Abstract:
An isolating circuit breaker ( 40 ) for pole mounting ( 12 ) in relation to a electrical distribution line, said isolating circuit breaker including: an isolating switch ( 50 ) with a pivoting link ( 52 ), said isolating switch having respective terminals ( 45, 44 ) for electrical coupling to load and supply conductors ( 54, 53 ) of the electrical distribution line; a circuit interrupter ( 55 ) disposed in series with the isolating switch ( 50 ), intermediate said load and supply conductors and preferably incorporated in the pivoting link of the isolating switch; an actuator ( 58 ) arranged for selectively opening and closing the circuit interrupter ( 55 ); an electronic controller ( 61 ) controlling operation of the isolating circuit breaker in response to electrical conditions in the distribution line; and said isolating switch, circuit interrupter, actuator and electronic controller arranged together on a mounting structure ( 41, 42, 43 ) adapted for pole mounting. A circuit protection arrangement comprising a plurality of isolating circuit breakers and an associated protection method are also described.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to electrical switchgear and protection arrangements for electric power distribution networks. More particularly, the invention relates to an improved isolating circuit breaker and a circuit protection system for medium voltage distribution networks utilising a plurality of the improved isolating circuit breakers.  
           [0003]    2. Discussion of the Background Art  
           [0004]    Existing electrical switchgear includes a dual purpose assembly that may function as both an isolating switch and a fuse, and is generally known in the electric power industry as a “dropout fuse”. Examples of a fuse and a dropout fuse holder are disclosed in Australian Patent No. 667953 and Australian Patent No. 683191 (respectively), both in the name of Cooper Power Systems, Inc. Some of the problems and drawbacks of conventional fuses and associated fuse holders are discussed in the introductory paragraphs of the Cooper Power Systems patent specifications.  
           [0005]    Electric power utilities generally use dropout fuses in relatively large numbers in medium voltage distribution networks, often in a pole-mounted configuration, for reasons of cost. However, the fuses must be manually replaced by linesmen, which delays restoration of supply subsequent to a fuse blowing. Isolating switches, with or without fuses, are used for operations in conjunction with separate line sectionalising circuit breakers or automatic re-closing type circuit breakers. In the present context, medium voltage includes the range from about 10 kV up to about 75 kV. There have also been previous attempts to combine on-load circuit breakers with isolating switches, as exemplified by U.S. Pat. No. 4,596,911 to Guery et al., wherein a common operating mechanism closes the isolating switch prior to the circuit breaker and vice versa during opening.  
           [0006]    More recently, U.S. Pat. No. 5,959,398 to McKean describes a sequential isolating circuit breaker and actuator wherein a vacuum interrupter provides fast interruption of load current and the isolator provides a large visual gap in the open position for safety reasons. The actuator mechanism in the McKean patent is a complex rotary driven arrangement including spring loaded levers and lost motion linkages.  
           [0007]    German Patent Application No. 3412399 in the name of Sachsenwerk, Licht &amp; Kraft AG discloses a high voltage switch with a vacuum switching tube incorporated into a rotary arm carrying an isolating contact on the end of the arm. The rotary mounting for the other end of the rotary arm, that is driven by an actuator, has a cam slot and guide pin arrangement for synchronising the opening of the vacuum interrupter contacts with the isolator contacts.  
           [0008]    The prior art arrangement of isolating switches and circuit breakers in series with common actuation does not allow for independent operation of the respective contact sets. In particular, this arrangement does not facilitate opening and automatic re-closing of the circuit breaker alone, to reconnect the load in the event of a transient fault on a protected line. Furthermore, the physical designs are generally suited to metal-clad or indoor use, rather than being more compact and suited to pole-mounting.  
           [0009]    In distribution networks with three phase circuits it is desirable to detect faults such as a phase-to-phase fault or a phase-to-ground fault, that may compromise distribution network security. Fault detection can be arranged to control operation of circuit breakers associated with the conductors in order to prevent network damage and when endeavouring to clear the fault.  
           [0010]    In obtaining measurements from phase conductors, substantial isolation arrangements are required in view of the potential difference which exists between individual phase conductors and the control equipment for circuit breakers. Conventionally control equipment for circuit breakers is mounted in a cabinet at ground potential.  
