Where should an isolator be installed?

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Detailed Solution

For maintenance and safety, an isolator must be ableto completely de-energize the circuit breaker. Therefore, in medium-voltage (MV) and high-voltage (HV) systems, isolators are installed on both sides of the circuit breaker (before and after).

Isolator Before the Breaker (Line Side): This isolator is on the power supply side. Opening it disconnects the circuit breaker from the upstream power source.
Isolator After the Breaker (Load Side): This isolator is on the side of the equipment being powered. Opening it disconnects the circuit breaker from the downstream load.

This "sandwich" configuration ensures that the circuit breaker can be completely and safely de-energized from both the power source and any potential back-feed from the load side, allowing for safe maintenance or replacement.

The correct sequence of operation is critical:

To disconnect: First, open the circuit breaker to stop the current flow. Then, open the isolators on both sides.
To connect: First, close the isolators on both sides. Then, close the circuit breaker to energize the circuit.

In some simpler low-voltage (LV) applications, a single isolator switch may be installed only on the incoming (line) side of the main circuit breaker of a distribution board.

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