Back to questionChoose “Save as PDF” in the print window.

SarkariResultOutAdditional Assistant Engineer (Electrical) · 2026

Previous Year Question

In a Buchholz relay, what does the mercury switch attached to the upper float do?
  1. It closes the alarm circuit when the upper float rises
  2. It closes the alarm circuit when the oil level drops
  3. It is responsible for activating the main circuit breaker
  4. It opens the alarm circuit when the oil level rises

Correct answer: B

Explanation

Slow internal faults generate gas that accumulates in the Buchholz chamber and displaces oil, causing the upper float to fall. The upper-float mercury switch then closes the alarm circuit; severe oil surge operates the trip element.

Step-by-step solution

Upper Float Alarm explanatory diagram

Correct Answer

B — It closes the alarm circuit when the oil level drops

Explanation

Slow internal faults generate gas that accumulates in the Buchholz chamber and displaces oil, causing the upper float to fall. The upper-float mercury switch then closes the alarm circuit; severe oil surge operates the trip element.

Key Concept

Upper Float Alarm

Exam Point

Buchholz: upper float = alarm; lower/surge element = trip.

Common Mistake

Alarm operation is associated with the float falling as gas accumulates, not rising.

Common mistake

Alarm operation is associated with the float falling as gas accumulates, not rising.

Exam tip

Buchholz: upper float = alarm; lower/surge element = trip.

Quick method

Buchholz: upper float = alarm; lower/surge element = trip.