ee018-earthing and earth electrode resistance-th-inst.pdf

7/25/2019 EE018-Earthing and Earth Electrode Resistance-Th-Inst.pdf

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

Earthing and EarthElectrode Resistance

Theory

No: EE 018

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit

Earthing and Earth Electrode Resistance

Theoretical Part

No.: EE 018

Edition: 2008Al l Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerst rasse 64 P.O.Box 36, A 4031 Linz / Aus triaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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EARTHING AND EARTH ELECTRODE RESISTANCE

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................3

1 EARTHING (GROUNDING) AND EARTH ELECTRODE RESISTANCE *..................4

1.1 Earth electrode resistance ...................................................................................4

1.1.1

Measuring the earth electrode resistance........................................................5

1.2

Measuring the earthing resistance with an earth measuring bridge.....................7

1.2.1 The functions of the earth measuring bridge....................................................7

1.3 Earthing practice ..................................................................................................8

2

INSULATION RESISTANCE........................................................................................9

2.1 Measuring the insulation resistance...................................................................11

2.1.1

General ..........................................................................................................11

2.1.2 Regulations for measuring Rinsin installations with a voltage up to 1000 Volts .

.......................................................................................................................11

2.1.3 Measuring the insulation resistance in an installation out of Operation with a

portable measuring instrument...................................................................................12

2.1.4 Monitoring the insulation resistance in a live installation................................17


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LEARNING OBJECTIVES

The student should

explain the meaning of earth electrode (grounding) resistance explain the procedures

followed when measuring earth electrode resistance

explain the function of an earth-measuring bridge

explain why insulation resistance has no fixed value

state the regulations valid when measuring insulation resistance in a system rated up

to 1000 Volts

draw the principle diagram of an insulation resistance tester with an generator

explain the function of an insulation resistance tester with battery converter

outline the procedures to be followed when measuring the insulation resistance of a

new installation


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1 EARTHING (GROUNDING) AND EARTH ELECTRODE RESISTANCE *

A properly installed earthing system is of fundamental importance for safety in electrical

installations and for lightning conductors.

The earth (system) is sometimes used for a return path for load currents as well as for

protection. As an example, plant and fault currents are returned through the earth.

The purpose of earthing (grounding) system is, therefore, to hold the voltage to earth at

any desired point in a circuit below a specified value. Should the fault current reach a

sufficiently high value in the event of an accidental earthing (grounding) in the circuit, the

circuit protective switch will operate and the circuit will be interrupted. An excessive fault

current is very often caused by low resistance to earth.

The earth electrode resistance is a very complex resistance, consisting not only of ohmic

resistance, but of inductive and capacitive components as well. Electrolytic resistance also

exists, which results from the organic ingredients in the earth. The earth electrode

resistance is not always constant, but is influenced by change in the weather and

chemical and biological factors.

1.1 Earth electrode resistance

The value of the earth electrode resistance (RE) must, from time to time, be re-measured,

the circuit will only have the required safety protection when RE is below the required

value.

To measure the REa bridge may be used.

* NOTE: Earthing may also be referred to as grounding.


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1.1.1 Measuring the earth electrode resistance

Measurements must be made using alternating current, as electrolytic action,

(polarization) could occur if direct current were used. Such action will cause an increase in

REand the result in a false measurement.

When making measurements two auxiliary earth (H1 and H2) are to be used in the

measurement. To avoid errors in measuring (false readings) the earth and auxiliary earths

must be minimum of 20 m apart (outside their resistance areas). The distance between H1

and H2must be such that there is no interference between them.

RE = Earthing resistance of the earth (ground)

RH1 = Earthing resistance of the auxiliary earth

RH2 = Earthing resistance of the auxiliary earth

To determine the earthing RE the following measurements are required:

1. Between E and H1 ... RI= RH1+ RE

2. Between E and H2 ... RII= RH2+ RE

3. Between H1and H2 ... RIII= RH1+ RH2


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If the values of the first two measurements are added together and the value of the third

measurement is subtracted from this total, then the result will be:

In the network with the earthing as a protective measure, the earthing resistance may

not exceed:

RE max = 65V___________________

operating current of the circuit breaker


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1.2 Measuring the earthing resistance with an earth measuring bridge

Circuit diagram

1.2.1 The functions of the earth measuring bridge

The connecting link between E1and E2is connected in series with the earthing resistance

RE.

Auxiliary earth H1 is connected at point S, auxiliary earth H2 is connected at point HE. A

transistor power converter in the instrument produces the alternating current. After power

has been applied to the circuit measuring current will flow through auxiliary earth RH, earth

REand the adjustable resistor R of the compensator. The voltage drop V1which appears

between earth and auxiliary earth H1will be opposed by the voltage drop V2 across the

adjustable potentiometer R. When the voltages are equal, zero (0) will be indicated on the

galvanometer. The earth resistance (indicated in Ohms) can be read on the scale of theadjustable potentiometer.


