Class 4 (4th Engineer) MET 📅 Dec 2022

Exam Question

(a) Describe the normal criteria used for setting thermal protection relays and their advantage compared to magnetic types.

(b) Calculate the r.m.s. value, the form factor and peak factor of a periodic voltage having the following values for equal time intervals changing suddenly from one value to next: 0, 5, 10, 20, 50, 60, 50, 20, 10, 5, 0, -5, -10, V etc. What would be the r.m.s. value of a sine wave having the same peak value?

Reference Answer

### (a) Thermal Protection Relays: Setting Criteria and Advantages
#### Concept and Setting Criteria
Thermal protection relays are designed to protect electrical equipment, primarily three-phase induction motors, from damage caused by overheating due to sustained electrical overloads or single-phasing conditions. Their operation is based on the heating effect of the current passing through the motor windings.
The normal criteria for setting these relays are as follows:

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