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Class 4 (4th Engineer) MET 📅 Sep 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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