Loose Fuse Tube Caps Impact Loose Fuse Tube Caps Impact ReliabilityReliability

How Improper Fuse Installation Causes Feeder How Improper Fuse Installation Causes Feeder Outages & Other IssuesOutages & Other Issues

Charlie WilliamsCharlie WilliamsPower System ServicesPower System Services

S&C Electric Co.S&C Electric Co.

Fuse InstallationFuse Installation

• Fuses are installed in cutout tubesFuses are installed in cutout tubes• The fuse is a current carrying deviceThe fuse is a current carrying device• Proper installation requires a Proper installation requires a TIGHTTIGHT cap on the fuse cap on the fuse

tube and a tube and a TIGHTTIGHT button on the fuse (if a button is button on the fuse (if a button is used)used)

• Typically 20 Ft-LBS torque required on fuse door capTypically 20 Ft-LBS torque required on fuse door cap

Capacitor CurrentsCapacitor Currents

• 100% Load Factor When Energized100% Load Factor When Energized• Typically energized 12 – 24 Hrs/DayTypically energized 12 – 24 Hrs/Day• Steady load current Steady load current • Magnifies heating of poor connections compared to Magnifies heating of poor connections compared to

cyclic loadscyclic loads

Current Path for a Fuse CutoutCurrent Path for a Fuse Cutout

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

Capacitor ConstructionCapacitor Construction

Capacitor Pack FailuresCapacitor Pack Failures

• Packs fail by shortingPacks fail by shorting• Usual cause is overvoltage or manufacturing defectUsual cause is overvoltage or manufacturing defect• When a pack fails it increases voltage on remaining When a pack fails it increases voltage on remaining

packspacks• This results in shorter life for remaining packsThis results in shorter life for remaining packs• The rsult is that pack failures cascade over timeThe rsult is that pack failures cascade over time

Capacitor Failures Increase CurrentsCapacitor Failures Increase CurrentsCapacitor Bank Currents

1.17

1.5

2.5

1.13

1.33

1.75

3

1.11.25

1.5

2

3.5

0

0.5

1

1.5

2

2.5

3

3.5

4

1 2 3 4 5

# Shorted Packs

Cu

rre

nt

Mu

ltip

lie

r

4 Series Packs 5 Series Packs 6 Series Packs

Typical Failure – One fuse door openTypical Failure – One fuse door open

Loose Fuse Door Cap and Fuse ButtonLoose Fuse Door Cap and Fuse Button

All Cutouts in Series All Cutouts in Series Equal current in each fuse cutout Equal current in each fuse cutout

Wrench Tight

Loose Cap

Loose Button & Cap

Result of Excess Cutout HeatingResult of Excess Cutout Heating

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

Fuse Sheath Incinerated

Increased Temperature of Cutout Door Increased Temperature of Cutout Door ConnectionConnection

• Can cause annealing of springs in the head of the Can cause annealing of springs in the head of the cutoutcutout

• Results in poor connectionResults in poor connection• Consequence is arcing and burning and possible Consequence is arcing and burning and possible

dropout of door without melting fusedropout of door without melting fuse• Arc is not interrupted and results in fault Arc is not interrupted and results in fault • Result is Feeder OutageResult is Feeder Outage

Heating and Arcing cause Cutout Failure

Cutout Current Interruption IssuesCutout Current Interruption Issues

• Difficult at current extremesDifficult at current extremes– Very High currentVery High current– Very Low currentVery Low current

• Low Current Interrupt is accomplished by use of tube on fuse Low Current Interrupt is accomplished by use of tube on fuse linklink– Compresses fuse gasses to provide sufficient pressure to Compresses fuse gasses to provide sufficient pressure to

expulse expulse – Assures Arc interruption in Cutout DoorAssures Arc interruption in Cutout Door– Failure results in external arcFailure results in external arc– Arc can then flow and cause external fault on pole Arc can then flow and cause external fault on pole

Current Interruption FailureCurrent Interruption Failure

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

Cure for Cutout HeatingCure for Cutout Heating

• Insure Fuse Buttons are TightInsure Fuse Buttons are Tight• Insure Cutout Door Caps are Insure Cutout Door Caps are Wrench TightWrench Tight

– Not Hand TightNot Hand Tight• Annual Infrared Inspections of Capacitor BanksAnnual Infrared Inspections of Capacitor Banks

B/L Fuse Upstream of TestB/L Fuse Upstream of Test

395 Deg F !!

Other Connection Issues

Failure can result in arcing and Fuse blowing

Branch Line ReliabilityBranch Line Reliability

• Loose Fuses result in increased heating of branch line Loose Fuses result in increased heating of branch line fusesfuses

• Low load factor on branch lines would lead to the Low load factor on branch lines would lead to the belief that this is a non-issue?belief that this is a non-issue?

• However, fault currents coupled with loose fuses However, fault currents coupled with loose fuses results in accelerated heating!results in accelerated heating!

Branch Line ReliabilityBranch Line Reliability

• The result is a failure of “fuse save” protection The result is a failure of “fuse save” protection schemes and unnecessary outages to branch linesschemes and unnecessary outages to branch lines

• When it occurs it appears as a temporary fault that no When it occurs it appears as a temporary fault that no cause could be found.cause could be found.

• Accelerated fuse heating acts like a large pre-load on Accelerated fuse heating acts like a large pre-load on the fuse and reduces the fuse blow time.the fuse and reduces the fuse blow time.

• Result is a branch line outage that should only have Result is a branch line outage that should only have been a momentary interruption!been a momentary interruption!

Fuse clears

Breaker Trips

Current

Voltage

Proper fuse installation procedureProper fuse installation procedure

• Tighten buttons well by handTighten buttons well by hand• Tighten fuse caps with a wrench to 20 ft-lbs!Tighten fuse caps with a wrench to 20 ft-lbs!

• ResultResult– Proper fuse operationProper fuse operation– Fuse save schemes that work!Fuse save schemes that work!– No feeder outages from capacitor banks!No feeder outages from capacitor banks!– Improved Reliability!Improved Reliability!