---
title: "Isolator operation with CB in HV installation"
subject: "Switchgear and Protection"
chapter: 4
key: "semester_6/switchgear_and_protection:isolator-sld-cb-operation"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-sld-cb-operation
site: BCT Engineering
---

# Isolator operation with CB in HV installation

Switchgear and Protection · Chapter 4: Isolators and Contactors

- Asked in: 2075 Baisakh, 2+4 marks
- Video: [EHV ISOLATOR GENERAL ARRANGEMENT, EARTH SWITCH, OPERATION AND SYMBOL IN SLD|HOW ISOLATOR OPERATES](https://www.youtube.com/watch?v=JxESvo2EmXY) — ELECTRICAL TECHNOLOGY and INDUSTRIAL PRACTICE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-sld-cb-operation
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4

## Question

What is isolator? With the single line diagram, explain the working of isolator with consideration of closing and opening of circuit breaker kept in high voltage installation.

## Short answer

An isolator is an off-load switch giving a visible gap. In HV, a CB is flanked by two isolators. Opening sequence: open CB first, then line and bus isolators; closing sequence: close isolators first, then CB. Interlocks enforce this order to prevent arcing.

## Answer

#### Isolator
An **isolator** (disconnector) is an off-load mechanical switch that separates a part of the circuit from the live supply and gives a **visible open gap** for safe maintenance. It has no arc-quenching device, so it is opened or closed only when the circuit carries no current.

#### Working of isolator with the circuit breaker
In HV installations a CB is always placed between two isolators, with an earth switch on the line side.
```text
 HV Bus
   |
  [I1]   bus-side isolator
   |
  [CB]   circuit breaker
   |
  [I2]   line-side isolator
   |-----[ES] earth switch --- earth
   |
 Outgoing line / transformer
```
**Opening (taking the feeder out of service):**
1. Open the **circuit breaker** first. It interrupts the load current and quenches the arc in its medium.
2. Now no current flows, so open isolator **I2** (line side) and then **I1** (bus side).
3. Close the **earth switch ES** to earth the dead feeder and drain trapped charge.
4. Lock and tag the isolators; work can now be done safely on the CB or the line.

**Closing (putting the feeder in service):**
1. Open the earth switch ES.
2. Close isolator **I1** and then **I2**; since the CB is open, they close without current.
3. Close the **circuit breaker** last; it makes the load current (and, if there is a fault, the fault current, after which it trips).

#### Why this order is important
- If an isolator is opened while the CB is closed, it breaks load current and draws a **long arc**, which can burn the contacts, cause phase-to-phase flashover and endanger the operator.
- The CB is designed to make and break current; the isolator only gives isolation.
- **Interlocks** (mechanical keys or electrical) between the CB, isolators and earth switch enforce this sequence: isolators can move only when the CB is open, and the earth switch can close only when the isolators are open.

## Check yourself

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

1. To take a feeder out of service, what is done first?
   A. Open the line isolator
   B. Open the circuit breaker
   C. Close the earth switch
   D. Open the bus isolator
   Answer: B. The CB interrupts the load current before any isolator moves. (Key point: Opening order)

2. When putting a feeder in service, which device is closed last?
   A. Circuit breaker
   B. Bus isolator
   C. Line isolator
   D. Earth switch
   Answer: A. Isolators close without current, then the CB makes the load current. (Key point: Closing order)

3. When is the earth switch closed?
   A. While the feeder is live
   B. After the CB and both isolators are open, to earth the dead feeder
   C. Before the CB is opened
   D. Only when the CB is closed
   Answer: B. It earths the dead section and drains trapped charge, allowed only with isolators open. (Key point: Earth switch rule)

4. What happens if an isolator is opened with the CB closed?
   A. Nothing, it quenches the arc
   B. It breaks load current and draws a long arc that can burn contacts and cause flashover
   C. The CB trips safely
   D. The earth switch closes
   Answer: B. Isolators have no arc-quenching ability, so opening on load causes a dangerous arc. (Key point: Why order matters)

## Related in Switchgear and Protection

- [Isolator vs CB and why both used](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-vs-cb-simultaneous) — Switchgear and Protection, chapter 4, asked 4×
- [HVDC circuit breaker operation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahvdc-circuit-breaker) — Switchgear and Protection, chapter 6
- [Vacuum circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-construction) — Switchgear and Protection, chapter 6, asked 2×
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