INTRODUCTION

• Cables form the artery system for the transmission

and distribution of electrical energy.
• Residential loads have a trend towards their growing
density.
• This requires a rugged construction, greater
reliability, increased safety and better appearance.
• The interference from external disturbances should be
reduced to a minimum.
• These are achieved by the use of underground cables.
• though the underground system is more costly as compared
with an overhead system for the same voltage, its more
acceptable because of the merits as stated previously.
• another advantage is that it requires lesser right of way.
• the main constraint being the temperature rise of the
insulating material used.
• Also if there is a fault in the cable it cannot be easily
located.
• Long distance transmission lines also have large charging
currents associated with it hence it also find restriction in
this aspect.
 Comparison between overhead transmission line and buried
cable
Advantage (Cable):-
 Not visible.
 Low failure rate
 Small route width
 No outer electrical field.
Advantage(OTL):-
 Low cost
 Long transmission length.
Disadvantage (cable):-
 Cost
 Limited transmission length
 Repair times.
Disadvantage (OTL):-
 External influence
 Low public acceptance
 Width of route.
 Cable construction
• Power cables consists of three main components:-
1. Conductor
2. Dielectric/insulation.
3. Sheath.
• Conductor: provides the conducting path for the current
• Insulation : withstands the service voltage and isolates the
conductor with other objects.
• Sheath: does not allows moisture to enter , and protects
the cable from chemical and all other external influences.
Conductor:
• The most acceptable metals for conductors are Copper or Aluminium
due to their high conductivity and ductility.
• As Copper gets higher affinity with Sulphur, it corrodes in the
atmosphere where Sulphur fumes are present. In these conditions,
tinned copper shall be used. Use of Copper has been restricted to
Control Cables, Signaling Cables and Mining Cables as per Govt.
Directives.
• Power Cables are mostly manufactured with Aluminium.
• The most economic construction for conductor is solid conductor. As
area of conductor increases, solid conductor becomes more stiff and
hence difficult to handle. In this connection Stranded Construction is
adopted i.e.made of number of strands, arranged in spiral layers in
1+6+12+18+24......formations.
 Design Aspects
 Current Flow – limited losses allowed.
 Short Circuit Current – limited temperature
allowed
 Bending forces during manufacturing and
laying – no break of conductor
 Pulling forces during laying - no break of
conductor.
 Thermomechanical forces during operation
load/no load – tension/pressure forces.
 Installation – easy installation of connectors.
Insulation Materials
PAPER INSULATION
 Polypropylene Paper Laminate
(PPL) insulation:
Material consists of 50% polypropylene and
50% paper .
Has a lower dielectric loss factor than paper
therefore heat generated within the insulation
at high voltages is reduced.
can operate at higher stress levels than paper
Polyvinyl Chloride (PVC) insulation
• Is non hygroscopic.
• Requires no metallic sheath.
• Absence of sheath simplifies joining
• Is lighter and more flexible than paper
insulated cables
• Cannot withstand the thermal effects of short
circuit currents
• Maximum operating temperature is 65° C -
70°C
 Cross Linked Polyethylene (XLPE)
insulation
• Has high dielectric strength
• Has good mechanical strength
• Is non hygroscopic
• Has no true melting point and remains elastic
at high temperatures
• Has greater current carrying capacity
 Ethylene Propylene Rubber (EPR)
insulation
• Has the same durable and thermal
characteristics of XLPE
• Has a higher degree of elasticity than XLPE
over a wider temperature range
 Sheath
• metal sheathing is required to provide an impervious layer to protect the cable
from moisture which would affect the insulation.
• Lead and lead alloy sheaths have been traditionally used to prevent ingress of
moisture into cables.
• These alloys can withstand the internal pressure of the pressurized cables.
Aluminum Sheaths:-
• Aluminum sheaths has a small weight and high mechanical strength then lead
sheath.
• It has greater conductivity and is easy to manufacture and install.
• It has good screening properties and eliminates the use of armour usually required
in lead sheath cables.
• An aluminum sheath can withstand the required gas pressure without
reinforcements.
• Owing to its greater conductivity the aluminum sheath of low voltage cable may be
used as neutral conductor. Thus there is no need of separate fourth conductor.
• Aluminium needs additional corrosion protection layers (bitumen, or similar) as it is
an electrochemical poor metal.
• Recently corrugated seamless aluminum (CSA) sheath is finding favor
these days .
• It has better bending property, reduced thickness and less weight.
• It is mainly been used in high voltage oil filled cables and telephone
cables.
• The corrugated cables are very flexible and can bent easily.
• Also by repeated bending the sheath is not distorted unduly and
therefore, it is not damaged.
 PROTECTIVE COVERING:-
• Lead sheath are subjected to mechanical damage, corrosion and
electrolytic action when laid directly in the ground.
• To protect from these actions protective covering are applied to
the sheaths.
• For protection against corrosion and electrolytic action, bitumen
and bituminized materials(paper, hessian etc.) or polyvinyl
chloride are used.
• Layers of fibrous permeated material with waterproof compound
applied to the exterior of the cable is called serving.
 ARMOURING:-
• Purpose of Armouring:-
– To carry Earth Fault current during fault condition
– To provide Earth path for Screen current
