Earthing System

CONTENTS



1.0. SCOPE

2.0 DEFINITIONS

3.0 STATEMENT OF COMPLIANCE

4.0 RESPONSIBILITY

5.0 SAFETY

6.0 METHODS

7.0 MEASUREMENT OF EARTH RESISTANCE










1.0 SCOPE

This scope of work covers the design, manufacturing, marking and packing, shipping, transportation to site, installation, testing and commissioning and final handover of the earthing system.



2.0 DEFINITIONS

SEE – Site Electrical Engineer, ST – Survey Team, S/s – Substation

3.0 STATEMENT OF COMPLIANCE

We hereby confirm that we shall comply all the procedures and execute the subject works as per the fullest requirement, satisfaction of end user and specifications

A constant supervision of works shall be done by qualified Electrical Engineer and certified safety officer.

Material shall be used for assembly of earthing system are designed in accordance with the standards and system characteristics as stated in corporate ADDC specification.

4.0 RESPONSIBILITY

This is the responsibility of the Site Electrical Engineer and the safety officer to ensure that this method statement and Safety procedures are followed during the execution of the work.



5.0 SAFETY

1) All work forces to be involved in execution of the Earthing system shall undergo safety induction training before starting the site activities. Prior to the commencement of any activities, the workforce will receive an induction talk on the project site safety requirements.
2) The necessary permits that may be required to work near existing services will be obtained. Any special conditions imposed upon these permits will be implemented.

3) The use or restriction of specific Electrical & mechanical equipment, whilst locating or excavating near the existing cables and other services will be implemented. The contractor shall utilize the manpower whoever obtains security passes. Company shall be responsible for all activities and interfaces related to this project.


6.0 METHODS


6.1 TOOLS & INSTRUMENTS

a. Earth meggar type AVO DET5/4D (for measuring earth resistance)

b. Multi meter types 0646171 Hioki (for measuring electrical continuity)

c. Hand tools & Hammers

All the testing instruments will have calibration certificate valid for minimum six months. Calibration certificate can be verified during testing.


6.2 REFERENCE VALUES

a. For electrical continuity = infinity

b. For earth resistance of Earthing System = less than 2 ohms (as per ADDC)


6.3 EARTHING PROCEDURE

A) MEASURING TECHNIQUES Measuring Soil Resistivity

TYPICAL VARIATIONS IN SOIL RESISTIVITY

The resistance to earth of an earth electrode is influenced by the resistivity of the surrounding soil. The resistivity depends upon the nature of the soil and its moisture content and can vary enormously as seen in the table below:-


Material Specific resistance in ohm-cms Information Source
Ashes 350 Higgs
Coke 20-800
Peat 4500-20000
Garden earth – 50% moisture 1400 Ruppel
Garden earth – 20% moisture 4800 Ruppel
Clay soil – 40% moisture 770 Ruppel
Clay soil – 20% moisture 3300
London clay 400- 2000
Very dry clay 5000-15000
Sand – 90% moisture 13000 Ruppel
Sand – normal moisture 300000-800000
Chalk 5000-15000
Consolidated Sedimentary rocks 1000-50000 Broughton
Edge & Laby


• Because it is impossible to forecast the resistivity of the soil with any degree of accuracy it is important to measure the resistance of an earth electrode when it is first laid down and thereafter
at periodic intervals. Before sinking an electrode into the ground for a new installation it is often advantageous to make a preliminary survey of the soil resistivity of the surrounding site. This will enable decisions to be made on the best position for the electrode (s) and to decide whether any advantage can be gained by driving rods to a greater depth. Such a survey may produce considerable savings in electrode and installation costs incurred trying to achieve a required resistance.

However, Soil resistivity could be carried by laboratory section.

B) DEEP DRIVEN EARTH ELECTRODE METHOD FOR MEASURING EARTH RESISTANCE:

In this method, Copper bond earthing electrode (approx. 16mm dia.), shall be deeply driven using suitable earth rod coupler till the depth of lower resistivity soil. (As per ADDC requirement Earth resistance shall be less than 2 ohms.). To obtain low soil resistivity (less than 2 ohms) earthing rods shall be driven 3mts deep or even more. Also, to obtain low overall resistance, current density should be as low as practicable in the soil which is in contact with the electrode.

After driving earth electrode in the respective pits, its shall be covered with inspection pits.

C) PARALLEL EARTH ROD METHOD FOR MEASURING EARTH RESISTANCE:

Where ground conditions make deep driving of earth rods impossible, a matrix arrangement of rods coupled to one another by conductors can be used. If possible, the earth rods must be spaced at least equal to their driven depth. No significant decrease in Earthing resistance, it will be obtained by spacing greater than twice their driven depth. These methods are commonly used for substation earthing where there is HV electrical equipment.

The ground condition is very hard and rocky, then earthing electrodes shall be driven by means of Drilling the soil with heavy drilling equipment.

After driving earth electrode in the respective pits, its shall be covered with inspection pits.

0 MEASUREMENT OF EARTH RESISTANCE:

a) General procedure:

Measurement of the resistance to earth of an earth electrode is not necessary a simple matter. While certain fairly simple rules can be laid down. Circumstances frequent arise which make it necessary to modify them. The resistance of an earth electrode is unique in that the terminal provided by the electrode itself is definite, the other terminal of the resistance being theoretically at an infinite distance. In practice a measurement has to be made which includes the greatest part, say 98% of the total resistance. There is no point in striving for a high degree of accuracy with such a measurement since within the volume of such a resistance, there may be considerable non-uniformity in the soil and other disturbing features. An accuracy of 2% is more than adequate, and accuracies of the order of 5% are usually quite acceptable.

A measurement current is passed between electrodes X, the one being tested and an auxiliary current electrode Y. The voltage drop between electrode X and a second auxiliary electrode Z is measured and the resistance of the electrode X is then the voltage between X and Z. The source of current and the means of metering either the current and voltage or their ratio are often, but not necessarily, combined in one device.


b) Measuring Techniques Testing Earth Electrodes:

FALL - OF - POTENTIAL METHOD

This is the basic method for measuring the resistance of earth electrode systems. However, it may only be practical on small, single earth electrodes because of limitation on the size of area available to perform tests.

Insert the Current Test spike into the ground some 30 to 50 meters away from the earth electrode to be tested. Firmly connect this spike to the instrument terminal ‘C2’.

Insert the Potential test spike into the ground midway between the Current test spike and the earth electrode. Firmly connect this spike to the instrument terminal ‘P2’.


Note:

1) It is important that the Current Spike, the Potential Spike and the earth electrode are all in a straight line. Also when running the test leads out to each remote spike, it is preferable not to lay the wires close to each other in order to minimize the effect of mutual inductance.

2) Firmly connect the ‘C1’ and the ‘P1’ instrument terminals to the earth electrode.

3) Operate the instrument as explained in the ‘Basic Test Procedure’ and note the resistance obtained.

4) Move the potential spike 3 meters further away from the earth electrode and make a second resistance measurement. Then moves the potential spike 3 meters nearer the electrode (than the original position) and make a third resistance measurement. If the three resistance readings agree with each other, within the required accuracy, then their average may be taken as the resistance to earth of the electrode.

5) If the readings disagree beyond the required accuracy then an alternative method should be used e.g. the 61.8% Rule or the Slope Method etc.