TT System Earth Fault Loop Impedance
You are commissioning a small rural workshop supplied by a TT system. The installation has a 100 mA 300 mA time-delayed RCD as the main protective device, a 30 mA RCD for socket outlets, and a consumer unit with MCBs. The earth electrode is a 1.2 m rod driven into clay soil, and the measured electrode resistance is 180 Ω. The supply is single-phase 230 V. The workshop has metal-clad distribution boards and steel conduit containing the circuits. The client wants to add a 7 kW electric vehicle charge point. You have a loop impedance tester and an RCD tester.
1. Calculate the prospective earth fault current for a direct contact fault to the earthed metalwork and assess whether the main RCD will operate within the required time.
Read the worked answer
For a TT system, the earth fault loop impedance Zs is dominated by the electrode resistance, approximately 180 Ω (plus supply and circuit impedances, which are small in comparison).
Prospective earth fault current I = U0 / Zs = 230 / 180 ≈ 1.28 A.
This is far below the tripping current of a 100 mA RCD (0.1 A), so the RCD will operate, but the fault current is small and the disconnection time depends on the RCD's time-current characteristic. A 100 mA time-delayed RCD will trip at 100 mA within its rated time, but the 1.28 A fault current is only about 13 times the rated residual current, so disconnection may be slow. For a TT system, the requirement is that the product of the RCD rated residual operating current and the earth electrode resistance does not exceed 50 V (RA × IΔn ≤ 50 V). Here 180 × 0.1 = 18 V, which is satisfactory. However, the 30 mA RCD for sockets gives 180 × 0.03 = 5.4 V, also satisfactory. The main concern is that the electrode resistance is high and should be improved if possible.
2. Evaluate whether the existing earthing arrangement is adequate for the proposed EV charge point and state any additional requirements.
Read the worked answer
An EV charge point requires a reliable earth connection, and for a TT system the electrode resistance must be low enough to ensure RCD operation. The existing 180 Ω is acceptable for RCD protection under the 50 V rule, but EV charging adds continuous loads and requires a 30 mA RCD (Type A or B depending on the charger) for the charge point circuit. The charge point must have its own 30 mA RCD protection, and the earthing system must be verified.
Additional requirements:
- The EV charge point circuit must be protected by a 30 mA RCD, typically Type A, and the charger may require Type B if it has DC leakage detection.
- The earth electrode resistance should be improved where possible (e.g., additional rods, longer rod, or better soil contact) to reduce Zs and improve fault clearance.
- The main protective bonding of the steel conduit and metal-clad boards must be verified, as they are extraneous conductive parts.
- The charge point must be supplied from a dedicated circuit with appropriate overcurrent protection and cable sizing for the 7 kW load (≈30 A at 230 V).
The existing arrangement is marginal and should be upgraded before adding the EV point.
3. Describe the test sequence you would use to verify the TT installation and the RCD operation, including the values you would expect.
Read the worked answer
- Earth electrode resistance test using a fall-of-potential or loop impedance tester: confirm RA ≈ 180 Ω and record.
- External earth fault loop impedance (Ze) measurement at the origin: for TT, Ze is essentially the electrode resistance plus supply, so expect ≈180 Ω.
- RCD tests on each RCD:
- 50% rated residual current (no trip).
- 100% rated residual current (trip within rated time, typically ≤300 ms for general, ≤40 ms for 30 mA).
- 5× rated residual current (trip within 40 ms for 30 mA).
- Record operating times and compare with BS 7671 requirements.
- Insulation resistance test on circuits (≥1 MΩ at 500 V).
- Polarity and continuity of protective conductors.
- Functional testing of the EV charge point and socket RCDs.
Expected values: RA ≤ 200 Ω for a 100 mA RCD under the 50 V rule (RA × IΔn ≤ 50 V gives RA ≤ 500 Ω for 100 mA, so 180 Ω is acceptable). For the 30 mA RCD, RA ≤ 1667 Ω, so 180 Ω is acceptable. Record all results and compare with BS 7671.