Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
SURGE ARRESTOR AND FUSE ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2018/055547
Kind Code:
A1
Abstract:
A combination surge arrestor and dropout fuse assembly is provided in which an electrically conductive supply bus-bar (1) carried by an insulator (2) supported on a mounting base (3) has a bus-bar (1) providing a supply contact for an operatively upper cooperating releasable terminal of a dropout fuse holder carried at its opposite end by an articulated contact assembly. The bus-bar (1) also has at least two electrically conductive surge arrestor contacts (13) for cooperation with terminals of two dropout surge arrestors (15) that operatively extend between the surge arrestor contacts (13) connected to the bus-bar (1) and earth contacts carried by a mounting base (3). In use, with a drop-out fuse and two dropout surge arrestors (15) in situ in the assembly, the dropout fuse and each of the two surge arrestors (15) are able to drop out independently of each other as and when it may become blown or otherwise ineffective.

Inventors:
VAN HEERDEN ANTON (ZA)
Application Number:
PCT/IB2017/055730
Publication Date:
March 29, 2018
Filing Date:
September 21, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
VAN HEERDEN ANTON (ZA)
International Classes:
H01H85/02; H02H3/04
Domestic Patent References:
WO2009127934A22009-10-22
Foreign References:
US5412526A1995-05-02
Attorney, Agent or Firm:
VON SEIDELS INTELLECTUAL PROPERTY ATTORNEYS (ZA)
Download PDF:
Claims:
CLAIMS:

1 . A combination surge arrestor and dropout fuse assembly comprising an electrically conductive supply bus-bar carried by a first insulator part supported on a mounting base wherein the bus-bar supplies an electrically conductive supply contact to supply an operatively upper cooperating releasable terminal of a dropout fuse holder carried at its opposite end by an articulated contact assembly supported by a second insulator part that is also carried by a mounting base, the bus-bar also having at least two electrically conductive surge arrestor contacts for cooperation with terminals of two dropout surge arrestors that operatively extend between the surge arrestor contacts connected to the busbar and earth contacts carried by a mounting base such that, in use, with a drop-out fuse and two dropout surge arrestors in situ in the assembly, the dropout fuse and each of the two surge arrestors are able to drop out independently of each other as and when it may become blown or otherwise ineffective.

2. A combination surge arrestor and dropout fuse assembly as claimed in claim 1 in which the surge arrestor contacts connected to the bus-bar and the earth contacts carried by the mounting base are arranged so that surge arrestors in situ therein are arranged electrically in parallel with each other such that both are connected to the surge arrestor contacts connected to the bus-bar and the earth contacts simultaneously and each is able to drop out if it becomes blown or otherwise ineffective with the other remaining in circuit as backup protection.

3. A combination surge arrestor and dropout fuse assembly as claimed in claim 1 in which the surge arrestor contacts that are connected to the bus-bar and the earth contacts that are carried by the mounting base are arranged so that surge arrestors operatively in situ therein are connected to earth by way of a switching assembly that electrically connects one earth contact at a time into the circuit between the surge arrestor contacts connected to the busbar and earth.

4. A combination surge arrestor and dropout fuse assembly as claimed in claim 3 in which a single electrical earth conductor is provided that is movable between two terminal positions in one of which one operatively installed surge arrestor is connected into the operative condition in which it is connected to earth and in the other of which the other operatively installed surge arrestor is connected to earth.

5. A combination surge arrestor and dropout fuse assembly as claimed in claim 4 in which the single earth conductor is associated with the earth contacts of the surge arrestors such that a disconnected surge arrestor earth contact is insulated at least sufficiently to isolate it from earth to a substantial extent with a single earth connection being made to the single earth conductor that sequentially connects one surge arrestor earth contact to earth and, should an installed surge arrestor drop out, the weight of the dropped out surge arrestor is arranged to move the movable single earth conductor to its other terminal position in which the other surge arrestor earth terminal is connected to the single earth connection.

6. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which the arrangement is such that installed dropout surge arrestors move in planes that are generally upright and parallel to a plane of movement of the dropout fuse holder.

7. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which the dropout surge arrestor contacts and co-operating earth contacts are orientated such that a surge arrestor installed therein extends transverse to the dropout fuse.

8. A combination surge arrestor and dropout fuse assembly as claimed in claim 7 in which an installed drop out surge arrestor extends in a generally transverse direction relative to the dropout fuse.

9. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which the dropout surge arrestor contacts are arranged one on each side of the dropout fuse holder contacts.

10. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which the electrically conductive surge arrestor contacts carried by the bus-bar are releasable with respect to co-operant terminals of an installed surge arrestor in which instance the other ends of an installed surge arrestor contacts, in use, the earth contacts carried by the mounting base by way of articulated dropout contact assemblies.

1 1 . A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which the dropout fuse holder is orientated such that the fuse holder in its operative position is generally upright but somewhat inclined towards the direction in which it is intended it should drop out if it should blow.

12. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which a single mounting base supports both insulator parts that carry at their opposite ends the supply contact and the articulated contact connecting, in use, to the two ends of the dropout surge arrestors.

3. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which the supply contact and the surge arrestor contacts that form releasable terminal assemblies each include an electrically conductive leaf in which instance the releasable contact includes a recess for receiving a domed contact surface provided on an input contact at one end of each surge arrestor or fuse holder as the case may be.

4. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which each of the electrically conductive articulated contact assemblies associated with the fuse holder and the surge arrestors include a bifurcated cradle for releasably receiving, in use, a relatively rotatable articulated contact assembly associated with an end of a surge arrestor to be grounded in its operative position.

5. A combination surge arrestor and dropout fuse assembly as claimed in any one of the preceding claims in which a dropout fuse holder and two surge arrestors are installed in the assembly in their operative positions.

Description:
SURGE ARRESTOR AND FUSE ASSEMBLY CROSS-REFERENCE(S) TO RELATED APPLICATIONS

This application claims priority from South African provisional patent application number 2016/06507 filed on 21 st September 2016, which is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a surge arrestor and fuse assembly suitable for providing surge protection to transformers within an electrical distribution network especially, but not exclusively, pole mounted surge arrestor and fuse assemblies. More particularly, the invention relates to a dropout surge arrestor assembly that embodies an expulsion fuse forming a part of the surge arrestor in addition to a fuse holder.

The assembly may be used in conjunction with a conventional drop-out fuse co-operating with upper and lower hardware to enable the drop out action in the case of a conventional fuse or a double sequential dropout fuse action in the case of a fuse assembly such as is described in international patent publication WO 2015/040541 entitled DROPOUT FUSE ASSEMBLY AND FUSE HOLDER. BACKGROUND TO THE INVENTION

Surge arrestors are used extensively in high voltage electrical networks, such as at distribution points, to discharge currents associated with over voltage surges within the network line length to afford voltage damping and reduce a high voltage spike effect on both insulation and plant. Surge protection is particularly used to protect transformers, cables and line equipment.

In order to protect electrical equipment in an electrical network from damage caused by the effects of over voltage surges through the system, generally occasioned by voltage spikes which include those resulting from lightning strikes, static and other causes, a surge arrestor is installed in the supply line to a consumer to prevent or at least minimize damage to the consumer's equipment by allowing a discharge of the over voltage to earth and thereby avoid to a large extent consequential fires; and also avoid possible bodily harm or even electrocution of personnel.

It is common practice to install a surge arrestor in parallel with an insulated input conductor passing through a transformer bushing in order to dissipate surges to ground that would otherwise damage the transformer that would typically by caused by a breakdown in the insulating properties of the usual insulating oil in the relevant transformer housing. A surge arrester is thus connected to ground and functions by routing energy from an over-voltage transient to ground if one occurs, while isolating the conductor from ground at normal operating voltages.

Commonly, the surge arrestor is of the metal oxide, in particular zinc oxide, a voltage dependent resistor type (varistor) that can become faulty if subjected to an excessive severity of surge or number of surges in which instance the normally high resistance of the arrestor to normal voltages becomes diminished and the input terminal of the surge arrestor may become effectively grounded. The purpose of the expulsion device is to disconnect the surge arrestor under such conditions.

