Isolator construction, principle and application

An isolator is an off-load switching device that provides a visible isolation gap. It is operated only after the circuit breaker has interrupted the current.

Construction

A common type is the double-break, centre-rotating isolator used in outdoor substations.

   Fixed contact      Moving blade      Fixed contact
   [==]======== ( rotates 90 deg ) ========[==]
    |                   |                   |
  Insulator      Rotating insulator     Insulator
    |                   |                   |
   ==== base channel (steel) + operating mechanism

1. Base frame: galvanised steel channel on which the insulator stacks stand.
2. Post insulators: porcelain or polymer insulators; the outer two are fixed and the middle one rotates (double-break type).
3. Moving contact (blade): copper or aluminium tube mounted on the rotating insulator.
4. Fixed contacts: spring-loaded finger contacts, silver plated, giving firm contact pressure.
5. Operating mechanism: hand-operated crank or motor drive, connected by rods to all three poles (gang operation).
6. Earth switch: attached blade that connects the line side to earth when the main isolator is open.
7. Interlocks: mechanical or electrical interlocks with the CB and earth switch.

Other types: single-break (centre-break), vertical-break, and pantograph (used for connecting to an overhead bus with vertical movement).

Operating principle

1. The isolator is a simple knife or rotating switch; it has no arc quenching medium.
2. Opening: after the CB is open (current is zero), the operating handle turns the middle insulator; the blade swings away from both fixed contacts, giving two visible air gaps.
3. Closing: with the CB open, the blade is turned back so that it enters the fixed finger contacts.
4. Since no arc control exists, opening on load would draw a heavy arc that can damage the contacts and injure the operator. Interlocking makes sure the CB opens first and closes last.
5. The open gap is designed for higher withstand voltage than the open CB, so the isolated part is completely safe.

Applications

• On both sides of circuit breakers in substations for maintenance isolation.
• Bus sectionalising, bus coupling and bus selection in double-bus systems.
• Transformer, PT, lightning arrester and capacitor bank isolation.
• Pole-mounted AB (air break) switches on 11 kV and 33 kV lines for feeder sectionalising.
• With earth switch, for earthing lines before work.