What's wrong with this picture?

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ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
JoeStillman said:
I believe it does, but low down, behind that box.
Click to expand...

Let's consider this scenario:
This being shielded cable (as Winnie inquired) and the shield connections to ground are loose/ bad and a ground fault occurred down stream then the heat from the high resistance path caused the shield to overheat and the tape discoloration. This, of course, would have to assume that the GF protective circuit did not open the breaker before the damage occurred. There are a lot of assumptions here but it warrants further investigation regarding shutting down the circuit and checking ground connections and the GF protection system. I would also find out what the history is of any faults or failures that may have occurred on this circuit thru a log book or talking with the old timers in the engineering department.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
These are interesting ideas, but note that the tape is over the unshielded part of the termination. I think the excessive length of that has something to do with it.

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mkgrady

Senior Member
Location
Massachusetts
I don't see where the cable shield is terminated and connected to ground. This should happen at the lower side of the stress cones.


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I

Ingenieur

Senior Member
Location
Earth
WAG
shield not properly prepared, not enough isolation from associated phase

leakage i phase to shield to common ground point back via shield to phase
high Z phase to phase
 
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mkgrady

Senior Member
Location
Massachusetts
ATSman said:
Let's consider this scenario:
This being shielded cable (as Winnie inquired) and the shield connections to ground are loose/ bad and a ground fault occurred down stream then the heat from the high resistance path caused the shield to overheat and the tape discoloration. This, of course, would have to assume that the GF protective circuit did not open the breaker before the damage occurred. There are a lot of assumptions here but it warrants further investigation regarding shutting down the circuit and checking ground connections and the GF protection system. I would also find out what the history is of any faults or failures that may have occurred on this circuit thru a log book or talking with the old timers in the engineering department.
Click to expand...

There should not be a shield between the bus terminal and the stress cone so the brown marks should not be from an overheated shield if the terminations were done correctly
 
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Ingenieur

Senior Member
Location
Earth
mkgrady said:
There should not be a shield between the bus terminal and the stress cone so the brown marks should not be from an overheated shield if the terminations were done correctly
Click to expand...

delta primary
a single line fault should not allow current flow unless a shield has a path to phase
???
 
T

topgone

Senior Member
mkgrady said:
I don't see where the cable shield is terminated and connected to ground. This should happen at the lower side of the stress cones.


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Click to expand...

Agree. There's no shield wire from the the widest portion of the cone up to the lug of the termination! IIRC, for a 15 kV cable, the creepage distance is 18 inches (bulge of the cone to the terminal lug).

Back to the topic, I guess the cable experienced overheating. Maybe the taping material at the lug portion is too thick which hid the discolored tape/ not showing.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
I really doubt that the discoloration is from overheating, since the cable is so lightly loaded.

I think what we are seeing is corona damage to the tape. There seems to be too much unshielded cable between the cones and the termination. That and the rough surface of the tape are contributing to atmospheric ionization, resulting in nitric acid condensation in times of high humidity.

Is there a way to measure for corona?
 
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mkgrady

Senior Member
Location
Massachusetts
JoeStillman said:
I really doubt that the discoloration is from overheating, since the cable is so lightly loaded.

I think what we are seeing is corona damage to the tape. There seems to be too much unshielded cable between the cones and the termination. That and the rough surface of the tape are contributing to atmospheric ionization, resulting in nitric acid condensation in times of high humidity.

Is there a way to measure for corona?
Click to expand...

I once saw an engineer use an am radio to see if there was excessive corona at terminations. He told me there would have a lot static if there was excessive corona
 
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