Temporary Construction Supply for a Circular Saw
A construction site office and workshop are fed from a 400/230 V TN-S temporary supply. The main temporary board has a 100 mA RCD at the origin and 30 mA RCDs on the socket-outlet circuits. A 3.0 kW 230 V circular saw is connected by a 75 m H07RN-F 4 mm² cable to a 16 A socket outlet. The cable runs along the ground through a doorway and is occasionally driven over. The saw is used intermittently, and the operator reports that it starts slowly and the cable is warm after long cuts. The site manager wants to know whether the cable and protection are adequate, and whether the RCD arrangement is acceptable for a construction site. The supply voltage at the board is 230 V; the saw has a power factor of 0.85 and efficiency of 0.88.
1. Calculate the saw current and the voltage drop in the 75 m 4 mm² cable. Use mV/A/m ≈ 11 for 4 mm² two-core. State whether the cable is acceptable for starting and running.
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The saw input current is I = P / (U × cos φ × η) = 3000 / (230 × 0.85 × 0.88) ≈ 17.4 A. A 16 A socket outlet and 16 A protection are therefore too small; the saw is already overloaded at full load. The voltage drop in the 4 mm² cable is approximately 17.4 × 75 × 11 / 1000 = 14.4 V, or 6.3% of 230 V, which exceeds the usual 5% limit for a final circuit. The 4 mm² conductor may have enough thermal ampacity in free air, but it is not acceptable here because the voltage drop is excessive and the 16 A socket is undersized. Use a 32 A industrial socket, a 20 A MCB, and 6 mm² cable: the drop becomes about 17.4 × 75 × 7.3 / 1000 = 9.5 V, or 4.1%, which is acceptable.
2. Assess the RCD arrangement and cable protection. What must be changed before the saw is used?
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A construction-site socket outlet up to 32 A requires 30 mA RCD protection; the 30 mA socket RCD is therefore correct in principle. The 100 mA RCD at the origin is not a substitute for the 30 mA protection, and it may not be selective with the downstream device. The 4 mm² H07RN-F cable is mechanically vulnerable: it lies on the ground, passes through a doorway, and is driven over. It must be raised, protected in conduit or cable ramp, or rerouted so that it is not subject to damage. Before use, replace the 16 A socket with a 32 A socket, fit a 20 A MCB appropriate to the 6 mm² cable, test the 30 mA RCD, and verify the cable and plug for damage. Do not bypass the RCD because of nuisance tripping.
3. If the 30 mA RCD trips repeatedly when the saw starts, what is the likely cause and what is the correct action?
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Repeated tripping at start suggests leakage from the saw motor, moisture in the plug or cable, damaged cable insulation, or excessive cable capacitance on the long run. It could also be a genuinely faulty RCD. The correct action is to isolate and test: inspect the cable and plug for damage, measure insulation resistance of the saw and cable, test the RCD trip time, and try a shorter or better-protected cable. Do not increase the RCD rating or bypass it. If the saw itself has high protective-conductor leakage, have it repaired or replace it. The temporary supply must remain protected by a 30 mA RCD for the socket circuit, and the cable must be protected from mechanical damage.