AU2017334269B2

AU2017334269B2 - Enclosed switch visible break isolator

Info

Publication number: AU2017334269B2
Application number: AU2017334269A
Authority: AU
Inventors: David Leslie Dart; Fahmid KHONDAKER; Muliyil K. MAHESH MUKUNDAN; Jukka Pekka MANNE; Michael Raymond MAURER; Neil Edward O'SULLIVAN; Oleg Vladimirovich SAMARSKI; Joni Sahak Tortian
Original assignee: NOJA POWER SWITCHGEAR Pty Ltd
Current assignee: NOJA POWER SWITCHGEAR Pty Ltd
Priority date: 2016-09-29
Filing date: 2017-09-28
Publication date: 2021-08-05
Anticipated expiration: 2037-09-28
Also published as: AU2017334269A1; WO2018058196A1

Abstract

An electrical switchgear assembly (100) including: a pair of electrical contacts (104a, 104b); a bushing (200) defining an internal chamber (201) in which the electrical contacts are located; a viewer (208), suitably including a window pane (620), in the bushing for enabling viewing an opened or closed status of the contacts in the internal chamber; and one or more electric field control screens (600, 602), suitably embedded in a dielectric body (206) of the bushing, for controlling electric field generated by the pair of contacts (104) to create a substantially uniform electric field (E) between the contacts to reduce electrical stress within the internal chamber (201).

Description

ENCLOSED SWITCH VISIBLE BREAK ISOLATOR

TECHNICAL FIELD

The present invention generally relates to electrical power switchgear, particularly although not exclusively, to air-filled, medium voltage enclosed switchgear for outdoor use.

BACKGROUND

Electrical switchgear is used outdoors on overhead power lines and substation applications, commonly for circuits operating in voltage ranges of about 10- 38kV, to connect and disconnect electrical circuits. Typical switchgear includes electrical isolators or disconnectors and interrupters or circuit breakers.

An interrupter, such as a vacuum interrupter, is used to interrupt the circuit and break current flow. The interrupted current is either normal load current in normal operating conditions, or short circuit current in fault conditions. Not all interrupters are considered as isolators that are capable of providing safe isolation between load and source sides after interruption. A significant problem with interrupters is the inability for an operator to readily and confidently confirm physical separation of the contacts after opening, in accordance with safety requirements. Additionally electrical isolators are conventionally used, in the circuit and in series to the interrupter, to safely disconnect the electrical circuit for service or maintenance and ensure isolation. Invariably, switchgear contacts need to be manually opened and open contact state verified during maintenance or replacement of equipment in the circuit. In one form isolating switches for pole- mounted equipment, such as knife-blade switches, do not include any housing. The absence of any housing allows ease of verifying open contact state by a switch operator. However, non-enclosed isolators are exposed to the environment (weather and pollution), including storm debris, and themselves incur additional maintenance for reliable operation. Loose contacts leading to compromised circuit continuity is a common problem for unhoused isolators.

US patent application publication No. 2007/0295694 discloses a gas-tight, medium voltage electric switch containing an insulating gas, such as SF6, under pressure. The switch contacts are contained in a tubular composite insulator surrounded by an elastomer casing that are arranged to define an observation window through which the position of the contacts is observable. However, environmental considerations are dictating discontinuation of the use of noxious gases such as SF6 for insulating purposes. Furthermore, the provision of an observation window adds to risks of gas tight sealing required to reliably contain the insulating gas, and adds to manufacturing costs.

Applicant's earlier Australian patent application AU 2016200621 A1 ("the '621 application") discloses an electrical switchgear assembly including an isolator having a pair of electrical isolator contacts and an interrupter including a pair of electrical interrupter contacts within a housing. A driving mechanism is provided for driving open each pair of electrical contacts. Advantageously, an operator can manually open the contacts of both the isolator and interrupter in the correct sequence using the driving mechanism.

The disclosures of the above-referenced earlier '621 application are incorporated herein by reference. The '621 application discloses an electrical switchgear assembly 50 as shown in Figure 1 , which is suitable for outdoor use, such as by mounting to an electricity pole typically around 10m above ground level, and is exposed to the elements (e.g. rain, wind).