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1.3 Earthing practice

As we have seen earthing is of fundamental importance in electrical engineering. Earthing

is used for the following purposes:

a) To prevent the outer casing of apparatus and conductors from reaching a

dangerous potential and which can result in ignition of flammable vapours or the

possibility of electric shock.

b) To allow sufficient current to pass to earth so that protective devices will operate.

The current path, on the occurrence of an earth fault, is from the fault to the associatedearth electrode, through the earth to the electrode of the supply transformer and back to

the fault.

If a low earth-loop resistance is required then a metallic earth-return path can be used.


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2 INSULATION RESISTANCE

Insulation resistance cannot be compared with metallic conductor resistance as the

current flow in a metallic conductor is exclusively concerned with the movement of free

electrons.

With insulation resistance there is a difference between the resistance within a material

and across the surface of the material. In the interior of an insulation material there are

practically no free electrons available and practically no current flow is possible.

Between the insulation sheets a charging current exists which builds a space charge in

the insulator. In the case of high voltages ionisation may also occur.

Condensed water and the acids and salts found in the surrounding atmosphere, together

with contamination by dust create the possibility of a current path over the surface of the

insulating material.The insulation material, its surface condition and the temperature all affect the insulation

resistance so, unlike metallic conductors, insulation resistance may vary.

It is possible to make several insulation resistance measurements on the same object and

the results may differ widely.

Also capacitive effects can cause changing readings during the actual measurement.

These effects can be shown more clearly with the aid of the equivalent circuit for the

installation resistance shown overleaf.


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a) pure ohmic resistance

b) capacitive resistance with a parallel-connected dropping resistor

c) voltage-dependent resistance

d) rectifier connected in series with a resistance

e) fixed voltage source with series resistance

Care must be taken when measuring insulation resistance that only the resistance which

is important for the active power loss is measured. By using a DC voltage as a measuring

voltage all of the capacitive effects may be eliminated (b).

To arrive at values for normal operating conditions, the measuring direct voltage should

be at least as large as the alternating voltage appearing during normal Operation.

Through this, the influence (c) is eliminated, while the electrolytic voltage (e) and theblocking voltage are only very small and, therefore, need not be taken into consideration.


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2.1 Measuring the insulation resistance

2.1.1 General

Before putting an installation into operation the soundness of the insulation must be

tested. It is not possible to insulate an installation completely.

For this reason, leakage currents will appear from time to time. However, with a good

insulation not more than 1 mA of current should occur in practice. The insulation

resistance (Rins) will be measured, restricts leakage current.

The following regulations are to be followed when measuring Rins in installations with a

voltage up to 1000 Volts.

2.1.2 Regulations for measuring Rinsin installations with a voltage up to 1000 Volts

a) When possible, the measurement should be made at the rated insulation voltage, and

not less than 500 Volts.

b) When measuring the insulation resistance of a conductor to earth, the negative pole of

the voltage source should be connected, where possible to the conductor being

measured.c) When not only the insulation resistance of the conductor to earth, but also the

insulation resistance between two conductors are to be tested, all light bulbs, motors

and other power users should be disconnected from the supply; all light sockets will be

connected, all fuses, circuit breakers and switches will be switched on.

d) Each final circuit (circuit directly connected from a fuse to a current consuming device)

must have Rinsof not less than 1000 Ohms per volt of the supply voltage.

Example: 100 V network Rins100 k

380 V network Rins380 k

500 V network Rins500 k


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2.1.3 Measuring the insulation resistance in an installation out of Operation with a

portable measuring instrument

The device most frequently used is the insulation resistance meter with hand generator.

a) Insulation resistance tester with hand generator and crossed coil instrument

The measured value is not dependent on the voltage (speed of turning hand

generator).


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b) Insulation resistance tester with hand generator and moving coil Instrument

To produce the proper measuring voltage so that the correct Mscale reading is obtained

the hand generator must be turned at a given rpm.

For the actual measurement of the insulation resistance, button S1 is pressed and the

hand generator is turned faster and faster until the indicator of the moving coil meter

movement indicates zero on the Mscale. When the button S1 is released the insulation

resistance is indicated. (When S1 is pressed the R is bypassed).

c) Insulation resistance tester with battery converter

Function

A DC voltage delivered from a flash light battery is chopped (mechanically or

electronically) and so transformed into a square alternating voltage.


