Isolators and Contactors

An isolator is an off-load disconnecting switch that gives a visible open gap so that a section of the circuit can be safely isolated for maintenance. A circuit breaker (CB) is an on-load switching device that can make, carry and break normal as well as fault currents automatically.

Differences between isolator and circuit breaker

Why an isolator is still needed

1. The contacts of a CB are inside a tank or chamber; the open position is not visible, so the maintenance crew cannot be sure the circuit is dead.
2. The CB itself needs maintenance (contact replacement, oil or gas filling). It must be isolated from both sides, which the CB cannot do for itself.
3. An open CB still has a small gap; a long, visible air gap from an isolator, plus an earthing switch, gives a proven safe condition.
4. Isolators are cheap, so they can be put at many points to section a bus or a feeder.

Using isolator and CB together

 Bus ---[ I1 ]---[ CB ]---[ I2 ]--- Feeder/line
                                  |
                                 [ES] earth switch
                                  |
                                 ===
 I1, I2 = isolators on both sides of CB

Opening sequence (to isolate):

1. Open the CB first; it breaks the load or fault current.
2. Then open isolator I2 (line side) and I1 (bus side) at no load.
3. Close the earth switch on the dead section.

Closing sequence (to restore):

1. Open the earth switch.
2. Close the isolators I1 and I2 (no current flows because CB is open).
3. Finally close the CB to energise the feeder.

Isolators are interlocked with the CB so that they cannot be operated while the CB is closed. In this way the CB does the switching under load and fault, and the isolator gives safe, visible isolation; each covers what the other cannot do.

A contactor is an electrically operated switch whose contacts are closed by an electromagnet (coil) and held closed as long as the coil is energised. It is designed for frequent making and breaking of normal load currents, so it is the standard device for starting, stopping and protecting motors when combined with an overload relay and fuses (a DOL starter).

Scheme: DOL starter using a contactor

 Power circuit                Control circuit
 L1 L2 L3                     L ---[F]---+
  |  |  |                                |
 [F][F][F]  fuses (SC)            [STOP NC]
  |  |  |                                |
 [K][K][K]  main contacts      +--[START NO]--+
  |  |  |                     |              |
 [OL OL OL] thermal relay      +----[K aux]---+
  |  |  |                                |
  (  M  )  3-ph motor              [OL NC contact]
                                         |
                                   (K coil)---N

Working

1. Starting: pressing the START button energises the contactor coil K. The armature is pulled in and the three main contacts close, so the motor gets full supply.
2. Holding (latching): an auxiliary NO contact of K, connected across START, closes at the same time. When the START button is released, the coil stays energised through this contact.
3. Stopping: pressing STOP (NC) breaks the coil circuit; the coil de-energises, the spring opens the main contacts and the motor stops.
4. Overload protection: the thermal overload relay (bimetal strips in each line) heats up if the motor current stays above its setting. Its NC contact in the coil circuit opens, the contactor drops out and the motor is disconnected. It is set at about 100–110% of motor full-load current with a time delay so that starting current does not trip it.
5. Short-circuit protection: a contactor cannot break large fault currents, so back-up HRC fuses or an MCCB ahead of it clear short circuits.
6. Under-voltage (no-volt) protection: if the supply fails or falls to about 70–80% of rated value, the coil cannot hold the armature and the contactor opens. When the supply returns, the motor does not restart by itself because the hold-in contact is open; the operator must press START again. This prevents accidents and a large simultaneous starting load.
7. Single phasing: a differential type overload relay or phase-failure relay opens the coil circuit if one phase is lost.

Other control functions

• Remote control: extra START/STOP buttons can be wired in parallel (START) and in series (STOP) from any location.
• Interlocking: in a reversing starter, two contactors are electrically interlocked by NC auxiliary contacts so that both cannot close together.
• Star-delta and auto-transformer starters use two or three contactors with a timer.

Thus the contactor gives frequent switching, remote and automatic control, and with its overload relay and fuses, complete protection of the motor.

