grounding techniques that save lives

National Utility Training Services

NATIONAL UTILITYNATIONAL UTILITYTRAINING SERVICESTRAINING SERVICES

GroundingGroundingTechniques that Techniques that

Save LivesSave Lives

This material was produced under grant (46K1-HT27) from the Occupational Safety and Health Administration, U.S. Department of Labor. It does not necessarily reflect the views or the policies of the U.S. Department of Labor, nor does the mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.

Special Thanks to:

Mike FitzpatrickMike Fitzpatrick Training

Pasco, Washingtonand

Steve Frost, ESCIKennewick, Washington

In Case of an EmergencyIn Case of an Emergency

If you hear an alarm:

Listen to the PA system.

Follow instructions.

AdministrationAdministration

Breaks

Smoking Areas

Restrooms

Lunches

This Course Will Cover:This Course Will Cover:

OSHA RegulationHistory of GroundingDalziel’s ResearchEffects of Current across the body

This Course Will Cover:This Course Will Cover:

Grounding methodsBracketSingle PointPersonal Protective

Specific Equipment used for equipotential zonesGrounding In Substations

OSHA RegulationOSHA Regulation

29 CFR 1910.269(n) Grounding for the protection of employees

29 CFR 1910.269(n)(3) “Temporary protective grounds shall be placed at such

locations and arranged in such a manner as to prevent each employee from being exposed to hazardous differences in electrical potential”

History of GroundingHistory of Grounding

Early on there were no methods established to safely work energized lines

Pioneers discovered if lines that were taken out of service were short-circuited to the earth, this could protect the lineworkers from shock

History of GroundingHistory of Grounding

First Personal Protective Grounds

Long metal chains pulled over conductorsBare stranded copper conductorCluster type grounds

HistoryHistory

In 1954, two engineers from Bonneville Power, E.J. Harrington and T. Martin published a paper called :

“Placement of protective grounds for the safety of linemen.”

Straw Man TestStraw Man Test

A set of coveralls was stuffed with straw. Inside the coveralls they ran a copper wire equal to 500 ohms of resistance.

The bracket grounds were installed and the line energized at 230.000 volts.

Dalziel’s ResearchDalziel’s Research

Current Flow and the Human Body

Research in 1941 by Professor Charles Dalziel/ University of California at BerkeleyEstablished general values regarding electrical current and reaction of the human bodyValues contributed to the evaluation of grounding proceduresResearch confirmed that individuals react differently to current values

Effects of CurrentEffects of Currentacross the bodyacross the body

1. The human body is a natural _________ of electricity.

2. _________ always takes the path of least resistance to ground and that path may be through a person’s body.

3. The effect of current on the human body depends on the _______ of current, the _________ of current flow, and the ______ that the current takes through the body.

Conductor

Current

Amount DurationPath

4. When current flow is directly through the _______, the chance of severe damage is the greatest.

5. Cardiac arrest or fibrillation can occur when current above ____________ passes through the heart.

6. The most severe electrical burns occur when the current flow is over _____________.

50 milliamps

200 milliamps

7. __________ can alter the effects of current flow and its path through the body.

8. Skin provides much more resistance to current flow than _______ _______.

9. With dry skin, current flow is ____ likely to travel across the skin surface and ____ likely to flow through muscle tissue.

Resistance

Muscle Tissue

LessMore

How Much VoltageHow Much Voltage Can We Take Can We Take ??

From Dalziel’s formula, we know the fibrillation threshold is:

164 ma, or 0.164 ampsor

Just a little more than 1/10 of an amp

How Much VoltageHow Much Voltage Can We Take Can We Take ??

V = I R

V = ( .164 ) x ( 500 )

V = 82 volts

Average human body has 500 to 1500 ohms of resistance

What Are The Two Things What Are The Two Things Current Will Always Do ?Current Will Always Do ?

Current will take the path of least resistance.

Current will take all paths to ground.

Grounding MethodsGrounding Methods

1910.269 (n) (2)General." For the employee to work lines or equipment as deenergized, the lines or equipment shall be deenergized under the provisions of paragraph (m) of this section and shall be grounded as specified in paragraphs (n)(3) through (n)(9) of this section. However, if the employer can demonstrate that installation of a ground is impracticable or that the conditionsresulting from the installation of a ground would present greater hazards than working without grounds, the lines and equipment may be treated as deenergized provided all of the following conditions are met:1910.269(n)(2)(i)The lines and equipment have been deenergized under the provisions of paragraph (m) of this section.1910.269(n)(2)(ii)There is no possibility of contact with another energized source.1910.269(n)(2)(iii)The hazard of induced voltage is not present.

Grounding Methods (cont.)Grounding Methods (cont.)

Bracket Grounds

“Old Style”Not a legitimate ground/better than noneNot highly recommendedCan have current flow under certain conditions

Single Point Grounding

EquipotentialIdentifyIsolate

Can’t have current flow at equal potential

Personal Protective Grounding

EquipotentialEquipotential ZoneZone

A method of grounding where a pole band is attached to the pole, then the common neutral or ground, and then jumpered to the conductor. The pole band is below the feet, creating a work zone that is the same potential as the line voltage.

Preferred Grounding PointsPreferred Grounding Points

Temporary driven ground rod.6

Structure ground.(pole ground rod, footing ground, etc.)

Multi-grounded system4

Common neutral3

Substation ground grid2

Designed ground point or ground bus.1Most Effective

Least Effective

Equipotential GroundingEquipotential Grounding

De-energize the cable and test.Install bracket grounds to bleed off any capacitive charge.After grounding, install the cable on insulated parking bushings.At the work-site install a ground mat.Tie the ground mat to the concentric of the cable that you are working on.

