protective grounding myths &...
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12010 IEEE, IAS Electrical Safety Workshop
Protective Grounding Myths & Facts
byWayne P. Blackley, PE
22010 IEEE, IAS Electrical Safety Workshop
Energy Levels for Injury Extremely small amounts of electrical
energy inflict major physical injury.
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• 10 mA - Paralysis Threshold - Cannot release hand; if no grip, victim may be thrown clear; may progress to higher current & be fatal.
• 30 mA - Respiratory Paralysis - Stoppage of breathing (Frequently fatal).
Current Range & Effect on 68 kG Man
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• 75 mA Fibrillation threshold 0.5 % (1 person out of 200)
• 250 mA Fibrillation threshold 99.5 % for exposure > 5 seconds
• Heart Fibrillation - Heart uncoordinated (probably fatal) We must have help!!!
Current Range & Effect on 68 kG Man
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Worker Protection• Protective grounding
– When it’s needed it’s• Our last level of protection• All else has failed• A lot of things went wrong
– It better be correctly installed– Lives depends on it
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Worker Protection• Protective grounding, misapplied!
– Installed on paint.– Cabinet inadequate.– To long
• (not shown)
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Home Made Protection
• Any Ground will work
• These were found in service!
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SELECTING GROUND SETS • Select only
approved devices designed specifically for protective grounding
• These grounds will fail! Inadequate
connection – Electrical and mechanical
Brazed connection - melts
Taped support – Inadequate for mechanical forces
#4 AWG copper – to small
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SELECTING GROUND SETS • These grounds
will also fail.• Not only
will they
fail, they
are not a
permitted
type
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IMPROPER GROUND SETS FAIL
• These grounds will fail!
• Man Made - Arc in a Box
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GROUND APPLICATIONS• Keep protective
ground length to a minimum
• This is WRONG
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GROUND SET LENGTH
• Long ground leads cause problems
• This is WRONG
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GROUNDING & JUMPERING• OBJECTIVES:
– GROUNDING, to provide an immediate and effective de-energization of a circuit.
– JUMPERING, to limit voltage across the worker’s body to a safe level. • Jumpering allows a worker to be
energized yet limits voltage across worker’s body.
• Two current paths, one through the worker’s body, the other, through the jumper.
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GROUNDING & JUMPERING• If jumper resistance is
much less than the resistance of the body, most current flows through the jumper and a very small current flows through the body.
• RJumper <<RHuman
• IJumper >>IHuman
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GROUNDING & JUMPERING• As jumper resistance
increases, more current is forced through the body.
• At 40 kA & Vworker = 50V limits are: –RJumper = .00125–RHuman = 1000–Worker should survive
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GROUNDING & JUMPERING• If connections are
dirty, over paint, galvanizing or loose, resistance increases!
• Remember a life may depend on low resistance connections.
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GROUND SET RESISTANCE
• Typical set of protective grounds or jumper.
• Draw a resistor every where resistance can enter the circuit.
Did you count 15 points?
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GROUND SET RESISTANCE • Fifteen points where
resistance can be introduced in a ground set.
• High resistance at any point can result in failed grounds and serious injury.
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GROUND SET RESISTANCE 40,000a
50ma39,999.95a
50v
1,000• THE CALCULATION– RJumper = 50v/39,999.95a– RJumper = 0.00125– RAverage = 0.00125/15– RAverage = 0.00008333
Do you clean & torque ground connections?
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Grounding Situations
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Air Switch Analyzed
• EPZ - A zone in which all objects are at nearly the same potential
• Mat & Switch handle are bonded together
Equal Potential
Zone
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Air Switch Analyzed
If switch flashes over where is the worker?
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Ground Level Mounted Equipment
• An area or zone in which all objects are at the same potential
• Portable Equal Potential Zones
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EPZ Boundaries
• Equal Potential Zones have BOUNDARIES
• One foot in & one foot out is a NO, NO!
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• If it’s not grounded it’s not dead
Therefore, if it’s grounded, it’s dead, right?
And if it’s effectively grounded there will never be a voltage present, right?
What We’ve Been Told?
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Work Situations
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Induced Voltages Have Increased!
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Metal Clad Switchgear
• Accessibility issues• Adequate
attachment• Cabinet and
conductors must be bonded
• Current Magnitude
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Metal Clad Switchgear
• Prudent work practices (and OSHA) require that hot sticks be used to install grounds
May require equipment modification to achieve
Alligator Clip
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Why did it happen?• Dec. 18, 2000 – Lineman touches static wire
on de-energized and grounded 345 kV line. • Circuit paralleled another 345 kV circuit.
Conductors Short Circuited & Grounded
Truck Grounded to Pole Ground
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Why did it happen?
• Mar. 27, 2001 230 kV line–Lineman contacting wire sling and chain hoist receives electrical shock.
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Why did it happen?
• Mar. 27, 2001 230 kV line–Lineman contacting wire sling and chain hoist receives electrical shock.
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• Worksite @ time of Accident
230 kV Line Structure
– Insulated Work Platform– Phase conductors
shorted & grounded to driven ground rod
– Metallic arms & wire sling bonded to static/shield wire
– Chain hoist bypasses insulator string between arm/sling and phase
– Worker removing hoist gets in series with hoist and wire sling
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Induced Voltages• Circuit is
disconnected from source and grounded
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230 kV Line Structure
• Now: Why the different potential from:–Metal X-arm–Phase
Conductor
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230 kV Line Structure
• Examining each part of the workplace
• And assuming all components are good– Truck, Temporary
Ground and Conductors are bonded
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230 kV Line Structure
• Phase conductors are short circuited and grounded
• Phases and truck are bonded together– Truck, Temporary Ground
and Conductors are bonded
– Static, X-arms & Ground system is bonded
392010 IEEE, IAS Electrical Safety Workshop
230 kV Line Structure
• Phase conductors are short circuited and grounded
• Static and truck are bonded together– Truck, Temporary Ground
and Conductors are bonded
– Static, X-arms & Ground system is bonded
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Grounding Accidents
• What creates these problems?– Luck (Good or Bad?)– Magnitude of hazard– Inattention– Misunderstanding– Lack of Knowledge– Improper Work Practices
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• People cause problems?– Luck (Good or Bad?)– Magnitude of hazard– Inattention– Misunderstanding– Lack of Knowledge– Improper Work Practices
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We All Fail to
Understand Problem
•Humans are injured at approximately 50 milliamps•Fault currents exceed 200,000,000 milliamps•At 40,000 amps we must protect workers from 99.999875% of the available fault current!•Induced voltages exceed 1000 volts
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• Current takes the path of least resistance?
We’ve told them
True False
Current does not just take the path of least resistance.It takes every available path!
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The Facts
Most current takes path of least resistance, but some doesn’t.
It only takes 0.000125% of 40,000a to injure a person.
What can 120 ma do to a body?
120v 1 1000
120a 0.120a
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What We’ve Told Them
• Proper grounding eliminates voltage
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What They Heard?
• Proper grounding eliminates all voltages.
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What We’ve Been Told?
• Proper grounding eliminates all voltages.
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• If it’s not grounded it’s not dead
Therefore, if it’s grounded, it’s dead, right?
And if it’s effectively grounded there will never be a voltage present, right?
What We’ve Said
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Induced Voltages Have Increased!
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Worker Knowledge
• Workers: –Use makeshift grounds–Fail to understand grounding
limitations–Underestimate fault currents–Supervision?
512010 IEEE, IAS Electrical Safety Workshop
For More Information ContactWayne P, Blackley, PE
ATC Consulting Servicesblackley@atc-trng.com
972-889-9075
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