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CONCEPTS OF EARTHING

(By R.K.Sinha)

E-Mail : rksinha59@yahoo.com

INTRODUCTION

Earthing is an integral part of any electrical installation.Earlier,very little was known about the requirements of earthing.Even now,when a wealth of information is available about the necessities and methodology of earthing systems,it still remains a complex and confusing subject.It is so common,yet so little understood.This article is an attempt to brush up the subject of earthing in the minds of electrical engineers and inspire them to dig further in the subject.

SOME FUNDAMENTALS

Earth : From electrical engineering point of view Earth is defined as the conductive mass of the earth,whose electric potential at any point is conventionally taken as zero.Being electrically neutral and at zero potential,the earth provides a common reference for voltage measurements.

Earthing : An electric connection to the general mass of earth,whose dimensions are very large in comparision to the electrical system being considered.

The terms ‘Ground’ and ‘Grounding’ are synonymous with ‘Earth’ and ‘Earthing’ and are more prevalent in some countries like North America.

Equipotential Bonding : Electrical connection putting various exposed conductive parts and extraneous conductive parts at a substantially equal potential.

Exposed Conductive Part : A conductive part of an equipment which can be touched and which is not a live part but which may become live under fault conditions.

Extraneous Conductive Part : A conductive part liable to transmit a potential including earth potential and not forming part of the electrical installation.

NEED FOR EARTHING

Earthing is of utmost importance for safety of plant,equipment,property and human as well as animal life.In the absence of a well designed effective earthing system,earth fault conditions may lead to tremendous loss of property and lives.The main objectives of earthing are as follows :

To ensure safety of life and property from hazards of electric shock and electric fires.
To ensure that system voltages on healthy lines remain within reasonable limits under fault conditions thereby prevending insulation breakdowns.
To provide a low impedance path to facilitate the satisfactory operation of protective devices under fault conditions.
To minimize arcing burn downs as in an earthed system arcing fault would produce a current in ground path thereby providing an easy means of detecting and tripping against phase to earth arcing fault breakdowns.
To provide an equipotential platform on which electronic equipments can operate.
To provide an alternative path for induced current and minimize the electrical noise in cables.

EARTHING CATEGORIES

There are two types of earthing.

SYSTEM EARTHING : This is primarily concerned with the protection pf electrical equipmen by stabilizing voltages with respect to ground.
EQUIPMENT EARTHING : This is primarily concerned with the protection of personnel from electric shock by maintaining the potential of non-current carrying equipment at or near ground potential.

SYSTEM EARTHING

This is basically achieved by earthing the neutral of the supply system.Ungrounded neutral supply systems are not in use mainly for the following disadvantages :

Under a single line to earth fault the voltage to earth of the two healthy phases rises from phase value to line value ie. Root 3 times.
The capacitive current in the two healthy phases increases to root 3 times the normal value.
The capacitive current in the faulty phase is 3 times its normal value.
The capacitive fault current gives rise to arcing ground condition.Under such condition the phase voltage may rise to 5 to 6 times its normal value causing insulation breakdown.
It is difficult to detect and isolate a single phase to earth fault.

Modern supply systems operate with their neutral points grounded.The advantages are :

There is no voltage rise in the healthy phases during phase to earth fault.
There are no unbalanced voltages with respect to earth.
Persistent arcing grounds are eliminated.
Earth fault currents can be utilized to operate protective relays to disconnect the fault.

METHODS OF NEUTRAL EARTHING

Solid or Effective Grounding : The use of solid grounding is limited only to systems where the normal circuit impedance is sufficient to prevent very high value of fault current.This is necessary to avoid excessive damage at the fault location.Experience shows that the combined impedance of the equipment,circuit and earth return path in systems, operating at voltages below 2.2 KV and above 33 KV, is sufficiently high so as to limit the value of fault current to a safe value.
Resistance Grounding : Neutral earthing is done through a resistor to limit the earth fault current.Neutral earthing resistors are normally designed to carry their rated current for a short period ,usually 30 seconds.Resistance grounding is normally employed on systems operating at voltages between 2.2 KV and 33 KV, when the total power source capacity exceeds 5000 KVA,as the current characteristics of such systems usually give rise to excessive currents under ground fault conditions.
Reactance Grounding : Another method of neutral grounding wherein the fault current can be limited is through reactance grounding.The reactance connected between neutral and earth provides a lagging current which neutralizes the capacitive current.Reactance grounding is preffered for circuits where high charging currents are involved such as transmission lines,underground cables etc.
Arc Suppresson Coil Grounding : Sometimes neutral earthing is done with an arc suppression coil.It is provided with tappings and the reactance of the coil can be tuned depending upon the length of the transmission line and the capacitance to be neutralized.