Although a surge is often of a very temporary nature, such as transients that may be a result of a lightning strike, a surge arrestor can fail as a result of supply follow through current. A surge arrestor is designed to be disconnected by an expulsion device comprising an explosive powder to forcefully and automatically disconnect a failed surge arrestor from the network to prevent a direct short circuit to earth, which would result in a lengthy network fault finding outage.

Once a surge arrestor has been disconnected after failure, there is no longer any protection of equipment such as transformers which may experience an internal flash over from windings to earth created by an over voltage. A transformer may therefore fail due to these over voltages occurring internally from winding to earth after an associated surge arrestor has failed. This phenomenon may eventually create an ionized, carbon current path through which the supply current can flow.

A failed surge arrestor needs to be replaced and, because it does not interrupt the power supply, it may go unnoticed and only be replaced during a periodic maintenance check that is frequently only conducted annually. The identification of a blown surge arrestor is solely dependent on visual inspection. In the intervening period a transformer and possibly other equipment would therefore be exposed to damage that may be caused by surges, for example to transients occasioned by lightning strikes that would otherwise be damped by the surge arrestor. Also, replacement of a surge arrestor often requires the electrical supply network to be isolated during replacement with accompanying inconvenience to all affected consumers. Newer versions of surge arrestors may have a dropout characteristic that makes them more identifiable should anyone look at them and some of them have an advantage in that they allow for live line replacement, with the use of elongate tools. Blown surge arrestors of the dropout type are also more easily identifiable although due to the size and length of a network, these too may remain disconnected for extended periods. In any event, during this intervening time the line may be subjected to excessive voltage spikes, due to an absence of active surge protection.

In one design that is called the Combi Unit a modification comprises an additional bus-bar which is extended over the live side of the surge arrestor, incorporating the same female contact receptor as is used for a standard drop out expulsion fuse and the male part of the connector is added to the connecting end of the surge arrestor. The design has the advantage of a single unit incorporating a surge arrestor and expulsion fuse, both of which can be identified easily and replaced under live conditions.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

SUMMARY OF THE INVENTION

In accordance with this invention there is provided a combination surge arrestor and dropout fuse assembly comprising an electrically conductive supply bus-bar carried by a first insulator part supported on a mounting base wherein the bus-bar supplies an electrically conductive supply contact to supply an operatively upper cooperating releasable terminal of a dropout fuse holder carried at its opposite end by an articulated contact assembly supported by a second insulator part that is also carried by a mounting base, the bus-bar also having at least two electrically conductive surge arrestor contacts for cooperation with terminals of two dropout surge arrestors that operatively extend between the surge arrestor contacts connected to the bus-bar and earth contacts carried by a mounting base such that, in use, with a drop-out fuse and two dropout surge arrestors in situ in the assembly, the dropout fuse and each of the two surge arrestors are able to drop out independently of each other as and when it may become blown or otherwise ineffective. In a first variation of the invention the surge arrestor contacts that are connected to the bus-bar and the earth contacts carried by the mounting base are arranged so that surge arrestors in situ therein are arranged electrically in parallel with each other such that both are connected to the surge arrestor contacts connected to the bus-bar and the earth contacts simultaneously and each is able to drop out if it becomes blown or otherwise ineffective with the other remaining in circuit as backup protection.

In a second variation of the invention the surge arrestor contacts that are connected to the busbar and the earth contacts are carried by the mounting base and are arranged so that surge arrestors operatively in situ therein are connected to earth by way of a switching assembly that electrically connects one earth contact at a time into the circuit between the surge arrestor contacts connected to the bus-bar and earth. This may conveniently be achieved by providing a single electrical earth conductor that is movable between two terminal positions, namely a first terminal position in which the associated earth contact is connected to earth in an operative condition and a second terminal position in which the other associated earth contact is connected to earth in an operative condition. The single earth conductor is preferably associated with the earth contacts of the surge arrestors such that a disconnected surge arrestor earth contact is insulated at least sufficiently to isolate it from earth to a substantial extent with a single earth connection being made to the single earth conductor that sequentially connects one surge arrestor earth contact to earth and, should an installed surge arrestor drop out, the weight of the dropped out surge arrestor is arranged to move the movable single earth conductor to its other terminal position in which the other surge arrestor earth terminal is connected to the single earth connection.

