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2

Grounding&

Testing

A Communications Perspective

Ed Rousselot

National Telecom Sales Engineer

3

4

Agenda

• What is a “Ground” and why is it important to have a

“good” one

• Soil resistivity and soil resistance

• Considerations with installing a Ground Electrode

System (for our purposes, “rods”)

• Measure the effectiveness of the ground electrode

system by means of ground testing

5

What is a Ground?

“A ground is a conducting connection by which an

electrical circuit or equipment is connected to the earth

or some conducting body.”

Source: IEEE Standard 81

Low impedance conductor used to provide a safe path

for the dissipation of:

- fault currents

- lightning strikes

- static charges

- EMF/RFI signals

Simply put….

6

Simply Put…

The characteristic of a grounded system is that there

is a steady flow of current that is going to the path of

least resistance.

We do not want that path of least resistance to be through

someone or some delicate equipment.

We want it to be through a

“ground electrode system”.

This current can be measured using

an ammeter that measures milliamps.

7

Brief Look at Benefits of

Proper Grounding

Safety – Us and our Equipment

Noise on our Circuits

8

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10

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14

Ground Resistance

vs.

Soil Resistivity

• Ground Resistance: The resistance (opposition to

current flow) of an installed electrode system

• Measured in Ohms

• Measured using three or four-point stake testers

or a clamp-on tester

• Soil Resistivity: The electrical properties of the soil for

conducting current

• Measured in Ohm-cm (Ohm centimeters)

• Measured using a four-point stake tester

15

Resistivity

• Soil Resistivity: The electrical

properties of the earth for conducting

current

16

Why Test Resistivity?

•Tells you how “good” (conductive) your soil is

•Good indication on whether or not a generic

ground specification design will work

•Helps reduce “surprises” at the end of the

installation

•An indication of the degree of corrosion to be

expected on components of the ground

system

17

Resistivity of Different Soils

Soil Type Resistivity Range Ohm cm

Loam 100 – 5,000

Clay 200 – 10,000

Shales 500 – 10,000

Limestone 500 – 400,000

Surface Limestone 10,000 – 1,000,000

Slates 1,000 – 10,000

Sandstone 2,000 – 200,000

Sand & Gravel 5,000 – 100,000

Granites, Basalts, etc. 100,000

Evershed & Vignoles

Bulletin 245

18

Soil Resistivity Ranges

•100 - 15,000 Ohm cm – Standard Design OK

•15,000- 25,000 Ohm cm - Standard Design Maybe

•25,000 - 50,000 Ohm cm - Special Circumstances

•50,000 + Ohm cm – Perhaps not practical

19

Measuring Soil Resistivity

• Use a 4-terminal earth resistivity tester

• Space the rods an equal distance apart –

“a” in next slide

• Insert the rods a distance of 1/20th “a”

into the ground

• Measures the average soil resistivity to a

depth equal to the rod separation

20

Measuring Earth Resistivity

a a a

C2P2P1C1

a a/20

21

Actual Site Testing Procedures

Test at multiple locations across the site

22

Four-Point Resistivity TesterAlso Tests Resistance

23

• To check that climatic conditions have not

changed the ground such that it no longer

meets the requirements

• To check for seasonal variations

• To check that changes to buildings, the lot, the

streets, etc. have not affected resistivity

Why Measure Soil Resistivity Periodically?

24

Ground Resistance

Ground Resistance: The resistance (opposition

to current flow) of an installed electrode

system

25

Typical Permissible Ground Resistance

Values

• Telecom - varies depending on the standard used:- Electronic equip mfg. vary from fraction of an ohm upward

-At premise must equalize with power ground which is 25Ω

• Typical values from an insurance company:- Industrial plant: 5

- Chemical plant: 3

- Computer system: 3

• Typical values for a power company:- Generating station: 1 maximum

- Large sub-station: 1 maximum

- Small sub-station: 5 maximum

Why 25 Ohms?

26

25 Ohm ground keeps US safe

NEC Section 250

IEEE STD 81

27

Ground Electrode System Components

• Ground “Electrodes”

• Ground Conductors

• Ground Bars

• Bonding Connectors

• Mechanical

• Compression

• Welded

28

Ground

Electrodes

29

Ground ElectrodesDo Not Have To Be Rods

Ground Electrode Types -

Ground Rods:

Copper Clad Steel

Solid Copper

Galvanized

Stainless Steel

Enhanced

Ground Plates

Ground Mesh

30

Ground Electrodes

Types of Grounding Systems

Single Rod

ElectrodeMultiple Pole Grounds

Multiple-Rod Electrode

31

Ground Electrodes

Ground

Wire

Ground

RodButt

Plate

Utility

Pole

≥ 12” below ground

?

