7.2 Prospective Fault Current (PFC) & RCD Testing

7.2 Prospective Fault Current (PFC) & RCD Testing

1. Prospective Fault Current (PFC) Determination

Prospective Fault Current (PFC) is defined as the maximum electric current that could flow in a circuit during a short-circuit fault of negligible impedance between live conductors (Line to Neutral or Line to Line) or an earth fault of negligible impedance between a live conductor and an exposed-conductive-part (Line to Earth).

PFC is a composite term comprising two distinct parameters:

1. Prospective Short-Circuit Current (PSCC): Measured between Line and Neutral ($L-N$) or Line to Line ($L1-L2-L3$).
2. Prospective Earth Fault Current (PEFC): Measured between Line and Earth ($L-E$).

The overall PFC recorded on the Electrical Installation Condition Report (EICR) or Schedule of Test Results MUST be the highest value between the measured PSCC and PEFC: $\text{PFC} = \max(\text{PSCC}, \text{PEFC})$

In single-phase TN-C-S (PME) installations, PEFC and PSCC are frequently identical because the neutral and earth conductors follow the same combined PEN path back to the supply transformer. In TN-S installations, PSCC is almost always higher than PEFC due to the higher impedance of the lead sheath or separate earth tape return path.

2. Measurement Procedures & Breaking Capacity Verification

PFC measurement is performed with the installation live, using a dedicated loop/PFC tester or multifunction installation tester connected at:

• The origin of the installation (main intake terminals).
• Every distribution board and consumer unit.

Verifying Protective Device Breaking Capacity ($I_{cn} / I_{cu}$)

Under BS 7671 Regulation 434.5.1, the rated breaking capacity ($I_{cn}$ for domestic/commercial BS EN 60898 MCBs or $I_{cu}$ for industrial BS EN 60947-2 MCCBs) of every protective device must be equal to or greater than the maximum PFC existing at the point where the device is installed.

If a main consumer unit receives a single-phase supply with a measured PFC of 4.5 kA, any installed circuit protective devices must have a minimum rated breaking capacity of 6 kA ($6,000\text{ A}$). Standard domestic MCBs are rated at 6 kA ($I_{cn} = 6000\text{ A}$), which safely clears a 4.5 kA fault without exploding or welding contacts closed.

Three-Phase PFC Estimation Rule

On a three-phase supply, short circuits between two line conductors ($L1-L2$) or three line conductors ($L1-L2-L3$) produce significantly higher fault currents than single-phase Line-to-Neutral faults.

Where a single-phase Line-to-Neutral PSCC measurement is taken on a three-phase supply intake, the maximum three-phase prospective short-circuit current ($PSCC_{3\phi}$) is estimated using the standard industry rule of thumb: $PSCC_{3\phi} \approx 2 \times PSCC_{1\phi(L-N)}$

For example, if the measured single-phase $L-N$ PSCC at a 400 V distribution board is 3.2 kA, the three-phase short-circuit fault level must be recorded as 6.4 kA. Consequently, protective devices rated at 6 kA would be inadequate, requiring 10 kA rated switchgear.

3. Residual Current Device (RCD) Verification Principles

RCDs operate on the core balance transformer principle, continuously vectorially summing the currents flowing through the live conductors (Line and Neutral). In a healthy circuit, $I_L - I_N = 0$. When an earth fault occurs, a residual current ($I_{\Delta n}$) leaks to earth, creating a net magnetic flux in the toroidal core that induces a voltage in the secondary sense winding, tripping the relay.

BS 7671 Regulation 415.1 defines 30 mA RCDs as providing Additional Protection against electric shock in high-risk areas (e.g., socket outlets $\le 32\text{ A}$, mobile equipment outdoors $\le 32\text{ A}$, cables concealed in walls at depth $<50\text{ mm}$, and all circuits in locations containing a bath or shower).

RCD Types & Current Waveforms

• Type AC: Designed for sinusoidal AC residual currents only. (Now restricted under BS 7671 Amendment 2 for general socket circuits due to DC electronic loads).
• Type A: Operates on sinusoidal AC and pulsating DC residual currents (produced by appliances with electronic rectifiers, washing machines, EV chargers).
• Type F: Operates on AC, pulsating DC, and composite high-frequency residual currents (variable speed drives, heat pumps).
• Type B: Operates on smooth pure DC, high-frequency, and AC residual currents (solar PV inverters, three-phase drives).

4. Current Minimum RCD Field Verification

Current BS 7671 verification uses the test appropriate to the RCD type, waveform and protective purpose. Use a suitable instrument and follow the manufacturer's instructions where checks beyond the minimum timed test are needed.

For the minimum AC operating-time test:

• General non-delay RCD: Test at rated residual operating current (IΔn). Maximum operating time is 300 ms.
• S-type time-delayed RCD: Test at IΔn. Acceptable operating time is 130 ms to 500 ms.

Do not import an older universal multi-step routine into the current minimum procedure. Extra diagnostic functions offered by an instrument are not automatically required field tests.

Inspectors must also confirm that the installed RCD type is suitable for the expected residual-current waveform and application. Disconnecting or controlling downstream loads may be necessary to avoid background leakage affecting a timed test.

5. S-Type RCDs and Selectivity

Where RCDs are installed in series, selectivity depends on the rated residual currents, time-delay characteristics and coordination of the actual devices. An S-type upstream RCD commonly provides a deliberate delay so that an appropriate downstream device can operate first.

Use product-specific instructions and the applicable design requirements for any further test or selectivity check.

6. Functional Test Button Manual Verification

Following instrument verification, the inspector must manually operate the integral test button on every RCD. Pressing the test button connects an internal current-limiting resistor across the live and neutral conductors (bypassing the core transformer on one side) to create an artificial residual imbalance.

• Requirement: The test button must cause the RCD to trip mechanically and open its contacts immediately.
• Limitation: The test button confirms mechanical trip linkage operation, but does NOT measure trip time or current sensitivity; it cannot replace instrument calibrated testing.