EQUIPMENT EARTHING

Under fault conditions the non-current carrying metal parts of an electrical installation such as frames,enclosures,supports,fencing etc. may attain high potential with respect to ground so that any person or stray animal touching these or approaching these will be subjected to potential difference which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal.

Safe value of current in amp(rms) which a human body can tolerate is

I = 0.165/root t for t < 3 secs

And I = 9 mA for t > 3 secs where ‘t’ is time duration in seconds of the flow of current.

To avoid this the non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground.The object of earthing is to ensure safety by discharging the electrical energy to the earth.The conductors may be in the form of a grid (also called mat) or multiple electrodes in the form of rods,plates,pipes etc.

Here,it is important to understand the concept of STEP and TOUCH voltage.

Touch Voltage : It is the potential difference between a grounded metallic structure and a point on the earth’s surface ,separated by a distance equal to the normal maximum horizontal reach,approximately one metre.

Step Voltage :It is the potential difference between two points on the earth’s surface ,separated by a distance of one pace,that will be assumed to be one metre in the direction of maximum potential gradient.

Consider that damage to a terminal bushing on a transformer (see fig.) has resulted in the faulting of the respective phase to the transformer body which is earthed by a pipe electrode driven into the earth.The earth fault current will flow from the phase conductor to the transformer body and through the earth electrode to earth.Around the electrode the current will flow outward in all directions.

As the current flows through a constantly increasing volume of earth ,its density drops as the distance from the electrode increases.The highest potential is at the electrode which is the same as that of the transformer tank.As the distance from the electrode increases,the less is the difference in earth surface potential between two points per unit length.

Now if a person happens to touch the transformer tank,the potendial difference between his hands and feet will be

E(Touch) = V1-V2

Where E(Touch) is termed as the Touch Potential . It is the voltage that exists between the hand and both feet of the person.

On the left side of the figure a person is shown walking towards the transformer tank.At any time,the earth surface potential between his feet will amount to

E(Step) = V3-V4

Where E(Step) is termed as the Step Potential and is the voltage between the two feet of a person.

It can be seen that

E(Step) = (Rk + 2Rf) Ik volts

And E(Touch) = (Rk + Rf/2) Ik volts

Where Rk is the resistance of the body,

Rf is the grounding resistance of one foot in ohms,taken for all practical purposes to be equal to 3 times the resistivity of the soil near the surface of ground in ohm-meter,

And Ik is the current in amps(rms) flowing through the body.

When a grounding system is installed,the objective is to obtain as low values of E(Step) and E(Touch) as possible in order to ensure full safety for human beings and stray animals.

CODES AND STANDARDS

The following are some of the important codes and standards which describe the earthing requirements and methods:

Indian Electricity Rules
National Electrical Code (India)
IS:3043 (Code of Practice for Earthing)
API RP 14F (Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities)
IEEE Std. 142 (Recommended Practice for Grounding of Industrial and Commercial Power Systems)

WHAT THE CODES SAY

Following are some excerpts from relevant codes :

INDIAN ELECTRICITY RULES

For Low and Medium Voltages :

Rule 61(1) states that neutral conductor of a 3phase,4 wire system shall be earthed by not less than two separate and distinct corrections with a minimum of two different earth electrodes or such large number as may be necessary to bring the earth resistance to a satisfactory value.The earth electrodes so provided,may be interconnected to reduce earth resistance.
Rule 61(2) states that the frame of every generator,motor and the metallic parts (not intended as conductors) of all transformers and any other apparatus used for regulating or controlling energy and all medium voltage energy consuming apparatus shall be earthed by two separate and distinct connections with earth.
Rule 61(4) states that all earthing systems shall –

a) consist of equipotential bonding conductors capable of carrying the prospective earth fault current and a group of pipe/rod/plate electrodes for dissipating the current to the general mass of earth without exceeding the allowable temperature limits in order to maintain all non-current carrying metal works reasonably at earth potential and to avoid dangerous contact potentials being developed on such metal works.

b) Limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.

c) Be mechanically strong,withstand corrosion and retain electrical continuity during the life of the installation.

