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 2

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 3

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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 K voltage) Ω (50 V output voltage) and 50 KΩ (25 V output ii)Current probe resistance  Resistance between the current stakes and it should be within range for accurate measurement  Rc limit: 100 K voltage) Ω (50 V output voltage) and 5 KΩ (25 V output 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 6

c) Then, ground noise voltage will be displayed by pressing TEST button FIG: Instrument connection for measuring ground noise voltage 7

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 8 C1-P1 and C2-P2

 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 9

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) 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 10

 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 11

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 It is necessary to locate current probe to the far way so that influential shell do not overlap 13

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 14

 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 15

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 16

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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 17

Any Queries

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THE END

Prepared By: Er. Niroj Bahadur Bhujel

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