Part P Inspection & Testing 4

Question 1

What is a 'prospective fault current' (PFC) test and why is it performed?

Accepted 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

Answer

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?

Accepted Answer

Zs increases (copper resistance increases with temperature) so the measured cold Zs must be corrected to operating temperature using a correction factor

Answer

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?

Accepted 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)

Answer

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?

Accepted 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)

Answer

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?

Accepted Answer

To ensure the test instruments used were calibrated and traceable, validating the accuracy of the test results

Answer

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.

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Inspection & Testing 4