A person received an electric shock at a commercial laundromat when they touched the metal rim of…
A person received an electric shock at a commercial laundromat when they touched the metal rim of a front loading washing machine during the wash cycle. Investigations revealed that the machine had a cracked spider bearing that caused the hub to be misaligned under load and rub through the wires connected to the internal 3-phase heater elements, as the primary cause of the fault. The heater elements were connected with star/delta capacity control (low/high), with 2 series elements per phase, with a total 3-phase capacity of 5.4kW, supplied from a 415V symmetrical 3-phase, 4-wire system with phase sequence (a-b-c). A number of RCD trips were detected prior to the person receiving the fatal shock, but static testing did not show any faults and the RCD disabled, as it was thought the heater elements inherent leakage was causing a false trip. (a) (60 marks) Draw a neatly labelled circuit diagram showing all voltages and fault paths. (b) (80 marks) Explain in your own words with the aid of a diagram, a logical sequence of events explaining how and when the dangerous voltage was present and what methods could have been used to detect/avoid these faults, protecting people from receiving such a shock. (c) (60 marks) For Vsn =120V /+45°, taking Van as reference, draw a neatly labelled phasor diagram for all of the above systems voltages. (N.B. on graph paper)
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