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This AC is stepped up across a transformer to the desired test voltage and then rectified

by a rectifying circuit, producing a DC voltage. This DC voltage is in turn filtered by a

capacitor. With this high DC voltage the insulation measurement is accomplished.

As an indicating device, a crossed-coil measuring instrument is used.

A direct indicating resistance-measuring instrument may also be used. This is

accomplished by measuring the current with known applied voltage across a resistance.

As an indicator, a normal moving coil instrument is used. The indicator is not dependent

on the condition of the battery. The battery voltage is held constant through the use of a

transistor-regulating circuit.

Measuring the insulation resistance of an installation not in operation

a) Insulation resistance to earth


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b) Insulation resistance between lines

c) Separate circuits - between lines


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d) Separate circuits - conductor to earth


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2.1.4 Monitoring the insulation resistance in a live installation

For this purpose an earth leakage indicator is used. The simplest method is to measure

the three conductor voltage in the power network to earth. This method is only suitable

when the neutral point is not earthed.

Circuit for three-conductor network (mains)

Circuit for two-conductor network

In a system without error the three voltage meters will indicate the same voltage V0/ 3.

Should accidental earthing occur, then the voltage indicated on the voltmeter connected

on the defective conductor will become less, while the voltage on the other two voltmeters

will increase.

Only for networks which are not earthed (grounded)!


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EE 018Earthing and Earth Electrode

Resistance

Theoretical Test


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EE 018


TEST 1

QUESTIONS:

1. What is the purpose of an earthing system?

2. What influences the value of the earth electrode resistance?

3. Draw a diagram and write the appropriate equations used to measure earthing

resistance.

4. What type of current is used for measuring the earthing resistance?

5. What is the maximum earthing resistance in a network with earthing as a protective

device?

6. State the current path in a earth measuring bridge when measuring the earthing

resistance.

7. Draw a schematic diagram to be used for measuring the earthing resistance with earth

measuring bridge.

8. Why has the insulation resistance no fixed value?


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Test 1 - questions continued

9. Using the equivalent circuit diagram for the insulation resistance given below, explain

how (b) and (c) can be eliminated.

10. State how the insulation resistance may be monitored in a live installation.


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EE 018


TEST 2

QUESTIONS:

1. State the value of Rins per volt of the supply between two lines in order for the

insulation resistance to be considered satisfactory and calculate Rinsfor:

a) 120 V network

b) 340 V network

c) 460 V network

2. Draw the circuit used to measure the insulation resistance to earth in an installation

not connected to the supply.

3. Draw a diagram in order to measure the Rinsusing an insulation resistance tester with

hand generator and moving coil instrument.

4. Draw a diagram in order to measure the insulation resistance between conductor and

conductor of an installation not in operation.

5. Draw a diagram to measure the Rinsusing an insulation meter with a hand generator

and cross-coil instrument.

6. Why is an earth leakage indicator built into an installation?

7. Draw a diagram for monitoring the insulation resistance in an installation in operation

with a two conductor (single phase) network.


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Test 2 - questions continued

8. In the diagram shown below, state what is the voltage indication of each meter for

a) Fault free condition

b) Accidental earthing of one line

9. Draw a diagram to be used for monitoring the insulation resistance in an installation in

operation with a three-conductor network.

10. State for what type of network the earth leakage indicator shown below is valid.


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EE 018


TEST 1

SOLUTION:

1. To hold the voltage at any desired point in a circuit below a specified value to earth.

2. It is influenced by chemical, biological and weather changes.

3.

4. Alternating current (AC).

5. REmax. = 65 V __

operating current of the circuit breaker

6. Through auxiliary earth resistance (RH) and earth resistance (RE) and the adjustable

resistor of the compensator (R).


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Test 1 - answers continued

7.

8. Because of contamination of the surface by dust and condensed water and also

because of the acids and salts in the surrounding atmosphere.

9. Through the use of DC voltage as a measuring voltage all of the capacitive influence

(b) can be eliminated. If the measuring voltage is at least as large as the alternating

voltage during normal operation, then (c) is eliminated.

10. An earth leakage indicator is built in.The simplest system is to measure the three conductor voltages to earth in the circuit.


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EE 018


TEST 2

SOLUTION:

1. Rins= 1000 ohms per installation Volt.

2.

3.


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Test 2 - answers continued

4.

5.

6. To monitor the insulation resistance in a live installation.


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Test 2 - answers continued ...

7.

8. a) Same voltage on each meter.

b) The voltage indicated on the voltmeter connected to the defective conductor will

become less, while the other two readings will increase.

9.

10. For networks which are not earthed.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

0 49 5

ee018-earthing and earth electrode resistance-th-inst.pdf

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