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Welcome to the presentation on

SUB STATION EARTHING

ByM.L.SHESHADRI, EE,KPTCL,

A.R.CIRCLE, BANGALORE

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HISTORICAL BACKGROUNDHISTORICAL BACKGROUND

Very early use of electricity was with ungroundedsystem.The electrical systems were generally localized systems.The first AC Transmission in 1886 paved the way for

growth of electrical systems.Fatal accidents were reported when persons came in

contact with live parts due to insulation damage.A need was felt to provide an alternate easy path for

fault currents to prevent accidents.

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HISTORICAL BACKGROUND

Earth being a huge mass (mean radius 6371 Km) wasthought to be the best sink.

Metal rods were driven into the earth and noncurrent carrying metal parts (enclosures) of electricalapparatus were connected to these rods calledelectrodes.

The famous Horse and Dairy accidents.

Investigation of these accidents revealed thatpotential gradients are the cause of these accidentsand needs to be addressed to prevent electricshocks.

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SUBSUBSTATION EARTHING AN INTRODUCTIONSTATION EARTHING AN INTRODUCTIONEARTH AS RETURN CONDUCTOR

The electrical behaviour of earth is heterogeneousand it behaves as differently as;

A conductor having resistance.An electrolyte.A dielectric.

The behaviour of a ground electrode buried in soilcan be analyzed by means of the circuit shown here

as soil model.The charging current is negligible in comparison tothe leakage current and the earth can berepresented by a pure resistance for powerfrequency currents.

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SUB STATION EARTHING ANSUB STATION EARTHING ANINTRODUCTIONINTRODUCTIONEARTH AS RETURN CONDUCTOR

The soil resistivity is not affected by a voltagegradient unless the latter exceeds a certain critical

value. If the critical value of gradient (KV/Cm) is exceeded

arcs would develop at the electrode surface andprogress into the earth so as to increase theeffective size of the electrode, until gradients are

reduced to values that the soil material canwithstand.

This condition is illustrated by the presence of gapsin the soil model circuit.

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EARTH AS RETURN CONDUCTOR

The soil resistivity

The concept of Remote Earth.

The earth resistance of an electrode.

The GI electrode as per IS 3043 1966.

The CI electrode as per IS 3043 1987.

The electrode used in KPTCL sub stations.

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THE ELECTRIC SHOCK

Effects of an electric current passing through the vital partsof a human body depend on the duration, magnitude andfrequency of this current.

The most dangerous consequence of such an exposure is aheart condition known as ventricular fibrillation resulting inimmediate arrest of blood circulation.

Humans are very vulnerable to the effects of electric currentat power frequency, currents of 0.1 Ampere can be lethal.

Still higher currents can be tolerated at high frequencies.

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THE ELECTRIC SHOCK The most common physiological effects of electric current on thebody, stated in order

of increasing magnitude, are;

1 mA Threshold of perception Just able to detect a slight tingling

sensation in the hands or finger tips.

1 to 6 mA Let Go currents Unpleasant to sustain. Generally do not

impair the ability of the person holding

the energized object to control his

muscles and release it.

9 to 25 mA Painful Difficult or impossible to release energized

objects grasped by the hand. Breathing may

be difficult. The effects disappear when

current is interrupted. Person may respond to

resuscitation. 60 to 100 mA Dangerous Ventricular fibrillation, stoppage of

heart, inhibition of respiration, injury,

burning or death.

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THE ELECTRIC SHOCK

THE SHOCK POTENTIALS (THE ACCIDENTAL GROUND CIRCUIT)

TOUCH VOLTAGE

Touch voltage is the potential difference between a grounded metallicstructure and a point on the earths surface equal to the normalmaximum horizontal reach, approximately one meter.

STEP VOLTAGE

Step Voltage is the potential difference between two points on theearths surface, separated by a distance of one pace which will beassumed to be one meter, in the direction of maximum potentialgradient.

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SB = (IB)2 ts

THE ELECTRIC SHOCK

If shock currents can be kept below fibrillating Threshold value by acarefully designed grounding system, Injury or death may beavoided.

The non fibrillating current of magnitude at durations rangingfrom 0.03 to 3 seconds is related to the energy absorbed by the bodyis described by the following equation;

Where,

= rms magnitude of the current through the body in Amps.

= duration of current exposure in seconds.

= is the empirical constant related to the electric shock energytolerated by a certain percent of a given population.

SB

ts

IB

IB

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THE ELECTRIC SHOCK

The magnitude and duration of the current conducted through ahuman body at 50 or 60 Hz should be less than the value that cancause ventricular fibrillation of the heart.

Duration formula: The tolerable body current limit that persons can safely withstand

without ventricular fibrillation, with passage of current withmagnitude and duration is determined by the following formula;

For a person weighing 50 Kg the value of SB is 0.0135 as per thefindings ofMr. Dalziel.

Thus, for 50 Kg body weight, K = 0.116Therefore

This results in a value of 116 mA for = 1 second and 367mA for= 0.1 seconds.

IB = K / ts where K = SB

IB = 0.116 / ts

tsts

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THE ELECTRIC SHOCK

Considering the significance of fault duration both in terms of theabove equation and implicitly as an accident exposure factor, highspeed clearing of ground faults is advantageous for two reasons;

The probability of exposure to electric shock is greatly

reduced by fast fault clearing time, in contrast to situationsin which fault currents could persist for several minutes orpossibly hours.

Tests and experience show that the chance of severe injuryor death is greatly reduced if the duration of a current flowthrough the body is very brief.

Research provides evidence that a human heart becomesincreasingly susceptible to ventricular fibrillation when the time ofexposure to current is approaching the heart beat period, but thedanger is much smaller if the time of exposure to current is in theregion of 0.06 to 0.3 seconds.

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THE PURPOSE OF EARTHING

A number of points from Generators to consumers installations areearthed (or Grounded).

In our country we have an effectively grounded system.

The purpose of earthing is:

To provide a low impedance path to facilitate satisfactory operationof protective schemes during fault conditions.

To ensure that living beings in the vicinity of sub stations are notexposed to unsafe potentials under steady state or fault conditions.

To retain system voltages within reasonable limits under faultconditions and ensure that insulation breakdown voltages are notexceeded.

To provide an alternative path for induced current and there byminimize electrical noise in communication circuits.

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THE PURPOSE OF EARTHING

GROUND POTENTIAL RISE (GPR)

During a ground fault the portion of the fault current that isconducted by a sub station grounding grid into the earthcauses the rise of the grid potential with respect to remoteearth.

The design of earthing system strives to control thesepotential gradients within the sub station for the safety of

personnel as well as to avoid damage to electricalequipment.

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INSTALLATION AND MAINTENANCE OF EARTHING SYSTEM

Care during installation Levelling, Welding,Sound electrical connections.

Equipment earth connections within the sub

station. Power Transformer. Lightning Arrester. Instrument Transformers.

Switchgear.Cables. Auxiliaries. Earth Resistance measurements.

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