The assembly 50 includes an isolator 52 including a pair of electrical isolator contacts 54 - including a fixed contact 54a and movable contact 54b - with relatively wide separation, and between which arcing does not occur. The assembly 50 further includes a vacuum interrupter 56 including a pair of electrical interrupter contacts 58a, 58b with comparatively narrow separation. A mechanical driving mechanism 60 is provided for opening and closing each pair of electrical contacts 54, 58, and is configured to carefully control the sequence of the opening and closing of the respective contact pairs. The isolator 52 and interrupter 56 are serially interconnected in an electrical circuit (not shown) for a load break application.

Advantageously, an operator can manually open and close the contacts 54, 58 of both the isolator 52 and interrupter 56 using the driving mechanism 60 during maintenance or replacement of electrical equipment in the circuit. The separation of the comparatively close interrupter contacts 58 alone would be insufficient, particularly if the vacuum interrupter 56 fails, such as by losing vacuum, in which case the contacts 58 may be bridged by arcing at atmospheric pressure. The electrical isolator contacts 54 have an air gap sufficient to withstand the rated isolation. The assembly 50 is typically rated at 15.5kV line voltage (nominally operating at 1 1 -12kV) wherein the isolator gap between contacts 54 can withstand a 1 10kV impulse (i.e. 10 times typical operating voltage). Whilst the isolator contacts may be enclosed in an air-filled housing, the housing is relatively bulky due to the significant separation of the isolator contacts, referred to as the "isolation gap", necessitated by a desired insulation voltage rating. This separation has a flow-on effect to the size of the driving mechanism, which must operate over the isolator contact spacing. Furthermore, an operator cannot readily confirm the state of the contacts, (e.g. separated) where required for reasons of safety. Verifying the state of enclosed switchgear is typically done by viewing an indicator that is mechanically linked to the contacts. In the event that there is a failure of the linkages, the indicator may provide false indication of the state of the switchgear. This situation poses a significant safety risk for operators working on the circuit.

Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge. It is an object of the present invention to provide an apparatus that addresses one or more of the problems of the prior art, or at least provides a useful alternative to existing electric switchgear arrangements.

SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided an electrical switchgear assembly including:

a pair of electrical contacts;

a bushing defining an internal chamber in which the electrical contacts are located;

a viewer in the bushing for viewing an opened or closed status of the contacts in the internal chamber from outside the bushing; and

one or more electric field control screens for controlling electric field generated by the contacts to create a uniform electric field between the contacts to reduce electrical stress within the internal chamber. Advantageously, an operator of the switchgear assembly may confirm that the contacts are open and that the associated circuit is safely isolated, by viewing an isolating break in a current carrying path incorporating the pair of contacts. Preferably, the viewer enables viewing of at least one of the contacts, for example in a closed state of the contact pair. If required, the viewer may enable viewing of the pair of contacts.

Each control screen may be of substantially annular or tubular configuration. Each control screen may be formed from conductive material (e.g. metal) or of a semi-conductive material. The screens may be provided on either side of the viewer. The viewer may not be at ground potential, suitably being at a floating potential.

The control screens may be embedded in a body of the insulating bushing. The control screens may create a uniform electric field in a gap between the contacts. One of the control screens may be located between the viewer and a tank upon which the bushing is mounted. At least one of the control screens may be at ground potential. The remaining control screens are preferably at a floating potential.

Preferably, the viewer includes a window. The window may include a window pane. The window pane may be transparent or translucent. The window pane may form a waterproof seal with the bushing. The bushing may include a body formed of solid dielectric (e.g. epoxy, silicone, plastic, etc.). The window pane may be assembled, bonded or molded within the body of the bushing.

At least one of the control screens may surround the window pane. At least one of the control screens may control the electric field around or at least proximal to a glass window pane of the viewer. The screen may electrically shield the window pane. At least one of the control screens may hold the glass window pane in place.

