ETO ETO Exam papers 📅 Jan 2024

Exam Question

(a) Describe the normal criteria used for setting thermal protection relays and their advantage compared to magnetic types. (6)

(b) The low-voltage release of an a.c. motor-starter consists of a solenoid into which an iron plunger is drawn against a spring. The resistance of the solenoid is 35Ω. When connected to a 220V, 50Hz, a.c. supply the current taken is at first 2A, and when the plunger is drawn into the "full-in" position the current falls to 0.7A. Calculate the inductance of the solenoid for both positions of the plunger and the maximum value of flux-linkages in weber-turns for the "full-in" position of the plunger. (10)

Reference Answer

(a) Thermal protection relays are designed for sustained overcurrent protection, typically set between 105-120% of the full load current with a time delay. They are not intended for momentary overcurrents.
The overcurrent setting depends on the following:
The electrical system's maximum continuous load capacity determines the relay settings to ensure the system operates without tripping under normal conditions.
The relay settings are influenced by the insulation class and its capacity to withstand elevated temperatures.

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