Insulation resistance testing measures how well a cable, motor or panel keeps current in its proper path. Falling insulation leads to leakage, shocks and fires. A simple meter reading catches the decline long before a fault appears.
Quick Answer
The test pushes a small voltage through insulation and measures leakage.
A high reading means sound insulation; a low reading warns of a fault.
Motors, cables and panels are all tested the same way.
Readings below the maker's limit mean the item needs attention or replacement.
Periodic testing builds a trend that predicts failure.
What is insulation resistance testing?
This test refers to applying a direct voltage across insulation and measuring the resistance to leakage current.
It means proving that cables, windings and panels still separate live conductors from earth.
insulation resistance testing in practice
Typical test voltages and minimum pass readings:
| Item | Test voltage | Minimum reading |
|---|---|---|
| Low voltage cable | 500 V dc | 1 megaohm |
| Motor winding | 500 V dc | 1 megaohm |
| Panel busbar | 500 V dc | 1 megaohm |
| New installation | 500 V dc | 1 megaohm |
| High voltage cable | 1000 V dc | 5 megaohm |
| Machine to earth | 500 V dc | 1 megaohm |
What does an insulation test cost in Nigeria?
A single-item insulation test costs about 20,000 to 60,000 naira in Lagos.
A full building inspection runs 150,000 to 500,000 naira and takes 4 to 8 hours on site.
Annual testing is recommended for most commercial installations.
Testing can cut downtime by spotting faults weeks before a trip.
A replacement cable or motor usually costs 2 to 3 times the test.
How is insulation resistance testing carried out?
- Isolate and lock off the circuit before testing.
- Discharge the item and confirm it is dead.
- Connect the meter between the conductor and earth.
- Apply the test voltage and hold for 1 minute.
- Record the reading after one minute and compare to the limit.
- Retest after any repair and keep the record.
Sources
https://www.nerc.gov.ng, https://nemsa.gov.ng, https://www.iec.ch, https://www.energy.gov
Why insulation fails
Heat dries and cracks insulation.
Moisture soaks into old cables.
Cables over 15 years in service deserve close attention.
Vibration rubs conductors together.
All of this lowers resistance over time.
Reading the numbers
A high reading is good.
A fall of more than 50 percent between yearly tests is a clear warning.
Compare today to last year.
Compare like items to like.
The trend predicts failure better than one test.
Test voltages
Too low a voltage misses weak spots.
Too high can damage good insulation.
Match the voltage to the system.
Follow the maker's guide.
Record the voltage with the result.
What to test
Cables between panels.
Motor and generator windings.
Distribution busbars.
Socket and lighting circuits.
Anything that has tripped before.
Periodic inspection
Inspect before you test.
Look for heat, smell and damage.
Test after the visual check.
Log every reading.
Repeat on a fixed schedule.
Acting on results
A low reading means investigate.
Clean and dry before condemning.
Retest after cleaning.
Replace what still fails.
Never leave a failed item live.
Related guides from this series
- Electrical test before touch procedure in Nigeria
- Electrical earthing rod installation and testing in Nigeria
- How to perform a residential electrical safety inspection in Nigeria
About the author
Written by the Illuminate Engineering technical team. We test and commission electrical installations across Nigeria.
Frequently Asked Questions
What is a good insulation resistance reading?
At least 1 megaohm for low voltage equipment, and 5 megaohm for many new installations.
How often should insulation be tested?
At least once a year for commercial sites, and after any repair or flood.
Can I test live equipment?
No. Isolate, lock off and confirm dead before every test.
What causes low readings?
Moisture, heat damage, dirt and ageing, which can often be cleaned and re-tested.
Conclusion
This test is cheap insurance against shocks and fires. Test on a schedule, watch the trend, and replace insulation before it fails.