The internal chamber may be an enclosed air-filled chamber in which the electrical contacts are located. Preferably, the viewer is located in the bushing such that, when the electrical switchgear is pole-mounted, an operator on ground level may view the pair of contacts from below the assembly. The assembly may further include another viewer in another bushing associated with another pair of contacts, and for enabling viewing of the other pair of contacts. The assembly may further include yet another viewer in a housing, such as the tank for housing a driving mechanism for driving open or closed the pair of electrical contacts.

The electrical contacts located within the chamber may be isolator contacts.

According to another aspect of the present invention, there is provided a bushing arrangement for a load break electrical switchgear assembly with a pair of electrical contacts, the bushing arrangement defining an internal chamber in which the electrical contacts are located and including a viewer for enabling viewing an opened or closed status of the contacts, the bushing arrangement further including one or more electric field control screens for controlling electric field generated by the electrical contacts to create a uniform electric field between said contacts to reduce electrical stress within the internal chamber. In a further aspect of the invention, there is provided an outdoor electrical switchgear assembly for a load break application, the assembly including: a first switch arrangement including a first pair of electrical contacts; a second switch arrangement including a second pair of electrical contacts;

a driving mechanism for driving open each pair of electrical contacts; a bushing defining an internal chamber in which one of the pairs of contacts is located and including a viewer for viewing an opened or closed status of said one of the pairs of contacts; and

one or more electric field control screens for controlling electric field generated by at least one pair of contacts, creating a uniform electric field between said at least one pair of contacts to reduce electrical stress within the internal chamber.

Preferably the first switch arrangement is an isolator and the second switch arrangement is an interrupter. Suitably, the electric field control screens control the electric field generated by contacts of the isolator switch.

The assembly may further include another bushing arrangement defining another internal chamber in which the other pair of contacts is located and including another viewer for enabling viewing of the other pair of contacts

In a still further aspect of the invention, there is disclosed an operation method for an outdoor electrical switchgear assembly for a load break application, the assembly including a pair of electrical contacts and a bushing defining an internal chamber in which the electrical contacts are located, the method involving viewing an opened or closed status of the contacts through a viewer in the bushing. In yet another aspect of the invention, there is disclosed an outdoor electrical switchgear assembly including:

a pair of electrical contacts;

a dielectric insulator defining an internal chamber in which the electrical contacts are located;

a viewer in the insulator and for viewing an opened or closed status of the contacts in the internal chamber; and

one or more electric field control screens for controlling electric field generated by the pair of contacts, creating a substantially uniform electric field between said pair of contacts to reduce electrical stress within the internal chamber.

Preferably, the viewer enables viewing of at least one of the contacts, for example in a closed state of the contact pair. The viewer may also enable viewing of a break in a current carrying path incorporating the contacts. If required, the viewer may enable viewing of the pair of contacts.

Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

Figure 1 is a sectional side view of a pole mounted electrical switchgear assembly 50 of the PRIOR ART, with both contact pairs in a fully open configuration; Figure 2 is a partially sectioned side view of an electrical switchgear assembly 100 including a bushing having a viewer in accordance with an embodiment of the present invention;

Figure 3 is a partially sectioned side view of the electrical switchgear assembly of Figure 2 with a contact pair in a closed configuration and visible through the viewer;

Figure 4 is a perspective view of a bushing of the assembly of Figure 2;

Figure 5 is a perspective view of a three-phase electrical switchgear assembly in accordance with another embodiment of the present invention; Figure 6 is a sectional side view of the switchgear assembly of Figure 5;

Figure 7 is an electric field plot of the switchgear assembly as shown in Figure 6; and

Figure 8 is an electric field plot of the switchgear assembly of Figure 6 after removal of the main electric field control screens. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning to Figure 2, the switchgear assembly 100 of the preferred embodiment includes an isolator bushing 200 defining an internal chamber 201 in which a pair of electrical isolator contacts 104a, 104b are located, a metal tank 202 in which a driving mechanism (not shown) is located, and an interrupter bushing 204 in which a pair of interrupter contacts (not shown) are located. The separate bushings 200, 204 are each mounted on the tank 202, and are sealed together to impede the ingress of moisture thereby suiting the switchgear assembly 100 to outdoor use.