– To provide mechanical Protection.
• As perIS :1554(Pt-I) -88, Round Wire armouring is provided in cable
where calculated diameter under armour is upto 13mm. Above this
the armour is either steel wire or steel strip of size 4.00X0.80mm. As
strip construction is economical, the manufacture always provides
steel strip armouring unless wire armouring is specified.
• In the case of double wire armour a fabric tape is used as a separator
between two layers of wires. Double wire armour is used for cables
with higher tensile strength for example along sloping routes , in
mines, under water etc.
• Armouring is not used over single core cables
because of large power loss and high resistance drop
• Presence of magnetic material within the alternating
magnetic field of a single core cable produces
excessive losses.
• For this reason single core cables are either left
unarmoured or if necessary they are armoured with
non magnetic materials like tin bronze or silicon
bronze tapes or wires.
• Aluminum has been used recently as an armour
material due to its non-magnetic properties, high
conductivity and mechanical strength.
• It is particularly used for single core cables working
on a.c.
 Purpose of Outer Sheath
• Provides Protection against
– Soil pressure
– Spiking of external Agencies
– Damages during Cable laying
• A final covering of PVC compound is applied over armouring
in case of armoured cable and over inner sheath in case of
unarmoured cable called as "Outer Sheath".
• I.S. 1554 specifies nominal and minimum thicknesses of outer
sheath for unarmoured cables and only minimum thickess of
outer sheath for armoured cables.
 Outer Sheath
• In the modern Power, Chemical, Fertilizer and Cement Plants
many PVC cables are bunched in the cable shaft or on cable
trays. In case of fire in these cables, the fire becomes self
sustaining. Moreover due to the burning of PVC a dense
corrosive smoke is emitted which makes fire fighting very
difficult, due to poor visibility and toxic nature of the smoke.
HCL content of the smoke, not only damages other costly
equipment lying nearby, but also penetrates the RCC and
corrodes the steel reinforcement. Due to this there is an
extensive damage to the property.
• To overcome these deficiencies FRLS i.e. Fire Retardant Low
Smoke PVC was developed.
 How to identify type of cable from
outside
• AS Per IS:1554(Part1)&7098(Part1)
'A' - Alluminium
'2X' - XLPE Insulation
'F' - Steel Strip Armour
'y' - PVC Insulation
'W' - Steel Round Wire Armour
'y' - PVC Outer Sheeth
'ww' - Steel Double round wire armour
'yy' - Steel Double Strip Armour
'Wa' - Non Magnetic round wire armour
'Fa' - Non Magnetic flat strip armour
YWY means CopperConductor, PVC insulated, Round Wire Armoured and PVC
Sheathed Cable
• 3 ½ core x 50 sq.mm AYFY: Aluminium conductor, PVC insulated, laid up,
innersheathed, Steel strip armoured and PVC sheathed cable having 3 cores of 50
sq.mm and 1 core of 25 sq.mm conductor size.
SINGLE CORE CABLE
THREE CORE CABLE (BELTED CABLE)
• Each three-conductor cores is wrapped with oil –impregnated paper, in a three core
cables.
• The cores are then assembled with a filler material. The assembled is enclosed by a
paper insulating belt.
• A sheath is provided above the belt
• The belted cables are usually upto 22 kV .
• The disadvantage is attributed to the tangential stress which the cable insulation
experiences.
• The field developed inside the cable is rotating one so a tangential stress along the
surface of the core insulation is developed.
• The tangential stress ultimately leads to development of partial discharge since
paper can withstand very little tangential stress.
• The discharge starting in the region gradually damage the whole of the paper
insulation.
• Moreover the loose fillers between the cores situated in electric fields are
electrically weak. With the variation of the load the cores expand and contract.
• This forms voids in the fillers particularly in the space between the cores.
• This voids become the position of discharge.
• If the discharges are sufficiently severe there may be local heating and charring of
paper with the result that there may be a breakdown of the cable insulation.
Screened/H-type cable:-
• Defects of belted cable can overcome in the screened cable.
• Here the tangential stresses can be eliminated by screening each core separately,
so that the cable in effect becomes three separate single core cables laid up within
the same protective covering.
• The H-type cable consists of three paper-insulated cores and over the insulation of
each is wound a perforated moralized paper.
• Perforated aluminum foil or copper tape is used so as to maintain homogeneity
between the outside impregnated paper and inside paper.
• By the use of metallic screen (which is maintained at earth potential) electric field
is made radial.
• These cables have greater breakdown strength, better heat dissipating properties
and reduced risk of core-to-core faults.
 LV Cables : 1.1kV
• Standard Sizes
– 400,240,150,70 & 25 Sq.mm
• Standard Construction
• Sector Shaped & Stranded Al Conductor
• PVC / XLPE Insulation
• PVC Inner Sheath
• Flat Stripped G.I Armour
• PVC Outer Sheath
 HV & EHV Cables
• PILC Cables (not added after 2000)
– 11KV & 33 KV
• Gas filled Cables
– 33 KV (not added after 1960)
– 132 KV (not added after 1985)
• XLPE Cables
– Up to 220 KV.
 Dielectric Stress in Cable
The stress on the dielctric at any point in a single core cable is as follows:-
Let r = radius of conductor or
inner radius of insulation.
R = internal radius of sheath
or outer radius of insulation.
R εo = permittivity of free space
εr = relative permittivity of
dielectric
r x
q = charge on the conductor
per unit length in coulombs.
V = operating phase to
neutral voltage in volts

