Part P Inspection & Testing 3 — Questions and Answers
Question 1: What is the purpose of an RCD test using a calibrated RCD tester?
- To verify the RCD trips within the required time (40 ms for 30 mA at 5×IΔn) and at no greater than its rated residual current (IΔn) (Correct answer)
- To test the mechanical operation of the trip lever only
- To measure the RCD's rated current
- To test the MCBs in the same group as the RCD
Correct answer: To verify the RCD trips within the required time (40 ms for 30 mA at 5×IΔn) and at no greater than its rated residual current (IΔn)
A calibrated RCD tester verifies the actual trip current and trip time of the RCD, confirming it will operate correctly at its rated sensitivity (IΔn) and within the maximum time specified by BS 7671.
Question 2: Why must the RCD test button be used regularly by the occupier?
- To confirm the mechanical trip mechanism remains operational — it should be tested quarterly (pressing the test button) (Correct answer)
- To reset the RCD after a fault
- To set the trip current
- To test the MCBs it protects
Correct answer: To confirm the mechanical trip mechanism remains operational — it should be tested quarterly (pressing the test button)
The RCD test button should be pressed quarterly to confirm the mechanical trip linkage is functioning; a stuck RCD may appear normal but will fail to trip in a real fault. The test button is the user's simple check.
Question 3: What is measured by the R1+R2 test?
- The combined resistance of the phase conductor (R1) and circuit protective conductor (R2) from the consumer unit to the furthest point of the circuit (Correct answer)
- The resistance of the phase conductor only
- The resistance between phase and neutral
- The resistance of the neutral conductor
Correct answer: The combined resistance of the phase conductor (R1) and circuit protective conductor (R2) from the consumer unit to the furthest point of the circuit
The R1+R2 test (also called the conductor resistance test) measures the total resistance of the phase and CPC in a circuit, used to calculate the maximum Zs at the furthest point from the supply.
Question 4: How is the maximum Zs of a circuit calculated from the R1+R2 test result?
- Zs = Ze + (R1+R2), where Ze is the external impedance measured at the origin (Correct answer)
- Zs = R1+R2 alone
- Zs = Ze only
- Zs = R1+R2 minus Ze
Correct answer: Zs = Ze + (R1+R2), where Ze is the external impedance measured at the origin
The total earth fault loop impedance Zs for any circuit is the sum of the external impedance Ze (from the supply) and the circuit's R1+R2 resistance; the result must not exceed the maximum Zs for the protective device.
Question 5: What does a low insulation resistance reading (below 1 MΩ) indicate?
- Damaged or deteriorated cable insulation, a fault between conductors, moisture ingress or incorrectly connected equipment (Correct answer)
- The supply voltage is too high
- The circuit is overloaded
- The earthing system is defective
Correct answer: Damaged or deteriorated cable insulation, a fault between conductors, moisture ingress or incorrectly connected equipment
An insulation resistance below 1 MΩ indicates a breakdown of the insulating properties between conductors — possibly caused by physical damage, overheating, moisture ingress, rodent damage or incorrect wiring.
Question 6: What safety precaution must be taken before performing an insulation resistance test?
- Disconnect all connected equipment and lamps (as the test voltage can damage electronic components and LED drivers) and open all switches to include all wiring in the test (Correct answer)
- Connect all equipment to increase the measured resistance
- Leave all switches closed to exclude the cabling from the test
- Test through the connected equipment to include it in the reading
Correct answer: Disconnect all connected equipment and lamps (as the test voltage can damage electronic components and LED drivers) and open all switches to include all wiring in the test
Electronic equipment (LED drivers, dimmers, electronic thermostats) must be disconnected before applying the 500 V DC test voltage, as it will damage sensitive components; lamps should also be removed.
Part P Inspection & Testing 3
What is the purpose of an RCD test using a calibrated RCD tester?