Case of the Faulted 4000A Bus Duct

[ATSman]

There are many stories out there about Bus Duct (Busway) failures. This is just one of them.
This occurred yesterday at one of our customer's sites. Someone walking by the building saw smoke coming from an underground vault. The FD was called, they came, looked around and called in the POCO. The lineman came, he looked around and checked the primary fuses on the padmount (12KV - 480V, 1500KVA) above the vault feeding the bldg. He replaces the 3 fuses, turns on the primary switch with his hot stick... then BOOM!!!!!!!
By this time all bldg load had switched over to their emer gen.
Turns out the vault had POCO switches that fed loads from other customers and the engineering/ transformer/ bus repair/ engineering could not be started until the MV is LOTO and the vault water pumped out. Bldg on gen for how many weeks?

A side note to this event: This transformer had the same fuses blow 5 years ago and the transformer was replaced.
By the looks of this pic it appears that this failure was a result of loose connections, water intrusion that produced heat and the degradation of insulation over a long period of time and finally developed into a phase to phase fault. Did this problem exist back then and was mis-diagnosed by the POCO? No inspection or meggering was done at that time. The system voltage was measured and found stable. Question: Can this problem exist for 5 yrs before showing it's ugly face?
Any comments?

 

[don_resqcapt19]

I doubt the bus duct would have lasted 5 years if was the cause of the original blown fuses.

I worked on a project to replace all of the bus duct at an industrial plant based on failures at that plant and sister plants from water intrusion into the duct. We installed cable bus to replace the bus duct.

 

[ATSman]

I doubt the bus duct would have lasted 5 years if was the cause of the original blown fuses.

I worked on a project to replace all of the bus duct at an industrial plant based on failures at that plant and sister plants from water intrusion into the duct. We installed cable bus to replace the bus duct.

Tks Don.
Cable Bus, is this the type where individual power cables are laid did by side and connected parallel in, in sections?

 

[ATSman]

Tks Don.
Cable Bus, is this the type where individual power cables are laid side by side in a metal trough or raceway

 

[don_resqcapt19]

Tks Don.
Cable Bus, is this the type where individual power cables are laid did by side and connected parallel in, in sections?

It is like cable tray, but engineered and the conductors are held in place with a couple of diameters of space between them. The manufacture specifies a specific arrangement of the cables.
http://www.mdfbus.com/

 

[JoeStillman]

Wondering when that piece of busduct last had its "annual" inspection. The key phrase for me in the OP was "pumped out". My experience with "wet-location-rated" busduct is that it just ain't so.

 

[ATSman]

Wondering when that piece of busduct last had its "annual" inspection. The key phrase for me in the OP was "pumped out". My experience with "wet-location-rated" busduct is that it just ain't so.

Good question, Joe.
No inspection of record since installation 30 yrs ago. Turns out the duct enclosure is NEMA 1 rated.
Question to all the code guys: Is this acceptable or does an underground vault require NEMA 3R rating?

 

[don_resqcapt19]

Wondering when that piece of busduct last had its "annual" inspection. The key phrase for me in the OP was "pumped out". My experience with "wet-location-rated" busduct is that it just ain't so.

That is the reason the busduct at the plant I worked on was replaced with cablebus.

Of course even wet rated busduct would not be suitable for submersion and that may have been the case here.

 

[electrofelon]

Good question, Joe.
No inspection of record since installation 30 yrs ago. Turns out the duct enclosure is NEMA 1 rated.
Question to all the code guys: Is this acceptable or does an underground vault require NEMA 3R rating?

I would think anything in an underground vault should be designed for submersion. In my experience utilities use submersible transformers in vaults.