Isolator vs CB and why both used

An isolator is an off-load disconnecting switch that gives a visible open gap so that a section of the circuit can be safely isolated for maintenance. A circuit breaker (CB) is an on-load switching device that can make, carry and break normal as well as fault currents automatically.

Differences between isolator and circuit breaker

Why an isolator is still needed

1. The contacts of a CB are inside a tank or chamber; the open position is not visible, so the maintenance crew cannot be sure the circuit is dead.
2. The CB itself needs maintenance (contact replacement, oil or gas filling). It must be isolated from both sides, which the CB cannot do for itself.
3. An open CB still has a small gap; a long, visible air gap from an isolator, plus an earthing switch, gives a proven safe condition.
4. Isolators are cheap, so they can be put at many points to section a bus or a feeder.

Using isolator and CB together

 Bus ---[ I1 ]---[ CB ]---[ I2 ]--- Feeder/line
                                  |
                                 [ES] earth switch
                                  |
                                 ===
 I1, I2 = isolators on both sides of CB

Opening sequence (to isolate):

1. Open the CB first; it breaks the load or fault current.
2. Then open isolator I2 (line side) and I1 (bus side) at no load.
3. Close the earth switch on the dead section.

Closing sequence (to restore):

1. Open the earth switch.
2. Close the isolators I1 and I2 (no current flows because CB is open).
3. Finally close the CB to energise the feeder.

Isolators are interlocked with the CB so that they cannot be operated while the CB is closed. In this way the CB does the switching under load and fault, and the isolator gives safe, visible isolation; each covers what the other cannot do.