DETECTION AND LOCALISATION OF CABLE FAULT

The detection and localization of cable fault is the major time consuming factor in the regular maintenance aspect due to various reasons. Detection is the first step in reaching a developing cable fault early. A timely detection & localization of cable fault will help to avoid and lessen the interruption period. Which ultimately give better satisfaction to the customer.

The procedures used for fault detection, diagnosis and location on subscriber lines are 1. Self supervision – is a permanent feature and fault is noticed physically also.

2. Performance monitoring – Gives the indication of the quality of the service.

3. Routine testing – Indicate the faults that are not noticed physically in normal course.

4. Demand testing – After detecting the side effect of fault.

Once the fault is reported by any a\of the above means The steps to follow are

• Verifying the fault

• Pinpointing the fault

• Determining the cause of fault.

• Rectification of fault with standard methods.

2.1 THE BASIC REQUIREMENTS FOR DETECTION AND LOCALIASATION OF CABLE FAULTS ARE

:-1. Thorough idea about the cable network.

2. Availability of up-dated cable records viz., cable diagram, cable plans, DP cards and cable route and joints indicators etc.

3. Availability of proper type of testing instruments in god working condition and skilled staff.

2.2 GENERALLY THE FOLLOWING ARE THE MEDIA THROUGH WHICH CABLE FAULTS ARE DETECTED

1. Complaints from the subscriber, testing of the circuits from test desk or OMC.

2. Routine testing from test-desk or OMC.

3. Non availability of pair at the time of putting through a new connection.

4. Alarm from the control panel of the pressurization system.

2.3 Location of a fault in working cable caused due to the digging of any agency can be easily identified / detected by cable route patrol or through information’s from local people. These types of faults and breakdowns are immediately brought to notice or identified by the complaints given by the effected subscribers.

2.4 Failure of weak and imperfect joint due to entry of moisture is a common phenomenon in this country. Presently usage of polythene insulated jelly filled cable has made this type of joint failure a rare phenomenon and detection an easy affair. If there is any damage in the sheath, the entry of the moisture will be avoided due to the presence of jelly. However even the entry of moisture can not create any fault as polythene insulation remaining unaffected by moisture or water.

The following is an example of the detection of joint failure (low insulation and earth fault)

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Conditions

:-1. Cable used is jelly filled

2. Cable diagram is very much up-to-date

The above diagram is a part of the cable diagram of a pillar. For example the telephones of DP nos. 1 and 2 are reported to be faulty. Now consulting the pillar diagram it can immediately be referred that joint - 2 is faulty. No necessity of localization testing is required. Now by consulting the cable plan and with the help of route and joint indicator the faulty joint - 2 can be physically localized.

For paper insulation cable, this type of localization is not possible because fault may arise any where in the body of the cable between joint - 1 and joint - 2 due the entry of moisture through sheath puncture. Hence the localization of this type of fault by proper instrument is unavoidable.

2.5 Localization of Fault by instruments

The following instruments are generally in use for the localization of cable fault.

2.5.1 Multimeter or A.V.O. meter: These instruments are used for finding out-(a) Resistance of wire or of a pair (loop)

(b) continuity of a wire between two points

(c) Numbering of the pairs, if there is reversal at any intermediate point.

2.5.2 BRIDGE MEGGAR : This instrument is extensively used in all the cable maintenance sections. This works on Whetstone Bridge principle and is used generally for the following purposes ::

(a) Accurate measurement of loop and wire resistances

(b) Measurement of cross insulation between the wires of the same pair or between the pairs of a cable.

(c) Earth insulation between a wire or pair and earth.

(d) Localization off dead-earth fault (when resistance between the faulty wire and earth is very low).

(e) Very rough localization of ‘Disc.’ fault (principle of condenser discharge).

2.5.3 EARTH FAULT LOCALISATION BY BRIDGE MEAGER

THE FOLLOWING CONDITIONS MUST BE SATISFIED :-1. The fault must be dead – earth

2. One good wire must be available between the testing station and the

point from where test is given at the looping end. the length and gauge of the good wire is not required to be the same as that the faulty wire. The good wire may have a separate route than the faulty wire.

3. No presence of foreign battery.

4. Useful for localizing four faults

(a) Condition of the fault (whether dead earth or not) (b) Earth insulation of the good wire

(c) Resistance of the faulty wire and good wire looped at the distant end (d) Valley loop test. –used for localizing earth or contact faults.

( constant arm radio Bridge )

(e) Murray test - used for localizing earth or contact faults on short lines.

(variable arm radio bridge)

The procedure for connections, testing and formula for manipulating the test-data to find out the distance of the fault from the testing end are available with each instrument.

3.0 PULSE ECHO TESTER OR ECHO METER

This is the modern method and reliable method for measurement of cables of any type.

This instrument provides reasonably an accurate distance measurement.

This instrument works on the pulse reflection principle. The pulses (sin2 form ) which are created by a generator and which are suitable for the location or places of error are transmitted through cable, the part of the pulse power is reflected at the fault according to magnitude of power. This is observed on the CRT screen. It makes use of characteristic impedance of the conductor pair whose deviation from the nominal characteristic impedance determines the intensity of reflection or the reflection co-efficient

The pulse echo test method provides quantitative as well as qualitative information on any transmission cable - impedance loss, Rise time, electrical length and discontinuities in a single measurement.

Pre-requisite :- while using this instrument for localization of fault, the gauge of the conductor and type of insulation of the cable under test must be known.

This instrument can detect and localize various types of faults in open wire and under Ground cable provided the impedance irregularity on the pair at the fault-point is large enough to cause a distinctly visible reflection of the screen. The type of the faults those can be localized by this instrument

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(g) Water logged section in PE insulated air core cable (h) Dent in outer conductor of coaxial cables etc.

In some situations it can also localize the multiple faults in a circuit. It can also display the condition of impendence irregularities i.e. the quality of a newly made joint.

This instrument can be very efficiently used for checking up the length of the cable in the drum before laying .

Operation of this instrument takes a very little time in some cases a few seconds only. One commercial brand of this instrument is of Brand and is known as APLAB cable fault locator- 3039 manufactured in our country having the facility of direct digital distance reach out display of the distance.

During the time of jointing the cable, or terminating the cable at terminal points like MDF, Cabinet, pillar, DP etc. a good number of wires become break or looped due to bad workmanship. The disconnection & loop faults may arise due to various other reasons. The localization of disc and loop fault simple with the help of the above meter.

3.1 FUNCTION OF CONTROLS AND INDICATORS ::