Inspection and Testing of a Modified Circuit
An electrician has added a new radial circuit for a utility room with three double sockets, protected by a 20 A MCB and a 30 mA RCBO. The circuit is 18 m of 2.5 mm² twin-and-earth, clipped direct, with a 4 mm² protective conductor run separately to a local earth bar. The existing installation is TN-S with a measured Ze of 0.8 Ω. The electrician carries out insulation resistance tests and records 0.9 MΩ between live conductors and earth on the new circuit, and 2 MΩ on the existing lighting circuit. The RCD test gives a trip time of 45 ms at 1×IΔn and 20 ms at 5×IΔn. The electrician proposes to issue a minor electrical installation works certificate.
1. Assess the test results and state whether the circuit can be certified, identifying any deficiencies.
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Insulation resistance. The minimum acceptable value for a new circuit at 500 V is 1 MΩ. The new circuit reads 0.9 MΩ, which is below the minimum and must be investigated before certification. Possible causes include moisture, damage, or a connected load; the test should be repeated with loads disconnected and the result should be at least 1 MΩ. The existing lighting circuit at 2 MΩ is acceptable but should be recorded.
RCD test. A 30 mA RCBO tripping at 45 ms at 1×IΔn and 20 ms at 5×IΔn is within the requirements (less than 300 ms at 1×, and less than 40 ms at 5×). The test results are satisfactory, but the RCBO should also be tested at half rated current to confirm it does not trip.
Earth fault loop impedance. With Ze 0.8 Ω (TN-S), the circuit's Zs must be within the maximum for the 20 A MCB. For a 20 A MCB, the maximum Zs is typically around 1.1–1.5 Ω depending on device type; the measured Zs must be recorded and compared. If Zs exceeds the limit, the circuit cannot rely on overcurrent disconnection and must be RCD-protected (it is) but the RCD is not a substitute for correct Zs where required.
Protective conductor. A separately run 4 mm² protective conductor must be continuous and connected to the main earthing terminal; its continuity and resistance must be tested.
Certification. A minor electrical installation works certificate is appropriate for the addition, but it cannot be issued until the insulation resistance is satisfactory and all results are recorded. The existing installation's condition should also be considered, and the certificate must include the schedule of test results and details of the work.
2. The electrician argues that because the RCBO is fitted, the low insulation resistance is acceptable. Evaluate this.
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The argument is incorrect. The RCBO provides additional protection against electric shock and earth fault current, but it does not compensate for defective insulation. A low insulation resistance indicates a potential fault - moisture, damage, or a connected appliance - and the circuit must be investigated and corrected before being put into service. The minimum insulation resistance of 1 MΩ is a fundamental requirement for the circuit's integrity, and a value of 0.9 MΩ is a fail. The RCBO may trip on a fault, but relying on it to clear a persistent insulation defect is not acceptable and could lead to nuisance tripping, fire risk, or failure to disconnect. The correct action is to isolate the circuit, disconnect loads, re-test, locate the defect, and rectify it before certification.