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Class 4 (4th Engineer) MET 📅 Apr 2024

Exam Question

With reference to the protection of electric motors explain EACH of the following:

(a) Fuse back up protection (6)

(b) How a motor fitted with fuse back up protection may exceed its rated temperature without being tripped by the primary protection (6)

(c) The value of current rating at which the over current relay should be set (4)

Reference Answer

### (a) Fuse Back Up Protection
Fuse back-up protection is a coordinated electrical protection scheme for motors that utilizes two distinct devices to provide comprehensive coverage against the full spectrum of overcurrent conditions, from mild sustained overloads to high-magnitude short-circuit faults.
The system comprises two key components whose time-current characteristics are carefully selected to ensure correct discrimination:
1. **Primary Overload Protection (Overcurrent Relay):**

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