dx

Fig.1.Single Core Cable

 Insulation Resistance
• Let ,
• r = radius of the core
• R = internal radius of the sheath or internal radius of the dielectric.
• l = cable length.
• ρ = specific resistance of the dielectric.
• The leakage current flows radially through the dielectric. Consider an element
of the dielectric radius x and the thickness dx as shown in fig1.
• The length through which the leakage current flows = dx. The area of cross-
section through which it is flowing is 2πxl.
• Therefore, the resistance of the element in the radial direction is
dx
dRi  
2 xl
• And the insulation resistance of the cable is
R  dx   R
Ri    ln xr  ln
R
r 2 xl 2 l 2 l r
• The above relation shows that the insulation resistance varies inversely as the
length.
 Dielectric Stress in Cable Contd….
• Flux passing through a cylinder of radius x and length one meter
surrounding the core is q.
• The electric flux density at a distance x from the centre is
q
Dx  C / m2
2 x
• The dielectric stress is given by
Dx q
gx   V / m...............eq (1)
 o r  2 o r  x
• The potential difference between the conductor and the sheath is equal to
the work done to move a unit positive charge from the conductor to the
sheath. Thus ,

R R q R q
V   g x dx   dx  ln volts.....eq(2)
r r  2 o  r  x 2 o  r r
 Dielectric Stress in Cable Contd….
• Combining eq(1) and eq(2) gives
V
gx  .....V / m...................eq  3
R
x ln  
r
• The maximum stress will occur at the smallest radius i.e x = r.
• The stress is a maximum at the surface of the conductor . The maximum
stress is given by
V
g max  ......V / m............eq  4 
R
r ln
r
• When x = R, the stress will be a minimum which indicates that stress has a
minimum value at the sheath. The minimum value is given by