Further features of the invention provide for the dropout surge arrestors to be arranged to move in planes that are generally upright and parallel to a plane of movement of the dropout fuse holder; for the dropout surge arrestor contacts and co-operating earth contacts to be orientated such that a surge arrestor installed therein extends transverse to the dropout fuse; for the dropout surge arrestors to be arranged one on each side of the dropout fuse holder; for the electrically conductive surge arrestor contacts carried by the bus-bar to be releasable with respect to co- operant terminals of the surge arrestors in which instance the other ends of the surge arrestors contact, in use, the earth contacts carried by the mounting base by way of articulated dropout contact assemblies; for the dropout fuse holder to be orientated such that the fuse holder in its operative position is generally upright but somewhat inclined towards the direction in which it is intended it should drop out if it should blow; and for the dropout fuse holder to be a double sequential dropout fuse action in the case of a fuse assembly such as is described in international patent publication WO 2015/040541 entitled DROPOUT FUSE ASSEMBLY AND FUSE HOLDER in which a second fuse is contained in piggy back manner within a piggy back recipient fuse holder that can be interchangeably installed in the dropout fuse holder part of the assembly to provide for a second fuse to be rendered active sequentially in the event that a first fuse holder drops out after blowing.

Still further features of the invention provide for there to be a single mounting base supporting both insulator parts that carry at their opposite ends the supply contact and the articulated contact connecting in use to the two ends of the dropout fuse holder; for the supply contact and the surge arrestor contacts that form releasable terminal assemblies to each include an electrically conductive leaf in which instance the releasable contact includes a recess for receiving a domed contact surface provided on an input contact at one end of each surge arrestor or fuse holder as the case may be; and for each of the electrically conductive articulated contact assemblies associated with the fuse holder and the surge arrestors to include a bifurcated cradle for releasably receiving, in use, a relatively rotatable articulated contact assembly associated with an end of a surge arrestor to be grounded in its operative position.

The invention also provides a combination surge arrestor and dropout fuse assembly as defined above in combination with a dropout fuse holder and two surge arrestors installed within the assembly in an operative position.

In order that the above and other features of the invention may be more fully understood, an expanded description of one embodiment of each of the first and second variations of the invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 is a schematic side view of one embodiment of the first variation of surge arrestor and fuse assembly according to the invention with a dropout fuse holder and surge arrestors in situ in their operative positions;

Figure 2 is an elevation of the embodiment of the invention illustrated in Figure 1 taken in a direction in which the bus-bar is foremost;

Figure 3 is an underneath view of the embodiment of the invention illustrated in Figure

1 ;

Figure 4 is the same as Figure 1 but illustrating one surge arrestor dropped out;

Figure 5 is the same as Figure 2 but illustrating the one surge arrestor dropped out as illustrated in Figure 4;

Figure 6 is the same as Figure 1 but illustrating the one surge arrestor dropped out and the fuse holder dropped out;

Figure 7 is the same as Figure 2 but illustrating the one surge arrestor dropped out as well as the fuse holder dropped out as illustrated in Figure 6;

Figure 8 is the same as Figure 1 but illustrating both the surge arrestors dropped out as well as the fuse holder dropped out; Figure 9 is the same as Figure 2 but illustrating both the surge arrestors and the fuse holder dropped out as illustrated in Figure 8; Figure 10 is the same as Figure 1 but showing a double fuse holder assembly according to WO 2015/040541 in situ in place of the single fuse holder illustrated in the other drawing Figures;

Figure 1 1 is a schematic elevation of an embodiment of the second variation of the invention taken in a direction in which the bus-bar is foremost as in the instance of Figure 2 and showing a single conductor that is movable between two terminal positions to connect the two surge arresters to earth sequentially;

