Soil Resistivity andGrounding System Testing

Roy “Chip” WhittenSenior Applications Engineer/Education SpecialistLyncole Grounding Solutions, LLC

What Is Grounding ?

An electrical connection , whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.

Reasons For Grounding

– Personnel safety and equipment protection by providing a path to safely dissipate any unwanted charges or potentials.

– Ensure equipment performance and protection

– Satisfy manufacturer’s warranty

Electrical Protection Pyramid®

RFAC Surge

Telco / DataLightning

Grounding

Resistance To Earth

20v 12v 10.6v11v16v 10.18v10.5v 10.45v

25 Ohms

0

Grounding System Components

Equipment

SoilResistivity

Basics

Soil Resistivity

Resistance of earth to current flow, measured in Ohms-Meter, between opposite faces of a cube of earth, one cubic meter in volume

Earth

1 Cubic Meter

Signal Generator

~

Soil Resistivity

Key Variable in System Design

– Determines grounding system resistance– Changes from Site to Site– Dependent on:

– Soil type– Moisture– Electrolytes– Temperature

Soil Resistivity Comparison

Surface Soils 100 - 5,000Clay 200 - 10,000Sand and Gravel 5,000 - 100,000Surface Limestone 10,000 - 1,000,000Limestone 500 - 400,000Shale 500 - 10,000Sandstone 2,000 - 200,000Granites, Basalts, etc 100,000Decomposed Gneisses 5,000 - 50,000Slates, etc 1,000 - 10,000

Soil Type Resistivity (ohm-cm)

Effect of Moisture on Resistivity

020406080

100120140160180

0 5 10 15 20 25 30Moisture Content (% by weight)

Res

istiv

ity (o

hm-c

m)

x Th

ousa

nd

Effect of Electrolytes (Salt)on Resistivity

Effect of Temperature on Resistivity

0

50100150200250300350

68 50 40 32 14Temperature F

32

25

Res

istiv

ity (o

hm-c

m)

x Th

ousa

nd

Water Ice23

SoilResistivity

Testing

4-Pt. WENNER METHOD

Soil Resistivity Testing

– Key Variable in System Design– 4 Point Wenner Method

– Most Accurate– Multiple Depth Testing

4-Pt. Wenner Method

– Visually Survey Lease Area to Determine Location and Direction For Test– Not parallel to buried metallic objects– Not parallel overhead power lines– Sufficient straight line distance to allow for test

– Minimal distance 300 feet

4-Pt. Wenner Method

15 ft 10 ft 5 ft 0 ft

C1P1P2C2

A

4-Pt. Wenner Method

Site Area

Direction of Test

Example’s of 4-Point Testing

4-Pt. Wenner Method

Drive test probes into soil0 ft; 5 ft; 10 ft; 15 ft0 ft; 10 ft; 20 ft; 40 ft0 ft; 20 ft; 40 ft; 60 ft

Connect wire leads from meter to probesC1 to 0 ftP1 to 5 ftP2 to 10 ftC2 to 15 ft

C1 to 0 ftP1 to 10 ftP2 to 20 ftC2 to 30 ft

C1 to 0 ftP1 to 20 ftP2 to 40 ftC2 to 60 ft

Soil Resistivity Testing4 Point Data Interpretation

ρ π=

++

−+

41 2

42 2 2 2

ARA

A BA

A B( ) ( )ρ = Resistivity A = Spacing of probesB = Depth of probes R = Resistance

(reading from meter)If A>20B, then ρ = 2π AR (ohm-ft)

= 1.915 AR (ohm-m)

(2π / 3.28 = 1.915 )

4-Pt. Wenner Method

Probe Spacing Meter Reading Calculated Resistivity(Feet) (Ohms) (Ohm-Meter)

5 52.00 497.90

10 19.68 370.87

15 10.16 292.00

20 6.53 250.10

30 4.30 247.04

40 10.80 827.28

60 7.40 850.26

80 5.58 855.60

100 4.44 850.26

ρ

= 1.915 AR

Typical Grounding Electrode System

Resistance Requirements

Typical Resistance Requirements

– NFPA 70 NEC 25 OHMS or Two Rods

– IEEE Standard 142 & 1100 Equipment Dependent

– Motorola Standard R-56 10 OHMS (Design Goal)

– Telecommunications 5 to 10 OHMS

– Emerson 3 OHMS

– Essilor 3 OHMS

– GE Medical Systems 2 OHMS

25Ω Grounding System

Lightning

Strike

18,000APotential Rise will be

~450KV at the site

5 Ohm Grounding System

Lightning

Strike

18,000APotential Rise will be

~90KV at the site

Grounding System Resistance Testing

Grounding System Testing

– Why Test Grounds?

