Test Before Touch Easier Said Than Done (final) · Test Ground to Phase for all Phases – Keeps...
Transcript of Test Before Touch Easier Said Than Done (final) · Test Ground to Phase for all Phases – Keeps...
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““Test Before TouchTest Before Touch””Easier Said Than DoneEasier Said Than Done
Ken Crawford, DuPontKen Crawford, DuPontKent Haggerty, DupontKent Haggerty, Dupont
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OverviewOverview
• Test Before Touch (TBT) Principles• Key Learnings by One Company• One Method for Performing a TBT• Induced Voltages • Medium Voltage Testing• Summary & Conclusions
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How Would You Verify Power is Off?How Would You Verify Power is Off?
• What would be the plan?• What could go wrong?• What instruments to use?• What PPE is needed?• How to minimize hazards?• Where are the shock and arc hazards?• Are you qualified to work on this equipment?
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Key Learnings in One CompanyKey Learnings in One Company• TBT Not Fully Understood• Inadequate Training
– No Planned Approach– Best Practices Not Documented– Did Not Warn of Common Mistakes
• Concepts Seemed Simple• Proper TBT Involves Knowledge and Planning
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• Consider All Circuits Energized Until Tested• Test EVERY Conductor To Be Touched• PPE Is Required Until Test is Completed• Test Instrument Must Be Verified• Knowledge of Equipment Is Essential• Test Circuit Again If Job Continuity Is Broken• Testing Must Be Done At Each Location
Where Conductors Are Going To Be Touched
““Test Before TouchTest Before Touch”” (TBT) (TBT) Key PrinciplesKey Principles
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Protect the PersonShock Protection
“#” indicates US ANSI voltage class“00” rated 500 volts; “2” rated 17,000 volts
Arc Flash Protection
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Selecting The Proper TesterSelecting The Proper Tester
• Zero Indication Must Mean No Voltage Present
• Testers With Limitations– Multi-meters Can Be on Wrong Setting – Some Solenoid Testers May Not Always Indicate– Proximity Testers May Give False Indication
• Voltage Only Tester Should Be Used for TBT• No Single Voltage Tester Will Cover All
Applications
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Solenoid Tester LimitationsSolenoid Tester LimitationsSolenoid testers may not indicate backfeed Solenoid testers may not indicate backfeed
from control power transformerfrom control power transformer
M
CA B
Control Power
Transformer
Ground Test Point120V480V
V
C
Solenoid Will Solenoid Will NOTNOT Activate Due to Impedance Activate Due to Impedance of Transformer and Testerof Transformer and Tester
Solenoid Testers With Voltage Indicating Solenoid Testers With Voltage Indicating Lights Are PreferredLights Are Preferred
Blown Fuse
Ground Test Point
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Limitations of Proximity TestersLimitations of Proximity TestersMultiMulti--Conductor or Shielded Cables Conductor or Shielded Cables
Require Special AttentionRequire Special AttentionProximity Tester Reads Energized Conductor
Ground Conductor Shields Energized Conductor
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Proximity Tester Limitations Proximity Tester Limitations for Low Voltage Testingfor Low Voltage Testing
• Detects AC Voltage Only• Will Not Indicate Voltage Inside
– Grounded Enclosures– Shielded Cables – Internally Wet Cables
• May Not Indicate Voltage if – Cable is Partially Buried– Insulated Test Point is Against Grounded Metal– Operator Isolated From Ground
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Testing Method on Testing Method on Low Voltage CircuitsLow Voltage Circuits
1. Test Ground to Phase for all Phases– Keeps Meter at Zero Volts for Most of the Test– Voltage Usually Lower (e.g. 277 Vs. 480)
2. Test Neutral to Phase (if available)– Provides Backup Test to Ground to Phase
3. Test Phase to Phase (if available)– Check Each Phase to All Other Phases (A-B,A-C,B-C)– May be Only Valid Test on Ungrounded Systems
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Steps in Performing Steps in Performing Absence of Voltage Absence of Voltage
Tests Tests
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Step 1 Step 1 -- Probing Position Probing Position • Place the Red Lead in the Meter
– Reduces Risk of Contact • Black Lead Placed on Ground First and
Removed Last
tester
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Step 2 Step 2 -- Check MeterCheck Meter• Verify Meter on Energized Source Before & After
Test• Source Must Be Same Type (AC or DC) As The
Equipment To Be Tested.