Figure 12 is a sectional view of a part of a fuse holder leaf contact showing the recess therein;

Figure 13 is a sectional view of a part of one surge arrester contact showing the recess therein;

Figure 14 is a schematic side view of an embodiment of a third variation of a surge arrester and fuse assembly according to the invention with a dropout fuse holder and surge arresters in situ in their respective operative positions;

Figure 15 is the same as Figure 14 but illustrating one surge arrester dropped out;

Figure 16 is a schematic diagram which illustrates an example releasable terminal assembly that may be used in aspects of this disclosure, the Figure showing a fuse holder being held in the terminal assembly;

Figure 17 is the same as Figure 16 but without the fuse holder;

Figure 18 is a schematic diagram showing a plan view of a retaining member of the releasable terminal assembly of Figure 16;

Figure 19 is a schematic diagram which illustrates the releasable terminal assembly of

Figure 16 in use, with the fuse holder in a first position;

Figure 20 is a schematic diagram which illustrates the releasable terminal assembly of

Figure 16 in use, with the fuse holder in a second position; and

Figure 21 is a schematic diagram which illustrates the releasable terminal assembly of

Figure 16 in use, with the fuse holder in a first position.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the following description, and in the embodiment of the assembly of the invention that follows, the description will be of the assembly with the dropout fuse holder and surge arresters in situ in the assembly although it will be understood by those of skill in the art that the assembly may be sold as a separate item of commerce for subsequent fitment of the required fuse holder and surge arresters as may be appropriate to a particular electrical network and business strategy.

In the embodiment of the invention illustrated in Figures 1 to 9, a combination surge arrester and dropout fuse assembly comprises an electrically conductive supply bus-bar (1 ) carried by a first insulator part (2) supported on a mounting base (3) wherein the bus-bar supplies an electrically conductive supply contact in a female recess (4) in a resilient leaf (5). The resilient leaf and supply contact are arranged to supply an operatively upper cooperating releasable terminal (6) of a dropout fuse holder (7). The other end of the fuse holder is carried by an articulated contact assembly (8) supported by a second insulator part (1 1 ) that is also carried at its opposite end by the same mounting base (3). In this embodiment, each of the first insulator part and the second insulator part is provided by a separate insulator. The electrically conductive articulated contact assembly includes a bifurcated cradle releasably receiving a relatively rotatable articulated contact assembly associated with the lower end of the fuse holder of a type that is well known in the art. The dropout fuse holder is orientated such that the fuse holder, when it is in its operative position, is generally upright but somewhat inclined towards the direction in which it is intended to drop out if it should blow. The dropout takes place therefore in a dropout plane that is roughly vertical. As provided by this invention, the bus-bar also has two electrically conductive surge arrestor contacts (13) for cooperation with releasable terminals (14) of two dropout surge arrestors (15) arranged electrically in parallel with each other between the surge arrestor contacts connected to the bus-bar and articulated earth contacts (16) carried by the mounting base (3). The dropout surge arrestors are arranged to move, when they drop out, in planes that are generally upright and parallel to the plane of movement of the dropout fuse holder. The dropout surge arrestors are orientated, in their operative positions, in a generally transverse direction relative to the dropout fuse and typically generally horizontally. Conveniently, the two dropout surge arrestors are arranged one on each side of the dropout fuse holder. The surge arrestor contacts thus form releasable terminal assemblies that each include an electrically conductive leaf in which instance the releasable contact includes a recess (18) for receiving a domed contact surface (17) provided on an input contact at one end of each surge arrestor as in the instance of the dropout fuse holder. The articulated earth contacts (16) typically include a bifurcated cradle for releasably receiving, in use, a relatively rotatable articulated contact assembly associated with a grounded end of the relevant surge arrestor.

In use therefore, with the drop-out fuse and two dropout surge arrestors in situ in the assembly, as described above, the dropout fuse and each of the two surge arrestors are able to drop out independently of each other as an when it may become blown or otherwise ineffective.

