Loose Fuse Tube Caps Impact Reliability How Improper Fuse Installation Causes Feeder Outages

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Loose Fuse Tube Caps Impact ReliabilityHow
Improper Fuse Installation Causes Feeder Outages
Other Issues

• Charlie Williams
• Power System Services
• SC Electric Co.

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Fuse Installation

• Fuses are installed in cutout tubes
• The fuse is a current carrying device
• Proper installation requires a TIGHT cap on the
  fuse tube and a TIGHT button on the fuse (if a
  button is used)
• Typically 20 Ft-LBS torque required on fuse door
  cap

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Capacitor Currents

• 100 Load Factor When Energized
• Typically energized 12 24 Hrs/Day
• Steady load current
• Magnifies heating of poor connections compared to
  cyclic loads

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Current Path for a Fuse Cutout

• Should be metal to metal
• Threaded joints are poor connections

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Capacitor Construction
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Capacitor Pack Failures

• Packs fail by shorting
• Usual cause is overvoltage or manufacturing
  defect
• When a pack fails it increases voltage on
  remaining packs
• This results in shorter life for remaining packs
• The rsult is that pack failures cascade over time

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Capacitor Failures Increase Currents
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Typical Failure One fuse door open
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Loose Fuse Door Cap and Fuse Button
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All Cutouts in Series Equal current in each
fuse cutout
Wrench Tight
Loose Cap
Loose Button Cap
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Result of Excess Cutout Heating

• Cutout doors were examined after 2 weeks
• All fuse sheaths were totally incinerated
• White ash residue found in cutout door

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Fuse Sheath Incinerated
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Increased Temperature of Cutout Door Connection

• Can cause annealing of springs in the head of the
  cutout
• Results in poor connection
• Consequence is arcing and burning and possible
  dropout of door without melting fuse
• Arc is not interrupted and results in fault
• Result is Feeder Outage

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Heating and Arcing cause Cutout Failure
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Cutout Current Interruption Issues

• Difficult at current extremes
• Very High current
• Very Low current
• Low Current Interrupt is accomplished by use of
  tube on fuse link
• Compresses fuse gasses to provide sufficient
  pressure to expulse
• Assures Arc interruption in Cutout Door
• Failure results in external arc
• Arc can then flow and cause external fault on
  pole

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Current Interruption Failure

• Results in Arc external to cutout
• Most capacitors are on mainline feeder
• Arc flows up on pole causing feeder fault
• Result is Feeder Outage

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Cure for Cutout Heating

• Insure Fuse Buttons are Tight
• Insure Cutout Door Caps are Wrench Tight
• Not Hand Tight
• Annual Infrared Inspections of Capacitor Banks

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B/L Fuse Upstream of Test
395 Deg F !!
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Other Connection Issues
Failure can result in arcing and Fuse blowing
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Branch Line Reliability

• Loose Fuses result in increased heating of branch
  line fuses
• Low load factor on branch lines would lead to the
  belief that this is a non-issue?
• However, fault currents coupled with loose fuses
  results in accelerated heating!

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Branch Line Reliability

• The result is a failure of fuse save protection
  schemes and unnecessary outages to branch lines
• When it occurs it appears as a temporary fault
  that no cause could be found.
• Accelerated fuse heating acts like a large
  pre-load on the fuse and reduces the fuse blow
  time.
• Result is a branch line outage that should only
  have been a momentary interruption!

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Fuse clears
Current
Breaker Trips
Voltage
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Proper fuse installation procedure

• Tighten buttons well by hand
• Tighten fuse caps with a wrench to 20 ft-lbs!
• Result
• Proper fuse operation
• Fuse save schemes that work!
• No feeder outages from capacitor banks!
• Improved Reliability!