           [0011]    Furthermore, circuit breakers for three-phase applications typically employ a common actuating mechanism for all three phases. This similarly calls for a relatively heavy and cumbersome device, resulting from the isolation requirements and the mechanical performance required to operate three sets on contacts. This also adds to the cost of acquiring and installing circuit breakers.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    Object of the Invention  
           [0013]    It is an object of the present invention to provide an isolating circuit breaker which ameliorates or overcomes at least some of the problems associated with the prior art.  
           [0014]    It is another object of the present invention to provide a compact isolating circuit breaker for pole mounted, medium voltage applications.  
           [0015]    It is yet another object of the present invention to provide a self-contained isolating circuit breaker which can be retrofitted to electrical distribution networks in place of conventional dropout fuses and fuse holders.  
           [0016]    It is a further object of the invention to provide a circuit protection arrangement including a plurality of isolating circuit breakers that effect coordinated protection of three phase circuits.  
           [0017]    Further objects will be evident from the following description.  
           [0018]    Disclosure of the Invention  
           [0019]    In one form the invention resides in an isolating circuit breaker for pole mounting in relation to a electrical distribution line, said isolating circuit breaker including:  
           [0020]    (a) an isolating switch with a pivoting link, said isolating switch having respective terminals for electrical coupling to load and supply conductors of the electrical distribution line;  
           [0021]    (b) a circuit interrupter disposed in series with the isolating switch, intermediate said load and supply conductors;  
           [0022]    (c) an actuator arranged for selectively opening and closing the circuit interrupter;  
           [0023]    (d) an electronic controller controlling operation of the circuit breaker in response to electrical conditions in the distribution line; and  
           [0024]    (e) said isolating switch, circuit interrupter, actuator and electronic controller arranged together on a mounting structure adapted for pole mounting.  
           [0025]    Most preferably, the circuit interrupter is incorporated in the pivoting link of the isolating switch.  
           [0026]    In one arrangement of the isolating circuit breaker the electronic controller is incorporated in the mounting structure; whilst in an alternative arrangement the electronic controller is incorporated in the pivoting link of the isolating switch.  
           [0027]    The actuator and electronic controller are suitably not isolated from the distribution line so as to float at line potential. If required however, the actuator and electronic controller could be isolated from the distribution line.  
           [0028]    Preferably the isolating circuit breaker further includes a sensor arrangement coupled to the electronic controller, the sensor arrangement arranged for producing signals in response to electrical conditions in the distribution line. In preference, the sensor arrangement is carried by the mounting structure. Suitably the sensor arrangement includes a current transformer for measuring current in the distribution line. The sensor arrangement may also include a voltage transformer for measuring voltage in the distribution line. If required, the sensor arrangement includes a resistive voltage divider or a capacitive voltage divider for measuring voltage in the distribution line.  
           [0029]    The electronic controller may derive power supply from the distribution line. The electronic controller may derive power supply from a separate transformer or capacitor coupled to the distribution line. In preference, the electronic controller derives power supply from a primary or secondary storage cell.  
           [0030]    The electronic controller preferably includes a integrated circuit device, such as a microprocessor, gate array or applications specific integrated circuit, with a memory for storing data. Preferably the data includes protection settings relating to operation of the isolating circuit breaker. Suitably the settings include parameters for an automatic re-closing sequence of the circuit interrupter. If required, the memory may also be arranged to store historical data relating to the electrical conditions in said distribution line.  
           [0031]    The circuit interrupter may further include at least one communications transceiver coupled to the electronic controller. Preferably the communications transceiver facilitates communications with other isolating circuit breakers for providing earth current and/or earth fault protection of a multi-phase distribution circuit. If required, the communications transceiver facilitates communications with a remote device for uploading protection settings and/or downloading of historical data.  
           [0032]    Preferably, the communications medium utilised by the communications transceiver is radio. If required the communications medium may be optical, suitably utilising optical fibre links. Alternatively, the communications medium may be acoustic, preferably in the ultrasonic range of frequencies.  
