The Shock at the Water Pump
At a farm pump shed near Erode, a 1.5 kW, 230 V single-phase motor is fed through 40 m of 1.5 mm² two-core copper cable protected by a 15 A fuse. The motor frame is not connected to earth. The operator complains of a tingling shock when he touches the frame, especially when the pump is running; a test lamp connected between the frame and a nearby earth pipe glows. The motor still runs and draws about 8 A. The owner wants to replace the 15 A fuse with a piece of thick copper wire because the fuse sometimes blows at start-up, and to continue using the pump until the end of the season. A multimeter shows low insulation resistance between one live terminal and the frame.
1. Explain how the frame becomes live and why the test lamp glows. Calculate the running current from the rated data, assuming a power factor of 0.8, and comment on whether the 1.5 mm² cable is suitable for the running load.
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The low insulation resistance between a live terminal and the frame means the winding insulation has failed. The live conductor is now connected, through that fault resistance, to the metal frame. Since the frame is not earthed, it rises to a potential close to the supply voltage. A person touching the frame and standing on damp ground provides a path through his body to earth, so current flows and he receives a shock. The test lamp glows because there is a voltage between the frame and the earth pipe; the lamp completes the circuit from the live frame to earth. Running current: I = P/(V × power factor) = 1500/(230 × 0.8) = 8.15 A, which agrees with the measured 8 A. For a 40 m run, the two-core loop resistance is R = ρ(2L)/A = 0.0175 × 80/1.5 = 0.933 Ω. The voltage drop at 8.15 A is about 7.6 V, or 3.3% of 230 V, which is acceptable for running but marginal when the motor starts and draws five to six times rated current. The cable is therefore adequate for running current but the installation is unsafe because it has no protective earth conductor and no residual-current device.
2. The owner proposes replacing the fuse with thick copper wire. Analyse the consequences and explain the correct protective measures for this installation.
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A fuse is the weakest link in the circuit and is chosen to melt before the cable or equipment is damaged. Replacing it with thick copper wire removes that protection entirely. If a short circuit or earth fault occurs, the copper wire will not melt; the fault current will overheat the cable, damage insulation and may start a fire. The motor winding can also burn out without the supply being disconnected. The fuse blowing at start-up is a symptom to investigate, not to defeat: the motor's starting current may be close to the fuse rating, the cable may be too long, or the motor may have a winding fault. Correct measures are: isolate the supply and take the pump out of service; replace the damaged motor or winding; provide a proper protective earth conductor using three-core cable and connect the frame to an earth electrode of low resistance; install a 30 mA RCCB (ELCB) on the circuit; use a correctly rated motor protective device, such as a 16 A type C MCB or a motor starter with overload protection; and have the work done by a licensed electrician and tested before reuse.
3. Explain why earthing alone may not be enough to protect the operator and why an RCCB is recommended in addition. Include the idea of earth resistance in your answer.
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Earthing connects the metal frame to an earth electrode so that, during an insulation fault, current flows to earth and the frame is held near earth potential. For the fuse or MCB to clear the fault quickly, the fault current must be high enough. That depends on the earth resistance and the supply loop impedance. In a rural installation the earth electrode resistance may be several ohms or more, so the fault current may be too small to blow the fuse promptly. The frame can then remain at a dangerous voltage for a long time, and a person touching it still receives a shock. An RCCB continuously compares the current in the live and neutral conductors. If the difference, which is the current leaking to earth, exceeds 30 mA, it trips within milliseconds. It therefore protects people even when the earth resistance is too high to operate the fuse. Earthing and RCCB protection are complementary: earthing limits the frame potential and provides a fault path, while the RCCB detects the imbalance and disconnects the supply quickly.