Workshop Circuit Tripping and Earthing Check
A small electronics workshop has a 230 V single-phase supply feeding a 3 kW geyser, a 1 kW lighting load and several soldering irons. The geyser circuit uses 2.5 mm² copper cable, 25 m long, protected by a 16 A MCB. The MCB trips after about 20 minutes of geyser operation. A clamp meter reads 15.8 A. The ambient temperature near the distribution board is 42 °C. The measured earth electrode resistance is 8 Ω. The geyser element insulation resistance is 20 MΩ. The cable voltage drop is measured as 7 V at 15.8 A. The owner wants to replace the 16 A MCB with a 32 A MCB to stop the tripping.
1. Calculate the load current of the 3 kW geyser at 230 V and assess whether the 16 A MCB is correctly sized, allowing for ambient derating.
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The geyser current is I = P/V = 3000/230 = 13.0 A. The measured 15.8 A includes the geyser plus other workshop load on the same circuit, or the element is drawing slightly high current. A 16 A MCB is nominally adequate for 13 A, but at 42 °C its thermal trip point is derated; a typical factor of about 0.9 gives an effective 14.4 A, so a steady 15.8 A will eventually trip it. The MCB is therefore not necessarily faulty, but the circuit is overloaded or the MCB is under-rated for the ambient. A 20 A MCB may be acceptable only if the 2.5 mm² cable's corrected current rating and the voltage drop are adequate; a 32 A MCB is not acceptable because it would not protect the cable.
2. Evaluate the 7 V voltage drop and the 8 Ω earth resistance against typical safety limits.
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The voltage drop is 7/230 × 100 = 3.0%, which exceeds the usual 2.5% recommendation for a final circuit. This indicates the 25 m run is long or the conductor is marginal; the geyser will run at reduced voltage and the cable will waste energy. The earth resistance of 8 Ω is high for a workshop. It may not allow enough fault current to trip the MCB quickly, and touch voltage on the metal enclosure could rise. A 30 mA RCD is required for additional protection, and the earth electrode and bonding should be improved, typically to a few ohms or less depending on the supply system. The 20 MΩ element insulation is good, so the tripping is not caused by an element-to-earth leak.
3. Recommend a safe corrective plan and explain why the proposed 32 A MCB is dangerous.
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First separate the geyser circuit from the lighting and soldering-irons circuit, and calculate the actual diversified load. Use a 20 A MCB only if the cable's derated capacity exceeds 20 A and the voltage drop is corrected, preferably by using 4 mm² cable for the 25 m run. Improve the earth electrode and bonding, and install a 30 mA RCD. Verify the MCB's breaking capacity and ambient derating. A 32 A MCB is dangerous because it allows up to 32 A to flow in a 2.5 mm² cable that may be rated around 24 A; the cable could overheat before the MCB trips, causing insulation damage or fire. Never up-rate a protective device to stop nuisance tripping without checking cable capacity, load, and fault-loop impedance.