Transcript DET4TC2

DET4TC2
Measurement of Earth electrode resistance and soil
resistivity……….
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Introduction
 Unique solution to the measurement of earth or
ground electrode resistance and soil resistivity
 Range of resistance 0.01 Ωto 200 K
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Features
IP54 rated
2,3 and 4 point testing
Attached rod technique
Stake less measurement
User selectable test frequency (94,105,111 and128
Hz)
Dry cell or non-rechargeable battery
User selectable output voltage
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i)Potential probe resistance
 Resistance between the potential stakes and it should be within
range for accurate measurement
 Rp limit: 100 KΩ (50 V output voltage) and 50 KΩ (25 V output
voltage)
ii)Current probe resistance
 Resistance between the current stakes and it should be within range
for accurate measurement
 Rc limit: 100 KΩ (50 V output voltage) and 5 KΩ (25 V output
voltage)
iii)Ground noise voltage
 The disturbance or interference caused by the formation of
unwanted ground loop due to external device and system
 Can be rejected upto 40 v peak to peak
 Calculated by following procedure
a) Connect the circuit as shown in fig
b) set the rotary switch to the V position
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c) Then, ground noise voltage will be displayed by
pressing TEST button
FIG: Instrument connection for measuring ground
noise voltage
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Principle of operation
i)Two terminal resistance measurement
 Required two earth points i.e. one earth electrode and
another is good ground connection
 DET injects an a.c. current into the system under test and
measures the voltage develop
When two terminal test is selected , DET instrument automatically connected
C1-P1 and C2-P2
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 Procedure
i) connect circuit as shown in fig
ii) Set the rotary switch to 2P position
iii) Set the required frequency
iv) Click Test button
v) Resistance will be displayed
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ii) Three terminal resistance measurement
 Fall of potential test
 Accurately measures the resistance of an earth electrode
 Uses auxiliary stake along with two terminal method
 DET injects an ac current of known magnitude and
measure voltage
Here, C1 and P1 terminals are connected together at the
electrode under test
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 In this case, the potential stake is moved by fixed
increment in a straight line between electrode under test
and current stake
 In each location, the resistance is calculated by R= V/I
then, a graph is plotted of resistance vs potential stake
position
 The resistance of the electrode under test is taken to be
the point at which the curve is flattest
 The earth electrode is consider to be surrounded by
concentric shells of earth shown in fig. The current probe
is also surrounded by earth shell but of smaller influence
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 It is necessary to locate current probe to the far way so
that influential shell do not overlap
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iii) Three terminal resistance measurement using ART
(Attached Rod technique)
 Classic three terminal test has the disadvantage that
electrode under test must be disconnected from the
system it is supposed to protect at the event of fault
 But here that system is connected
 Current I splits into two paths i.e. I = I1 +I2
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 The current I2 is calculated using Iclamp
 Then, Resistance is calculated by R= V/I2 or V/(I-I1)
 Like previous away, graph is plotted to obtain exact electrode
resistance
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iv) Two clamp stake less resistance measurement
 If electrode under test is connected to a network of other
electrodes as shown in figure and unsafe to disconnect
 Requires both Iclamp and Vclamp
 Using Vclamp defined test voltage is injected into the
system inducing current I which is measured by Iclamp
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v) Four terminal resistivity measurement
 Used for measuring resistivity of the soil
 Stakes are kept as shown in figure with equal distance
 As usual, resistance R is calculated
 Then, soil resistivity is calculated as 2*π*A*R
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THE END
Any
Queries??
Prepared By:
Er. Niroj Bahadur Bhujel
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