           [0033]    Suitably, the isolating switch is an air gap isolator. Preferably the circuit interrupter is a vacuum interrupter.  
           [0034]    In a second form, the invention resides in an isolating circuit breaker for pole mounting in relation to a electrical distribution line, said isolating circuit breaker including:  
           [0035]    (a) a mounting arrangement comprising an insulating housing;  
           [0036]    (b) a first line terminal on the housing for connection to a supply conductor of the distribution line;  
           [0037]    (c) a first isolating contact electrically coupled to the first line terminal;  
           [0038]    (d) a sensor arrangement located in said housing, the sensor arrangement producing signals in response to electrical conditions in the distribution line;  
           [0039]    (e) a control module located in said housing, the control module receiving signals from the sensor arrangement for controlling operation of an actuator in response to said signals;  
           [0040]    (f) a link pivotally mounted in relation to said housing, the link having a second isolating contact on a free end thereof for engagement with the first isolating contact;  
           [0041]    (g) the link incorporating a circuit interrupter selectively opened and closed by the actuator; and  
           [0042]    (h) wherein said circuit interrupter has a terminal electrically coupled to the second isolating contact and has an opposite terminal electrically coupled to a second line terminal for connection to a load conductor of the distribution line.  
           [0043]    Suitably the sensor arrangement is associated with a portion of the electrical coupling between the first line terminal and the first isolating contact, which portion is also located in said housing.  
           [0044]    Electronic circuitry for the control module may be disposed in a separate cartridge that is selectively removable from the insulating housing for adjusting the operating parameters and/or downloading data logs.  
           [0045]    Preferably, the actuator includes an operating arm protruding from the insulating housing for opening and closing said circuit interrupter.  
           [0046]    The actuator may comprise a solenoid mechanically coupled to the operating arm. Most preferably the operating arm is manually operable, suitably by a separate lever which may also functions as a position indicator for the circuit interrupter.  
           [0047]    In a third form the invention resides in an isolating circuit breaker for pole mounting in relation to a electrical distribution line, said isolating circuit breaker including:  
           [0048]    (a) a mounting arrangement comprising two insulating members carrying respective support arms;  
           [0049]    (b) a first line terminal on an upper support arm for connection to a supply conductor of the distribution line;  
           [0050]    (c) a first isolating contact on the upper support arm electrically coupled to the first line terminal;  
           [0051]    (d) a link pivotally mounted on a lower support arm, the link having a second isolating contact on a free end thereof for engagement with the first isolating contact;  
           [0052]    (e) the link incorporating:  
           [0053]    (i) a sensor arrangement for producing signals in response to electrical conditions in the distribution line,  
           [0054]    (ii) a controller receiving signals from the sensor arrangement for controlling operation of an actuator in response to said signals, and  
           [0055]    (iii) a circuit interrupter selectively opened and closed by the actuator;  
           [0056]    (f) wherein said circuit interrupter has a terminal electrically coupled to the second isolating contact and has an opposite terminal electrically coupled to a second line terminal for connection to a load conductor of the distribution line.  
           [0057]    Suitably the sensor arrangement is associated with the electrical coupling between the circuit interrupter and the second isolating contact.  
           [0058]    The actuator is preferably a magnetic actuator with an axially movable operating member.  
           [0059]    The circuit interrupter is suitably a vacuum interrupter having fixed and movable contacts coupled to respective fixed and movable terminals.  
           [0060]    Suitably, the fixed terminal of the circuit interrupter is pivotally mounted on the lower support arm and the movable terminal is coupled to the actuator. The circuit interrupter may also be manually operated by an operating lever mechanically coupled to the actuator.  
           [0061]    If required the operating lever operates the circuit interrupter during a first arc of travel, and opens the first and second isolating contacts of the isolating switch during a second arc of travel. Desirably, the circuit interrupter is only able to be closed after the first and second isolating contacts of the isolating switch are closed.  
