Atlanta IAS Presentation
November 2015
Presentation by:
Hilton Mills
Hood-Patterson & Dewar
850 Center Way, Norcross GA 30071
hmills@hoodpd.com
770-543-1452
Testing Grounding Systems
Why
How to Specify
How to Test
Why do we test Grounding
Systems?
We all know that exposure to electrical
shock can kill you; is that not enough?
We have design engineers doing their
best to design a safe installation, but is it
safe?
It's safe for humans, but what about the
installed equipment?
Why do we test Grounding
Systems?
Per IEEE Std 81-2012:
To verify adequacy of new grounding system
To determine if there are any changes to an
existing grounding system
To identify hazardous touch and step
voltages
To determine Ground Potential Rise (GPR) for
protecting communications circuits.
How to Specify
We see numerous testing specifications
come though our office
Some specify Ohm values for selected items
of interest
99% of the specifications detail the method of
test as Fall of Potential method
So what's the problem?
How to Specify some
examples
How to Specify some
examples
How to Test
What are the problems with the existing
specifications?
Are there any problems with the tried and
tested Fall of Potential (FoP) method?
Specification Issues
Typically only mentions the FoP testing
methodology
Fails to include a requirement that would rule
out the FoP method; is the grounding system to
be tested totally isolated from any external
grounds?
Specifying specific values for a grounding
system might not result in a totally safe
installation
Methods for Testing
Fall-Of-Potential Method
Computer Based Grounding Multimeter
Method
10
Fall of Potential
Developed in the 1950s when we were less congested
Does not work well on large ground systems or odd
shaped grounding areas
Ground under test must be isolated no O/H Statics
or Neutrals bonded to ground grid
No connection from construction supply neutral to
site ground
No error correction for induced voltages and noise
Small data set
No statistical analysis
11
Fall of Potential Set Up
D
I
V
d
G PR
0
d = 62% D
D = 5 x L
12
Fall of Potential Set Up
Isolate the ground to be tested.
Set up the various probes.
Rule 1: Current at 5 times
diagonal of site
Rule 2: Voltage probe
varied from 50% to 70%
of distance to current
probe
13
Fall of Potential Results
Ideal results from an isolated ground FoP
test
14
Fall of Potential Results
Typical results from an isolated ground
FoP test
15
FoP Limitations
Downtown congested area
16
FoP Limitations
Large site - already bonded and energized
17
Alternative Test Method
IEEE Std 81-2012 addresses the
shortcomings of FoP
Annex E.3 Computer-based ground meter Modified fall-of-potential
The fall-of-potential method is the most popular
method of measuring the resistance of ground
electrodes and has been widely used for many
years. However, there are many variables and
situations that can distort test results and greatly
reduce the accuracy of these measurements. In
recent years, a computer-based ground meter has
been developed that can moderate the effect the
variables have on test results and improve the
18
Computer Based Multimeter
Method
Grounding system does not have to be
isolated
Current return distance is minimized
Large quantity of samples taken
Data corrected for noise, etc.
Calibration is performed for every case
Probe performance is quantified
Statistical analysis is performed
19
Equipment Set Up
Black
Blue Voltage Probe Assembly
(Triplex Shielded Cable)
Green
Ground System Under Test
Current
Return
Electrode
Yellow Voltage Probe Assembly
(Triplex Shielded Cable)
20
General Probe Placement
Recommended Area
for Current Probe
Current Return
Electrode
R > 2D
R
2D
Recommended Area
for Voltage Probes
L
3Y
2Y
1Y
3B
2B
1B
21
Site Specific Grounding
Geometry
22
Site Specific Ground Grid
Model
23
Site Test View
24
Probe Performance
SGM
Case:
Probe
Probe Performance Report
HOHENWALD_SGM-S-X-X-X
Resistance
Capacitance
(pF)
Inductance
(mH)
Error
Soil Resistivity
- m)
1Y
57.13
28.3
0.00
0.57
5.71
R em ove
2Y
126.97
62.9
0.00
2.09
3.80
R em ove
3Y
102.45
50.8
0.00
1.34
3.84
R em ove
1B
258.67
128.2
0.01
0.00
1.95
R em ove
2B
128.79
63.8
0.00
0.96
3.00
R em ove
3B
230.27
114.1
0.00
0.24
2.07
R em ove
Average*
Average*
Cancel
Proceed
129.12
Hood P atterson & D ewar
64.0
* NOTE
Maximum value
is omitted
(%)
F orm P R O BE _P E R F - C opyright A. P . Meliopoulos 1992-2013
25
Data Performance
SGM
Data Acquisition Performance
Case Name: HOHENWALD_SGM-S-X-X-X
Injected Current: 2.54 Amperes RMS
Resistance
Coherence
Prb #
%Valid
%Error
1Y
57.28
6.59
57.1
0.8968
Acceptable
Remove
2Y
55.34
4.51
127.0
0.8890
Good
Remove
3Y
52.75
5.05
102.4
0.8812
Acceptable
Remove
1B
58.90
4.28
258.7
0.8988
Good
Remove
2B
55.66
4.46
128.8
0.8909
Good
Remove
3B
55.02
4.53
230.3
0.8867
Good
Remove
(Ohms)
Cancel
Hood P a tte rson & D e wa r
Average-Squared
Quality
Proceed
F orm D AQ_ P E RF - C opyright A. P . Me liopoulos 1 9 9 2 -2 0 1 3
26
Data Spectrum Analysis
27
Ground Impedance
SGM
Ground Impedance Report
Case: HOHENWALD_SGM-S-X-X-X
Plot Mode
Magnitude / Phase
Resistance / Reactance
Series R-L
Parallel R-L
Plot Cursor
Frequency (Hz)
60.000
Magnitude (Ohms)
0.14130
Phase (Degrees)
43.198
Statistical Analysis
Return
Hood P a tte rson & D e wa r
F orm GR_ RE P _ 1 A - C opyright A. P . Me liopoulos 1 9 9 2 -2 0 1 3
28
Statistical Analysis
SGM
Statistical Analysis
Case: HOHENWALD_SGM-S-X-X-X
Return
Error Vs Confidence Level
Conf.%
0.00
100.00
100.00
100.00
100.00
Error %
4.0%
8.0%
12.0%
16.0%
20.0%
Probe Performance Index
1Y
0.04
1B
0.03
2Y
0.02
2B
0.02
3Y
0.03
3B
0.02
Hood Patterson & Dewar
Form E RR_CONF - Copyright A. P. Meliopoulos 1992-2013
29
Hypothetical Voltages
30
Hypothetical Voltages
31
Hypothetical Voltages
32
Ground Impedance Problems
High probe index (poor probe
performance)
High error/low confidence
Insufficient injected current
High earth voltage harmonics
33
Ground Mat Impedance
Whats the difference between the
Ground Impedance test and the Ground
Mat Impedance test?
Is it performed differently?
Can the Fall-Of-Potential method be
compared to the Computer Based
method?
34
Ground Mat Impedance
Case
SGM
Ground Mat Impedance Report
Case: HOHENWALD_SGM-M-X-X-X-X
Plot Cursors
Frequency
(Hertz)
60.000
Magnitude
(Ohms)
1.7491
Phase
(Degrees)
0.53873
Statistical Analysis
Return
Hood P atterson & D ewar
F orm GM_R E P _2 - C opyright A. P . Meliopoulos 1992-2013
35
Summary
36
What to Specify?
Replace
with
Perform tests by method as described in IEEE
81-2012.
37
Questions?
Thank you
38
