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Electrical Failure
InvestigationsPresented byPresented by
Ashley HarknessAshley Harkness
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Forensics, the Mystery
• Our Forensics Work
• Project Examples
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Why Do Things Fail?
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Because of...• Installation
• Application
• Operation
• Maintenance
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What we will look at...
• When the installer does it wrong…
• When the unexpected happens...
• When the design has problems...
• When People get involved…
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Poor Installation...
First:
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A High Voltage Bushing
• 27 KV bushing installed on a commuter train system.
• Steamed and Smoked when energized.
• Client wanted to know: What happened.
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Manufacturer’s Information
• Cut the Cable• Strip off insulation• Strip away insulation
shield• Wrap tape stress cone• Attach top connector• Assemble components• Fill with sealant
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The first step: X-ray the Sample
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What the X-ray revealed:
• The connector had not been properly soldered.
• The conductor strands were clearly visible.
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How bad was the connection?
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It was not the only problem!
• Question: Where did the steam come from?
• Further investigation showed a second problem.
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The cable seal was improper.
The seal is in the wrong place.
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The Correct Seal Position
The seal is against the outer jacket material.
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The Report:
• The poor connection caused heat and steam.• The improper seal allowed water to
penetrate the bushing.• This was caused by poor workmanship.
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Next Example:
Contractor: “It’s bad equipment!”
Manufacturer: “Poor workmanship!”
Owner: “Help!”
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Arcing between phases
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Mfg. Limits:
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Conclusion:
• Improper installation
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“Well, we had a limited budget…”
Let’s take another example..
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Conclusion:
• Mismatch of equipment• Poor installation
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Something “Off the Wall”...
Weird ones:
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An Airport Maintenance Facility in Texas
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The Problem:
• Blow-outs between A Phase and Ground• Several in the first months of installation.• No previous history of such faults.• Was it bad product?• Was it bad installation?• What was the cause?
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Power Bus Duct
• Reduced impedance.
• Reduced cost.• Increased inherent
strength.• Manufacturing
simplicity.• Reliable.
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Buss Duct End View
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Fault Tree
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Insulation Porosity
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The Report:
• Rain water entered the product.• Insulation porosity was present.• The installation was correct.• The product was properly produced.• The transportation was incorrect.
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A defective product.
Undetected, until suspicions were aroused
Another weird one:
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The Problem:
• The installing splicersdid not like the cable.
• Client wanted it tested to Specifications.
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5 KV Cable Sample
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Suspicious gaps in the strand shield system.
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Close-up of the defect.
• The strand shield had separated during extrusion.
• The mark did not follow the strands.
• A longitudinal impression was present.
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The Culprit
• A cut-off knife caused the gash in the conductor stranding.
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The Report:
• A manufacturing defect was found.• The life of the cable had been shortened. • The manufacturer offered the client an
extended warranty.
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There was no fault!
–A Different Type of Case:–A Critical Area—EDP–The Fuse Blew!–The mystery…
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Melted element
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Intact Elements
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Mechanical Fractures
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Hey, wait a minute...
• Earlier testing records noted this fuse--
• One fuse had high resistance
• Replacement had not been done
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Conclusion:
• Work hardened elements fractured• Fuse opened on less than full load
• Operation and Maintenance Issue• Preventable Outage
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TIME OUT!
How do you know that?
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The Forensic Process...
• Things we know:
• Arc Behavior• Fire Directions • Expected Performance
• Things we Learn:
• Circuit Loading• Environmental Factor• Discovery
– Inspection– Testing
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The Fault Tree
• Initiating Event• Failure Mechanism• Findings
– Test, inspections, measurements• Conclusion:
– Not Likely Cause– Most Probable Cause
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A Design Problem:
•“Well, I’m not sure what the load is...”
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Why did it fail?
Better question:
What is it?
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Conclusion:
• Bus Overload– 4000 Amp Bus– 5000 Amp Load!
– Application
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The Human Factor...
And then:
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What People will do...
• Working “Hot”
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“I don’t need no stinking safety procedures.”
“Hey, loan me your wiggie.”
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Conclusion:
• Defective meter initiated fault• Poor Work Practices• Lack of Protective Equipment
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But we paid extra to
keep the power on !
(Human Factor)
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The Attorney's
Office
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480 Volt Generator
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208 Volt Generator
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Conclusion:
• Improper wiring of generator– No connected neutral or ground
– Metal Oxide Varistor could not withstand the voltage.
– Fire was result
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When maintenance goes wrong…”
“What’s this green wire for?”
“It’s only a street light!”
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Conclusion:
• Poor Installation and Maintenance:– Water Damage– Pinched Wire– Not Grounded
• Child’s death:– She touched the energized light pole and a
grounded fence.
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In Summary:
• Forensics finds the strangest things.– Poor Workmanship– Defective Product– Off-the-Wall Problems