CABLE TESTING USING A MEGGER

Purpose • Testing Procedure • Test Voltages • Acceptable Values • Safety

1. What is a Megger and why is it used?

A Megger (insulation resistance tester) applies a specified DC test voltage to electrical insulation and measures its resistance, usually in megaohms (MΩ), gigaohms (GΩ) or higher.

· Detect deteriorated, damaged, wet or contaminated cable insulation.

· Identify leakage paths between conductors and earth.

· Verify insulation condition before energizing a newly installed cable.

· Monitor insulation deterioration during preventive maintenance.

· Support investigation of earth-leakage and insulation-related faults.

Important: A Megger measures insulation resistance. It does not directly verify current-carrying capacity, conductor continuity, phase sequence or the cable’s ability to withstand every operating surge.

2. Step-by-step cable testing procedure

Step 1 — Isolate the cable: Switch off and isolate the cable from every source. Apply lockout/tagout (LOTO) and verify absence of voltage with an approved voltage detector. Follow the site electrical safety procedure.

Step 2 — Disconnect sensitive equipment: Disconnect the cable from PLCs, electronic equipment, VFDs, LED drivers, surge protection devices and other components that could be damaged by the test voltage. Isolate both ends wherever required.

Step 3 — Select the test voltage: Choose the DC test voltage according to cable rated voltage, insulation type, applicable standard and the manufacturer’s instructions. Do not automatically use 1000 V on every cable.

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Step 4 — Test each core to earth: Connect the Megger LINE (+) lead to the conductor under test and the EARTH (−) lead to a suitable earth reference such as the cable screen or armour, as specified by the test procedure. Keep other conductors grounded as required by the approved method.

Step 5 — Test between conductors: For multicore cables, test each required conductor-to-conductor combination. For a three-phase cable, this may include R–Y, Y–B and B–R, as well as phase-to-neutral tests where applicable.

Step 6 — Record the reading: Record the cable ID, test voltage, test duration, resistance, temperature and test conditions. A one-minute reading is commonly used when specified by the applicable procedure.

Step 7 — Discharge the cable: Allow the tester’s discharge function to operate, then discharge and earth the cable using the approved method. Long cables can retain stored charge. Verify safe conditions before touching conductors or reconnecting equipment.

3. Common DC test voltages

The values below are common starting points only. The applicable standard, cable manufacturer and approved project/site procedure take precedence.

Cable / circuit type	Common DC test voltage
SELV / PELV circuits	250 V
230/415 V low-voltage circuits	500 V
LV power cables where procedure permits	500–1000 V
3.3 kV / 6.6 kV / 11 kV cables	Often 2.5–5 kV, subject to cable-specific procedure
Higher-voltage cables	Specialist test plan and manufacturer guidance

Caution: For MV/HV cables, do not select test voltage from this table alone. XLPE cables require cable-specific procedures; insulation-resistance testing is not a substitute for appropriate VLF, withstand or diagnostic tests.

4. Minimum insulation resistance values

IEC 60364-6:2016 provides the following minimum values for general low-voltage installation verification. These are installation verification thresholds, not universal acceptance criteria for every isolated power cable.

Nominal circuit voltage	DC test voltage	Minimum insulation resistance
SELV / PELV	250 V	0.5 MΩ
Up to and including 500 V	500 V	1 MΩ
Above 500 V	1,000 V	1 MΩ

A healthy new cable may measure far above these minimums. The required acceptance value must be taken from the applicable cable standard, project specification or manufacturer. There is no single resistance value that proves every cable is perfect.

5. Practical interpretation of readings

· High and stable reading — Generally encouraging. Compare with the commissioning baseline and previous readings taken under similar conditions.

· Low or falling reading — Investigate moisture, dirt, damaged insulation, poor terminations, connected equipment and temperature differences.

· Below the required minimum, unstable or near-zero reading — Do not energize until the cause is investigated, corrective action is completed and the cable passes specified tests.

A reading of 100 MΩ is not automatically proof of safety compared with every 10 MΩ reading. Cable length, temperature, insulation material and test setup matter. A sudden decline from the established baseline warrants investigation.

6. Example: 3-phase, 415 V cable test matrix

Example only: a disconnected four-core cable containing R, Y, B and N conductors, with armour connected to the earth reference. Confirm the exact connections against the approved site procedure.

Test	Connection (example)	Purpose
R–Earth	R to LINE; earth reference to EARTH	R-core insulation to earth
Y–Earth	Y to LINE; earth reference to EARTH	Y-core insulation to earth
B–Earth	B to LINE; earth reference to EARTH	B-core insulation to earth
N–Earth	N to LINE; earth reference to EARTH	Neutral insulation to earth
R–Y	R to LINE; Y to EARTH	Insulation between phases
Y–B	Y to LINE; B to EARTH	Insulation between phases
B–R	B to LINE; R to EARTH	Insulation between phases
Phase–N	Each phase tested against N	Insulation between phase and neutral

For a 415 V circuit, 500 V DC is a common installation-test setting when permitted by the approved procedure. Disconnect connected loads and sensitive devices first.

7. Illustrative test report

The values below are illustrative only and are not actual measurements or guaranteed acceptance criteria.

Test combination	Reading at 500 V DC	Example assessment
R–Earth	250 MΩ	Encouraging
Y–Earth	220 MΩ	Encouraging
B–Earth	245 MΩ	Encouraging
N–Earth	300 MΩ	Encouraging
R–Y	400 MΩ	Encouraging
Y–B	380 MΩ	Encouraging
B–R	410 MΩ	Encouraging

These readings are high and reasonably consistent. Final acceptance still depends on the applicable cable specification, test duration and any other required tests.

8. Factors affecting Megger readings

· Cable length: longer cables can show different measured resistance and charging behaviour.

· Temperature: insulation resistance generally falls as temperature rises. Compare at similar temperatures or apply the appropriate correction factor.

· Moisture and contamination: water, dirt and damp terminations can reduce readings.

· Insulation material: PVC, XLPE and other insulation materials have different properties.

· Test duration: record readings at the specified time and compare like-for-like results.

· Connected equipment: loads, electronics and surge protection can distort results or be damaged by test voltage.

9. Final checklist before energizing

☐ Cable isolated and absence of voltage verified.

☐ Cable ends and connected equipment appropriately disconnected.

☐ Correct Megger test voltage selected.

☐ Core-to-earth insulation resistance measured.

☐ Core-to-core insulation resistance measured as required.

☐ Readings recorded and checked against approved acceptance criteria.

☐ Cable safely discharged after testing.

☐ Conductor continuity, earthing, phase identification and other required commissioning tests completed.

☐ All connections restored, inspected and cleared for energization.

10. Engineering conclusion and references

A good cable should meet the specified minimum insulation resistance, show no abnormal variation between comparable cores, and produce stable results consistent with its baseline. A Megger reading alone cannot certify a cable as completely fault-free.

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Wednesday, October 14, 2026

CABLE TESTING USING A MEGGER