Isolator in SLD; application of contactors

An isolator is an off-load switch that provides a visible break so that equipment can be safely separated from live parts after the circuit breaker has interrupted the current.

Function of isolator in a single line diagram

 Incoming 132 kV line
        |
      [LI]--ES      LI = line isolator + earth switch
        |
      [CB1]
        |
      [BI]          BI = bus isolator
        |
 =======+========[BS]========= 132 kV bus
        |        sectionaliser
      [BI]
        |
      [CB2]
        |
      [TI]
        |
     ( T/F ) 132/33 kV
        |
       ...

1. Isolation of CB: isolators BI and LI on both sides of CB1 allow the breaker to be taken out for maintenance. Sequence: open CB1 → open LI and BI → close ES.
2. Line isolation and earthing: the line isolator with earth switch earths the incoming line during line work and discharges trapped charge.
3. Bus sectionalising: isolator BS divides the bus into two sections, so one half can be maintained while supply continues from the other.
4. Transformer isolation: TI isolates the transformer after its breaker opens.
5. Bus selection: in double-bus arrangements, isolators transfer a feeder from main to reserve bus.
6. Visible open gap for confirming safe working conditions.

Isolators are interlocked so they can be operated only when the related CB is open.

Application of contactors in power system

A contactor is an electromagnetically operated switch for frequent on-load switching of normal currents. Applications:

1. Motor starters: DOL, star-delta, reversing and auto-transformer starters for pumps, fans, compressors and conveyors.
2. Capacitor bank switching in automatic power factor correction (APFC) panels.
3. Lighting control: street lighting with timers or photocells, large building lighting.
4. Heating loads: furnaces, ovens and heaters controlled by thermostats.
5. Automatic transfer switches (ATS) between mains and diesel generator.
6. Remote and automatic control with PLCs, level switches and pressure switches (e.g. water pumping stations).
7. Auxiliary supply in substations and battery charger switching.
8. No-volt and overload protection of motors together with overload relays and back-up fuses.

Unlike isolators, contactors switch load current many times a day; unlike CBs, they cannot interrupt short-circuit currents and need fuse or MCCB back-up.