           [0062]    In further form, the invention resides in a circuit protection arrangement for a multi-phase circuit of an electrical distribution network, said arrangement comprising:  
           [0063]    (a) a plurality of isolating circuit breakers for connection in respective phase lines of the multi-phase circuit, wherein each isolating circuit breaker includes:  
           [0064]    (i) an isolating switch with a pivoting link, said isolating switch having respective terminals for electrical coupling to load and supply conductors of a phase line;  
           [0065]    (ii) a circuit interrupter disposed in series with the isolating switch, intermediate said load and supply conductors;  
           [0066]    (iii) an actuator arranged for selectively opening and closing the circuit interrupter;  
           [0067]    (iv) a sensor arrangement producing signals in response to electrical conditions in a respective phase line;  
           [0068]    (v) a communications transceiver for transmitting said signals to others of the plurality of isolating circuit breakers and for receiving respective phase line condition signals transmitted by other isolating circuit breakers of said plurality;  
           [0069]    (vi) an electronic controller receiving signals from both the sensor arrangement and from the other isolating circuit breakers via the communications transceiver; and  
           [0070]    (vii) said isolating switch, circuit interrupter, actuator and electronic controller arranged together on a mounting structure adapted for pole mounting;  
           [0071]    (b) whereby the electronic controllers process the line condition signals from each phase in substantially real time to control operation of individual actuators of respective circuit interrupters.  
           [0072]    Preferably, the communications transceiver uses an electromagnetic communications medium, suitably employing an analogue modulation scheme. The communications medium may comprise radio, or alternatively, the communications medium may comprise optics. In an alternative arrangement, the communications transceiver uses an acoustic communications medium such as ultrasonic band frequencies.  
           [0073]    Suitably, the signals produced by the sensor are indicative of the instantaneous value of the current and/or voltage on each phase line. Preferably, the control modules sum the instantaneous values whereby a residual vector sum is indicative of a fault condition in the multi-phase circuit.  
           [0074]    In a yet further form the invention resides in a circuit protection arrangement for a multi-phase circuit of an electrical distribution network, said arrangement including:  
           [0075]    (a) a plurality of isolating circuit breakers for connection in respective phase lines of the multi-phase circuit, wherein each isolating circuit breaker comprises:  
           [0076]    (i) an isolating switch with a pivoting link, said isolating switch having respective terminals for electrical coupling to load and supply conductors of a phase line;  
           [0077]    (ii) a circuit interrupter disposed in series with the isolating switch, intermediate said load and supply conductors;  
           [0078]    (iii) a sensor arrangement producing signals in response to electrical conditions in a respective phase line;  
           [0079]    (iv) an electronic controller receiving signals from the sensor arrangement,  
           [0080]    (v) an actuator arranged for selectively opening and closing the circuit interrupter in response control signals from the electronic controller; and  
           [0081]    (vi) said isolating switch, circuit interrupter, actuator and electronic controller arranged together on a mounting structure adapted for pole mounting;  
           [0082]    (b) the actuators of respective isolating circuit breakers being mechanically coupled such that all circuit interrupters of said plurality may be opened substantially simultaneously in response to a predetermined fault condition in at least one of said phase lines.  
           [0083]    The mechanical coupling may be arranged such that all circuit interrupters are opened simultaneously only as a result of a persistent fault condition, for example at the end of an automatic re-closing sequence. Suitably, the operating mechanisms are mechanically coupled by insulating coupling rods extending between respective actuators.  
