Low Insulation Resistance: Causes and Fault-Finding
A low insulation-resistance result is a fault-finding exercise: divide the circuit methodically and protect equipment before applying a test voltage.
Poor insulation resistance can be caused by connected equipment, moisture, damaged cable, contamination or incorrect isolation. The first job is to confirm the test setup and preserve equipment that should not be subjected to the selected test voltage.
This is practical guidance for competent electricians. Assess the actual installation, follow the current edition of BS 7671 and manufacturer instructions, and stop where the work cannot be completed safely.
Key takeaways
- Agree the scope and make the installation safe before starting work.
- Use a controlled sequence rather than relying on a single reading or visual assumption.
- Record the actual result, limitation or action taken while it is still traceable.
- Check the result against the design, protective measures and equipment instructions.
- Issue the appropriate certificate or report when the work is complete.
Scope the work first
Poor insulation resistance can be caused by connected equipment, moisture, damaged cable, contamination or incorrect isolation. The first job is to confirm the test setup and preserve equipment that should not be subjected to the selected test voltage.
Method
Isolate, prove dead, disconnect or protect sensitive loads and test the circuit in sections. Start at the distribution board, separate outgoing conductors and accessories where necessary, then narrow the fault without repeatedly energising a suspect circuit.
Doing this on a job? EICR Certificate — part of TradePlanr. Free to start, no card needed.
Checks and common pitfalls
Look for damp external accessories, damaged flexes, trapped cable, borrowed neutrals and electronic controls left connected. Never dismiss a low result as nuisance leakage without locating the source or recording a clear, justified limitation.
Record and hand over
Record the voltage used, conductors tested, result, disconnected equipment and corrective work. Re-test after repair and ensure every circuit is correctly reconnected before restoring supply.
Keep the evidence with the job
Record readings, limitations and remedial actions as you work. A clear certificate or report protects the client and makes the next visit easier to manage.
From guidance to action
Frequently asked questions
Can I insulation-test with electronics still connected?
Only where the chosen test method and manufacturer guidance permit it. Sensitive equipment can be damaged, so disconnect or use an appropriate alternative method and record the limitation.
Can I rely on a standard rule of thumb?
No. The installation method, supply characteristics, equipment instructions and the current requirements all affect the correct decision. Use the guide as a workflow, then assess the actual job.
What should be recorded?
Record the scope, results, limitations, observations, remedial work and the certificate or report appropriate to the work carried out.
Related guides
Insulation Resistance Testing
Test voltages, minimum acceptable values, and how to test insulation resistance safely.
RCD Keeps Tripping: Causes and Fixes
A structured method for tracing why an RCD trips — from nuisance tripping to real faults.
Safe Isolation Procedure
The step-by-step safe isolation procedure, proving dead with a GS38 tester, and locking off.
