A circuit breaker and an isolator (disconnector) both open a circuit, so beginners often confuse them. In reality they do opposite jobs: the breaker interrupts current — including huge fault current — while the isolator only proves, with a visible air gap, that a dead section is really dead. This post compares the two and explains why an isolator is fitted on both sides of a circuit breaker.
Circuit breaker vs isolator — the differences
| Feature | Circuit Breaker (CB) | Isolator (Disconnector) |
|---|---|---|
| Function | Makes and breaks load current and short-circuit (fault) current | Disconnects a de-energised section for maintenance; provides a visible isolation point |
| On-load or off-load | On-load — can open under full load and under fault | Off-load only — must never be opened while carrying current |
| Arc quenching media | Has an arc-quenching medium — oil, vacuum, SF6 or air-blast | No arc-quenching medium; contacts separate in air with no arc control |
| Control | Operated by protective relays (automatic trip) as well as manually | Manually operated (or motor-operated), only after the circuit is dead |
| Withstand capacity | High breaking and making capacity; rated short-time current alongside high short-time thermal ratings | Carries continuous current and withstands through-faults when closed, but cannot interrupt current |
| Visible gap | Contacts are enclosed inside an opaque interrupting chamber; the gap is not visually verifiable | Provides a clearly visible physical air gap in the circuit to guarantee personnel safety |
Figure 1.1: Isolators on both sides of a circuit breaker — single-line arrangement with the opening and closing interlock sequences. (Schematic, not to scale.)
Why isolators on both sides of a circuit breaker?
During routine maintenance, inspection or repair of a circuit breaker, the breaker must be completely disconnected from both sides. The busbar side is energised from the substation, and the line side may also be energised — from the remote substation end, or by induced and capacitive voltages on a long line. One isolator alone would leave the breaker connected to a live circuit on the other side.
Placing an isolator on the line side and on the bus side creates a visible double air-break isolation barrier. With both isolators open, the breaker sits in a dead, visibly isolated section, and the earthing switch can then be closed to discharge any trapped or induced charge.
The mandatory interlocking sequence
Isolators have no arc-quenching medium, so opening one under load would draw a dangerous arc across its contacts. The sequence is therefore interlocked so the breaker — the only device that can interrupt current — always acts first:
- Opening (to isolate for maintenance): Open the circuit breaker first → open both isolators → close the earthing switch.
- Closing (to restore service): Open the earthing switch → close both isolators → close the circuit breaker.
Reversing either sequence is dangerous: closing the earthing switch onto a live circuit, or opening an isolator under load, can cause a flashover and serious injury. That is why the interlock is mandatory, not optional.
Key points to remember
- A circuit breaker interrupts load and fault current on-load; an isolator only disconnects a dead circuit off-load and has no arc-quenching medium.
- The isolator's real job is safety: it gives a visible air gap that proves the section is dead.
- Isolators go on both sides of a breaker because the line side can stay energised from the remote end even when the bus side is dead.
- Opening sequence: CB first → both isolators → earthing switch. Closing sequence: earthing switch → both isolators → CB.
- Never open an isolator under load — it has no medium to quench the arc.
Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q1(a).