How to measure and check Underground Cable

Underground cable is laid on the ground over a great distance length. Tests on these large cable to be completed before buried into the ground. Identify two(2) site point to perform tests.

1.1 Type of test on underground cable

The following test are to conducted by competent person.

polarity test
continuity test
pressure test
insulation resistance test

Other types of tests on an electrical power cable.

Persulphate test (for copper)
Annealing test (for copper)
Tensile test (for Aluminium)
Wrapping test (for Aluminium)
Conductor resistance test (for all)
Test for thickness of insulation (for all)
Measurement of overall diameter (where specified) (for all)

1.3 Cable Testing Requirement

Testing of underground cable before cover up the ground is very important. Make sure all the cable conditions achieved standard requirement.

Type of test and measurement for underground cable:

Insulation test. --> Purpose: To ensure that the cable has no fault, standard acceptable value is more than 1 Mega-ohm, test voltage at least 1k Vdc for one minutes duration.
Continuity test --> Purpose: To ensure that the cable has no open line. This can use multi-meter or ohm-meter.
Phasing test or cable identification test --> Purpose: To identify the cable conductor. This can use multi-meter or ohmmeter.
Resistor board test --> Purpose: To identify conductor using resistor measurement between conductor and ground.
Pressure test --> Purpose: To inject high voltage into the cable to ensure the cable can handle voltage pressure.

2.0 Using an Insulation Resistance Testset or 'meggar'

2.1 Insulation Resistance Testset - Analog Type

Insulation resistance is a measurement to detect a cable conductor is having fault condition like short circuit to another conductor.

Test equipment is known as 'meggar test'.

Example of picture of IR test equipment from Japan.

(For reference only)

2.2 Standard acceptable test for IR test

Reading result must be greater than 1MΩ.

Test voltage is commonly set to:

500V for 3phase system
250V for single phase system
1000V for underground cable test

Test voltage for LV cable is set to : 500 volts

Duration test: 1 minute (60 second)

Test voltage for HV cable is set to : 1000 volts

Duration test: > 2 minute

Note: Some bigger size cable are tested at 5k volt, refer to cable manufacturer specs.

2.3 Insulation Resistance Test Result

Example of analog type IR test set as shown, when needle points infinity, this means cable is healthy.

Megger or IR test set works on the principle of electromagnetic attraction. When a primary coil that is carrying current is placed under the vicinity of a magnetic field it experiences a force.

This kind of force generates a torque that is made to deflect the pointer of the device which gives some reading.

Figure 1 above showing the upper scale (blue) reads infinity or > 2000 MΩ.

This scale reading is suited for HV power cable. (>1000v )

Figure 2 above showing the upper scale (red) reads infinity or > 200 MΩ.

This scale reading is suited for LV power cable. (<1000v )

2.4 Insulation Resistance testing sequence order

When perform this insulation resistance test, no jumper cable is required.

If you using the jumper cable similar with continuity tests, all the results will show 0Ω or short circuit.

Note:

Do not touch any terminal conductor , LIVE 500V is generated from IR test equipment .

2.5 Underground cable - Insulation Test 1

Example 1

Refer to figure on left, no cable jumper is used on site A. Do not touch any conductor on this site.

Site B the meggar is done on Y-E, the result reads infinity. This means that Y-E is healthy or NOT SHORT CIRCUIT fault.

Example 2

Refer to figure on left, where a cable jumper is used on site A.

Site B the meggar is done on B-N, the result reads 0Ω.

This means that B-N is NOT HEALTHY or POSSIBLE SHORT CIRCUIT fault if this cable is energized.

2.6 Underground cable - Insulation Test Results

Identify cable faulty

Refer to table result below, you can identify the faulty cable.

Below is another example result table, you can identify the faulty cable.

3.0 Continuity Test

Purpose of this test are:

identified cable phase
ensure each conductor are not crossed due to color of conductor.
ensure connection to load or circuit is known.

Standard test shall be using multi-meter for LV cable. If the cable are HV then insulation resistance test is sufficient to perform this test.

3.1 Test equipment for perform continuity

Test equipment

a digital multi-meter or ' analog meter'
digital clamp meter

Additional item required to perform the test:

cable jumper sized 2.5mm² with two clips on both ends.

Meter result reading meaning

∞ symbol means infinity or open line 'O.L'

0.7Ω - ohm value means conductor is good.

multi-meter Clamp multi-meter Cable jumper (cable sized 1.5/2.5 mm² )

digital meter (analog) two end point with crocodile clip

( DM ) (DCM) (AM) (any color choice)

Basic continuity test is illustrated below.