The isolator bushing 200 is a dielectric insulator and includes a molded body 206 formed of opaque and solid resin, silicone, epoxy or other like solid dielectric material. A viewer 208 having an opening in the body 206 is provided in the bushing 200 to enable viewing of the isolator contacts 104a, 104b, typically moving contact 104b. Advantageously, an external operator can view the internal contacts 104a, 104b to verify their status (e.g. open or closed). In particular, the operator can visually confirm that the contacts 104a, 104b are open (i.e. separated) and that the associated circuit is thus safely isolated.

The viewer 208 has a window including a floating-potential window pane 207 provided in the molded (e.g. resin) body 206. The window pane is composed of an insulating material that is at least translucent, and preferably transparent (e.g. glass, plastics). The pane 207 can be molded within the body 206 of the bushing 200 to form a waterproof seal with the bushing 200 and impede ingress of water. Alternatively, the pane 207 can be assembled by press fitting in the body 206, held in compression by the resilient body 206 or fixed thereto by screwing to the body 206. The assembly 100 thereby defines an internal sealed air-filled chamber 102 in which the contacts 104a, 104b are located and enclosed. The air chamber is filled at about atmospheric pressure and is SF6 gas free.

In practice, an operator can view the contacts 104a, 104b through the viewer 208 thereby readily determining the opened or closed state of the switchgear assembly 100. In particular, the operator standing on the ground adjacent to an electricity pole carrying the switchgear assembly 100 in an elevated position (e.g. 10m above ground level) can directly view the movable contact 104b upwardly through the viewer 208. As shown in Figure 2, the contacts 104a, 104b are separated in an "open" configuration indicating that the circuit is isolated. The viewer thus enables an operator to view an isolating break in the current carrying circuit or path incorporating the contact pair. As shown in Figure 3, the contacts 104a, 104b are together in a "closed" configuration, indicating that the circuit is not isolated (and thus may be live) and that caution must be exercised by the operator.

Turning to Figure 4, the bushing 200 may include multiple viewers 208 with respective window panes 207 so that the contacts 104a, 104b can be viewed from several angles. In use, a flashlight beam can be directed through the pane of one viewer 208 to illuminated contacts 104a, 104b whilst they are conveniently viewed through the pane of another adjacent viewer 208.

A three phase electrical switchgear assembly 100 of another embodiment of the invention is shown in Figures 5-7. The switchgear assembly 100 includes an individual isolator bushing 200 for each phase line of the three phases, wherein each bushing includes at least two opposed viewers 208. Conveniently, where the switchgear assembly is pole mounted, the opposed window panes 207 of the viewers may be disposed on a substantially vertical axis in order to capture ambient light for viewing contact state by an operator from ground level.

Turning to Figure 6, the isolator bushing 200 and therefore the overall switchgear assembly 100 can advantageously be reduced in size with the inclusion of strategically placed electric field control screens 600 - 608. Typically, the switchgear assembly would be up to three (3) times larger in the absence of control screens, including in order to provide an adequate isolation in an air-filled chamber. Each control screen 600 - 608 is annular or tubular, some including substantially cylindrical or frustro-concial portions in a body of revolution. The electric field control screens are suitably shaped in an iterative design process with the aim of achieving substantial uniformity of the local electric field to reduce peak electrical stresses within the internal chamber, including between the contacts and on adjacent insulation, such as the body 208 of the bushing 200. Each control screen 600 - 608 is here formed from conductive material such as a metal (e.g. aluminium or copper) and formed by standard manufacturing processes such as casting, machining or forging. The bushing 200 is then moulded from dielectric material such as epoxy, silicone or plastic. During manufacture, the control screens are suitably placed in a moulding tool and over-moulded with the dielectric material during the moulding process.

The main electric field control screens 600, 602 for the contacts (hereafter "contact screens") are disposed adjacent to the contacts 104a, 104b being embedded in portions of the insulating material body 206a, 206b that receive respective isolator contacts 104a, 104b. The contact screens 600, 602 control the electric field in the gap between the contacts 104a, 104b. An embedded tank screen 604 is located between the viewer 208 and tank 202 (shown in phantom) and is of annular configuration receiving the tubular contact screen 602 therein. The tank screen 604 electrically contacts the tank 202 and can be at ground potential.