           [0084]    In a still further form, the invention resides in a method for protecting a multiphase circuit of an electrical distribution network, said network comprising:  
           [0085]    (a) a plurality of isolating circuit breakers for connection in respective phase lines of the multi-phase circuit, wherein each isolating circuit breaker includes:  
           [0086]    (i) an isolating switch with a pivoting link, said isolating switch having respective terminals for electrical coupling to load and supply conductors of a phase line  
           [0087]    (ii) a circuit interrupter disposed in series with the isolating switch intermediate said load and supply conductors;  
           [0088]    (iii) a sensor arrangement producing signals in response to electrical conditions in the phase line;  
           [0089]    (iv) a communications transceiver associated with the sensor arrangement; and  
           [0090]    (v) the isolating switch, circuit interrupter and sensor arrangement carried by a mounting structure adapted for pole mounting;  
           [0091]    (b) wherein the method includes the steps of:  
           [0092]    (i) sensing the instantaneous values of an electrical parameter of a phase line at a respective isolating circuit breaker;  
           [0093]    (ii) transmitting the instantaneous value signals produced by the sensor arrangement to others of the plurality of isolating circuit breakers and receiving respective instantaneous value signals transmitted by the other circuit breakers;  
           [0094]    (iii) summing the instantaneous value signals from the sensors in each circuit breaker in substantially real time; and  
           [0095]    (iv) controlling operation of individual actuators of respective circuit interrupters in accordance with the sum of said instantaneous value signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0096]    To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which:  
         [0097]    [0097]FIG. 1 is a diagrammatic side elevational view of an isolating circuit breaker of a first embodiment in a closed position;  
         [0098]    [0098]FIG. 2 is a diagrammatic side elevational view of an isolating circuit breaker of the first embodiment in an open position;  
         [0099]    [0099]FIG. 3 is a diagrammatic top plan view of an array of isolating circuit breakers of a second embodiment connected in a multi-phase electrical distribution network;  
         [0100]    [0100]FIG. 4 is a diagrammatic side elevational view of an isolating circuit breaker of a third embodiment in a closed position;  
         [0101]    [0101]FIG. 5 is a diagrammatic side elevational view of an isolating circuit breaker of the third embodiment in an open position;  
         [0102]    [0102]FIG. 6 is a diagrammatic top plan view of an array of isolating circuit breakers of a fourth embodiment connected in a multi-phase electrical distribution network; and  
         [0103]    [0103]FIG. 7 is block diagram of a preferred electro-mechanical arrangement for isolating circuit breakers of the embodiments. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0104]    In the drawings, like reference numerals refer to like parts. Referring to FIG. 1, there is an isolating circuit breaker  10  of a first embodiment shown mounted on the cross arm  11  of a utility pole  12 . The isolating circuit breaker has a mounting structure in the form of an insulating housing  13 . The insulating housing is constructed of plastics, suitably being moulded from an epoxy resin. A first line terminal  14  is provided on the top of the housing for connection with a supply conductor of a electrical distribution line (not shown), whilst a second line terminal  15  is provided for connection with a load conductor of the distribution line. The physical arrangement of the housing  13  and terminals  14  and  15  in the embodiment is such that the isolating switch may be conveniently retrofitted to the pole  12  in place of a conventional dropout fuse and fuse holder.  
         [0105]    A first conductor  16  extends from the first line terminal  14  through the insulating housing  13  and protrudes from a front portion of the housing. The front end of conductor  16  carries a first isolating contact  17  of an isolating switch  18 . A second isolating contact  19 , arranged for engagement with the first contact  17  (as shown), is carried adjacent the free end of a pivotally mounted link member  20 . A ring  21  is provided on the end of the link  20  for hooking with a linesman&#39;s “hook stick”, such that the isolating switch may be manually operated as required. The isolating switch is depicted in the “open” position in FIG. 2, wherein the link member  20  depends from the pivot  22 .  
         [0106]    The link member  20  carries a circuit interrupter suitable for breaking load current, formed by a vacuum interrupter  23  in the embodiment. The vacuum interrupter may be of a conventional type including contacts in a sealed evacuated container. A series of sheds of elastomeric material are provided on the exterior of the sealed container. The vacuum interrupter  23  has a fixed terminal associated with an internal fixed contact (not shown) and a movable terminal, at an opposite end of the container, associated with an internal movable contact (not shown). The fixed terminal  24  of the interrupter is coupled to the first isolating contact  17 , whilst the movable terminal  25  on the opposite end of the vacuum interrupter is pivotally coupled  22  to an operating arm  26 . The movable terminal  25  of the vacuum interrupter  23  is electrically coupled to the second line terminal  15 .  
