---
title: "Short note: Protective devices and sizing"
subject: "Basic Electrical and Electronics Engineering"
chapter: 4
key: "semester_1/basic_electrical_and_electronics_engineering:short-note-protective-devices-sizing"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/short-note-protective-devices-sizing
site: BCT Engineering
---

# Short note: Protective devices and sizing

Basic Electrical and Electronics Engineering · Chapter 4: Electrical Installations

- Asked in: 2081 Kartik (new course), 3 marks
- Video: [Electricians' Guide: Circuit Protective Device Selection](https://www.youtube.com/watch?v=y5mQPAHajQI) — eFIXX
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/short-note-protective-devices-sizing
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-4

## Question

Write a short note on Protective Device and their sizing.

## Short answer

Protective devices include fuses, MCBs, MCCBs, RCCBs/ELCBs and surge protectors; they disconnect faulty circuits automatically. Size them so the device rating In lies between the design current Ib and cable ampacity Iz, with breaking capacity above the prospective short-circuit current and correct voltage/curve; e.g. a 3 kW geyser at 230 V uses a 16 A MCB, 4 mm² cable and 30 mA RCCB.

## Answer

Protective devices disconnect a faulty circuit automatically to prevent damage to cables, appliances and persons. Their sizing must match the load and the cable.

**Common devices:**
1. **Fuse:** wire that melts at overload. Rated in amperes.
2. **MCB:** thermal-magnetic breaker for overload and short circuit; ratings 6, 10, 16, 20, 32, 40, 63 A.
3. **MCCB:** moulded-case breaker for higher currents (up to a few hundred amperes).
4. **RCCB/ELCB:** detects earth leakage (residual current, typically 30 mA for people) and trips quickly.
5. **Isolators and surge protectors** for switching and voltage surges.

**Sizing rules:**
- Rated current of the device $I_n$ must be at least the design (load) current $I_b$, and not more than the current-carrying capacity of the cable $I_z$:

$$
I_b\le I_n\le I_z
$$

- The breaking capacity must be greater than the prospective short-circuit current at the point of installation (e.g. 6 kA for a house).
- The rated voltage must match the system (230/400 V).
- Select the curve type: B for resistive loads, C for mixed/inductive, D for motors with high inrush current.
- For a motor, the fuse/breaker is chosen for about 1.5-2.5 times the full-load current to allow for starting.

Example: a geyser of 3 kW at 230 V draws 13 A, so a 16 A MCB with a 4 mm² cable is chosen, and a 30 mA RCCB is added.

## Check yourself

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

1. Which condition must the rated current I_n of a protective device satisfy?
   A. I_n <= I_b <= I_z
   B. I_z <= I_n <= I_b
   C. I_n >= I_z always
   D. I_b <= I_n <= I_z
   Answer: D. The device rating must be at least the load current I_b and not more than the cable capacity I_z. (Key point: Rating rule)

2. Which curve type suits motors with high inrush current?
   A. B
   B. D
   C. A
   D. None, motors need no curve
   Answer: B. B is for resistive loads, C for mixed/inductive, and D for motors with high starting current. (Key point: Curve selection)

3. For a 3 kW geyser at 230 V (about 13 A), what is chosen and what extra protection is added?
   A. 10 A MCB with 1.5 mm^2 cable
   B. 16 A MCB with 4 mm^2 cable and a 30 mA RCCB
   C. 63 A MCB with 4 mm^2 cable and no RCCB
   D. 32 A MCB with 1.5 mm^2 cable
   Answer: B. A 16 A MCB is just above 13 A and below the cable rating; the 30 mA RCCB protects people from earth leakage. (Key point: Geyser example and RCCB)

## Related in Basic Electrical and Electronics Engineering

- [Importance and types of protective devices](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Aprotective-devices-importance) — Basic Electrical and Electronics Engineering, chapter 4

## Related in other subjects

- [Memory devices; PAL vs PLA](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apal-vs-pla) — Digital Logic, chapter 4
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- [Digital device applications; frequency counter](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Adigital-devices-frequency-counter) — Digital Logic, chapter 11