wire caught on fire the welder figured there might be a problem with where he attached his groung lead.
This post caught my eye, because on occasion we have had welding work done in my buildings, most of the time the welder cord gets run into a
480v. panel, to a spare breaker, then off they go. As a building engineer, what precautions should I take in allowing this?
 

kwired

Electron manager
Location
NE Nebraska
This post caught my eye, because on occasion we have had welding work done in my buildings, most of the time the welder cord gets run into a
480v. panel, to a spare breaker, then off they go. As a building engineer, what precautions should I take in allowing this?
Where the supply side of the welder gets connected is not the issue.

Make sure whoever is welding connects the ground clamp of the welder output near the work. If they move the work the ground clamp moves with them. This is especially true when moving to an entirely different piece of equipment - the only current path for the welder secondary could easily be a small control conductor between two pieces of equipment.


A good welder should know this.
 
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