Live_Tests Prospective Fault Currents

Measuring Prospective Fault Current (PFC) at the Origin of a Single Phase Supply

• Objective: Measure and confirm Prospective Fault Currents (PFC) for safety and adequacy of protective devices.

Purpose of Measurement

• Establish maximum fault current for installations.

• Ensure protective devices can withstand fault current and disconnect safely.

• Devices have rated maximum current to prevent explosion or failure.

  • Rated short-circuit capacity (fuses, circuit breakers, RCBOs).

  • Breaking capacity sometimes used for fuses.

• Verifying breaking/short-circuit capacity against possible fault currents.

Equipment Needed

• Use of a prospective fault current tester.

• Familiarize with test instrument and read instructions.

  • Some meters display prospective fault current when set to test earth fault loop impedance.

• If meter requires three leads, ensure correct connections (L-N or L-E).

  • If unsure, combine earth and neutral leads.

Safety Considerations

• Avoid exposing self to live terminals unnecessarily.

• Comply with GS38 standards during live testing.

Testing Methodology

• Conduct tests at the distribution board near the supply, on the supply side of the main switch:

  • Single-phase installations: Test L-N (PSCC) and L-E (PEFC).

  • Three-phase installations: Test L1-N, L2-N, L3-N (all PSCC).

  • PEFC test is unnecessary for three-phase due to voltage considerations.

Test Results

• Single-phase: Record highest value from two tests in kilo-amperes (kA).

• Three-phase: Take highest of three tests and multiply by two for a 400 V fault simulation.

• Document the final result on the Electrical Installation Certificate (EIC) and schedule.

Verification of Results

• Recorded PFC must not exceed short-circuit capacities of protective devices.

• Consult Table 7.2.7 in IET On-Site Guide for rated capacities; confirm capacity is visible on device.

• If downstream device capacity is lower than measured, further testing needed.

Key Considerations

• PFC cannot increase beyond the value at installation's origin.

• If unable to test, estimations can be obtained from distribution network operator.