A contactor is an electromagnetically operated switch used to make and break a power circuit many times, under normal load conditions, by a small control signal. It is used for motors, heaters, lighting and capacitor banks.

Construction

        +-----------------------+
        |  fixed contacts (3)   |
        |  ==  ==  ==           |
        |  moving contacts      |
        |  --  --  --  <- bridge |
        |  [ armature ]         |
        |      ^  spring        |
        |  [  coil on core ]    |
        +-----------------------+
   A1  A2 = coil terminals

1. Electromagnet (coil and core): a laminated E- or U-shaped iron core with a coil. The coil works on AC or DC (24 V, 110 V, 230 V, 415 V). AC cores carry a shading ring to stop chatter.
2. Armature: the movable iron part attracted by the magnet; the moving contacts are fixed to it through an insulating carrier.
3. Main contacts (power contacts): usually three NO poles, made of silver alloy (silver–cadmium oxide or silver–tin oxide) for low resistance and resistance to welding. Double-break bridge contacts are common.
4. Auxiliary contacts: small NO and NC contacts for holding, interlocking and signalling.
5. Return spring: opens the contacts when the coil is de-energised.
6. Arc chutes / arc splitters: splitter plates or blow-out coils in larger contactors cool and lengthen the arc.
7. Enclosure and frame: moulded insulating housing with terminals.

Operating principle

1. When the control voltage is applied to the coil, current in the coil sets up flux in the core.
2. The magnetic pull exceeds the spring force; the armature moves and closes the main contacts (and changes over the auxiliary contacts). The load is connected.
3. The coil current is small compared to the load current, so a small switch or PLC output can control a large load.
4. When the coil is de-energised (STOP button, overload relay or supply failure), flux collapses and the spring opens the contacts. The arc formed is quenched in the arc chute at the next current zero.
5. Because pick-up needs about 85% and drop-out happens below about 60–70% of coil voltage, a contactor gives built-in no-volt protection.

Main features

• Designed for very high mechanical life (millions of operations) and frequent switching.
• Breaking capacity is limited to a few times rated current (overload/motor stall current), not short-circuit current; fuses or MCCBs give short-circuit back-up.
• Utilisation categories (IEC 60947-4-1) such as AC-1 (resistive), AC-3 (squirrel-cage motor start and run), AC-4 (inching, plugging) define the duty.

A contactor is an electrically operated switch in which a magnetic coil closes the main contacts and a spring opens them when the coil is de-energised. It is used for frequent on-load switching and remote control of motors and other loads.

Construction

• Coil and laminated core (electromagnet), AC cores with a shading ring.
• Armature carrying the moving contacts through an insulated carrier.
• Main contacts (normally 3 NO, silver alloy) for the power circuit.
• Auxiliary contacts (NO/NC) for latching, interlocking and indication.
• Return spring, arc chutes/splitters, moulded housing and terminals.

Operating principle

1. Control voltage on the coil produces flux; the armature is attracted against the spring.
2. Main contacts close and the load is energised; auxiliary contacts change state.
3. Removing coil voltage lets the spring open the contacts; the arc is extinguished in the arc chute at current zero.
4. Since the coil drops out at low voltage, the contactor also gives no-volt protection.

Circuit for remote control of a contactor

START buttons are connected in parallel and STOP buttons in series, so the motor can be started or stopped from the local panel or from a distant point. Only thin control wires go to the remote station.

 L --[Fuse]--[STOP local]--[STOP remote]--+--------+
                (NC)          (NC)        |        |
                               [START local] [START remote]
                                  (NO)        (NO)
                                  |            |
                                  +-----+------+
                                  |     |
                              [K aux NO] |
                                  |     |
                                  +--+--+
                                     |
                              [OL relay NC]
                                     |
                                 ( K coil )
                                     |
 N ----------------------------------+

 Power: L1,L2,L3 -> fuses -> K main -> OL -> Motor

Working of the remote scheme

1. Pressing any START button energises coil K; main contacts close and the motor runs.
2. The auxiliary contact K (in parallel with START buttons) closes and holds the coil after the button is released.
3. Pressing any STOP button opens the series path; coil drops out and the motor stops.
4. The overload relay NC contact also opens the coil path on overload.
5. Indicating lamps through auxiliary contacts show ON/OFF status at the remote point.