         [0107]    The operating arm  26  protrudes from the insulating housing  13  and is arranged to open and close the vacuum interrupter  23 . The operating arm  26  is also constructed of an insulating material, such as glass filled epoxy resin. The second line terminal  15  mounted on the operating arm  26  includes a rotating joint in the present embodiment to accommodate movement during operation of the vacuum interrupter  23 . Actuation of the operating arm may be effected by a solenoid  27  located in the housing  13  and mechanically linked to the operating arm  26 . Alternatively, the operating arm may be manually operated by a lever  29  which protrudes from a lower portion of the housing in the embodiment. The lever includes a ring  29  on its free end which may be hooked by a hookstick. It will be appreciated from FIG. 2, that the lever  29  also indicates the position of the vacuum interrupter contacts to a linesman.  
         [0108]    The operation of the solenoid  27 , a low power magnetic latching type in the embodiment, is controlled by a control module  30  also located in the housing  13 . The control module  30  is disposed in a separate cartridge to facilitate convenient removal from the housing when desired. The isolating circuit breaker  10  further includes a sensor arrangement for monitoring electrical conditions in a distribution line coupled to the first and second line terminals  14 ,  15 . The sensor arrangement of the embodiment comprises a current transformer  31  associated with conductor  16 , and a resistive voltage divider  32  coupled between the first line terminal  14  and a mounting bracket  33  which is electrically coupled to ground potential. Accordingly, both the instantaneous current and voltage of the distribution line may be monitored by the control module  30 .  
         [0109]    A further solenoid  34  is also provided for ejecting the second terminal  19  of the isolating switch  18  from the first terminal  17 . This ejection occurs subsequent to the control module opening the vacuum interrupter to a “lock-out” condition, that is the vacuum interrupter will no longer be re-closed automatically. This typically occurs after the vacuum interrupter has interrupted load current on two immediately previous occasions, generally caused by a fault condition which is not able to be cleared. Upon ejection, the link  20  carrying the vacuum interrupter  23  rotates to the position shown in FIG. 2. This arrangement mimics the operation of a dropout fuse when the fuse blows, clearly indicating the lock-out condition, providing a visible point of isolation and requiring manual intervention to re-set the isolating circuit breaker.  
         [0110]    Power is supplied to the control module  30  and a capacitor arrangement  35  for driving the solenoids  27 ,  34  from the current transformer  31 . In an alternative arrangement, the voltage divider  32  may be employed to supply the control module  30  separately. The capacitor arrangement for the actuator solenoid  27  is maintained fully charged, with enough energy to effect the maximum number of open (“trip”) and close operations of the vacuum interrupter to lockout. The power supply may further include a rectifier and voltage regulator (not shown). The sensor arrangement, the electronics contained in the control module  30  and the solenoids all float at line voltage, thereby reducing the requirement for voltage isolation. The interior of the insulating housing  13  is filled with a hydrophobic material, such as silicone grease to prevent ingress of moisture.  
         [0111]    The cartridge of the control module  30  may not be removed from the housing  13  unless the vacuum interrupter is open, whereby the manual operating lever  28  swings away to allow the cartridge to be unplugged. When removed the cartridge may be manipulated to adjust operating parameters of the isolating switch, such as protection settings and time delays. The cartridge may be manipulated directly, such as with a screw-driver to adjust potentiometers or, more desirably, can interface with a portable computer or hand-held programming device as in the present embodiment. The control module of the embodiment includes a micro-controller and associated memory, and data logs of electrical conditions on the distribution line may be downloaded in addition to adjustment of protection settings for the isolating circuit breaker. The control module may also be removed for repair or electronics upgrade.  
         [0112]    A second embodiment of the invention is illustrated in FIG. 3 of the drawings, wherein a series of three isolating circuit breakers  10 A,  10 B and  10 C are provided for respective phase lines A, B and C of a multi-phase distribution circuit. Supply conductors  36 A,  36 B and  36 B are connected to respective first line terminals  14 A,  14 B and  14 C; whilst load conductors  37 A,  37 B and  37 C are connected to respective second line terminals  15 A,  15 B and  15 C.  
         [0113]    The three isolating circuit breakers  10 A,  10 B,  10 C of the second embodiment are similar to those described above in relation to the first embodiment, except that a communications transceiver is incorporated into each housing. Signals indicative of electrical conditions in one phase, for example phase A, are transmitted via the communications interface to control modules located in the remaining phases, for example phases B and C. This allows the control modules to process line condition signals from each phase in substantially real time to control operation of individual actuators.  
