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UNSW Australia
Earthing
Trevor Blackburn
School of Electrical Engineering
1 INTRODUCTION
Functions & benefits of earthing (grounding):
Limitation of touch, step and transferred potentials
to prevent electric shock
Equipotential bonding of exposed metal conductors
to prevent electric shock and static charge buildup
Promote rapid and reliable operation of electrical
protection and to limit earth fault damage
Limitation of over-voltages (eg lightning and
switching) on equipment to prevent damage to
insulation and to electronic components
TO EARTH OR NOT TO EARTH?
Unearthed system
Earthed system
Electrical systems: with earthing
Electrical supply substations (utility operated)
 Transmission and distribution
 SWER systems
HV utilization substations (owner operated)
 Industrial sites
 Mining sites
Commercial sites
Domestic sites
From
Hpls;s;s
(Taken from From Ausgrid Standard NS116)
Electrical earthing
Electrical supply substation
 Earth grid or electrode_supply earth reference
HV utilization substations
 Industrial and mining sites
 HV earth system_extended grid and/or electrodes
 LV earth systems_local busbar or wiring interconnection
Commercial sites
 LV earth system mainly busbar & wiring interconnection
Domestic sites
 Totally LV earth systems: mainly wiring interconnection
Design requirements of earthing
Traditional approach has been prescriptive and regulatory
 This approach is now changing to an acceptable risk-based
probabilistic process for utility systems and for large
consumers with their own operations:
 based on the approach given in ENA EG-0 (Power System Earthing Guide)
 Applies to:
 major substations (AS2067), T and D networks, power stations and
large industrial systems
 Does not apply to:
 LV earthing on customer premises, DC systems, railways, mining
installations and equipment, ships and off-shore installations
 Australian Standards
 AS 1824.1, Insulation co-ordination – Definitions, principles and rules
 AS 1824.2, Insulation co-ordination (phase to earth and phase to phase, above 1 kV) –
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Application guide
AS 2067, Power installations exceeding 1 kV a.c.
Australian/New Zealand Standards
AS/NZS 1768, Lightning protection
AS/NZS 3000, Electrical Installations
AS/NZS 3835 Earth potential rise – Protection of telecom network users, personnel
and plant
AS/NZS 3931, Risk Analysis of Technological System – Application Guide
AS/NZS 4360, Risk management
AS/NZS 60479.1, Effects of current on human beings and livestock, Part 1: General
Other Documents
ENA C(b)1, Guidelines for design and maintenance of overhead distrib and trans lines
ENA EG1, Substation earthing guide
IEEE Std 80 „IEEE Guide for Safety in AC Substation Grounding‟.
Risk of death in various activities: (from AS1768)
Electric shock: design
constraints on LV earthing
2
METHODS OF
POWER SUPPLY SYSTEM EARTHING
General requirements of an earthing system:
low impedance path (resistance and reactance) to the
earth of the local supply system and thence to earth at
the main substation
Equipment/appliance items needing earthing must be
connected to earth by a low resistance/impedance to
provide equipotential bonding between all items. Avoid
earth loops.
earth potential rise (EPR) associated with any conductor
carrying fault current must be limited to safe levels
earth conductors must capable of handling all prospective
fault currents without thermal or mechanical damage
METHODS OF EARTHING
Unearthed system
Solidly earthed system
Resistance earthed system
Reactance earthed system
Use of an earthing transformer
METHODS OF EARTHING
Unearthed system
Solidly earthed system
Resistance earthed system
Reactance earthed system
Use of an earthing transformer
Advantages:
Solidly earthed systems
High fault current and fast protection operation
Better personnel and equipment safety
Earth fault current is easy to detect
Unearthed systems
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Line to earth fault will not interrupt supply thus improves
production line reliability
Low earth fault current (limited by the line capacitance to
earth) will not cause damage
Disadvantages:
Solidly earthed systems
 Line to earth fault causes loss of supply
 Line to ground fault current may be high enough to cause
significant damage but be undetectable by protection (HIAF)
Unearthed systems
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Line-line fault current may be low and may not trip protection
May have prolonged arc faults
Line-line voltage imposed on phase insulation with earth fault
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Overvoltages may cause increased insulation stress if there is an earth
fault
Finding earth faults may be difficult – requires monitoring
Regular maintenance is required
3
PERSONNEL PROTECTION
REQUIREMENTS FOR
EARTH SYSTEM DESIGN
STANDARDS
AS/NZS 60479.1:2002 : Effects of current on
human beings and livestock - General aspects
 Uses Biegelmiers (IEC) stepwise graphical methods
 Not easy to apply for step and touch potentials
In Nth America: IEEE Std.80 – 1986
 Uses Dalziel‟s “Electrocution equation”
 Much simpler to incorporate in earthing design
From AS60479.1
HUMAN BODY ELECTRICAL IMPEDANCE
(REF. AS/NZS 60479.1:2002)
Dalziel’s “Electrocution equation”:
Defines a specific energy (I2t) as the determining factor
for potential electrocution (fibrillation).
Uses the I2t value to determine a fibrillating current level
Values are based on extensive tests carried out on
humans (his students!) and animals.
Comparison of the IEC and IEEE approaches
IEEE
4
SUBSTATION EARTH:
DESIGN REQUIREMENTS
EARTH POTENTIAL RISE (EPR [GPR])
Hazardous situations which can occur in
substations when there is an earth
surface potential rise (EPR) in the vicinity
of an earth electrode due to fault current
flowing to earth through that earth
electrode.