This arrangement allows control of large motors (pumps, fans, cranes) from a control room using low-power control wiring, and safe automatic disconnection on overload or supply failure.

An isolator (disconnector) is a mechanical switch that opens or closes a circuit only when no current, or negligible current, is flowing. It provides a visible, safe air gap between a live part and the part to be maintained. It has no arc-quenching arrangement, so it must never be opened on load.

Functions of an isolator

1. Safety isolation: disconnects equipment (CB, transformer, line, bus section) from all live parts so maintenance can be done safely.
2. Visible break: the open blades give an obvious, visible gap that confirms the circuit is dead.
3. Sectionalising: divides a busbar or a feeder into sections so that one part can be taken out while the rest stays in service.
4. Change-over / bus selection: in double-bus schemes, isolators select which bus a feeder is connected to.
5. Earthing: many isolators are combined with an earth switch to earth the dead line and discharge trapped charge.
6. Breaking small currents: can safely break the small charging current of busbars or short unloaded lines and the magnetising current of small transformers (where permitted).

Applications in power system

• On both sides of every circuit breaker in substations, so the CB can be maintained.
• At the incoming and outgoing line bays of substations (line isolator with earth switch).
• Bus coupler and bus sectionaliser isolators in single and double busbar arrangements.
• Transformer isolation on HV and LV sides of power and distribution transformers.
• Pole-mounted (gang-operated) AB switches on 11 kV and 33 kV distribution lines in Nepal for sectionalising feeders.
• Isolating potential transformers, lightning arresters and capacitor banks.
• Bypass isolators used to bypass a CB during maintenance.

Types (brief)

• By construction: double-break, single-break (centre-break), pantograph, vertical-break.
• By location: bus-side, line-side, transfer-bus isolators.
• By operation: manual (hook stick), gang-operated, motorised.

Example: in a 33/11 kV substation, the operator first opens the 33 kV breaker, then opens the isolators on both sides and closes the earth switch before work is done on the breaker.

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.

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.

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.

Taking the question as a high-voltage feeder (e.g. 33 kV) fed from a bus through isolators and a circuit breaker.

 Bus ==+==
       |
     [I1]  bus isolator
       |
     [CB]
       |
     [I2]  line isolator --[ES] earth switch
       |
     Feeder

Guideline for disconnecting the feeder

1. Inform the control room and load end; reduce load if possible.
2. Open the circuit breaker by the control switch; confirm "OFF" on the indicator and ammeter reading zero.
3. Open the line isolator I2, then the bus isolator I1 (no-load operation).
4. Check visibly that all three blades are open.
5. Test for absence of voltage, then close the earth switch ES (and apply portable earths at the work site).
6. Lock the isolators and CB, and attach "Do not operate – men at work" tags (permit-to-work).

Guideline for connecting the feeder

1. Cancel the work permit; make sure all persons and portable earths are removed.
2. Open the earth switch ES.
3. Check that the CB is open; close the bus isolator I1, then the line isolator I2.
4. Close the circuit breaker last; watch the ammeter and relay flags. If a fault exists, the CB trips safely.
5. Record the operation in the log book.

Rule to remember: CB opens first and closes last; isolators move only when the CB is open. Interlocking enforces this.

Can a contactor replace the circuit breaker?

No, not on an HV feeder. Reasons:

• A contactor is built to make and break normal load currents (and motor starting currents), with breaking capacity of only about 8–10 times rated current. A feeder short circuit can be tens of kA; the contactor would fail to interrupt it and could weld or explode.
• A contactor has no tripping by protective relays in the usual sense; it opens only when the coil is de-energised, and it also drops out on voltage dips, causing unwanted outages.
• Contactors are mostly made for LV and up to about 11 kV (vacuum contactors); they do not have the voltage rating, insulation level or making capacity of a 33 kV CB.
• A contactor needs back-up fuses for short circuits; HV fuses large enough for a feeder are not practical and must be replaced after each fault.

