Underground Cables (UG) Test Procedure

This document explains the standard test procedure for underground (UG) power cables prior to energization. 

Objective

The objective is to verify the electrical integrity of 

• Cable insulation, 
• Screen continuity and 
• Sheath condition 

following installation thereby ensuring safe and reliable operation of the cable system.

Scope

This procedure applies to all types of underground power cables installed for

• Low Voltage (LV), 
• Medium Voltage (MV) and 
• High Voltage (HV) 

distribution and transmission systems which includes:

• Single core and multi core XLPE (or) EPR insulated cables,
• Armoured & unarmoured cable systems,
• Direct buried, ducted and trough installed cables and
• Cable joints, terminations & sealing ends.

References & Standards

Standard / Code	Description
IEC 60502	Power cables with an extruded insulation & their accessories.
IEC 60229	Tests on cable oversheath / outer sheath.
IEEE 400	Guide for the field testing of shielded power cable systems.
IS 7098	Cross linked polyethylene insulated thermoplastic sheathed cables.
OSHA 1910.333	Electrical safety — Selection and use of work practices.

Safety Precautions

Qualified Personnel

All testing shall be performed by a qualified electrical engineer (or) a licensed cable testing technician. 

A minimum of 2 persons should be present at all times during high voltage (HV) testing.

Personal Protective Equipment (PPE)

• Insulating rubber gloves rated for the test voltage,
• Safety goggles / face shield,
• Arc flash rated clothing (minimum Category 2),
• Insulating rubber mat at the test station and
• Safety boots with electrical hazard rating.

Safety Precautions

• Isolate all cable ends from the live conductors and confirm the absence of voltage using an approved voltage detector.
• Post warning signs as ‘High Voltage (HV) Testing in Progress’ at all cable ends & access points.
• Erect temporary barriers to avoid unauthorised access within the test zone.
• Ensure the cable is discharged and earthed before connecting (or) removing any test equipment.
• Maintain radio communication between the team at the test set & the team at the far end.
• Obtain & display a valid Permit to Work (PTW) before the process of commencing.

Test Equipment Required

Equipment	Specification / Range	Calibration Required
Insulation Resistance (IR) Tester	500 V / 1 kV / 5 kV / 10 kV DC	Yes / Annual
High Voltage DC Test Set	0-80 kV DC, 20 mA	Yes / Annual
Very Low Frequency (VLF) Test Set	0.1 Hz, 0-52 kV AC	Yes / Annual
Continuity Tester / Low-Ohm Meter	Resolution: 0.01 mOhm (mΩ)	Yes / Annual
Sheath Fault Locator	Pulse reflection / TDR type	Recommended
Calibrated Measuring Tape	As required	No
Earth Resistance Tester	0-2000 Ohm (Ω)	Yes / Annual

Test Procedures

Visual Inspection

Before any electrical test is performed a thorough visual inspection shall be conducted:

• Confirm the cable route markers, joint bays and termination chambers are accessible & correctly identified.
• Inspect cable terminations and joints for any physical damage, moisture ingress, oil leaks (or) incorrect assembly.
• Check armour wire bonding & earthing connections at both ends.
• Verify the cable identification labels/tags match the single line diagram.
• Record any defects with photographs & report immediately for the rectification before testing.

Continuity Test

To verify that conductor connections are complete with no open circuits.

• Short circuit and earth all the conductors at the far end of the cable.
• Using a continuity tester / low ohm meter measure the conductor resistance for each phase (L1, L2, L3) & neutral at the near end.
• Compare the measured resistance against the calculated value: R = rho x L / A (where rho – resistivity of conductor material, L – cable length, A – cross sectional area).
• Measured the values must not exceed the manufacturers specified resistance by more than 5%.
• Record all the results in the test datasheet.

Insulation Resistance (IR) Test

To assess the condition of cable insulation before applying a high voltage.

System Voltage	Test Voltage (DC)	Min. Acceptable IR
Up to 1 kV	1000 V DC	100 MΩ
1 kV – 11 kV	5000 V DC	1000 MΩ
11 kV – 33 kV	10000 V DC	10000 MΩ

• Ensure all the conductors are isolated and deenergised.
• Connect the IR tester across each conductor and earth by testing each combination: L1-E, L2-E, L3-E & L1-L2, L2-L3, L3-L1.
• Apply the test voltage for a time duration of 1minute (60 seconds) & record the reading.
• Polarisation Index to be calculated (PI = IR at 10 min/IR at 1 min). PI must be greater than 2.0 (> 2.0) for the acceptable insulation.
• Discharge the cable for at least 4 times the test duration before proceeding.

High Voltage DC / VLF Withstand Test

To confirm the dielectric integrity of the cable insulation under a stress conditions.

Method A: DC Withstand (for cables up to 33 kV)

• Apply DC voltage at a ramp rate not exceeding 1 kV/s up to the specified test voltage (typically 3 x U0 for 15 minutes).
• Monitor leakage current throughout. A stable or decreasing current is acceptable. A rising (or) fluctuating current indicates a fault.
• If the cable withstands the testing voltage for the full complete duration without breakdown, it passes.

Method B: VLF Withstand (preferred for XLPE/EPR cables)

• Connect the VLF test set at 0.1 Hz. Apply 3 x U0 AC voltage for 1 hr (60 minutes).
• Monitor for partial discharge activity and if PD measurement is integrated in the test set.
• Failure (flashover/breakdown) at any point constitutes a test rejection.

Sheath Integrity Test

• To verify that the outer sheath & bedding are intact & provide adequate protection for the metallic screens and armour.
• Disconnect all the screen and armour earth connections at both ends.
• Apply 5 kV DC between the metallic screen/armour & earth for 1 minute (60 seconds).
• Minimum acceptable insulation resistance of the outer sheath: 1 MΩ per km of cable length.
• If a sheath fault is indicated utilize a TDR (or) sheath fault locator to pinpoint the fault location for repair.

Acceptance Criteria 

Test	Parameter	Acceptance Criteria
Continuity	Conductor Resistance	Within 5% of calculated value
IR Test	Insulation Resistance	Per voltage class (see 6.3)
IR Test	Polarisation Index	PI > 2.0
HV / VLF Withstand	Withstand (no breakdown)	No flashover / leakage stable
Sheath Integrity	Sheath IR	Min. 1 MΩ/km

Documentation 

All test results shall be recorded on the UG Cable Test Report Form. 

The report shall include:

• Cable identification, rating, length and installation date.
• Test equipment details, serial numbers and calibration certificates.
• Ambient temperature & humidity at time of testing.
• Individual test readings for every phase & each test type.
• Pass (or) Fail status for each test.
• Name & signature of Test Engineer & Witnessing Authority.

Completed test reports shall be submitted to the Project Engineer within 24 hours of testing & archived in the project quality file for the duration of the cables service life.