---
title: "Selecting CB for 6.6 kV, 650 MVA"
subject: "Switchgear and Protection"
chapter: 6
key: "semester_6/switchgear_and_protection:select-cb-6-6kv-650mva"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/select-cb-6-6kv-650mva
site: BCT Engineering
---

# Selecting CB for 6.6 kV, 650 MVA

Switchgear and Protection · Chapter 6: Circuit Breakers

- Asked in: 2071 Magh, 8 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/select-cb-6-6kv-650mva
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6

## Question

Select a circuit breaker for the following application: 6.6 kV, 650 MVA (breaking capacity) breaker. It is supposed to be connected to primary of 6600/433 V, 2000 kVA transformer. Justify your choice and prepare the specifications for the breaker.

## Short answer

Rated current is 175 A (standard 630 A), breaking current 56.9 kA, making current 145 kA. Specify a 7.2 kV, 630 A, 63 kA vacuum circuit breaker.

## Answer

The breaker controls the 6.6 kV primary of a 2000 kVA, 6600/433 V distribution transformer and must interrupt 650 MVA at 6.6 kV.

#### Calculations

```text
Rated (normal) current:
I_n = 2000 × 10³ / (√3 × 6600) = 174.95 A

Symmetrical breaking current:
I_b = 650 × 10⁶ / (√3 × 6600) = 56 860 A ≈ 56.86 kA

Making current (peak):
I_m = 2.55 × I_b = 2.55 × 56.86 = 144.99 kA ≈ 145 kA

Short-time current (1 s or 3 s) = I_b ≈ 56.86 kA
```

#### Choice of breaker and justification
A **vacuum circuit breaker** (indoor, withdrawable type) is chosen:
- 6.6 kV is in the medium voltage range, where vacuum breakers are standard and have the lowest lifetime cost.
- Vacuum gives fast recovery of dielectric strength, very low contact erosion and long life (10,000+ operations) with almost no maintenance.
- No fire hazard (no oil) and no gas handling (unlike SF6), so it suits indoor switchgear near the transformer.
- Compact size; suitable for transformer switching. An SF6 breaker of the same rating is an acceptable alternative; older practice would have used a minimum oil breaker.
- Transformer magnetising current is small; modern vacuum interrupters have a low chopping current, and surge arresters at the transformer terminals take care of switching surges.

#### Specification of the breaker (as per IEC 62271-100 standard values)

| Parameter | Value |
|---|---|
| Type | Indoor, 3-pole vacuum CB, withdrawable |
| Rated voltage | 7.2 kV (system 6.6 kV) |
| Rated frequency | 50 Hz |
| Rated normal current | 630 A (required 175 A) |
| Rated short-circuit breaking current | 63 kA symmetrical (required 56.86 kA) |
| Rated breaking capacity | ≥ 650 MVA at 6.6 kV |
| Rated making current | 157.5 kA peak (2.5 × 63 kA; required 145 kA) |
| Rated short-time withstand current | 63 kA for 3 s |
| Rated operating sequence | O – 0.3 s – CO – 3 min – CO |
| Power frequency withstand | 20 kV rms for 1 min |
| Lightning impulse withstand | 60 kV peak |
| Opening / break time | about 3 cycles (60 ms) |
| Operating mechanism | Motor-wound spring, 110 V DC trip and close coils |
| Protection | IDMT overcurrent + earth fault relay through CTs 200/5 A |

**Answer:** rated current 174.95 A (630 A standard), breaking current 56.86 kA, making current 145 kA; a 7.2 kV, 630 A, 63 kA vacuum circuit breaker is specified.

## Check yourself

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

1. What is the normal current of the 2000 kVA, 6.6 kV transformer primary?
   A. 303 A
   B. 175 A
   C. 87 A
   D. 350 A
   Answer: B. I = 2000x10^3/(sqrt(3) x 6600) = 174.95 A. (Key point: Rated current)

2. How is the symmetrical breaking current obtained from 650 MVA at 6.6 kV?
   A. 650 MVA / 6.6 kV
   B. 650 MVA x sqrt(3) x 6.6 kV
   C. 650 MVA / (sqrt(3) x 6.6 kV) = 56.86 kA
   D. 650 MVA / (3 x 6.6 kV)
   Answer: C. MVA = sqrt(3) V I, so I = 650x10^6/(sqrt(3) x 6600) = 56.86 kA. (Key point: Breaking current)

3. What making current is required for the breaker?
   A. 56.86 kA
   B. About 145 kA peak (2.55 x breaking current)
   C. About 28 kA
   D. About 22 kA
   Answer: B. Making current is 2.55 times the symmetrical breaking current: 2.55 x 56.86 = about 145 kA. (Key point: Making current)

4. Why is a vacuum circuit breaker chosen at 6.6 kV?
   A. It needs a compressor plant
   B. It has high contact erosion
   C. It is the only type that works at MV
   D. It is standard at MV, compact, fire-free, with long life and little maintenance
   Answer: D. Vacuum breakers offer fast recovery, low erosion, 10,000+ operations and no oil or gas handling. (Key point: Choice of breaker)

## Related in Switchgear and Protection

- [Breaking MVA and making kA (800 A, 33 kV)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-breaking-making-800a) — Switchgear and Protection, chapter 6
- [Specifying breaker on 132/66 kV transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abreaker-x-specification) — Switchgear and Protection, chapter 6
- [CB ratings: 2500 A, 1500 MVA, 33 kV](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-2500a-1500mva) — Switchgear and Protection, chapter 6
- [CB ratings: 33 kV, 1500 A, 3000 MVA](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-1500a-3000mva) — Switchgear and Protection, chapter 6
- [Breaking, making, short-time ratings (500 MVA)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-500a-500mva) — Switchgear and Protection, chapter 6
- [CB terms: making, breaking, operating duty](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-terms-making-breaking-duty) — Switchgear and Protection, chapter 6

## Related in other subjects

- [Design: 25 kVA 11000/433 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-25kva-11000-433) — Electric Machine Design, chapter 3
- [Short circuit MVA of feeder circuit breaker](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afeeder-cb-short-circuit-mva) — Power System Analysis II, chapter 3, asked 2×
- [Design: 100 kVA 4000/433 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-100kva-4000-433) — Electric Machine Design, chapter 3, asked 4×
- [Design: 150 kVA 6600/400 V distribution transformer](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adesign-150kva-6600-400-regulation) — Electric Machine Design, chapter 3, asked 5×