TT System Motor Fault and RCD Disconnection
A small metalworking workshop is supplied from a TT system at 400/230 V. The main board has a 63 A three-phase incoming breaker and a 300 mA type AC RCD at the origin. A 15 kW, 400 V three-phase motor is fed by a 63 A C-curve MCB and a 4 mm² cable. The motor frame is connected to a local earth electrode with resistance Ra = 8 Ω. The measured earth-fault loop impedance Zs at the motor terminals is 8.5 Ω. The motor circuit has no additional RCD. During a phase-to-frame fault, the motor MCB does not trip immediately, and the operator receives a shock from the frame before the origin RCD operates. The owner asks whether the 63 A MCB alone provides the required disconnection and whether a 30 mA RCD should be added to the motor circuit.
1. Calculate the prospective earth-fault current and the touch voltage on the frame before the RCD operates. Show why the 63 A C-curve MCB cannot be relied on.
Read the worked answer
The prospective earth-fault current is If = U0 / Zs = 230 V / 8.5 Ω ≈ 27.1 A. The touch voltage on the frame, using the local electrode resistance, is Uc = If × Ra = 27.1 × 8 ≈ 217 V, which is lethal. A 63 A C-curve MCB has a magnetic trip threshold of about 10 × In = 630 A. The 27.1 A fault current is far below that, so instantaneous magnetic tripping will not occur. The thermal element may eventually trip, but the disconnection time is not predictable and is far too long for a 230 V touch voltage. The MCB alone therefore does not satisfy the disconnection requirement in this TT system; the fault must be cleared by an RCD or by a suitably low electrode resistance that gives a much higher fault current.
2. Determine the maximum RCD rated residual current permitted for this TT electrode using the 50 V touch-voltage criterion. Is the existing 300 mA origin RCD acceptable, and what type/rating would you specify for the motor circuit?
Read the worked answer
For a TT system, the touch-voltage criterion is IΔn ≤ 50 V / Ra. Here IΔn ≤ 50 / 8 = 6.25 A. The existing 300 mA RCD is therefore well within the permitted residual current for touch voltage: 0.3 A × 8 Ω = 2.4 V, below 50 V. At the 27.1 A fault current, the RCD sees about 90 times its rated residual current and should trip quickly, well within the required disconnection time. The origin RCD can provide the TT disconnection function, provided it is tested and not defeated by DC leakage. For the motor circuit, a 30 mA RCD is not required by the touch-voltage calculation and may nuisance trip because of motor leakage and cable capacitance. A 300 mA type A device, or a type B if the motor is fed through a frequency converter, is more appropriate; verify selectivity and trip time.
3. Explain why fitting a 30 mA RCD on this motor may cause problems and what else must be checked before energizing.
Read the worked answer
A 30 mA RCD on a 15 kW motor can trip on normal leakage current from the motor windings, long cable capacitance, moisture, or starting transients. The result may be repeated nuisance tripping, tempting someone to bypass the device, which is unacceptable. Before energizing, verify the electrode resistance and the main protective bonding, confirm the RCD trip time at its rated and five-times rated residual current, check motor insulation resistance and cable insulation, and confirm the fault loop path. Also check that the 4 mm² cable is adequate for the motor current and starting duty, and that the 63 A MCB protects the cable. If the motor is inverter-fed, the RCD type must be suitable for the DC and high-frequency leakage present.