Auxiliary embedded viewer screens 606 surround the viewer 208 through which the contacts 104a, 104b can be viewed from two sides via window panes 620, here composed of glass. The viewer screens 606 control the electric field around the glass window panes 620 of the viewer 208 and electrically shield the panes 620. Cap screens 608 hold respective glass window panes 620 in place and provide further electric field control. The viewer and cap screens 606, 608 surrounding the window panes 620 can also reduce the risk of electrical failure and improve the longer term performance and life of the assembly 100 by reducing the risk of partial electrical discharge within the internal chamber. In an alternative embodiment, the window pane 620 may be interference press fit into the bushing 200. In a further embodiment, the window pane 620 could be bonded on to the bushing 200 using an adhesive or may be moulded into the bushing during the bushing moulding process. The electrical field control screens 600 - 608 together control the electric field generated by the pair of contacts 104 across the viewer 208 to reduce the risk of electrical breakdown between the contacts or within the chamber. As well as preventing immediate failure under fault conditions, the electric field control also aids in the prevention of failures due to surface contamination (i.e. pollution build up), degradation of material over time (e.g. UV and exposure to the elements), and/or geometric imperfections introduced during manufacture. It is preferable that the control screens 600, 602 are embedded in the dielectric body portions 206a, 206b of the bushing rather than in the form of an internal surface layer. This is particularly desirable where the bushing 206 and discontinuities - such as marginal edges of window panes 207 of the viewer 208 - are at a floating potential which contributes to electrical stress. The diagram in Figure 7 shows that the screens 600 - 608 control the electric field to be substantially uniform between the contacts 104a, 104b as depicted by the evenly spaced electrical field lines E. The field lines are generally closer together in the insulating material when compared with air. By way of comparison, the assembly 100' with screens 600 - 608 removed is shown in Figure 8. The electric field is not uniform between the contacts 104a, 104b, and the extremely dense spacing between the electrical field lines at moving contact 104b (typically at line potential) results in increased electrical field gradient that increases the likelihood of breakdown and arcing failure. A person skilled in the art will appreciate that many embodiments and variations can be made without departing from the ambit of the present invention. In one embodiment, the interrupter bushing 204 may take the same form as the isolator bushing 200 for enabling viewing of the interrupter contacts 108a, 108b.

In another embodiment, the assembly 100 may further include yet another viewer 208 in the tank 202 (i.e. housing) allowing inspection of at least a portion of the operating mechanism.

In another embodiment, the bushing 208 may include an internal background screen such that the contacts 104a, 104b are located between the screen and the viewer to facilitate viewing of one or more of the contacts 104a, 104b. The background screen may be brightly colored in a contrasting hue. In a variation to this, the contacts may also or alternatively be brightly colored to facilitate improved visibility.

In yet another embodiment, the bushing 200 may be integrally formed with the tank 202. In some limited applications such as deflection of relatively minor electric fields, the electric field screens may be composed of a semi-conductive or a high permittivity material (e.g. high permittivity silicone). Advantageously, the operator of the switchgear of the embodiment can utilise the viewer to inspect the electrical contacts to verify the status (e.g. opened or closed) of the switchgear assembly. In particular, the operator may confirm that the contacts are open and that the associated circuit is safely isolated. The electric field control screens may provide for a more compact assembly by allowing the contacts to be more closely spaced in the open condition, also reaping further benefits by allowing a more compact driving mechanism. In the absence of the control screens or increasing the physical size of the switchgear assembly, the contact spacing (when open) would not provide the rated basic insulation level and the assembly would need to rely upon another break - such as a vacuum interrupter - to achieve an equivalent basic insulation level of a switch including the electric field screens.

In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. The term "comprises" and its variations, such as "comprising" and "comprised of" is used throughout in an inclusive sense and not to the exclusion of any additional features.

It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.

Reference throughout this specification to One embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Throughout the specification and claims (if present), unless the context requires otherwise, the term "substantially" or "about" will be understood to not be limited to the value for the range qualified by the terms.

