---
title: "Static vs electromagnetic relays; components"
subject: "Switchgear and Protection"
chapter: 8
key: "semester_6/switchgear_and_protection:static-vs-electromagnetic-relays"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/static-vs-electromagnetic-relays
site: BCT Engineering
---

# Static vs electromagnetic relays; components

Switchgear and Protection · Chapter 8: Static and Digital Relays

- Asked in: 2070 Magh, 8 marks; 2074 Bhadra, 8 marks; 2079 Chaitra, 8 marks; 2080 Chaitra, 8 marks (asked 4 times)
- Video: [Static relay and electromagnetic relay](https://www.youtube.com/watch?v=dumIqqsvI7s) — MADHURA Institute
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/static-vs-electromagnetic-relays
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8

## Question

Discuss the advantages and limitations of static relays over electromagnetic relays. Also explain the main components of a static relay and their role with the help of block diagram.

## Short answer

Static relays offer low CT/PT burden (≈0.05 VA), fast operation (≈1 cycle), no moving parts, flexible characteristics. Limitations: sensitive to transients and temperature, need auxiliary DC, low overload capacity. Main blocks: input scaling, rectifier, measuring unit (comparator/timer), amplifier, output relay/thyristor, DC supply.

## Answer

**Static relays** use solid-state electronic components (transistors, diodes, ICs) for measurement and decision, unlike **electromagnetic relays** that use moving armatures, discs or cups.

#### Advantages of static relays over electromagnetic relays

| Feature | Static relay | Electromagnetic relay |
|---|---|---|
| Burden on CT/PT | Very low (≈ 0.05 VA) | High (2–10 VA) |
| Moving parts | None in measuring unit | Disc, armature, bearings |
| Speed | Fast, about 1 cycle | Slower, several cycles |
| Reset/overshoot | Quick reset, no overshoot | Overshoot of disc |
| Characteristics | Any shape (mho, quad, etc.) | Limited by construction |
| Maintenance | Little | Regular cleaning, adjustment |
| Size | Compact; multi-function | Bulky |
| Shock/vibration | Not affected | Can mal-operate |

Other advantages: high sensitivity (amplifiers), good accuracy, easy to combine several functions, suitable for fast schemes like carrier and distance protection.

#### Limitations

- Sensitive to **voltage spikes/transients** – need surge suppressors and screening.
- Characteristics drift with **temperature**; components age.
- Need an **auxiliary DC supply**.
- Low **overload capacity** of semiconductor devices.
- Expensive for simple single-function relays; needs skilled staff for repair.
- Affected by electrostatic discharge and electromagnetic interference.

#### Main components of a static relay

```text
CT/PT→[Input/aux CT]→[Rectifier/smoothing]
      →[Measuring unit: comparator,
         level detector, timer]
      →[Amplifier]→[Output relay/SCR]→Trip coil
               ▲
        [Aux DC supply]
```

1. **Input circuit:** auxiliary CTs/PTs, transactors; scale down signals, isolate electronics, filter surges.
2. **Rectifier/smoothing:** converts AC to DC or square waves for comparison.
3. **Measuring unit:** comparators (amplitude/phase), level detectors, timing circuits – decides if a fault exists.
4. **Amplifier:** strengthens the weak decision signal.
5. **Output unit:** auxiliary relay or thyristor energises the CB trip coil.
6. **Auxiliary supply:** stabilised DC for the electronics from station battery.

## Check yourself

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

1. How does the CT/PT burden of a static relay compare with an electromagnetic relay?
   A. Much lower (about 0.05 VA vs 2-10 VA)
   B. Much higher
   C. Identical
   D. Zero for both
   Answer: A. Static relays draw very little from CTs/PTs, so smaller CTs can be used. (Key point: Burden)

2. Which is true of static relay speed?
   A. Slow, with disc overshoot
   B. Fast (about 1 cycle) with quick reset and no overshoot
   C. Always slower than electromagnetic
   D. Needs several cycles to start
   Answer: B. No moving mass means fast operation and no overshoot. (Key point: Speed and reset)

3. Which is a limitation of static relays?
   A. Affected by vibration
   B. Sensitive to voltage transients, temperature drift and need an auxiliary DC supply
   C. High burden on CTs
   D. Cannot shape characteristics
   Answer: B. Surges, temperature drift, ageing and the need for DC supply are the main drawbacks. (Key point: Limitations)

4. In the static relay block diagram, what energises the CB trip coil?
   A. The input aux CT
   B. The rectifier
   C. The output unit (auxiliary relay or thyristor)
   D. The level detector directly
   Answer: C. The output relay or SCR is driven by the amplifier and energises the trip coil. (Key point: Output block)

## Related in Switchgear and Protection

- [Essential components of a static relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-relay-essential-components) — Switchgear and Protection, chapter 8, asked 5×
- [Types of electromagnetic relays and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aelectromagnetic-relay-types) — Switchgear and Protection, chapter 7, asked 2×
- [Directional static overcurrent relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adirectional-static-overcurrent) — Switchgear and Protection, chapter 8, asked 2×
- [Static MHO relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-mho-relay) — Switchgear and Protection, chapter 8
- [Static differential relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-relay) — Switchgear and Protection, chapter 8, asked 3×
- [Static distance relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-distance-relay) — Switchgear and Protection, chapter 8, asked 3×

## Related in other subjects

- [Magnetic material for electromagnetic relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Arelay-magnetic-material) — Electrical Engineering Material, chapter 4
- [How symmetrical components help analysis](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Asymmetrical-components-unbalance-help) — Power System Analysis II, chapter 4
- [OFC block diagram vs cable](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-block-diagram-vs-cable) — Propagation and Antenna, chapter 6
- [OFC block diagram vs microwave link](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-block-diagram-vs-microwave) — Propagation and Antenna, chapter 6