         [0114]    The communications transceiver uses radio as the communications medium, whereby the signals produced by the sensor are indicative of the instantaneous value of the current and/or voltage on each phase line. Each of the control modules sums the instantaneous values whereby a residual vector sum is indicative of a fault condition in the multi-phase circuit.  
         [0115]    Alternative communications arrangements may be optical, involving optical fibres, or utilise an acoustic medium such as ultrasonic band frequencies.  
         [0116]    [0116]FIG. 4 is a diagrammatic view of an isolating circuit breaker  40  of a third embodiment. The mounting assembly for the circuit breaker includes insulating members  41 ,  42  having upper and lower support arms which incorporate respective line terminals  44  and  45 . The insulating members are supported by a bracket  43  which is mounted on the cross-arm  11  of a utility pole  12 . The upper support arm  46  is a conductor and carries a first isolating contact  48  of an isolating switch  50 , whilst the second isolating contact  49  is formed on an upper free end of a pivotally mounted link member  52 . The pivot for the link member  52  is provided on an end  51  of the lower support arm  47 .  
         [0117]    [0117]FIG. 5 shows the link  52  in a pivoted position whereby the isolating switch  50  is in an open position, providing visible air gap isolation between first isolating contact  48  and second isolating contact  49 . In the embodiment, a supply conductor  53  of a distribution line may be coupled to the first line terminal, whilst a load conductor  54  may be coupled the second line terminal. As with the previous embodiments, the link  52  also includes a circuit interrupter disposed both in series with the isolating contacts  49 ,  50  and intermediate the supply conductor  53  and load conductor  54 .  
         [0118]    The circuit interrupter is a component of link  52  and takes the form of a vacuum interrupter  55  in the present embodiment. The fixed terminal  56  of the vacuum interrupter is pivotally coupled to the lower conductive support arm  47  for the link. The movable terminal  57  of the vacuum interrupter is mechanically coupled to an actuator, that here takes the form of a magnetic actuator  58 . The fixed terminal  56  of the vacuum interrupter is electrically coupled to a conductor  59  which carries the second isolating contact  49  of the isolating switch  50 .  
         [0119]    The conductor  58  is associated with a sensor arrangement for producing signals in response to electrical conditions in the distribution line. The sensor arrangement includes a current transformer  60  for detecting current flowing therein, and current flow signals produced by the current transformer  60  are provided to an electronic controller  61 . The electronic controller  61  suitably includes an integrated circuit device, in the form of a microprocessor and a memory for storing both operating programs and related data. The data includes protection settings relating to operation of the isolating circuit breaker  40 , in particular parameters for an automatic re-closing sequence for the vacuum interrupter  55 .  
         [0120]    In addition, the vacuum interrupter may be manually tripped by way of an operating lever  62  which protrudes from a housing containing the electronic controller  61 , the current transformer  60  and the magnetic actuator  58  to which the operating lever is mechanically coupled. When the vacuum interrupter  55  is opened or “tripped”, the operating lever  62  can then be used to open the isolating switch  50  to a position as shown in FIG. 5. This provides a visual break in the distribution line, in accordance with safety requirements for work by linesmen.  
         [0121]    In the third embodiment, the housing containing the magnetic actuator  58 , the current transformer  60 , associated conductor  59  and electronic controller  61  is incorporated into the link  52 . A mechanism (not shown) mechanically links the movable terminal  57  of the vacuum interrupter  55  to the manual operating lever  62 . In a first arc of operation the operating lever can open the vacuum interrupter  55 . In a second arc of operation, the operating lever can open the first and second contacts  48 ,  49  of the isolating switch  50 .  
         [0122]    The housing  63  may be constructed of a metallic material, such as aluminium. Bulky and sometimes unreliable insulation arrangements are not required for the housing of the embodiment since it is suitably arranged to float at the line potential of the distribution circuit when in operation. Supply for the electronic controller  61  is provided by the current transformer  60 , although other arrangement may also include a battery back-up arrangement for added reliability of operation.  
