Touch potential

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Transcript Touch potential

GROUNDING SYSTEMS
The objective of a grounding system are:
1. To provide safety to personnel during normal and fault
conditions by limiting step and touch potential.
2. To assure correct operation of electrical/electronic
devices.
3. To prevent damage to electrical/electronic apparatus.
4. To dissipate lightning strokes.
5. To stabilize voltage during transient conditions and to
minimize the probability of flashover during transients.
6. To divert stray RF energy from sensitive audio, video,
control, and computer equipment.
A safe grounding design has two objectives:
1. To provide means to carry electric currents
into the earth under normal and fault
conditions without exceeding any operating
and equipment limits or adversely affecting
continuity of service.
2. To assure that a person in the vicinity of
grounded facilities is not exposed to the
danger of critical electric shock.
The PRIMARY goal of the grounding
system throughout any facilities is
SAFETY.
Why ground at all?
PERSONNEL SAFETY FIRST
EQUIPMENT PROTECTION SECOND
What are the three main types
of grounding?
The three main types are:
 EQUIPMENT GROUNDING (SAFETY)
 SYSTEM GROUNDING
 LIGHTNING/SURGE GROUNDING
Earth / Ground Basics
Types of Grounding Systems

Many different types
available
 Choice depends on local
conditions and required
function

Simplest form is a single
stake

Mostly used for:
 Lightning protection
 Stand alone structures
 Back-up for utility ground
Ground rod
Earth / Ground Basics
Types of Grounding Systems

ground rod group

typically for lightning
protection on larger
structures or protection
around potential hotspots
such as substations.
Ground rod group
Earth / Ground Basics
Types of Grounding Systems

For areas where there is
rock (or other poor
conducting material) fairly
close to the surface ground
plates are preferred as
they are more effective
Ground plate
Earth / Ground Basics
Types of Grounding Systems

A ground mesh consists of
network of bars connected
together, this system is
often used at larger sites
such as electrical
substations.
Ground mesh
Soil Characteristics
 Soil
type. Soil resistivity varies widely
depending on soil type, from as low as 1
Ohmmeter for moist loamy topsoil to almost
10,000 Ohm-meters for surface limestone.
 Moisture content is one of the controlling
factors in earth resistance because electrical
conduction in soil is essentially electrolytic.
Cable(Earthing conductor)
Clamp
Test link
Rod(Earthing electrode)
Rod coupler
Recommended values of earth resistance
system
Recommended earth
resistance(ohm)
Light current
0.5-1
Low voltage
5
Medium
voltage
2.5
High voltage
0.5
Substation earthing system
•Step & Touch voltage
•Grounding grids
Step and touch voltages
Step potential
 “Step potential”
is the
voltage
between the feet of a person standing
near an energized grounded object.
 It is equal to the difference in voltage,
given by the voltage distribution curve,
between two points at different
distances from the “electrode.”
 A person could be at risk of injury
during a fault simply by standing near
the grounding point.
Touch potential
“Touch potential” is the voltage between
the energized object and the feet of a
person in contact with the object.
 It is equal to the difference in voltage
between the energized object and a point
some distance away.
 The touch potential could be nearly the
full voltage across the grounded object if
that object is grounded at a point remote
from the place where the person is in
contact with it.

Ground Testing Methods (1)
Resistivity Measurement
The purpose of resistivity measurements is to quantify the
effectiveness of the earth where a grounding system will be
installed.
Differing earth materials will affect the effectiveness of the
grounding system.
The capability of different earth materials to conduct current
can be quantified by the value E (resistivity in W.m).
Resistivity measurements should be made prior to installing a
grounding system, the values measured will have an effect on
the design of the grounding system.
Ground Testing
Methods (1)
Resistivity Measurement ( Wenner method)
Resistivity measurements are performed by using a
four wire method.
Used to determine
which KIND of
earthing should be
used, so BEFORE
placing earth stakes
Ground Testing
Methods (1)
Resistivity Measurement
From the indicated resistance value RE, the soil
resistivity is calculated according to the equation :
 E = 2  . a . RE
E
RE
a
...... mean value of soil resistivity (W.m)
...... measured resistance (W)
...... probe distance (m)
Resistance of driven rods:

The Ground Resistance (R) of a single rod, of diameter (d) an
driven length (i) driven vertically into the soil of resistivity (ρ), can
be calculated as follows:
   8l  
R
ln    1

2l   d  
ρ
Soil Resistivity in m
l
Buried Length of the electrode in m
d
Diameter of the electrode in m
The rod is assumed as carrying current uniformly along its rod.
 Examples
(a) 20mm rod of 3m length and Soil resistivity 50 Ω-m .....R=16.1 Ω
(b) 25mm rod of 2m length and Soil resistivity 30 Ω-m .....R=13.0 Ω
where:
 The
resistance of a single rod is not sufficiently
low.
 A number of rods are connected in parallel.
 They should be driven far apart as possible to
minimize the overlap among their areas of
influence.
 It is necessary to determine the net reduction in
the total resistance by connecting rods in
parallel.
 The rod is replaced by a hemispherical
electrode having the same resistance.
Rod Electrodes in Parallel

If the desired ground resistance cannot be
achieved with one ground electrode, the overall
resistance can be reduced by connecting a
number of electrodes in parallel.

These are called “arrays of rod electrodes”.

The combined resistance is a function of the
number and configuration of electrodes, the
separation between them, their dimensions and
soil resistivity.

Rods in parallel should be spaced at least twice
their length to utilize the full benefit of the
additional rods.
 If
the separation of the electrodes is much
larger than their lengths and only a few
electrodes are in parallel, then the resultant
ground resistance can be calculated using the
ordinary equation for resistances in parallel.
 In practice, the effective ground resistance will
usually be higher than this.
 Typically, a 4 spike array may provide an
improvement of about 2.5 to 3 times.
 An 8 spike array will typically give an
improvement of may be 5 to 6 times.
Earth clamping 1
AT-090H
AT-090H
Earth clamping 2
AT-087J
AT-089J
AT-093J
METHODS OF DECREASING GROUND
RESISTANCE
 Decreasing
the ground resistance of a
grounding system in high resistivity soil is
often a formidable task.
 Recently,
some new methods have been
proposed to decrease ground resistance.
1-Chemical Rods

Chemical rods are electrodes with holes along
their length, filled with mineral salts.
 The specially formulated mineral salts are
evenly distributed along the entire length of the
electrode.
 The rod absorbs moisture from both air and soil.
 Continuous conditioning of a large area insures
an ultra-low-resistance ground which is more
effective than a conventional electrode.
 If
the conductive salts are running low, the
rod can be recharged with a refill kit.
 These rods are available in vertical and
horizontal configurations.
 They may be used in rocky soils, freezing
climates, dry deserts, or tropical rain
forests.
 They provide stable protection for many
years.
Disadvantages are:
 Chemicals concentrated around
electrodes will cause corrosion
 Chemicals leach through the soil and
dissipate
 Scheduled replacement may be required
 May be prohibited because they may
contaminate the water table
Soil Treatment Alternatives
 Ground enhancement material
Cement-like compound
 Non-corrosive
 Extremely conductive
 Installed around the electrode
 Easy installation
 Permanent

Installing an EARTHLINK 101 earthling strip is
simple:
Dig a trench and lay in the wire.
Pour EARTHLINK 101 conductive cement, using the handy
applicator bag, and shovel in a thin protective layer of soil.
Backfill the remaining soil using a front-end loader
and restore the surface to grade.