32

Another type of butt ground

33

Soil Resistivity - Some soils, (such as sandy soils), have

such high resistivity that conventional ground rods or

ground electrode systems may be unable to attain the

desired ground resistance.

•Enhanced ground electrodes or ground enhancement

materials may be required to meet the grounding

specification.

Some of these are…

Ground Electrodes… Considerations

34

Ufer Grounds - Concrete

encased electrode tying

into tower footing or

building-pad rebar

Enhanced Ground Electrodes

35

Enhanced Grounding Material

Low Resistance

Reduces seasonal variation

Carbon-based

Not permanent

Washes away over time

Only consider when deep or

multiple rods are not practical

36

Enhanced Grounding Material

Soil treating material

placed in circular trench

and covered with earth

Ground rod

Appx 1 ft.

Appx 18 in.

37

Enhanced Grounding Material

38

Soil Ph - affects the rate of corrosion of metal ground

components that are in contact with the soilcont.

Ground Electrodes… Considerations

39

Soil Characteristics - Some sites may have only a few

inches of soil (or none) sitting on top of bedrock.

Consider-

Ground mesh & plates

Horizontal rods

(Don’t drill into bedrock such as granite. Such rock does

not make a good ground.)

cont.

Ground Electrodes… Considerations

40

Ground Rods… Considerations

Ground Rod Diameter –

•Doubling diameter of ground rod reduces

resistance only 10%.

•Using larger diameter ground rods is mainly a

strength issue. In rocky conditions, a larger

diameter ground rod might be advantageous.

Cont.

41

Ground Rods… Considerations

Ground Rod Length - Doubling length theoretically reduces

resistance 40%.

Actual reduction depends on soil resistivities of multi-layered

soils.

Cont.

42

Ground Rods…Considerations

• Multiple Ground Rods – Two well-spaced rods driven

into the ground provide parallel paths. They are, in

effect, two resistances in parallel.

• The rule for two resistances in parallel does not apply

exactly. (The resultant resistance is not one-half of one

of the rod’s resistance.)

• The reduction for equal resistance rods is about:

• 40 percent for 2 rods

• 60 percent for 3 rods

• 66 percent for 4 rods

–Spaced apart greater than their length

43

What Else Can YOU

Control?

Torque

Bullet Bonds

44

Which kind does your

locate contractor use?

45

Causes of Ground System Deterioration

• Weather influences exert mechanical strain on

ground rods

• Cable locates

• Corrosion over time

• Catastrophic events like lightning strikes or large

fault currents can cause instant degradation that

may not be visible

• Soil resistivity can change over time due to

environmental conditions

• Facility expansion can create different needs in

the ground system

46

Risks from Ground System Deterioration

• Potentially deadly electrical shock situations

• Plant-wide equipment damage

• Disruption in the performance of sensitive

equipment with tight voltage parameters

• Heat build-up on a single piece of electrical

equipment and, eventually, fire

47

Ground Electrode System Testing

• Ok, the system is designed and

installed.

• Let’s Test!

48

Choose the Proper Instruments

Use a dedicated ground tester

- designed to measure grounds

• Don’t use a generalized ohmmeter or multimeter

• Don’t use an insulation resistance tester

• Don’t use the ground test of a telecom multi-

function test set

49

Safety First!

This is not a

safety course

50

Three-Terminal Ground-Resistance Testing

51

Review – Ohm’s Law

• Resistance = Voltage ÷ Current

• Ohms = Volts ÷ Amps

• If we know Voltage and Current, we can calculate

Resistance

• So, If we have 50 Volts and 2 Amps, we have 25

Ohms (25 = 50 ÷ 2)

52

Three-Terminal Ground Resistance Tester

Current Supply

Ammeter (I)

Voltmeter (E)

P C

X

EarthEarth

CurrentProbe

PotentialProbe

GroundRod

Under Test(Isolated)

53

Fall of Potential

(Isolated)

Ground Rod

Under Test (X)

