---
title: "Short note: Level detectors"
subject: "Switchgear and Protection"
chapter: 8
key: "semester_6/switchgear_and_protection:short-note-level-detectors"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/short-note-level-detectors
site: BCT Engineering
---

# Short note: Level detectors

Switchgear and Protection · Chapter 8: Static and Digital Relays

- Asked in: 2073 Magh, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/short-note-level-detectors
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8

## Question

Write a short note on level detectors.

## Short answer

A level detector is a comparator circuit that outputs a logic high when an input signal exceeds a set reference level. It uses a Schmitt trigger for hysteresis to prevent chatter, and serves as the pick-up element in static relays for overcurrent, voltage, and frequency protection.

## Answer

A **level detector** is a static relay circuit that compares an input signal with a fixed reference level and gives an output (logic 1) only when the input exceeds that level. It acts as the pick-up element of a static relay.

```text
            +Vcc
             │
 V_in ──►(+)╲
            │ ▷──── V_out → next stage
 V_ref ─►(−)╱   (high when V_in > V_ref)
             │
       (positive feedback R for
        hysteresis – Schmitt trigger)
```

- **Construction:** an op-amp (or transistor) comparator; V_ref set by a Zener diode or potentiometer, which fixes the relay setting.
- **Hysteresis:** positive feedback (Schmitt trigger) gives different pick-up and drop-off levels, so the output does not chatter when the input is near the threshold.
- **Input:** rectified and smoothed CT/PT signal, or the output of a comparator/timer.
- **Uses:** pick-up of overcurrent, over/under-voltage and frequency relays; detecting when a timer capacitor voltage reaches its set value; conditioning comparator outputs.
- **Requirements:** accurate, stable threshold with temperature; high reset ratio (≈ 0.95); fast response.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A level detector in a static relay gives an output (logic 1) when:
   A. The input falls to zero
   B. The input exceeds a fixed reference level
   C. Two inputs are in phase
   D. A capacitor is discharged
   Answer: B. It compares the input with a reference level and outputs logic 1 only when the input exceeds it, acting as the pick-up element. (Key point: Definition)

2. What does positive feedback (Schmitt trigger) provide in a level detector?
   A. Different pick-up and drop-off levels, avoiding output chatter
   B. A higher gain at the reference level only
   C. A reduced reset ratio
   D. A variable time delay
   Answer: A. Positive feedback gives hysteresis so the output does not chatter near the threshold. (Key point: Hysteresis)

3. How is the relay setting (pick-up level) fixed in a level detector?
   A. By the supply frequency
   B. By the CT ratio alone
   C. By the reference voltage from a Zener diode or potentiometer
   D. By the output relay coil resistance
   Answer: C. V_ref set by a Zener or potentiometer fixes the threshold, hence the relay setting. (Key point: Setting the relay)