A circuit is disconnected by isolators when

Line is energized
There is no current in the line
Line is on full load
Circuit breaker is not open

The correct answer is: A. Line is energized.

Isolators are used to disconnect live electrical circuits. They are designed to be opened and closed when the circuit is energized, and they should not be used to disconnect a circuit that is not energized. If an isolator is opened when the circuit is not energized, it can create a spark that could ignite flammable materials.

Option B is incorrect because isolators are designed to be opened and closed when the circuit is energized.

Option C is incorrect because isolators are designed to be used on circuits that are on full load.

Option D is incorrect because isolators are designed to be used on circuits that are energized.