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Step 3 Step 3 -- Verify Circuit Is Verify Circuit Is ““OffOff””• Verify the Circuit Requiring Work is Turned "Off"
And Any Disconnect Knife Blades Are OpenKnife Blades Are Open
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Step 4 Step 4 -- Ground to Phase TestGround to Phase Test• Must be Absolutely Certain of a Reliable Ground
– If Not Sure, Test Ground Point
• Reliable Ground Point Depends on Equipment
t
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Step 4Step 4-- Ground to Phase Test Ground to Phase Test -- Cont.Cont.Ground Test Point Depends on the EquipmentGround Test Point Depends on the Equipment
Reliable Ground Test PointReliable Ground Test Point
Not a Reliable Ground Test Point
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Reliable Ground Test Point
Not a Reliable Ground Test PointNot a Reliable Ground Test Point
Step 4 Step 4 -- Ground to Phase Test Ground to Phase Test -- Cont.Cont.Ground Test Point Depends on the EquipmentGround Test Point Depends on the Equipment
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Test PointsTest Points• The Load Side of Switch Should be Tested First• Never Use Fuse Caps (Ferrules) as a Test Point
– Some Fuses Have Insulated Ferrules
Don’t Test Here
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Test Points (contTest Points (cont’’d)d)
Phase Test Points
Test on the Fuse Holder Not the Fuse
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Step 5 Step 5 -- Phase to Phase TestPhase to Phase Test
• Check Phase to Phase Even if Ground to Phase Checks Indicate "0" Volts– Provides Backup Check – May Be Only Valid Test on Ungrounded Systems
• Phase to Phase Test Alone Is Not Sufficient
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CA B
Blown Fuses or faulty switch
PhasePhase--toto--Phase Test LimitationsPhase Test Limitations
Meter Meter Does NotDoes Not Indicate VoltageIndicate Voltage
xxxxxxxxx
voltmeter
voltmeter
VV
Proximity Tester DoesIndicate Voltage
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Task: Replace Motor TTask: Replace Motor T--Leads in Leads in a a
3 Phase Motor Starter3 Phase Motor Starter
What Are The Minimum What Are The Minimum Number of Tests Required?Number of Tests Required?
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CA B
Control Power
Transformer
120V480V
Motor
Test Points
M
Testing to Remove Motor Leads for Testing to Remove Motor Leads for 480V Starter480V Starter
A Total of 12 Tests Are Required A Total of 12 Tests Are Required 6 Ground To Phase & 6 Phase to Phase6 Ground To Phase & 6 Phase to Phase
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Voltage Measured on Voltage Measured on ““DeDe--energizedenergized””LV Circuit, What Do You Do?LV Circuit, What Do You Do?
• Verify Proper Circuit & Equipment– Check Prints, Labels, Resources
• Determine if Voltage is Induced– Detected by Meters With High Input Z ~ 10 M Ω– Low Input Z Meters Collapse Induced Voltage Z~ 2 kΩ– Verify Voltage Collapses to “0” With Low Z Tester
• Voltage Still Present After Loading Circuit– Check for Backfeed, Equipment Failure
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Close Coupling in Cable TrayClose Coupling in Cable Tray
V
High Z Digital Meter Reading 95VHigh Z Digital Meter Reading 95VAnalog Meter Reading 80V Analog Meter Reading 80V Low Z Digital Meter Reading 0VLow Z Digital Meter Reading 0V
Induced Voltage Value Depends Induced Voltage Value Depends on Meter Impedanceon Meter Impedance
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Testing Insulated
Conductors
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Testing Insulated ConductorsTesting Insulated Conductors• Use a Proximity Tester for a First Test• Should Not Be Used as the Only Test• If No Voltage Indicated
– Wearing PPE, Expose Conductor For Testing– Verify “0” Volts With Contact Meter
• Voltage Indicated with Proximity Tester– Verify Proper Circuit is Locked out– Re-Check source for Induced Voltage Using a High
Impedance Meter– If Voltage is Present, Load Circuit at Switch with a Low
Impedance Meter and Re-check Insulated Conductor
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V
Verifying Induced Voltage on Insulated Verifying Induced Voltage on Insulated ConductorsConductors
Low Impedance Meter
Proximity Tester Indicates Induced Voltage on Insulated Conductor
Proximity Tester Does Not Indicate Voltage Due to Circuit Loading by Low Impedance Meter
Proximity Tester
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Testing Testing Medium Medium Voltage Voltage CircuitsCircuits
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Considerations in Testing Considerations in Testing Medium Voltage SystemsMedium Voltage Systems
• Proximity Testers Often Preferred Due to Strong Electric Fields– Direct Contact Not Required– Only One Probing Stick Needed– Can Test Taped Connections
• Shielded Conductors Cannot Be Tested Except at Terminal Connections– Spiking Tool May Be Necessary to Verify Shielded
Conductors Are De-energized
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Shielded Cable - Proximity Tester Will Not Indicate
tttT
Proximity Tester Will Work Here
Medium Voltage Medium Voltage Shielded Cables Require Shielded Cables Require
Special AttentionSpecial Attention
Cable Shield Terminates Here
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Which Shielded Cable Is DeWhich Shielded Cable Is De--Energized?Energized?
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Gauss Meter For Cable IdentificationGauss Meter For Cable Identification
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Spiking ToolSpiking Tool
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Summary & ConclusionsSummary & Conclusions
• Consider All Circuits Energized Until Tested• Training and Knowledge Required • Plan for Testing• Use Proper Voltage Tester • Use Appropriate PPE• Test Before Touch Not Always Simple
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TestTestEvery Circuit, Every Every Circuit, Every
Conductor, Every Time Conductor, Every Time BeforeBefore You You Touch!Touch!
It Could Save It Could Save Your Your Life!Life!
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“Test Before Touch”Easier Said Than Done
Questions?