For HV/EHV Systems

Rule 67(1) states that all non-current carrying metal parts associated with HV/EHV installation shall be effectively earthed to grounding system or mat in order to :

a) limit the touch and step potential to tolerable values.

b) limit the ground potential rise to tolerable values so as to prevent danger due to transfer of potential through ground,earth wires,pipe lines etc.

c) maintain the resistance of the earth connection to such a value as to make operation of the protective device effective.

d) Rule 67(1A) states that the neutral point of every generator and transformer shall be earthed by connecting it to the earthing system as defined in Rule 61(4) by not less than two separate and distinct connections.The neutral point of a generator may be connected to the earthing system through an impedance to limit the fault current to the earth.

Additional Precautions to be Adopted in Mines and Oil-Fields

Rule 116(1) states that in the interest of safety,appropriate switchgear with necessary protective equipment shall be suitably placed for automatically disconnecting supply to any part of the system where a fault including an earth fault occurs.
Rule 117(5) states that no switch,fuse or circuit-breaker shall be inserted in any earth conductor.

NATIONAL ELECTRICAL CODE (INDIA)

Clause 2.0.2 states that the object of an earthing system is to provide, as nearly as possible, a surface under and around a station which shall be at a uniform potential and as nearly zero or absolute earth potential as possible.The purpose of this is to ensure that all parts of apparatus,other than live parts,shall be at earth potential,as well as to ensure that operators shall be at earth potential at all times.Also by providing such an earth surface of uniform potential under and surrounding the station,as nearly as possible,there can exist no diffrence of potential big enough to shock or injure an operator when fault conditions occur.
Clause 2.0.3 states that earthing associated with current-carrying conductor is normally essential to the security of the system and is generally known as system earthing,while earthing of non-current carrying metal work is essential to the safety of life and property and is generally known as equipment earthing.
Clause 2.0.9 states that each earth system shall be so devised that the testing of individual earth electrode is possible.It is recommended that the value of any earth system resistance shall not be more than 5 ohms,unless otherwise specified.
Clause 2.0.10 states that a drawing shall be prepared for each installation showing the main earth connection and earth electrodes.
Clause 2.0.11 states that no addition to the current-carrying system shall be made which will increase the maximum available earth fault current or its duration until it has been ascertained that the existing arrangement of earth electrodes,earth bus-bar etc. is capable of carrying the new value of earth fault current.
Clause 2.0.11 states that no cut-out,link or switch other than a linked switch arranged to operate simultaneously on the earthed or earthed neutral conductor and the live conductors shall be inserted on any supply system.
Clause 2.1.1.2 states that the earth system resistance should be such that when a fault occurs against which the earthing system is designed to give protection,the protective gear will operate to isolate the faulty portion of the plant and render it harmless.

API RP 14 F

Clause 6.10.3 states the following :

Grounding of electrical equipment on fixed and floating offshore petroleum facilities in a positive manner is of particular importance because personnel standing on steel decks or in contact with steel framing present a low impedance path to ground ,effectively grounded.In addition,the dampness and salt deposition contribute to the breakdown of insulation and the possibility of leakage on the surface of insulators and similar devices.It is recommended that all metal equipment,such as skids,vessels etc. be grounded to the steel structure.Exposed,noncurrent-carrying metal parts of fixed equipment that may become energized because of any condition shall be grounded.The physical contact obtained when equipment is bolted to a steel structure is not necessarily an adequate effective ground because of paint and possible corrosion.

To provide the desired safety ,equipment grounding should accomplish the following :

a) Grounding shall limit the voltage (normally to 42 V maximum) that may be present between the equipment in question and any other grounded object with which personnel may be in contact at the same time.

b) For solidly grounded systems ,grounding should present a low impedance path for short circuit current to return to the source of power,thus opening a fuse or tripping a circuit breaker.This requires that the equipment ground be bonded to the system ground.

Monday 8 July 2013

CONCEPTS OF EARTHING (By R.K.Sinha)