---
title: "Fuse time-current factors and coordination"
subject: "Switchgear and Protection"
chapter: 3
key: "semester_6/switchgear_and_protection:fuse-tcc-factors-coordination"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-tcc-factors-coordination
site: BCT Engineering
---

# Fuse time-current factors and coordination

Switchgear and Protection · Chapter 3: Fuses

- Asked in: 2073 Magh, 6 marks
- Video: [Fuses Explained](https://www.youtube.com/watch?v=ot-_PJEja7c) — Eaton
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-tcc-factors-coordination
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3

## Question

What factors govern the time-current characteristics of a fuse link? Explain what is meant by "fuse coordination".

## Short answer

Time-current characteristic depends on element material, cross-section/notches, length, M-effect, filler, ambient temperature, and circuit voltage/power factor. Fuse coordination selects series fuses so only the branch fuse nearest a fault blows; conditions are non-crossing T-I curves, minor fuse total I²t below major pre-arcing I²t, and about 1.6:1 to 2:1 rating ratio for gG fuses.

## Answer

#### Factors governing time–current characteristic of a fuse link
The time–current characteristic shows the operating time of a fuse for various currents (inverse curve). It depends on:
1. **Element material:** melting point, resistivity, specific heat and thermal conductivity (silver, copper, tin, zinc, lead). Silver gives stable, predictable operation.
2. **Cross-section and shape of element:** thinner sections melt faster; **notches (reduced sections)** concentrate heating and decide pre-arcing time.
3. **Length of element:** a longer element has more resistance and less heat lost to end caps.
4. **Number of parallel strips:** affects current sharing and speed.
5. **M-effect (tin/solder bead):** lowers melting temperature at overload, giving time delay at small overloads.
6. **Filler and enclosure:** silica sand, ceramic body and its thermal conductivity determine heat dissipation and arcing time.
7. **Ambient temperature and previous loading:** a hot fuse operates faster.
8. **Contact resistance and mounting:** poor contacts add heat.
9. **Circuit voltage and power factor:** affect the arcing time (and thus total operating time) for high currents.
10. **Ageing / oxidation** of the element in non-HRC fuses.

#### Fuse coordination (discrimination)
**Fuse coordination** means selecting fuses in series (e.g. main fuse upstream, branch fuse downstream) so that for any fault on a branch, **only the branch fuse nearest the fault blows** and the upstream (major) fuse remains intact, keeping other branches in service.
```text
 Supply ──[F1 major 200 A]──┬──[F2 minor 63 A]── load 1
                            └──[F3 minor 63 A]── load 2
 Fault on load 1 → only F2 should blow
```
**Conditions:**
- The T–I curves of the two fuses must not intersect over the whole range of fault current.
- For short-time (high) faults, the **total I²t of the minor fuse must be less than the pre-arcing I²t of the major fuse**.
- A rating ratio of about **1.6:1 to 2:1** between successive gG fuses usually ensures discrimination (as per manufacturer tables).

Coordination is also needed between a fuse and a circuit breaker or overcurrent relay (fuse curve below the relay curve with a time margin).

## Check yourself

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

1. Which affects T-I characteristic?
   A. Element material, cross-section/notches, length, filler, ambient temperature and contacts
   B. Colour of body only
   C. Voltage rating only
   D. Position in panel
   Answer: A. Notches concentrate heating and decide pre-arcing time. (Key point: Element factors)

2. What does fuse coordination mean?
   A. Both blow together
   B. Only the branch fuse nearest the fault blows; the upstream fuse stays intact
   C. Main blows first
   D. Fuses are connected in parallel
   Answer: B. Keeps other branches in service. (Key point: Coordination)

3. Which condition ensures coordination at high faults?
   A. Major fuse smaller
   B. Curves intersect
   C. Total I2t of minor fuse < pre-arcing I2t of major fuse
   D. Equal ratings
   Answer: C. Also T-I curves must not cross. (Key point: Condition)

## Related in Switchgear and Protection

- [Cut-off and time-current characteristics of fuse](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-cutoff-time-current) — Switchgear and Protection, chapter 3
- [Prospective current, cut-off current, time delay fuse](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprospective-cutoff-time-delay-fuse) — Switchgear and Protection, chapter 3
- [HRC fuse operating characteristics and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-operating-characteristics) — Switchgear and Protection, chapter 3
- [Fuse vs MCB; HRC fuse time terms](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-vs-mcb-hrc-terms) — Switchgear and Protection, chapter 3
- [HRC fuse I²t and fuses in series](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-i2t-series) — Switchgear and Protection, chapter 3
- [Time-current grading for relays X, Y, Z](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-grading-xyz) — Switchgear and Protection, chapter 7

## Related in other subjects

- [Cut-off current characteristics of HRC fuse](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Ahrc-cutoff-characteristics) — Industrial Power Distribution and Illumination, chapter 3
- [Factors for residual soil characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aresidual-soil-characteristics-factors) — Soil Mechanics, chapter 1
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