protective grounding myths &...

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

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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?

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For More Information ContactWayne P, Blackley, PE

ATC Consulting Servicesblackley@atc-trng.com

972-889-9075