protective grounding myths &...

12010 IEEE, IAS Electrical Safety Workshop

Protective Grounding Myths & Facts

byWayne P. Blackley, PE


Energy Levels for Injury Extremely small amounts of electrical

energy inflict major physical injury.


• 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


• 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


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


Worker Protection• Protective grounding, misapplied!

– Installed on paint.– Cabinet inadequate.– To long

• (not shown)


Home Made Protection

• Any Ground will work

• These were found in service!


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


SELECTING GROUND SETS • These grounds

will also fail.• Not only

will they

fail, they

are not a

permitted

type


IMPROPER GROUND SETS FAIL

• These grounds will fail!

• Man Made - Arc in a Box


GROUND APPLICATIONS• Keep protective

ground length to a minimum

• This is WRONG


GROUND SET LENGTH

• Long ground leads cause problems

• This is WRONG


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.


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


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


GROUNDING & JUMPERING• If connections are

dirty, over paint, galvanizing or loose, resistance increases!

• Remember a life may depend on low resistance connections.


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?


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.


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?


Grounding Situations


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


Air Switch Analyzed

If switch flashes over where is the worker?


Ground Level Mounted Equipment

• An area or zone in which all objects are at the same potential

• Portable Equal Potential Zones


EPZ Boundaries

• Equal Potential Zones have BOUNDARIES

• One foot in & one foot out is a NO, NO!


• 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?


Work Situations


Induced Voltages Have Increased!


Metal Clad Switchgear

• Accessibility issues• Adequate

attachment• Cabinet and

conductors must be bonded

• Current Magnitude


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


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


Why did it happen?

• Mar. 27, 2001 230 kV line–Lineman contacting wire sling and chain hoist receives electrical shock.


• 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


Induced Voltages• Circuit is

disconnected from source and grounded



• Now: Why the different potential from:–Metal X-arm–Phase

Conductor



• Examining each part of the workplace

• And assuming all components are good– Truck, Temporary

Ground and Conductors are bonded



• 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



• 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


Grounding Accidents

• What creates these problems?– Luck (Good or Bad?)– Magnitude of hazard– Inattention– Misunderstanding– Lack of Knowledge– Improper Work Practices


• People cause problems?– Luck (Good or Bad?)– Magnitude of hazard– Inattention– Misunderstanding– Lack of Knowledge– Improper Work Practices


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


• 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!


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


What We’ve Told Them

• Proper grounding eliminates voltage


What They Heard?

• Proper grounding eliminates all voltages.


What We’ve Been Told?

• Proper grounding eliminates all voltages.


• 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


Induced Voltages Have Increased!


Worker Knowledge

• Workers: –Use makeshift grounds–Fail to understand grounding

limitations–Underestimate fault currents–Supervision?


For More Information ContactWayne P, Blackley, PE

ATC Consulting [email protected]

972-889-9075

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