Claims

CLAIMS:

1 . An electrical switchgear assembly including:

a pair of electrical contacts;

one or more electric field control screens for controlling electric field generated by the contacts to create a uniform electric field between the contacts to thereby reduce electric stress within the chamber.

2. The electrical switchgear assembly as claimed in claim 1 wherein the viewer enables viewing of at least one of the contacts of the contact pair.

3. The electrical switchgear assembly as claimed in either claim 1 or claim

2, wherein each electric field control screen is of annular or tubular configuration.

4. The electrical switchgear assembly as claimed in any one of claims 1 to

3, wherein each electric field control screen is formed from a conductive material or a semi-conductive material.

5. The electrical switchgear assembly as claimed in any one of claims 1 to

4, wherein said one or more electric field control screens are provided on either side of the viewer.

6. The electrical switchgear assembly as claimed in any one of claims 1 to

5, wherein the viewer is not at ground potential.

7. The electrical switchgear assembly as claimed in any one of claims 1 to

6, wherein the electric field control screens are embedded in a dielectric body of the bushing.

8. The electrical switchgear assembly as claimed in any one of claims 1 to 7, wherein the electric field control screens create a uniform electric field in a gap between the pair of contacts.

9. The electrical switchgear assembly as claimed in claim 1 , wherein one of the electric field control screens is located between the viewer and a tank upon which the bushing is mounted.

10. The electrical switchgear assembly as claimed in any one of claims 1 to

9, wherein at least one of the electric field control screens is at ground potential.

1 1 . The electrical switchgear assembly as claimed in any one of claims 1 to

10, wherein at least one of the electric field control screens surrounds the viewer.

12. The electrical switchgear assembly as claimed in any one of claims 1 to

1 1 , wherein at least one of the electric field control screens controls the electric field proximal to a window pane of the viewer.

13. The electrical switchgear assembly as claimed in claim 12, wherein said at least one electric field control screen electrically shields the window pane.

14. The electrical switchgear assembly as claimed in any one of claims 1 to

13, wherein at least one of the electric field control screens holds a window pane of the viewer in place.

15. The electrical switchgear assembly as claimed in any one of claims 1 to

14, wherein the viewer is located in the bushing to enable an operator to view at least one of the contacts or an isolating gap in a current path including the contacts from below the assembly.

16. The electrical switchgear assembly as claimed in any one of claims 1 to

15, wherein the internal chamber in which the contacts are located is an enclosed air-filled chamber.

17. An assembly as claimed in claim 1 , wherein the electrical contacts located within the enclosed chamber are isolator contacts.

18. An assembly as claimed in claim 1 , wherein the viewer enables viewing of the contacts from more than one orientation.

19. An assembly as claimed in claim 1 wherein the viewer enables viewing of an isolating break in a current carrying path in the opened state.

20. A switchgear arrangement for a load break application, the arrangement including:

the electrical switchgear assembly as claimed in any one of claims 1 to

19;

another pair of electrical contacts arranged in series with said pair of electrical contacts; and

a manually operated mechanical driving mechanism for driving open each pair of electrical contacts.

21 . The switchgear arrangement as claimed in claim 20 wherein the other pair of electrical contacts is comprised by a vacuum interrupter.

22. A bushing arrangement for an electrical switchgear assembly with a pair of electrical contacts, the bushing arrangement defining an internal chamber in which the electrical contacts are located and including a viewer for viewing an opened or closed status of the contacts in the internal chamber, the bushing arrangement further including one or more electric field control screens for controlling electric field generated by the electrical contacts to create a substantially uniform electric field between said contacts to reduce electrical stress within the chamber.

23. A bushing arrangement for an electrical switchgear assembly wherein the viewer enables viewing of at least one of the electrical contacts.

24. The bushing arrangement as claimed in claim 22 wherein the internal chamber is defined by dielectric body portions surrounding respective contacts, and said electric field control screens are embedded in said body portions.

25. The bushing arrangement of either claim 22 or claim 23 wherein the viewer includes a window pane held in place by an electric field control screen.

26. The bushing arrangement of any one of claims 22 to 25 wherein the viewer enables viewing of an isolating gap in a current path incorporating the pair of contacts.