V
g min  ......V / m........eq  5 
R
R ln
r
 Dielectric Stress in Cable Contd….
• Also g max R

g min r
• The electric stress in a belted cable cannot be calculated accurately due to non
uniformity of the dielectric and the distortion in the electrostatic field.
• The present day tendency is to design high voltage cables on the basis of a fixed
maximum value of operating stress. The stress is expressed in kilovolts per
millimetre.
• Eq (4) is then utilized to determine the thickness of insulation necessary for a
given diameter of conductor.
• It is evident from eq(4) that greater the value of permissible stress lesser will be
the insulation thickness. It is desirable to choose a higher value of operating
stress in order to have a reduced thickness of insulation and therefore reduced
size of cable.
• Since smaller cables size affords more economy there is a tendency to increase
the operating stresses to their highest values without failures to cable either
under actual operating conditions or during its approval specified tests.
 Most economical size of cable
• Maximum potential gradient,
V
g max 
R
r ln
r
• If V and R are constant and r is made variable the expression above has a
minimum value when the denominator is a maximum. This occurs when
d  R
 r ln   0;
dr  r
• Solving this we get,
R
 e  2.718
r
• This is the condition that would make the voltage gradient at the
conductor surface a minimum.
 Grading of Cables
• The process of achieving uniform electrostatic stress in the dielectric of
cables is known as grading of cables.
• It has been observed that the stress is greatest at the surface of the
conductor and goes on decreasing as we move towards the sheath.
• If the dielectric be chosen according to the maximum stress the thickness
of the cable would change considerably.
• Moreover the benefit of the chosen dielectric would not be achieved fully
near the sheath.
• If all the dielectric were equally stressed the thickness of insulation would
be considerably reduced. This would reduce the cost of cable insulation.
• Moreover the heat generated in the conductor can be more easily
conducted to the sheath.
• For this reason stress has to be equally distributed in the cable.
• There are two methods of grading:-
• Capacitance grading
• Intersheath grading.
 Grading of Cables Contd…..
• Capacitance Grading: the process of achieving uniformity in the dielectric
stress by using layers of different dielectrics is known as capacitance
grading.
• Intersheath Grading: in this method a homogenous dielectric is used but it
is divide into various layers by placing metallic intersheaths between the
core and lead sheath. The intersheaths are held at suitable potentials which
are in between the core potential and the earth potential. This
arrangement improves voltage distribution in the dielectric of the cable and
consequently more uniform potential gradient is obtained.
• Disadvantages:
1. Cable grading is of theoretical interest only. The main disadvantage of the
capacitance grading is that the range of permittivity values of insulating
materials available for cable insulation is limited. Moreover the permittivity's of
the layers may not remain constant during the service period of the cable.
Consequently the stress distribution may change and even cause insulation
breakdown at the normal operating voltage.
2. The intersheaths , being very thin, are liable to damage during transportation
or installation. They are not able to carry the charging current of long cable
lines and thus the current carrying capacity of the cable is reduced.
 Cable Capacitance
• The capacitance of cable transmission line is very much than that of an overhead
line of the same length due to the following reasons:-
1. the distance between the conductors is very small.
2. The distance between the cores and the earthed sheath is also small.
3. The permittivity of the cable insulation is usually 3 to 5 times greater than that of air
insulation around the conductors of overhead lines.
• It is easier to calculate the capacitance of overhead system accurately if its
configuration be known, but for a cable such a calculation is only approximate.
• The approximate method of cable capacitance is based on the assumption that the
cable dielectric is perfectly homogeneous. The insulation of a cable is however far
from being homogeneous or uniform.
 Charging Current or Capacitive Current
• An underground power cable has the capability to store and release
electrical energy with the voltage variation; it works as a shunt
capacitance generating a capacitive current which is in quadrature with
the resistive or load current.
• The capacitive or charging current has a limiting effect on cable rating
capacity.
• This effect is quantified by the fact that when intended to supply energy to
an active load consumer in a radial network, via a power cable circuit, it is
needed to inject a higher current at the source to compensate for cable
capacitance.
• The charging current is calculated with the following equation:

