Isolator and Circuit Breaker both are switches to isolate a section of power system. But Isolator is off load device whereas Circuit Breaker is on load device. Off load device means, Isolator cannot be operated when load is connected i.e, current is flowing through the circuit. Therefore, Isolator shall only be operated when that circuit is disconnected from load and current is not flowing through the circuit.
Isolators don’t have any arc quenching mechanism and hence as mentioned above, it should be operated in off load condition. Basically Isolator is provided in a section of power system to provide physical isolation for maintenance purpose.
Carefully see the figure below.
As shown in the figure, Isolator is always connected on the two sides of a Circuit Breaker. Suppose we want to take maintenance on the Circuit Breaker. How will we provide Isolation?
We will open the Circuit Breaker and will start working? No, there will be potential at either end of the Circuit Breaker.We cannot take risk as it might happen that while we are working someone may close the Breaker and people working there on the Circuit Breaker will get directly promoted to heaven. Therefore we will open the Circuit Breaker and then will open the Isolators on the both side of the Circuit Breaker. See opening the Isolators physically isolates the CB from the circuit.
So we have physically isolated the CB. Shall we start working?
No, we shall not start working as the maintainer may get shock. Why?
Because of the static charge on the line, there will be dangerously high voltage on the line between the two isolators on the both sides of CB. So we need to discharge the static charge to ground. For this purpose, Earth Switch is provided.
Therefore, once we have opened the Isolators we shall close the Earth Switch first and then start working.
Thus Isolators are provided to physically isolate equipment from the circuit. Now coming to CB, CB is on load device. It can be operated irrespective of whether load is connected or disconnected from the circuit. As CB, has arc quenching mechanism, therefore it can isolate faulty section from the healthy section in case of fault.
A Circuit Breaker (CB) is equipment which can open or close a circuit under normal as well as fault conditions and it can be operated manually under normal conditions and automatically under fault conditions. For the latter operation a relay is used with a CB.