CABLE FAULT LOCALISATION

The efficiency of the maintenance of external plant largely depends on the proper planning, execution, constructional practices and maintenance of cable network i.e. the primary, secondary and distribution cables, cabinets, pillars, DPs etc. One cable fault normally affects a large number of circuits.

GENERAL TYPES OF CABLE FAULTS Earth fault:: When the insulation between the earth and the conductor in the cable becomes very low. Low insulation fault:: When the insulation between conductors in the cable or between the pairs or between pair and earth falls below a prescribed limit (normally 0.5 meg ohm) This may be due to entry of moisture or due to failure of wire insulation. Disconnection Fault :: When the Conductor is cut then the fault is called break fault or is called High Resistance fault.

Contd.. Short Circuit Fault: When the resistance between the wires or between the conductors becomes very low, even without any loop in the circuit on the pairs. This is also called contact fault. Foreign potential: The existence of potential, even when the circuit is idle or isolated from the potential of exchange and subscriber premises equipment is foreign potential or contact of the conductor with other circuit having potential.

FAULTS OCCUR DUE TO Corrosion :- Chemical and electro chemical action, which damages the outer sheath and the conductor of the cable. Inter-crystalline fracture :- Sheath is effected due to mechanical stresses, longitudinal, transverse, tensional oscillation or periodical variation of temperature or repeated overheating of the sheath. Faulty materials in manufacturing of the Cable Bad constructional practices like in-efficient, nonstandard and careless work-man ship, specially during the time of laying & jointing of cable. Natural causes and accidents :- Due to termites, rodents borers etc., & due to earth quack, floods, penetration by tree roots, contact with power lines etc. Digging operation by other public utility authorities and individual public.

DETECTION AND LOCALISATION OF CABLE FAULT The detection and localization of cable fault is a major time consuming factor in the regular maintenance aspect. Detection is the first step in reaching a developing cable fault early. Timely detection & localization of c/f will help to avoid and lessen the interruption period, which ultimately gives better satisfaction to the customer.

Procedures used for fault detection, diagnosis & location on subscriber lines Self supervision – is a permanent feature and fault is noticed physically also. Performance monitoring – Gives the indication of the quality of the service. Routine testing – Indicate the faults that are not noticed physically in normal course. Demand testing – After detecting the side effects of fault.

Once the fault is reported by any 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.

BASIC REQUIREMENTS FOR DETECTION AND LOCALIASATION OF CABLE FAULTS Thorough idea about the cable network. Availability of up-dated cable records viz., cable diagram, cable plans, DP cards and cable route and joint indicators etc. Availability of proper type of testing instruments in good working condition and skilled staff.

MEDIA THROUGH WHICH CABLE FAULTS ARE DETECTED Complaints from the subscriber, testing of the circuits from test desk or OMC. Routine testing from test-desk or OMC. Non availability of pair at the time of putting through a new connection.

Localization of Fault by instruments Multimeter or A.V.O. meter: 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.

BRIDGE MEGGAR This works on Wheatstone 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 of dead-earth fault (when resistance between the faulty wire and earth is very low).

EARTH FAULT LOCALISATION BY BRIDGE MEAGER THE FOLLOWING CONDITIONS MUST BE SATISFIED :- 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 of the faulty wire. The good wire may have a separate route than the faulty wire. No presence of foreign battery. 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 Valley loop test. –used for localizing earth or contact faults. ( constant arm radio Bridge ) 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.

PULSE ECHO TESTER OR ECHO METER Modern and reliable method for measurement of cables of any type. Provides reasonably an accurate distance measurement. Works on the pulse reflection principle. Pulses created by a generator & which are suitable for the location of 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.

Pre-requisite: while using this instrument for localization of fault, the gauge of the conductor & type of insulation of the cable under test must be known. Echometer 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. It can be efficiently used for checking up the length of the cable in the drum before laying .

The type of the faults that can be localized by this instrument are:- Open circuit/disc Short circuits High resistance point Partial contacts Low insulation fault Split pairs

APLAB cable fault locator -3039 facility of direct digital display of the distance. The localization of disc and loop fault is simple with the help of the above meter.

APLAB cable fault locator- 3039

PROCEDURE TO TEST Check Battery level before taking to the place of testing. Battery charge LED should be off on the panel. There should be no voltage on the pair to be tested. Set the V/2 value for the cable using velocity selector switch (one half the velocity of propagation value for the cable under test). Adjust the vertical gain sensitivity control to obtain pulse height of two divisions height. Adjust the horizontal position control so that the foot of pulse is aligned with vertical line.

Contd… Connect the meter to the pair to be tested at L1 and reference pair at L2. Adjust the horizontal expansion control to show the reflected pulse on the right side of the screen. Depress the alternate trace selector switch. Adjust the trace shift control, so that the foot of the transmitted pulse is observed. Depress the SET / READ switch. Read the distance in display. Subtract the length of the test leads to obtain the actual distance.

OPERATING CONSIDERATIONS A good cable with a matched termination displays a flat trace with no reflection. If the measured fault location is near a splice, or if near a construction site where recent digging has been done, it is quite probable, the fault is located there. Water existing in a splice case or in a jelly filled cable may produce a very small reflection due to the confined area of water. A cable pair may have more than one fault, so the first fault can easily mask a more remote fault. Then locate the closest fault then locate and repair the more distant faults.

INDICATION OF TYPICAL FAULTS When the fault is with in few meters from the testing point it is difficult to get an accurate reading. This can be overcome by connecting a dummy 50 to 100 meter length of cable between meter and pair to be tested. The final actual measurement is got by subtracting the length from reading shown in the meter. Open circuit :: Open circuit gives a positive reflection at the point of break. If both conductors are open, reflection amplitude is greater and reflection at open far end of cable is not seen. Short circuit ::The contact or short gives a negative reflection and far end reflection is not seen.

Sheath fault ::A short or low resistance between one or both conductors shows a negative reflection, which is smaller as resistance increases. A small reflection is seen at the far end. Connect the meter between the sheath and one or both conductors. Sheath should not be grounded when making this test. Open sheath :: Connect as many good pairs as possible to one test lead and the sheath to the other lead. A positive reflection is seen. Resistive splice :: Cable splice shows positive reflection, which is higher as resistance increases. Good splice :: Even good splices show a reflection because of capacitance change. This shows as a small positive reflection followed by a negative one. Split pair.:: Wires between two pairs are split. The reflection is a positive pulse, but is smaller than that for a resistive splice. Water in cable :: Water in section of cable affects dielectric. A negative reflection shows where water begins and positive where water ends. Amount of reflection shown by degree of saturation. To better pinpoint watered section, make tests from each end of cable.

Cable Measurements if V/2 is known :: It is recommended that voltage not to be present on the cable. If far end of cable is terminated, temporarily remove the termination. Connect the instrument to the cable and measure the length. To find V/2 if cable length is known :: Connect the instrument to a cable pair of good insulation but with out far end termination. Measure the cable in the normal manner by aligning the reflected pulse with the transmitted pulse. Adjust the value on the V/2 velocity selector switches until the distance readout shows the actual cable length.

Comparison of faulty cable or pair with one known to good cable or pair Connect the instrument to a cable pair of good insulation to the L2 jacks, and faulty one to the L1 jack. With the line select pushbutton in the out position, measure the fault location in the normal manner. Press the line select pushbutton. Note that the screen provides a simultaneous display of the two cables and shows the fault location relative to the far end of the good cable.