ChoicesChoices

As with overhead distribution systems, there are three acceptable methods:

InsulateIsolate Equipotential zone

Underground Equipotential Underground Equipotential GroundingGrounding

Underground equipotential Underground equipotential grounding uses the same theory as grounding uses the same theory as

overhead.overhead.

Equipotential GroundingEquipotential Grounding

The ground mat, when properly installed will work in the same way as a cluster bar on a pole.

TransmissionTransmission LinesLines

Transmission

There are potential problems when working around high-voltage transmission lines.

Electrostatic induction.

Electromagnetic induction.

Electrostatic induction occurs when an ungrounded open conductor acts like a capacitor and picks up a charge from wind blowing across large expanses of wire.

Electromagnetic induction occurs when an ungrounded open conductor picks up an induced voltage from an energized line paralleling or perpendicular to the line.

Electromagnetic Induction

When grounds are installed where there is a transmission line paralleling the de-energized lines, a potentially dangerous condition exists due to the induced electromagnetic field from the high-voltage lines.

The solution is to remove the grounds at the switches and ground at the location closest to where the work is being performed.

In this case, by grounding at the source (open switches in the switch yard), we produce the same condition that exists in a transformer.

Placement of Personal Protective Placement of Personal Protective GroundsGrounds

Proper placement of personal protective grounds is essential. PPG's should be placed as close to the work location as possible to dissipate induced voltages.

In Order to Provide True In Order to Provide True Equipotential Protection for the Equipotential Protection for the

WorkerWorker

All Mechanical Equipment Working Near Power Lines

Must Be Grounded

Mechanical EquipmentMechanical Equipment

Tests were conducted using a 1969 ford cab over digger/derrick.

The truck had two sections of steel, and one fiberglass section.

A Total of Four Tests Were A Total of Four Tests Were Completed.Completed.

Truck ungrounded.Truck grounded to a temporary ground rod.Truck grounded to the pole ground.Truck grounded to the common neutral.

Truck UngroundedTruck Ungrounded

00000Tires not insulatedStifflegs on woodNylon winch line

00000Tires not insulatedStiff legs on woodFiberglass boom

6.055676.0551672Tires not insulatedStiff legs on wood

6.358566.2572818Tires not insulatedStiff legs insulated

5.854316.05516182Tires not insulatedStiff legs not insulated

6.0amps

5601volts

6.1amps

5601volts

120amps

Tires InsulatedStiff legs not insulated

Front Rear

Truck UngroundedTruck Ungrounded

6.056015.95431402Tires InsulatedStiff legs not insulated

5.854316.05474614Tires not insulatedStiff legs on wood

6.055676.0551672Tires not insulatedStifflegs on wood

6.358566.2572818Tires not insulatedStiff legs insulated

6.0amps

5601volts

6.1amps

5601volts

120amps

Tires InsulatedStiff legs not insulated

Front Rear

Truck to Ground RodTruck to Ground Rod

5.854316.05474614Tires not insulatedStifflegs on wood

6.0amps

5601volts

5.9amps

5431volts

402amps

Tires insulatedStiff legs not insulated

Front Rear

Truck to Pole GroundTruck to Pole Ground

.26246.282555100Tires not insulatedStiff legs on wood

.26238.282555487Tires not insulatedStiff legs insulated

.24amps

221volts

.24amps

221volts

4422amps

Tires not insulatedStiff legs not insulated

Front Rear

Truck To Common NeutralTruck To Common NeutralFront Rear

00.221994633Tires not insulatedStiff upSteel plate on leather shoes

00000Tires not insulatedStiff legs on woodNylon winch line

00000Tires not insulatedStiff legs on woodFiberglass boom

.23212.232075336Tires not insulatedStiff legs on wood

.22amps

204volts

.23amps

212volts

5040amps

Tires not insulatedStiff legs not insulated

OSHA 1910.269OSHA 1910.269Mechanical EquipmentMechanical Equipment

Mechanical equipment shall be operated so that the MAD (Minimum.approach distance) is maintained.However, the insulated portion of an aerial lift operated by a qualified employee in the lift is exempt from this requirement.If the MAD is maintained from exposed objects at a different potential.

Minimum Approach DistanceMinimum Approach Distance

7 ft. – 6 in.230

4 ft. – 3 in.115

3 ft. – 11 in.69

2 ft. - 10 in.21

2 ft. - 3 in.12

2 ft. - 3 in.4

Avoid ContactSecondary

Minimum Approach Distance

System Voltage(KV)

Boom and Digger/DerricksBoom and Digger/Derricks

Energized lines exposed to contact shall be covered.Equipment shall be insulated for the voltage involved, and be positioned so the uninsulated portion cannot come within the MAD.Equipment shall be grounded to the best available ground.

Working Outside of MADWorking Outside of MAD

Exposed energized lines shall be covered.Equipment shall be insulated for the voltage.Equipment shall be grounded to the best available ground.Ground mats used around the equipment.Insulating protective equipment. (rubber gloves and boots).

Working Within MADWorking Within MAD

The exposed energized lines and equipment shall be covered with protective materials.

The equipment shall be insulated for the voltage involved.

SubstationsSubstations

Substation GroundingSubstation Grounding

A substation is one big equipotential grid.

Install grounds from the preferred ground point to the conductors,bus or bushing using hot line tools.Do not ground through fuses, interrupters, power circuits breakers, switches, power transformers and other type of devices.Install ball studs whenever possible for future grounding.

Step and touch potential is provided within one foot of the grounded device.

Grounding cables should be sized according to the available fault current.

Program End

grounding techniques that save lives

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