Isolator operation with CB in HV installation

Isolator

An isolator (disconnector) is an off-load mechanical switch that separates a part of the circuit from the live supply and gives a visible open gap for safe maintenance. It has no arc-quenching device, so it is opened or closed only when the circuit carries no current.

Working of isolator with the circuit breaker

In HV installations a CB is always placed between two isolators, with an earth switch on the line side.

 HV Bus
   |
  [I1]   bus-side isolator
   |
  [CB]   circuit breaker
   |
  [I2]   line-side isolator
   |-----[ES] earth switch --- earth
   |
 Outgoing line / transformer

Opening (taking the feeder out of service):

1. Open the circuit breaker first. It interrupts the load current and quenches the arc in its medium.
2. Now no current flows, so open isolator I2 (line side) and then I1 (bus side).
3. Close the earth switch ES to earth the dead feeder and drain trapped charge.
4. Lock and tag the isolators; work can now be done safely on the CB or the line.

Closing (putting the feeder in service):

1. Open the earth switch ES.
2. Close isolator I1 and then I2; since the CB is open, they close without current.
3. Close the circuit breaker last; it makes the load current (and, if there is a fault, the fault current, after which it trips).

Why this order is important

• If an isolator is opened while the CB is closed, it breaks load current and draws a long arc, which can burn the contacts, cause phase-to-phase flashover and endanger the operator.
• The CB is designed to make and break current; the isolator only gives isolation.
• Interlocks (mechanical keys or electrical) between the CB, isolators and earth switch enforce this sequence: isolators can move only when the CB is open, and the earth switch can close only when the isolators are open.