How To Locate Faults In Cables? | Types of Cable Faults

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How To Locate Faults In Cables?
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What is Faults in Cables?

• When electrical energy is generated in the
  generations stations, it is distributed to the
  different loads, i.e. cities, towns and villages
  for consumption then.
• The process involves stepping up the voltage to
  minimize the loss of energy in the form of heat.
  The stepped up voltage is distributed to grid
  stations where it is stepped down for
  distribution to the local transformers where it
  is finally stepped down and distributed to the
  consumers.
• Distribution of the electrical energy is done
  via electrical cables. The cables are either
  insulated or uninsulated.

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Types of Cable Faults

1. Open-Circuit Faults Open circuit fault is a kind
   of fault that occurs as a result of the conductor
   breaking or the conductor being pulled out of its
   joint. In such instances, there will be no flow
   of current at all as the conductor is broken
   (conveyor of electric current).
2. Short-circuit or cross fault This kind of fault
   occurs when the insulation between two cables or
   between two multi-core cables gets damaged. In
   such instances, the current will not flow through
   the main core which is connected to load but will
   flow directly from one cable to another or from
   one core or multi-core cable to the other
   instead. The load will be short circuited.
3. Ground or earth faults This kind of faults
   occurs when the insulation of the cable gets
   damaged. The current flowing through the faulty
   cable starts flowing from the core of the cable
   to earth or the sheath (cable protector) of the
   cable. Current will not flow through the load
   then.

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Types of Tests to locate faults in cables

1. Blavier Test (For a Single Cable Faults)
2. Loop Test
3. Open Circuit Test
4. Potential Fall Test

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Blavier Test (For a Single Cable Faults)

• When a ground fault occurs in a single cable and
  there is no other cables (without faulty one),
  then blavier test can be performed to locate the
  fault in a single cable.
• In other words, in the absence of a sound cable
  to locate fault in the cable (to make a loop by
  connecting both cable as we do in the Murray loop
  test), then measurement of the resistance from
  one side or end is called blavier test.
• In blavier test, resistance can be measured by
  two ways.
• To insulate the far end of the cable
• To ground (earthed) the far end of the cable

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Fault to ground resistance rResistance from
the Far end to the cable fault r1Resistance
from the testing end of the cable to the fault
r2
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Loop Tests to finding Cable faults

• These kinds of tests are carried out on short
  circuit faults or earth fault in underground
  cables.
• Cable faults can be easily located if a sound
  cable runs along with the grounded cables.
  Following are the types of loop tests.
• Murray loop Test 
• Varley loop Test.
• Earth Overlap Test

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Murray Loop Test

• The connection on how a cable faults can be
  located using Murray loop test method is shown
  below.
• Wheatstone bridges principle is used in murray
  loop test to find the cable faults.
• Ra and Rb are the two ratio arms consisting
  of resistors. G is a galvanometer. The cable
  having fault (Rx) is connected to the second
  cable (Sound cable Rc) through low resistance
  link at the far end.
• The Wheatstone bridge is kept in balance by
  adjusting resistance of the ratio arms Ra and Rb
  until the galvanometer deflection is zero.

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where l length of a single cable (In meters of
yards)2l total length of two cablesx
distance from the upper side to the fault
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Varley Loop Test
The only difference between Murray loop test
and Varley loop test is that Varley loop test
provision is made for measurement of total loop
resistance instead obtaining it from the relation
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Earth Overlap Test

• In earth overlap test, two measurements are
  performed (instead of one as in Blavier test).
  The first measurement of resistance is R1
  (between Line to ground i.e. from the testing end
  to the far (earthed) end).
• The second measurement of resistance is R2
  (between Line to ground i.e. from the far end and
  the testing (earthed) end).

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As in the Blavier test, we also suppose
that we know the actual resistance of the cable
before the cable fault which is R. R
 r1r2                                     
                                            
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Open Circuit Test

• Open circuit fault can be occurs when cable is
  pulled out of its joint or a break occurs in the
  cable.
• Such a fault can be traced by carrying out
  capacity test.
• The capacitance of the faulty cable is measured
  from both ends of the cable either by means of
  ballistic galvanometer or by bridge method.
• Capacitance of the cable to the ground is
  proportional to the length of the cable.

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Potential Fall Test
In Potential fall test, Ammeter, voltmeter,
Variable resistor (rheostat) and battery are
connected as shown below to find the fault
location in the cable. This test is carried out
with the help of a sound cable that has no fault
running along the faulty cable
Where V1 and V2  the voltmeter readings at point
A and BL length of the faulty core X length
of core between fault and testing end A.
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