         [0123]    [0123]FIG. 6 illustrates a group of three isolating circuit breakers  40 A,  40 B,  40 C of a fourth embodiment arranged in array in relation to a multi-phase distribution circuit. The isolating circuit breakers of the fourth embodiment are similar to those described above in relation to the third embodiment, except they further include a mechanical coupling arrangement for their respective circuit interrupter actuating mechanisms. In particular, the coupling arrangement allows the mechanisms to be linked by insulating coupling rods  64  such as depicted in FIG. 6.  
         [0124]    In the present embodiment, the coupling rods  64  are coupled between the actuating mechanisms in close vicinity to the manual operating levers  62 A,  62 B and  62 C. Accordingly, automatic operation of a circuit interrupter initiated by at least one of the electronic controllers may be arranged to cause all three phases to open. Similarly, a manual operation by a linesman hooking one of the operating levers  62  may be arranged to trip all three circuit interrupters, if desired. It will be appreciated that it is desirable, for safety reasons, to ensure that the circuit interrupters are closed only after the isolating switches have been closed.  
         [0125]    [0125]FIG. 7 is a block diagram illustrating a preferred electro-mechanical arrangement suitable for isolating circuit breaker of the above embodiments. An electronic controller  70  receives supply from a power source  71 , which may be derived from the distribution line and/or a battery or similar storage cell, via a power supply circuit  72 . The power supply circuit  72  conditions power from the power source  71  for all the electronic circuitry in the controller  70 .  
         [0126]    The sensor arrangement  74  includes transducers, such as current and voltage transformers, for monitoring the distribution line. The transducers provide electrical signals to respective analogue circuits  73 , which signals are indicative of electrical conditions in the distribution line. The electrical signals from the transducers are conditioned an converted into digital form, using an analogue to digital converter (ADC) for example, and passed to the microcontroller  76 .  
         [0127]    The micro-controller  76 , which includes a microprocessor and associated memory in the embodiment, processes the digital signals in accordance with protection settings contained in the memory. Other embodiments may utilise gate arrays or an applications specific integrated circuit in the controller. The processing may also take account of other digital data received from other electronic controllers via the peer communications interface  77 . This data typically relates to controllers associated with other phases in a multi-phase distribution circuit. The interface includes a communications transceiver which may utilise radio, fibre optic, ultrasonic or other convenient communications medium.  
         [0128]    The micro-controller  76  also controls an actuating mechanism  79  via an actuating mechanism interface  78 . Signals to trigger operation of actuators for driving the circuit interrupter  80  and (if required) isolating switch are supplied via the interface  78 . Signals from position indicating switches are also received from the actuating mechanism  79  via the interface  78 .  
         [0129]    A first mechanical linkage is provided to drive the circuit interrupter  80 , whilst a second linkage may be provided to mechanically gang circuit interrupters in other phases, as required. In one arrangement, the second linkage is operated only when a persistent fault condition is not cleared by an automatic re-closing sequence. Under these conditions all interrupters are operated substantially simultaneously to a lock-out condition.  
         [0130]    An external communications interface  75  may also be provided to facilitate remote or near field operations or programming tasks, as required. These tasks include varying the protection settings in the memory or upgrading or renewing processing software. Remote operations may include selective tripping from a remote control centre to facilitate maintenance of associated distribution system equipment.  
         [0131]    It will be appreciated that the first embodiment of the invention may be modified to include a mechanical, rather than electronic, ganging arrangement as required. Similarly, the electronic ganging arrangement could be omitted from the third embodiment if desired for reasons of cost savings.  
         [0132]    The isolating circuit breaker of the invention can suitably be retrofitted in place of dropout fuses and configured to provide a re-closing function at a significantly lower cost than installation of a separate recloser. The capability of manual operation of the isolating switch is also preserved. When used in a three phase configuration, the added protection provided by detection of voltage or current imbalances between phases, such as caused by earth faults, is available,.  
         [0133]    Throughout this specification, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers or steps.  
         [0134]    Although illustrative embodiments of the present invention, and various modifications thereof, have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and the described modifications, and that various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.