I c  CV  106
• where,
Ic = Charging Current (A/km)
w = 2πf; f = system frequency
C = capacitance per unit length (μF/km)
V = Applied voltage (Volts)
 Charging Current or Capacitive Current Contd…..
• The charging current generates heat losses in cable which are of a
significant magnitude in very long high and extra high A.C. underground
power cable connections.
• In addition to the heating effect of charging current the cable capacitance
may have an impact on steady-state voltages across the power system to
which the cable is connected.
• The voltage may rise, especially at low loads, due to charging current
flowing through cable series inductances and through system inductances;
phenomenon known as Ferranti effect.
• The Ferranti effect is significant with long cables energized from one end.
• In this kind of situations the network voltage stabilization is carried out by
adjusting the voltage magnitude at generator by reducing the field
excitation or by lowering the voltage taps on transformers.
• However in case of long cable circuits, the compensation of charging
current requires installation of shunt reactors connected at one end or
both end of cable circuits which are selected based on specialized system
studies by taking into consideration cable specific data and system
parameters.
 Capacitances in a three Core Belted Cable
• The conductors in a cable are separated from each other by dielectric.
Similarly, there is dielectric between the conductors and the sheath.
• When a potential difference is applied between the conductors the cable
in effect is a combination of six capacitances. The capacitances between
the conductors are represented by Cc, while those between conductor
and sheath is represented by Cs.
• Thus a three phase belted cable may be represented by a system of
capacitances connected in star and delta as shown in fig 2.
• The delta connected Cc may be replaced by equivalent star connected
capacitances C1 as shown in fig 3.. The capacitance between the pair of
terminals will be the same in the two systems.
• Capacitance between A and B in the delta system = Cc  0.5Cc  1.5Cc

• Capacitance between A and B in the star system = 0.5C1

• For the two system to be equivalent

1.5Cc  0.5C1, C1  3Cc

 Capacitances in a three Core Belted Cable
• the cable may therefore be represented by fig4. if the neutral point N of
the system be earthed and the sheath be also at zero potential , N and S
will become equipotential and Fig 4 becomes equivalent to that shown in
Fig5.
• Since C1 and Cs are in parallel they are combined into a single capacitance
(C1+Cs). This is seen in fig 6.
• The capacitance of each conductor to neutral or equivalent capcitance is
given by

C0  C1  Cs  3Cc  Cs
• If VL be the line voltage , Vp be the phase voltage , the charging current
per phase is
VL
I c  V pCo    3Cc  Cs  ... A
3
• Note: Co is the capacitance between any conductor screen for a 3 core
screened cable.
 Measurement of Cc and Cs
• The non uniformity of the insulation material, the variation in the shape of
conductors and the use of filler makes it difficult to estimate the capacitance of a
cable from its diameter. The following tests are generally performed:-
a) One conductor is say C is connected to the sheath or insulated and the
capacitance is measured between the remaining two conductors A and B.
Fig7(a) reduces to fig 7(b).
The total capacitance CL measured between the cores A and B is
C  CS 1 1
CL  CC  C   3Cc  Cs   Co
2 2 2
The single measurement is sufficient for calculating the charging current per
conductor.
b) The three conductors are connected or bunched together Fig8 and the
capacitance is measured between this bunch and the sheath . Let it be denoted
by Cb. Here Cc becomes zero and Cb = 3 Cs.
c) Two conductors, say A and B are joined together and the capacitance is
measured between them and remaining conductor.(fig 9) The capacitance
between B and C is = 2 2 2
CC  CC  CS   3CC  CS   CO
3 3 3
d) Two conductors B and C are connected to sheath and the capacitance is
measured between these and the third conductor A (as shown in fig 10).
The capacitance measured in this case is

 CS  CC  CC  2CC  CS
• From the above tests the values of Cc and Cs can also be determined
separately.
Fig2 Fig3
 Losses in Cable
• Losses in Conductor
• Losses in dielectric
• Losses in sheath.
 Losses in Conductor
• Resistive Losses :
• Skin Effect: current pressed to the outer region of the conductor due to its
own current.

• Skin Effect , depth δ depends on frequency and material

• Due to skin effect
– no use of full conductor cross section
– increased conductor and cable diameter and weight
 Losses in Conductor
• Proximity Effect: current pressed to one side of conductor due to
neighboring current.

• The phenomena of skin and proximity effect is responsible for increase in

the AC resistance of the conductor.
 Dielectric Loss
• Power loss occurs in the dielectric of a cable due to the three main reasons:-
1. Conductivity of insulation.
2. Dielectric hysteresis or dielectric absorption.
3. Ionization or corona.

