Part P Inspection & Testing 4 — Questions and Answers
Question 1: What is a 'prospective fault current' (PFC) test and why is it performed?
- It measures the maximum fault current at the consumer unit origin to verify the breaking capacity of the main fuse and all protective devices exceeds the available fault current (Correct answer)
- It measures the normal operating current
- It tests the prospective voltage drop
- It measures the prospective load current
Correct answer: It measures the maximum fault current at the consumer unit origin to verify the breaking capacity of the main fuse and all protective devices exceeds the available fault current
The PFC test measures the maximum available short-circuit and earth fault current at the origin; this value must be within the breaking capacity of the protective devices, and must be considered when selecting devices throughout the installation.
Question 2: What happens to Zs as cable temperature increases?
- Zs increases (copper resistance increases with temperature) so the measured cold Zs must be corrected to operating temperature using a correction factor (Correct answer)
- Zs decreases with temperature
- Zs is not affected by cable temperature
- Zs only changes with load current, not temperature
Correct answer: Zs increases (copper resistance increases with temperature) so the measured cold Zs must be corrected to operating temperature using a correction factor
Copper conductor resistance increases with temperature (approximately 0.4% per °C); Zs measured at ambient (cold) conditions must be multiplied by a temperature correction factor to obtain the Zs at operating temperature for comparison against BS 7671 limits.
Question 3: What is the purpose of the 'verification of voltage drop' in a new installation?
- To confirm that the voltage drop in circuit cables under design load current does not exceed the BS 7671 limits (typically 3% for lighting, 5% for other circuits) (Correct answer)
- To measure the supply voltage at the meter
- To test the accuracy of the electricity meter
- To confirm the correct fuse rating
Correct answer: To confirm that the voltage drop in circuit cables under design load current does not exceed the BS 7671 limits (typically 3% for lighting, 5% for other circuits)
Excessive voltage drop in long cables can affect the performance of connected equipment; BS 7671 limits are 3% for lighting circuits and 5% for other circuits from the origin of the installation.
Question 4: What does the term 'electrode resistance testing' involve?
- Measuring the resistance between an earth electrode and the general mass of earth using a dedicated earth electrode resistance tester (3-probe fall-of-potential method) (Correct answer)
- Testing the continuity of the CPC within the installation
- Testing insulation resistance of earth conductors
- Measuring Ze at the supply terminals
Correct answer: Measuring the resistance between an earth electrode and the general mass of earth using a dedicated earth electrode resistance tester (3-probe fall-of-potential method)
Earth electrode resistance testing uses the fall-of-potential method, injecting current between the electrode under test and a remote current electrode, measuring voltage between the electrode and a probe electrode, and calculating resistance from V/I.
Question 5: What is the purpose of recording serial numbers and test instrument calibration dates on an EIC or EICR?
- To ensure the test instruments used were calibrated and traceable, validating the accuracy of the test results (Correct answer)
- To identify the test instruments for insurance purposes
- To meet a Health and Safety requirement unrelated to accuracy
- To record the purchase cost of the instruments
Correct answer: To ensure the test instruments used were calibrated and traceable, validating the accuracy of the test results
Recording calibration details provides traceability and confidence that the test instruments were accurate at the time of testing; calibration certificates must be current, typically renewed annually.
Question 6: How often should test instruments used for electrical inspection and testing be recalibrated?
- At minimum every 12 months (or as specified by the instrument manufacturer and the engineer's certification scheme) (Correct answer)
- Every 5 years
- Only when they are dropped or damaged
- No recalibration is required for electrical test instruments
Correct answer: At minimum every 12 months (or as specified by the instrument manufacturer and the engineer's certification scheme)
BS 7671 and Competent Person scheme requirements specify that test instruments should be calibrated regularly (typically annually) to ensure measurement accuracy; the calibration certificate provides traceability of measurement.
Part P Inspection & Testing 4
What is a 'prospective fault current' (PFC) test and why is it performed?