A contactor is suitable only for frequent switching of a motor or capacitor bank; the feeder needs a circuit breaker with relays to clear faults.

A contactor is an electromagnetic switch whose coil can be energised by any small control signal (push-button, timer, sensor, PLC). Because a low-power signal switches a high-power load, contactors are the basic building block of automation of electrical systems.

Example: automatic water pumping system

Assumption: a pump fills an overhead tank. A float/level switch in the tank and a dry-run sensor in the sump control the pump through a contactor.

 Power: L1 L2 L3 --[MCB/fuse]--[K]--[OL]--( Pump M )

 Control circuit:
 L --[F]--[OL NC]--[Sump level OK]--+
                                    |
                     [Tank LOW float NO]---+
                                    |      |
                     [K aux NO]--[Tank HIGH NC]
                                    |      |
                                    +--+---+
                                       |
                                   ( K coil )-- N

Working

1. When the tank level falls below the LOW float, its contact closes; the contactor coil K is energised and the pump starts.
2. The auxiliary contact of K, in series with the HIGH float (NC), holds the coil, so the pump keeps running as the water rises above the LOW point.
3. When the water reaches the HIGH level, the HIGH float contact opens; the coil drops out and the pump stops.
4. If the sump runs dry, the sump-level contact opens and stops the pump (dry-run protection).
5. The overload relay stops the motor on overload; the contactor gives no-volt protection on supply failure.
6. No operator is needed; the system runs automatically.

Other automation done with contactors

• Star-delta starter: a timer switches the star contactor off and the delta contactor on after the motor speeds up.
• APFC panels: a power factor controller switches capacitor steps through capacitor-duty contactors.
• Street lighting: a photocell or timer energises the lighting contactor at dusk.
• Automatic transfer switch: two interlocked contactors change load from mains to generator on supply failure.
• Conveyors and lifts: limit switches and PLC outputs drive contactors for start, stop and reversal.
• Temperature control: a thermostat switches heater contactors in furnaces.

The contactor thus acts as the power output stage of the control system: sensors and logic decide, and the contactor carries out the switching safely and repeatedly.

An isolator is an off-load switch for visible, safe isolation, while a contactor is an electromagnetic switch for frequent on-load switching of normal currents. Their construction follows directly from these different duties.

Key components

Isolator

• Steel base frame and post insulators (porcelain/polymer) sized for the full system voltage and BIL.
• Blade (moving contact) of copper/aluminium tube and spring-loaded finger fixed contacts.
• Manual or motor operating mechanism with gang rods; no stored-energy trip.
• Earth switch and mechanical/electrical interlocks with the CB.
• No arc chute, no coil, no spring for fast opening.

Contactor

• Electromagnet: coil on a laminated core, with shading ring for AC.
• Armature and return spring for fast, automatic opening.
• Silver-alloy main contacts, often double-break bridge type.
• Arc chutes / splitter plates / blow-out coils, or a vacuum bottle in MV contactors.
• Auxiliary contacts for latching and interlocking; moulded enclosed housing.

Comparison of design

Design considerations

• Isolator: high dielectric strength across the open gap (greater than across an open CB) so the isolated side is safe; contacts must carry rated and short-time fault current without welding or being blown open by electromagnetic forces; outdoor weather and ice resistance; visible break; interlocking.
• Contactor: low coil power and reliable pick-up/drop-out voltages; light moving parts for fast operation and long life; contacts and arc chute rated for the utilisation category (AC-1, AC-3, AC-4) to resist erosion and welding during motor starting; compact enclosed design for panels.

In short, the isolator is built for insulation and safety, the contactor for speed, repeated operation and arc control.