4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
(OP)
The electric utility I work for recently experienced a 4 kV Overhead Distribution Circuit line down, where all 3 phases of 4 kV circuit burned clear mid span. Span where 4 kV circuit burned clear had double-ended construction on both sides, so there was no significant tension on this span. All 3 phases of 4 kV circuit that burned clear were 336 All Aluminum Conductor (AAC) with low voltage tree insulation.
Initiating event was a 34 kV over-build circuit dropping a single phase bare conductor onto this 4 kV under-built circuit. 4 kV circuit went through a series of reclose shots and then subsequently held with normal load current of approximately 50 Amperes per phase for 60 seconds.
After 60 seconds, 4 kV circuit relayed out and reclosed again. Circuit held, but load current was now 0 Amperes. Circuit was then cleared from SCADA due to closed circuit breaker with no current indicating an energized line down condition.
Most interestingly, the distance between initiating 34 kV circuit event and where under-built 4 kV circuit phase conductors burned clear is approximately 1,000 feet apart, separated by multiple pole spans.
I am having trouble grasping how it would be possible to burn clear 336 AAC conductors that far away from initiating event.
Any thoughts would be greatly appreciated.
Initiating event was a 34 kV over-build circuit dropping a single phase bare conductor onto this 4 kV under-built circuit. 4 kV circuit went through a series of reclose shots and then subsequently held with normal load current of approximately 50 Amperes per phase for 60 seconds.
After 60 seconds, 4 kV circuit relayed out and reclosed again. Circuit held, but load current was now 0 Amperes. Circuit was then cleared from SCADA due to closed circuit breaker with no current indicating an energized line down condition.
Most interestingly, the distance between initiating 34 kV circuit event and where under-built 4 kV circuit phase conductors burned clear is approximately 1,000 feet apart, separated by multiple pole spans.
I am having trouble grasping how it would be possible to burn clear 336 AAC conductors that far away from initiating event.
Any thoughts would be greatly appreciated.






RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
Bill
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"Why not the best?"
Jimmy Carter
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
Distance from substation to location where 34 kV bare conductor dropped is approximately 3,000 feet.
Distance from substation to where all 3 phases of 4 kV circuit burned clear is approximately 2,000 feet.
Reclose sequence when 34 kV over-built conductor dropped was:
Trip #1
- 34 kV bare conductor de-energized when it makes contact with under-built 4 kV circuit
- Starts as 1-phase fault
- O/C elements picking up / dropping out over the course of seconds
- Final trip is on 3-phase fault
- Total time from initial o/c element pickup to circuit breaker trip is 7.54 seconds
Reclose # 1
- 3 second delay
- Circuit breaker closed
- Starts as 1-phase fault
- Final trip is on 3-phase fault
- Significant fault current
- Total time from initial o/c element pickup to circuit breaker trip is 0.238 seconds
Reclose #2
- 15 second delay
- Circuit breaker closed
- Circuit breaker holds for approximately 51 seconds with normal load current (~ 50 Amperes per phase)
- Reclose relay resets after 30 seconds
Trip #2
- Starts as phase-to-phase fault
- Final trip is on 3-phase fault
- Total time from initial o/c element pickup to circuit breaker trip is 0.813 seconds
Reclose #3
- 3 second delay
- Circuit breaker closed
- 1-phase fault
- Total time from initial o/c element pickup to circuit breaker trip is 1.303 seconds
Reclose #4
- 15 second delay
- Circuit breaker closed
- No load current (0 Amperes across all phases)
- Circuit breaker tripped from SCADA due to likely energized line down condition
My assumption is that 4 kV circuit at location of burn down got wrapped together during initial Trip / Reclose sequence.
If so, while wrapped together for the 51 seconds with normal load current (~ 50 Amperes per phase), is it possible that enough heat was generated to burn clear the 336 AAC conductor?
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span
The catch is that where the 34 kV over-built cable was laying on under-built 4 kV was approximately 1,000 feet from where all 3-phases of 4 kV circuit burned clear.
RE: 4 kV Overhead Distribution Circuit Burned Clear - All Phases Downed Mid Span