– Confirm Design Spec Satisfied

– Satisfy Warranty Reqs

– Determine Baseline

– Ensure Equip Protection & Performance

– Validate Construction

– Two Test Methods

− Fall of Potential (Three Point) Test

− Clamp-on Test

Testing Methods

Required Equipment3 / 4 Pole Tester

Test KitProbesConductorTape MeasureHammer

Ground System Testing

Fall-of-Potential Method

P2 Auxiliary Voltage ElectrodeGround Rod

Under Test

Earth

C2

RemoteCurrent

ElectrodeC1P1

1 ft 10%20% 30% 40% 50%60%70%80% 90%

10 x depth of electrode

P2 Distance P2 Distance Resistance Amt. of Change( %) ( feet ) ( Ohms ) (Ohms)

1 1 ft 67.0 -10 % 10 ft 114.5 47.520 % 20 ft 119.6 5.130 % 30 ft 121.3 1.740 % 40 ft 122.2 0.950 % 50 ft 122.5 0.360 % 60 ft 123.1 0.6 70 % 70 ft 123.7 0.680 % 80 ft 126.6 2.990 % 90 ft 141.8 15.2

100 % 100 ft C2 -

Fall-of-Potential Test Results

Res

ista

nce

(ohm

s)

5565758595

105115125135145

0 10 20 30 40 50 60 70 80 90 100 P2 Probe Spacing

Fall of Potential

– Why 10+ Samples?– Single Point Could Be Misinterpreted– Data Must Be Plotted

– Visual Plateau– Confirms Test Validity

– #1 Reason– Not Isolating System Under Test

– Meter is a constant amperage meter– Part of the current travels through the connection – The ground system is part of a parallel network

– Test Is Invalid Unless Disconnected

Fall Of Potential Test

Why Invalid?

Ground Resistance Testing Data Plot

Invalid Test Graph

Ohms

Distance (Ground-Probe)

Test With Neutral Connected

Plot For 5 Ohm Ground

Plot For 25 Ohm Ground

Plot For 100 Ohm Ground

Plot For 1000 Ohm Ground

5

15

10

20

Fall Of Potential Test

Why Invalid?

– #2 Reason– Insufficient Probe Spacing

– Req’d to Avoid the Spheres of Influence

Spheres of Influence

Readings Not Impacted by Spheres of Influence

2025

15100500

Plateau System Resistance

Fall of Potential Test

CurrentProbe

50-100ft Spacing

Fall of Potential Test

– Spacing For Current Probe? – Single Electrode

– Minimum 5X Length of Rod– Ideal, 10X Length of Rod

– 10 Foot Rod, 50-100 Feet Away – 200 Foot Well, 1000-2000 Feet Away

Ground Resistance Testing

Clamp-on Resistance Testing Clamp-on Ground Resistance Meter– Convenient, Quick, Easy– Does Not Require Disconnecting

Equipment– Measures Current on the Ground

May Read Ground Loops vs. Ground Resistance

Clamp -on Meter Operation

Current Flow

Current Flow

R = E / I??? ohms

2 Control Xformers

Service Meter

Ground Conductor

Ground Rods

3 Phase Utility Line

Induced Current Flow

Current From Other Grounds Current From Other Grounds

Clamp-On Resistance Testing Example

Clamp-On Resistance Testing Example

Rx R2R1 R3 Rn-1 Rn

VI

Rx R1

Rn-1 Rn

Neutral-GroundBond

Grounding Conductor

Clamp-On Meter Operation

???Current Flow

Utility Neutral Line

Series Loop

Parallel Paths

Current Path

Reading: < 1 OhmInvalid

Invalid Clamp-on Reading

- 48vdcSwitchGear

Use Extreme Caution

Qualified Electricians Only

Clamp-On Use

Service Entrance

THANK YOU FOR YOUR ATTENTION Please enjoy the rest of your time at the

2015 BICSI Fall Conference & ExhibitionRoy “Chip” Whitten

Senior Applications Engineer/Education SpecialistLyncole Grounding Solutions, LLC

800-962-2610