The jumper cable is act to as a loop joint, so that current measured from meter able to flow through between the two conductor.

3.2 Continuity test sequence order

3.3 Underground cable outlook

A jumper cable with a clip at both ends is best tool to be used during continuity test. An armor cable SWA or GWA are used as grounding purpose. This earth help reduce magnetic hazard when the cable is energized with live supply.

3.4 Underground cable testing - faulty conductor line or cable

Example figure on the above showing on site A put jumper cable is placed between phase conductor B to E.

While on site B, the meter is measuring the point between E to Y, the result reading on meter should be infinity or 'O.L' . This means that the cable is not connected.

3.5 Underground cable testing - good cable conductor

Another example, on site A, the jumper cable is clipped between R-B.

At point site B, when place the meter test lead on between R-B, the meter reads 1.6Ω. This means that the cable between R-B is good condition.

3.6 Underground cable - How to identify cable fault

Refer to the continuity test result table on the right, which of the conductor is having fault ?

The answer shall be when measure between all the phase to neutral, the result showing infinity, this means that the neutral conductor is faulty.

Polarity Test or ' Phasing test '

Polarity tests conducted on underground cable is simple.

There are three (3) methods basically method to identify the polarity of these UG cable.

looking at the color of each color conductor inside the underground cable.
using resistor to identify correct conductor
using earth as reference test (for armour type underground cable)
using neutral conductor as reference test (if UG is none armor type)

Pressure Test

Pressure Test or 'voltage injection' testing

Purpose of the pressure test to an underground cable are:

ensure high voltage inject to cable to identify the dielectric strength of these cable
the insulation material can withstand high voltage by 1.5
measure the temperature of the cable during injection.

Commonly this test is conducted by cable manufacturer, and tested at factory before sending the cable to site.

Important of tests on power cable:

Product warranties are limited
Testing is less expensive than repair
Periodic testing will future proof the infrastructure

Voltage range of HV power cable:

6.6kv ~11kV
11kv ~33kV
125kv~175kV
275kv ~385kV

High Voltage Test or 'hi-pot'

The intention of the hot pressure cable test is to determine if the cable insulation and sheathing material can withstand a degree of pressure exerted on the cable at elevated temperatures without causing significant damage and a potential safety risk.

The high voltage test (also called dielectric strength test or hi-pot test) can be made in AC or DC. If the high voltage test is made in DC, it is then combined with insulation resistance test. If the high voltage test is made in AC, it is then, this is then, more stressful.

Measurement of high voltage test under alternating current is performed using an alternating voltage (50Hz) adjustable to an effective 50V to 1,500V. As is the case with direct current, the high voltage test detects any sudden rise of current up to a programmed threshold. The short circuit test is maintained by default. The rise time is more than 500 ms and the application time at least one period.

Warning: The high voltage test under alternating current is penalized by the capacitive value of the tested equipment. It must be remembered that the generator power is limited to 5 mA.

Test guide on HV power cables:

voltage set 1000V max for LV cable, test duration about <5 minutes.
voltage set 5 kVdc for HV cable, test durations depends on test equipment.

Requirement for testing HV cable:

Competency H1 or Competent Engineer.

HV cable test requirement

set voltage test to 1000V dc for LV cable and test duration at least more than 5 minutes.
Wear correct PPE when perform HV insulation test.

Sarawak Electricity Rules and Regulation Rule.22

Rule 22 : Underground mains and connections

(1) An underground supply line shall be continuously insulated and protected to the satisfaction of the Chief Electrical Inspector, and where it is attached to a bridge or culvert it shall be protected in such a manner as may be directed by the Chief Electrical Inspector.

(2) Subject to the State Roads Ordinance, 1994 [Cap. 9], and any other written law, the breaking up of a street, road or public place for the purpose of laying an underground main and the connections thereto and the refilling and making good thereof shall be carried out by the owner, management, licensee, or his servant or agent, as the case may be, in the manner as may be required by the relevant authority charged with the responsibility for matters relating to streets,roads or public places.

(3) Except for the purpose of making immediate repairs, no underground main shall be laid by the owner, management, licensee, or his servant or agent, as the case may be, in any road unless reasonable notice has been given to the relevant authority.

Manual Hand operated type 'meggar '

Figure above on the left is an old type hand held operated IR test equipment.

While on the right is the battery operated insulation resistance testset.

(For reference only)

This old type of meggar is obsolete and not used for testing. Not user friendly compare to modern type IR test with digital reading.

Figure above is basic diagram of hand held type meggar used in the 1960s.

(For reference only)

Download Cable Testing Sheet

Testing Sheet LV Underground cable | Filename: ugc_testingsheet.pdf >> click here .

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