• Conductivity of Insulation:-
The resistance of dielectric is very high but not infinite and therefore, a small
current flows due to the conductivity of the dielectric. This current is called
the leakage current. The leakage current through the insulation resistance
gives rise to a power loss in the dielectric. This power loss is termed as
leakage loss.
 Dielectric hysteresis or dielectric absorption.
• If a constant d.c voltage is applied across a cable the current in the
beginning is very high. The current decreases gradually to a low steady
state value determined by the cable insulation. This steady state value of
current is called the conduction current.
• On removal of applied voltage the current does not become zero
instantaneously, but the cable is discharging for practically the same
length of time as it took previously to attain its steady state value before
the dielectric is fully discharged.
• However the amount of charge released is somewhat lesser than that
stored in dielectric during charge. A part of the absorbed charge
recombines with the dielectric.
• If an alternating voltage be applied to the cable, the dielectric is subjected
to several cycles of charge and discharge per second. A power loss takes
place in the material of the dielectric due to constant changing of
absorbed charge. This loss due to absorption is much greater than that in
the insulation resistance.
 Corona or Gaseous Ionization in Cables
• The spaces or voids in the cables are filled up with the vapours of the
impregnating oil and air.
• The dielectric strength of the voids or gaseous spaces within the insulation
is very small compared with that of insulation.
• If the applied potential gradient in the insulation is large there is a
concentration of field intensity in the spaces resulting in the ionization or
corona discharge.
• Thus a breakdown of insulation may occur at relatively low stresses and
temperatures.
• The life of the cable is thus affected.
 Dielectric Power Loss
• The cable behaves as an imperfect capacitor. The equivalent circuit is
shown if Fig11.

R1
IR1
I

Ic C

Fig 11
• Let
• V = line – to – neutral supply voltage.
• IR1 = Current through R1.
• Ic = charging current.
• I = supply current.
 Dielectric Power Loss Contd….
• The current I leads the applied voltage Ic I
by an angle φ.
• Dielectric angle = δ= (90-φ).
• For a single phase line ,
the dielectric power loss
Pd  VI cos   VI R1 δ

φ
• From the phasor diagram IR1 V
I R1
 tan  ;
IC
I R1  IC tan   V  Co tan 

 Pd  VI R1  V 2 Co tan 
 Dielectric Power Loss Contd….
• Since δ is usually very small
tan   

 Pd  V 2Co ......watts

• In a three phase cable , if Co be the equivalent star capacitance or

capacitance to neutral, the dielectric power loss is given by

Pd  3V p 2 Co

• Where Vp is the phase voltage.

 Current Rating of Cables
• In a cable, the factor which ultimately limits the current carrying capacity
is the maximum operating temperature which may be sustained by the
cable throughout its life without risk of damage or deterioration.
• The cause of temperature rise is the losses that occur in a cable which
appears as heat. These losses are:
(a) ohmic loss in the conductor,
(b) the dielectric loss in the insulating medium, and
(c) the sheath and intersheath losses.
• The maximum steady temperature prevail when the heat generated in the
cable is equal to the heat dissipated. The heat dissipation of the conductor
losses is by conduction through the insulation to the sheath from which
the total losses may be conducted to earth. Therefore , in order to find the
permissible current loading, the thermal resistivities of the insulation,
protective covering and the soil must be known.
 Types of Cable Faults
• Cables are generally laid directly on the ground or in ducts in the
underground distributions systems. For this reason there is less chance of
faults occurring in a underground cable system. However if a fault does
occur it is difficult to locate and repair the faults as the cables are not
visible.
• The following are the most common faults occurring in the cables:-
1. Open Circuit Fault:- when there is a break in the conductor of a cable it is
called open circuit fault. This can be checked with the help of a megger.
For this purpose the three conductors of a three phase core cable at the
far end are shorted and earthed. Then resistance between each
conductor and earth is measured by megger. The megger will indicate
zero resistance in the circuit of the conductor that is not broken. However
if the conductor is broken, it will indicate infinite resistance in its circuit.
2. Short Circuit Fault:- when two conductors of a multi core cable come in
electrical contact with each other due to the insulation failure, it is called
a short circuit fault. For this purpose two terminals of the megger are
connected to any two conductor. If the megger gives zero reading, it
indicates short circuit fault between these conductors. The same step is
repeated for other conductors taking two at a time.
 Types of Cable Faults
3. Earth Fault:- when the conductors of a cable comes in contact with earth,
it is called earth fault or ground fault. To identify this fault one terminal
of the megger is connected to the conductor and the other terminal
connected to earth. If the megger indicated zero reading, it means the
conductor is earthed. The same procedure is repeated for other
conductors of the cable.
• Treeing is an electrical pre-breakdown phenomenon in solid insulation. It
is a damaging process due to partial discharges and progresses through
the stressed dielectric insulation, in a path resembling the branches of a
tree. Treeing of solid high-voltage cable insulation is a common
breakdown mechanism and source of faults in power cables.
 Loop test for location of faults in Underground Cables
• The two most popular loop test are :-
1. Murray Loop Test.
2. Varley Loop Test.

