A Review of the IEEE Guide for
Grounding System Characterization
and
Application in Touch and Step Potential
Estimations
Ahmad Shahsiah, Ph.D., P.E.
March 18, 2015
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Purpose of Grounding
• Grounding is one of the means of safeguarding
employees and the public from injury
– Other means include, but are not limited to, guarding,
adequate clearance above ground, proper burial depth, etc.
• Grounding also allows protective devices to operate
during a fault condition
• The basic theory behind grounding is to keep the
voltage of grounded parts as close as possible to the
potential of earth, so that a voltage difference does not
exist between a person and a grounded metal object
(NESC 090)
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Grounding and Bonding
• Normally non-current carrying conductive
materials enclosing electrical conductors and
equipment, or forming part of such equipment,
shall be connected to earth so as to limit the
voltage to ground on these materials.
• Normally non-current carrying conductive
materials enclosing electrical conductors or
equipment, or forming part of such equipment
shall be connected together and to the electrical
supply source in a manner than establishes an
effective ground-fault current path.
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Examples of the Secondary Grounding
Grounding
Y-connected,
3-phase, 4-wire
e.g. 120/208 V
and 277/480 V
Single-phase, 3-wire system
e.g. 120/240V, 1φ, 3-W
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Delta-connected
3-phase, 4-wire
e.g. 120/240V,
3φ, 4-W, center-tap
Why Measure Earth Resistivity?
• Earth resistivity is used to estimate:
– Ground impedance of a grounding system
– Earth potential gradients to estimate step and
touch voltages
• Earth resistance data provides a quick
estimate of the potential rise of the ground
electrode at the time of a fault
• Ground grids are designed to limit the surface
voltage gradient
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How to Measure Earth Resistivity?
• The fall-of-potential method is commonly used to measure the
ground resistance with respect to a grounding electrode
• The method involves passing current between a “Ground Electrode”
(E) and a “Current Probe” (G) and measuring voltage between the
electrode (E) and a “Potential Probe” (P)
• Distance of the P to the electrode G is 62% of the distance of G to
the electrode E
– This method assumes soil with uniform resistivity
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Clamp-on Method
• This method measures resistance of a grounding electrode by clamping
onto the down-lead-wire
• It induces voltage at higher frequencies (~1kHZ). Induced voltage
circulates back and is measured by a second meter probe
• Is widely used but has limitations:
– The grounding electrode must have relatively low impedance
– Large error can be introduced if the reactance of the path is large compared to the
resistance because of the higher frequency measurements
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Touch and Step Potentials
• Dangerous levels of voltage can develop on
grounded equipment and on the ground
surface due to a high-current short-circuit fault
• OSHA regulation 1910.333(a)(1):
…Live parts that operate at less than 50 volts to
ground need not be de-energized if there will be no
increased exposure to electrical burns or to explosion
due to electric arcs.
• Lower voltage levels of concern may be defined
by the user based on the perception level as
determined by IEC60479-1-2005
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Measuring Step and Touch Voltages
• Current injection method:
– Involves injecting lower level currents into the ground and measuring
touch and step potentials
– Scaling the measured potentials to values that would be encountered
during a fault based on the ratio of the calculated fault current to the test
current
• Examples of locations at which touch voltages can be created:
–
–
–
–
Steel structures
Grounded equipment housings
Fences
Gates
• Measurement methods:
– Inject current between a remote point and simulated fault location
– Measure the touch potentials using twisted wire pairs
– Measurement equipment should have sufficient accuracy to distinguish
created potentials as a result of the test current from noise
– Estimated step and touch potentials can be compared with tolerable
voltages defined by OSHA or minimum perception levels defined by the
IEC
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Human Body Resistance
Values for hand-to-hand paths in saltwaterwet conditions are listed from IEC60479-12005 (Table 3)
•
•
•
Touch
Voltage
5th
Percentile
50th
Percentile
95th
Percentile
25
960 Ω
1300 Ω
1755 Ω
50
940 Ω
1275 Ω
1720 Ω
75
920 Ω
1250 Ω
1685 Ω
100
880 Ω
1225 Ω
1655 Ω
The heart-current factor allows
calculation of current through other
body pathways that represent the
same danger of ventricular fibrillation
as the left hand to feet pathway.
The heart current factor for left-hand to
right hand is 0.4 (IEC Table 12).
This means the estimated body current
for the pathway from left-hand to righthand must be multiplied by a factor of
0.4 to get the equivalent heart effect
between the left hand and both feet.
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IEC60479-1-2005 (Figure 5)
Dependence of the Total Impedance ZT of one living
person on the surface area of contact in dry condition
and at touch voltage (50Hz)
Electric Shock Hazard vs Current
Through the Body and Exposure Time
• IEC time/current zones for the
left hand to feet pathway
(IEC60479-1-2005 Table 11):
– AC-1: Perception possible but
usually no ‘startled’ reaction.
– AC-2: Perception and involuntary
muscle contractions likely but
usually no harmful electrical
physiological effects.
– AC-3: Strong involuntary muscle
contractions. Difficulty in
breathing. Reversible disturbances
of heart function. Immobilization
may occur. Effects increasing with
current magnitude. Usually no
organic damage to be expected.
– AC-4: Patho-physiological effects
may occur such as cardiac arrest,
and burns or other cellular
damage. Probability of ventricular
fibrillation increasing with current
magnitude and time.
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Source: IEC60479-1 – 2005 (Figure 20)
Stray Voltage Measurements
• Stray voltages and
currents are unavoidable
side effects of the
grounding system
• Stray voltages and
currents should be low
enough to avoid affecting
livestock
• There has been
considerable research
performed in this area.
Some of the results of
this research were used
to create laws in states
such as Wisconsin and
Idaho
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Effect of the Load Resistor
• Voltage measurements without a load resistor will result in overstated
measurements of stray voltage
• Parasitic capacitances exist due to the presence of leakage and space
charges, and can transfer small amounts of energy into the meter input
• The digital multi-meter leads and surfaces being measured can act as
antennas and result in invalid readings
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Effect of the Load Current
• Some of the load current imbalance returns through the ground
creating stray voltage
Distribution
Transformer
Neutral
Isolator
Primary
Neutral
Wire
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Secondary
Neutral
Wire
Downground
Wires
Typical Grounding and Bonding Techniques May Not
Limit the Ground Voltage for Fast-Changing Currents
IEEE C62.41.1
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Questions
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