Potential rise around an earth electrode taking fault
current: (from IEEE 80)
HAZARDOUS POTENTIALS NEAR EARTHS
 Step potential : voltage difference between a person's
feet when spaced 1m apart (radially w.r. to the earth).
 Touch potential: voltage difference between exposed
metal object, connected directly to earth electrode,
and ground surface potential where feet are placed
(usually distance of 1m is used).
 Grid (mesh) potential: maximum possible touch voltage
in an earth grid area.
 Transferred potential: voltage difference between earth
surface potential and exposed metal object connected
to remote earth (effectively at true earth potential of
zero volts).
VT: Touch
VS: Step
Vtrans: Transferred
Potential hazards due to Earth Potential Rise from a fault (From AS1768).
CALCULATED GROUND SURFACE POTENTIAL IN AN EARTH GRID
POTENTIAL PLOTS AND PERMISSIBLE LEVELS FOR AN EARTH GRID
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EARTHING CONDUCTOR
- Current carrying capacity
ADIABATIC SHORT CIRCUIT HEATING
From IEEE Std. 80
6
SYSTEMS OF EARTHING
IN LOW-VOLTAGE INSTALLATIONS
Direct earthing system:
system relies on current flow through the ground
thus requires low earth resistivity and good earth
electrode. Not always possible.
MEN system is preferred as it utilizes supply utility‟s
neutral to provide an additional earth return path.
MEN system:
earth connections to neutral at consumer‟s installation
and along route to supply substation
neutral provides the return path
while in direct earth system the metallic path is
provided by water pipes, cable sheaths or by special
earthing connections if provided
balancing of load to utilize phase current cancellation
in return neutral to minimize voltage drop
neutral conductor must be earthed at the substation
and at other locations as necessary to ensure that
total impedance between neutral and earth does not
exceed 10 ohms
conductors used to earth neutral conductor of
distribution system must have a cross-section area of
at least 20% that of the smallest size of neutral used in
system
CMEN system:
Common Multiple Earthed Neutral
extension of MEN system
high voltage and low voltage equipment are bonded
(via a neutral conductor) to a single common earth
impedance to ground of this interconnected system of
earthing is very low, typically 1 ohm or less.
7
TYPES OF EARTHING SYSTEMS
IN CONSUMER’S INSTALLATIONS
TN-C
TN-S
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TN systems
TT systems
IT systems
TN-C-S
In practice, only TT and TN systems are commonly used.
1st letter (I or T)
gives relationship of supply to earth
T (terra): direct connection of one point of supply
system to earth
I (insulation): all live parts of supply isolated from
earth or one point connected to earth through an
impedance
2nd letter (T or N)
gives relationship of exposed conductive parts of
the general installation to earth
T (terra): direct connection of exposed conductive
parts to earth, independent of earthing of supply
system
N (neutral): direct connection of exposed
conductive parts to earthed point of supply
(neutral point).
TT system: one point directly earthed, exposed
conductive parts connected to earth via separate earth
electrode, no direct connection between live parts and
earth, exposed conductive parts connected to earth
TN systems: source side directly earthed, exposed conductive
parts connected to that point by the protective conductor (PE)
TN-S system: separate neutral (N) and PE throughout
 TN-C system: N and PE combined into a single
conductor throughout
 TN-C-S system: N and PE combined into a single
conductor in a part of the system, separated in another
part
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TN-S
TN-C-S
TN-C
IT system: no direct connection between live parts and
earth, exposed conductive parts connected to earth
8
EARTH RESISTANCE OF
BURIED ELECTRODES
Earth resistance determined by:
shape of electrode(s)
extent of electrode(s)
electrical resistivity of the soil
Resistance to
ground for
various
electrode
configurations
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EQUIVALENT HEMISPHERE MODEL
OF AN EARTH ELECTRODE:
Use for touch, step and transferred
potential calculations
The most common form of earth electrode is a vertical
driven rod or horizontal pipe or a complex distributed
mesh in the ground.
These are not simple for analytic calculation of touch
and step potentials
An approximate estimation used is to determine an
equivalent hemisphere and then used for potential
distribution calculations
Example:
V(r) = ground potential w.r.t earth electrode
10
MEASUREMENT OF
EARTH RESISTANCE
Three-electrode method
Fall-of-potential method
potential
probe
remote
current
probe
STAKELESS TEST METHOD FOR MULTIPLE
ELECTRODES (FROM FLUKE MANUAL)
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EARTH ELECTRICAL RESISTIVITY
not a good conductor compared to metals.
Extremely variable depending upon physical nature of
soil/ground material and chemical composition
heavily influenced by moisture content, temperature
and dissolved salts
Can be improved by use of electrolytes at the earth
stake (eg bentonite)
Rule of thumb resistivities of some materials
Mud (compressed coal):
Wet soil:
Moist soil:
Dry soil:
Rock:
1 Ωm,
10 Ωm,
100 Ωm,
1000 Ωm,
10000 Ωm.
Typical general values are 100 - 300 Ωm
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ELECTRIC SHOCK EFFECTS
IN SUBSTATIONS
r = soil resistivity
body
resistance
foot contact
resistance
Equivalent circuits for touch and step potentials
Assume bare feet or conducting footwear:
Tolerable touch and step potentials