(c) A single phase motor circuit is protected by a 63 A circuit breaker. A fault occurs and causes a current of 98 A to

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C A Single Phase Motor Circuit Is Protected By A 63 A Circuit Breaker A Fault Occurs And Causes A Current Of 98 A To 1 (75.08 KiB) Viewed 66 times

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C A Single Phase Motor Circuit Is Protected By A 63 A Circuit Breaker A Fault Occurs And Causes A Current Of 98 A To 2 (87.7 KiB) Viewed 66 times

(c) A single phase motor circuit is protected by a 63 A circuit breaker. A fault occurs and causes a current of 98 A to flow through the earth continuity path. Due to the poor contact of lock nut and bush connecting a steel conduit to a metal box, the resistance of this conduit connection alone is 0.5Ω. Regulation D22 regarding the basic earthing requirements is given in Table Q2(c). (i) Predict whether the protective device will be damaged. (3 marks) (ii) Estimate the amount of heat produced at the metal box. (2 marks) (iii) Suggest the degree of risk of a fire developing. (1 mark)
Table Q2(c) Regulation D22 (Basic Earthing Requirements) States that earth leakage protection may be provided by means of fuses or excess current circuit breakers if the earth fault current available to operate the protective device and so make the faulty circuit dead exceeds: 1. 3 times the current rating of any semi enclosed fuse or any cartridge fuse having a fusing factor exceeding 1.5, used to protect the circuit, or 2. 2.4 times the rating of any cartridge fuse having a fusing factor not exceeding 1.5, used to protect the circuit, or 3. 1.5 times the tripping current of any excess current circuit breaker used to protect the circuit.