Testing of Cables|What are the test for cables?

 Testing of Cables|What are the test for cables?|Quad  cable testing

 

Contents:

1. TESTING OF CABLES
2. Why cable testing is needed?
3. What are the test for cables?
4. How many types of cable testing are there?
5. How do you test the quality of a cable?
6. Detailed Testing Procedure

 

 

Why cable testing is needed?

The cable must be tested for continuity, insulation, and attenuation. The test should be carried out before and after cable laying. For maintenance purposes, these tests shall be performed

every year before and after the monsoon.

 

How many types of cable testing are there?

 

Types of tests on quad cables

 

Test of quad cable may be classified into four parts.

1.    Test during laying of cable.

2.    Test after cable laying.

3.    Routine tests

4.    Tests during localization of fault.

 

What is the test for cables?

 

Types of tests

Test of quad cable may be classified into four parts.

Test during laying of cable.

Test after cable laying.

Routine tests

Tests during localization of fault

 

1.    Tests during laying of cable

􀂾 Test the cable for proper end ceiling. Check up the cable throughout the length for any physical

Damages during transportation and handling and for manufacturing defects.

􀂾 Tests for continuity and insulation before laying the cable.

 

2.    Tests after cable lying:

a.    Test for continuity.

b.    Test for insulation.

c.    Test for attenuation.

d.    Test for crosstalk (FEXT & NEXT ).

 

Precautions while testing a cable

 All working circuits and power supply shall be disconnected from the cable at both ends.

􀂾 Communication between the ends of the cable under test shall be established by magneto telephones with other cable/ cable under test if possible otherwise by VHF sets.

Competent staff required instruments and material shall be available at both the ends.

􀂾 Testing shall be carried out when conductors and insulated parts like terminal blocks are clean and

dry.

􀂾 Cable conductors shall be earthed momentarily to discharge the accumulated charge, if any, before

the commencement and after the end of insulation testing.

 

Detailed procedure for testing of cables

1.    CONTINUITY TEST:

This test is carried out to confirm that core under test is either showing break or not. Testing can be done as follows:

􀂾 Set the knob of multimeter to check resistance at suitable lower range at one end.

􀂾 Loop the pair at the other end.

􀂾 Connect the probes of mulitmeter to the both limbs of the pair.

􀂾 Deflection of mulitmeter needle shows the limb is ok, otherwise there is a break in the limb under test.

􀂾 Repeat the procedures for testing of other pairs

􀂾 The standard value is 56 ohms /loop km or 28 ohms per km ( each conductor)

The above procedure is adopted for testing of continuity of terminated cable including armour continuity.

 

2.    Insulation test of cables

This test is carried out to measure insulation resistance of the cable.

Insulation resistance measured

(i) Between conductors to conductor,

(ii) Between conductor to earth.

Procedure:

(i) Insulation resistance Between Conductor to conductor

1. A 500-volt insulation tester or megger is used for this test.

2. Pair of Conductor for which insulation to be measured shall be connected to line and earth terminal of the megger.

3. Now rotate the handle of the megger or press the push button of the megger, Reading of the megger shows the insulation between conductors.

4. Replace the conductor connected to the earth terminal of megger by other conductor of cable and take measurement.

5. Repeat process (4) for remaining conductors.

6. Now connect next conductor to line terminal of the megger

& connect the remaining conductors one by one to earth terminal of the megger and take measurements as per procedure 3, 4& 5.

7. The standard value is 5,000 mega ohms/km.

Insulation resistance Between Conductor to Earth

By this we can measure individual insulation of conductors with respective to earth.

􀂾 Connect conductor under test to the line terminal of the megger.

􀂾 Connect earth terminal of the megger to the earth.

􀂾 Rotate the handle of megger or press push button of megger. The reading of meter will show the insulation resistance of the conductor. Insulation reading shall be recorded after applying the test voltage for about a minute till a steady reading is obtained.

􀂾 Replace the conductor at Line terminal of the megger by another conductor under test and repeat the

procedure as above.

􀂾 The above measurements can be adopted for terminated cable.

3.     Attenuation test of cables

Instruments to be used:. Transmission measuring set (TMS)

􀂾 Two TMS sets are required for this measurement.

􀂾 One set is kept at one end of the cable and the other at other end.

􀂾 Before commencing test the zero errors of the both meters to be checked and corrected.

􀂾 Feed 0 db at 800 Hz tone from one end(sending end) and measure the receive level at the other end

(receiving end)across 600ohms impedence.

􀂾 The transmission loss, return loss, insertion loss are measured using this method.

􀂾 The standard attenuation values are

(a) 4.4 dB/KM at 150KHz( unloaded cable)

(b) 2dB /KM at 300-3400HZ ( unloaded cable)

( C ) 0.25 db /km at 800hz (loaded cable)

 

 

4.    Cross talk test : Far end Near end

Instrument used- Cross talk measuring set. Oscillator 150 KHz or 800Hz.

􀂾 Feed test tone of 0 db at 150 kHZ on one pair and measure the Near end and far end cross talk terminated at 600 ohms load impedence on the other pair of the same quad.

􀂾 Also the cross talk has to be measured in adjacent quad.

􀂾 Cross talk is measured at near end and also at far end. Cross talk attenuation between any two pair

space insulated VF pair at 150KHz shall not be less than -67.8 dB

􀂾 The near-end cross talk shall not be less than – 55 dB.

5.     Routine tests

􀂾 Under ground cables shall be tested once in every year with megger for insulation only and the

results of tests shall be submitted to DSTE/ASTE

􀂾 Apart from the testing to be performed during laying and after laying the cable, routine tests

shall also be conducted on the cables to ensure that the cable is in good condition.

This will provide the data to decide as when a cable has served its life and to replace the same in time, to

avoid complete breakdown.

􀂾 All spare pairs in a cable shall be tested periodically once a year to ensure that they are in good condition. This will help in using the same pairs whenever a working pair has been faulty and the circuit carried by it has to be transferred to one of the spare pair.