• These simple tests can be used to locate the earth fault or short circuit
fault in underground cables provided that a sound cable runs parallel
along the faulty cable. Both these tests employ the principle of
Wheatstone bridge for fault location.
 Murray Loop Test
• This test is the most common and accurate method of locating earth fault
or short circuit faults in underground cables.
Earth Fault:
Test End Far End

• Here AB is the sound cable and CD A

R
Sound Cable B

is the faulty Cable; P

G
Low
resistance
Path

• Earth Fault occuring at point F. K2

Faulty Cable

• The far end D of the faulty cable is Q C D

X
K1

Connected to the far end B of the sound +

B
Earth Fault

cable through a low resistance link. -

E Earth Path

• Two variables P and Q are joined to ends

A and C respectively and serves as the ratio arms of the Wheatstone
bridge.
• Let R = resistance of the conductor loop upto the fault from the test end.
• X = resistance of the other length of the loop.
• Note: P,Q,R and X are the four arms of the Wheatstone bridge. The
resistances P and Q are varied till the galvanometer indicates zero
deflection .
 Murray Loop Test
• In the balanced condition of the bridge we have
P R

Q X
P R
• Or, 1  1
Q X

• Or, PQ R X

Q X
• If r is the resistance of each cable , then R+X = 2r

P  Q 2r
 
Q X

Q
or , X   2r
PQ
 Murray Loop Test
• If l is the length of each cable in metres, then resistance per metre length
of cables = r/l.
• Distance of fault point from the test end is

X Q l Q
d   2r    2l
r /l P Q r PQ
• Or , d = (Q/(P+Q))× (loop length).
• Thus the position of the fault is located.
• Note that resistance of the fault is in the battery circuit and not in the
bridge circuit . Therefore fault resistance does not affect the balancing of
the bridge. However if the fault resistance is high the sensitivity of the
bridge is reduced.
 Murray Loop Test
Short Circuit Fault:
Test End Far End

• Note: the fault resistance is in the A

R

Sound Cable B

• Battery circuit and not in the P

G
Low
Resistance
Path

• Bridge circuit. The Bridge is K2

balanced by adjusting the +

Q

C Faulty Cable D

resistances P and Q. -
B X

Short Circuit Fault

• For the balanced position of the K1

Bridge:
P R

Q X
PQ R X 2r
or ,  
Q X X

Q
X   2r
PQ
• X = (Q/(P+Q))× (loop length) metres
• Thus the position of the fault is located.
 Varley Loop Test
• This test is also used to locate earth fault or short circuit fault in
underground cables. This test also employs Wheatstone bridge principle. It
differs from Murray Loop test in that here the ratio arms P and Q are fixed
resistances. Balance is obtained by adjusting the variable resistance S
connected to the test end of the faulty cable.

Earth Fault Short Circuit Fault

 Varley Loop Test
• For earth fault or short circuit fault, the key K2 is first thrown to position 1.
the variable S is varied till the bridge is balanced for resistance value of S1 .
• Then, P R

Q X  S1
P  Q R  X  S1
or , 
Q X  S1
Q  R  X   PS1
or , X  ..............(i )
PQ

• Now key K2 is thrown to position 2 (for earth fault or short circuit fault)
and bridge is balanced with new value of resistance S2. Then,
P R X

Q S2
or ,  R  X  Q  PS2 .............(ii )

• From eq(i) and eq(ii) we get, P( S2  S1 )

X 
PQ
• Loop Resistance = R  X 
P
S2
Q
 Varley Loop Test
• If r is the resistance of the cable per metre length the distance of fault
from test end is

X
d metres
r