Ground Rod

Position

54

Resistance Curve

CurrentProbe

Position

Distance of Potential Probe from X Ground

RodPosition

X C

Resis

tan

ce i

n O

hm

s

True Resistance

55

“Rules of Thumb” on “C” Probe Spacing

From Ground Rod “X”

Single ground rod - 50 feet

Small grid of 2 ground rods - 100-125 feet

Large system (several rods or plates in parallel)

>200 feet

Complex systems (large number of rods or other

electrodes and other metallic structures bonded

together) - far greater distances are required

56

Insufficient Probe Spacing

Distance of Potential Probe from X

Resis

tan

ce i

n O

hm

s

CurrentProbe (C)

PotentialProbe (P)

GroundRod

Under Test (X)

(Isolated)

No Flattening

No True Resistance

57

Advantages/Disadvantages of

Fall of Potential Testing

Advantage:

Conforms to IEEE 81 - only approved method.

Disadvantage:

Time consuming

58

61.8% Rule

(Isolated)

61.8%

Ground Rod

Under Test (X)

Ground Rod

Position

59

61.8% Rule/Method

• Based on the full Fall of Potential method

• Take measurement at only one point

• Advantage – Quick and easy

• Disadvantage – Assumes that conditions

are perfect with respect to:

• Adequate spacing of C and P probes

• Resistivity of the soil being the same

60

Ground Testing on Asphalt“Lazy Spike”

MEASURE

3 POLE 4 POLE

MEGGER® DET5/4R

(Isolated)

Does not have to be wet

61

Ground Testing Methods

• Fall of Potential Method*

• 61.8% Rule Method*

• Simplified Fall of Potential

• Four Potential Method

• Intersecting Curves Method

• Slope Method

• Dead Earth Method

• Star-Delta Method*Covered here

62

Stake-less or Clamp

Ground Testing

63

Review – Ohm’s Law

• Resistance = Voltage ÷ Current

• Ohms = Volts ÷ Amps

• If we know Voltage and Current, we can calculate

Resistance

• So, If we have 50 Volts and 2 Amps, we have 25

Ohms (25 = 50 ÷ 2)

64

65

CAUTION

Ammeter does not measure the value of the ground

66

Considerations When Using

Stake-less/Clamp-On Method

Effective only in situations with multiple

grounds in parallel such as pole grounds

67

Multiple Grounds in Parallel

68

Considerations When Using

Stake-less/Clamp-On Method

Effective only in situations with multiple grounds in

parallel such as pole grounds

Requires a good return path so it cannot be

used on isolated grounds

69

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

No Return Path

Clamp Won’t Work

70

Considerations When Using

Stake-less/Clamp-On Method

Effective only in situations with multiple grounds in

parallel such as pole grounds

Requires a good return path so it cannot be used on

isolated grounds

Cannot be used if an alternate, lower resistance,

return path exists not involving the soil

71

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

Must Measure at the Correct Part of the Loop

Lightning RodsConnecting Wire

72

Multiple ground rods

bonded underground

To power company

ground (Has many

parallel connections to

power company feed.)

Power Company Feed

What is being measured?

NID

6’ Rods are 6’ to 8’ apart

Clamp-on set must be

BELOW this point.

73

Reality

74

What is Being Measured?

Grounding

Conductor –

Bonded to MGN

Ground

Rod

Butt

Plate

Utility

Pole

75

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

What is Being Measured?Only Connectivity – Not Ground Resistance

Ground Rods

At Each Leg

Buried Wire

Connecting the Rods

Wire Connecting

Each Leg to Rod

76

SHIELD CONNECTION

PedestalGround

Bus

Ground

Bar

77

Ground

bar

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

CA

TIV

n600V

Meg

ger®

Measuring

continuity

78

This is what the

ground wires look like

that we have to

clamp around to test.

Right?

79

Sometimes It’s Not Easy

80

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What Now?

Could be worse…

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Quick Review

84

Addressing Ground System Problems

• Use longer ground rods

• Use multiple ground rods

• Chemically treat the soil

• Place the system in lower resistivity soil if

possible

85

• So, we tested our ground system and we

passed!

• Do we ever need to test it again?

86

• (You have seen this before but…)

• To determine the effectiveness of ground rods and

connections

• Seasonal changes

• Water table changes

• Changes in the site and/or building

• To check that standards set by codes are still being met

• To check that specific design parameters have been met

• To check that the ground rods and bonds are still present

Why Measure Ground Resistance Periodically?

87

Questions