Accordingly, the assembly described will effectively protect a transformer or other plant that is being fed by an electrical supply passing through the assembly against both overcurrent that would cause the dropout fuse to blow and overvoltage that would cause typically one of the surge arrestors to possibly become ineffective and dropout in consequence of its expulsion fuse activating.

As regards the arrangement of the surge arrestors, it should be noted that each of the two surge arrestors should be of a rating such that each could independently of the other serve the function of damping a voltage surge by a controlled leakage to earth. It is highly unlikely that both surge arrestors would be rendered inoperative at the same time and it would be expected that only one would dropout at any one time and the other could serve its purpose for possibly extended periods of time. The objective achieved by the invention is that substantially more time would be available for the dropped out surge arrestor to be identified and replaced, which would be done with the system live in known manner.

It is not envisaged that the parallel arrangement of the two surge arrestors will cause any problems as the voltage across parallel connections would be the same without adversely affecting either of the surge arrestors. The design makes use of a known understanding that one surge arrestor will fail prior to the other. Thus if one surge arrestor does drop out any change resulting would automatically keep only one surge arrestor in circuit at a time.

It is envisaged that it may be possible to apply a lower rating surge arrestor in the practice of this invention and that would be advantageous from a weight perspective. A lighter surge arrestor could be more easily manipulated using long reach tools than the standard surge arrestor. This may also result in cost efficiencies where, for example, two lower rating (e.g. distribution class) surge arrestors in parallel cost less than a single, higher rating (e.g. station class) surge arrestor. Also, in the event that the mechanical strength and the bus-bar is unable to maintain proper contact with a remaining surge arrestor after one has dropped out and in consequence of the other still placing pressure on the female dome receptor that is carried by the bus-bar, the busbar could be supported by one or more additional insulators secured to the mounting base in order to provide the required stability.

It should also be noted that the surge arrestors could be of any suitable type and that there is no restriction on the construction of the surge arrestors other than to render them suitable for inclusion in an assembly as described above. Commonly, the surge arrestor is of a metal oxide, in particular zinc oxide and is of a voltage dependent resistor type (varistor) that can become faulty if subjected to an excessive severity of surge or number of surges. In such an instance, the normally high resistance of the arrestor to normal voltages becomes diminished and the input terminal of the surge arrestor may become effectively grounded by way of the voltage dependent resistors. The purpose of the expulsion fuse or other device is, as indicated above, to disconnect the surge arrestor under such conditions. It should be noted that the surge arrestors protect the dropout fuse.

It should also be noted that a change-over mechanism may be implemented such that only one surge arrestor is connected into the circuit at any one time. If one surge arrestor blows, the other will automatically become engaged.

It is to be mentioned that, as illustrated in Figure 10, the combination dropout fuse and surge arrestor assembly could be enhanced by using a dropout fuse holder of the double sequential dropout fuse action as is described in international patent publication WO 2015/040541 entitled DROPOUT FUSE ASSEMBLY AND FUSE HOLDER. In that instance, a second fuse (21 ) is contained in piggy back manner within a piggy back fuse holder (22) that can be interchangeably installed in the dropout fuse holder part of the assembly to provide for a second fuse to be rendered active sequentially in the event that a first fuse holder drops out after blowing. Further details of that aspect of the dropout fuse holder may be obtained by referring to that published patent application.

Turning now to the second variation of the invention and referring to Figure 1 1 of the drawings, the surge arrestors are arranged electrically to be connected one at a time into the circuit between the surge arrestor contacts connected to the bus-bar and the earth contacts at the opposite end of the surge arrestors that are not shown in the Figure for reasons of clarity. They are the same as is described above other than for the following. In this instance the earth contacts take the form of articulated earth contacts that include a bifurcated cradle (31 ), as in the instances described above for releasably receiving, in use, a relatively rotatable articulated contact assembly associated with the co-operating end of the relevant surge arrestor that is to be grounded in use.

Each bifurcated cradle has an earth contact (32) electrically connected to it and that earth contact is arranged to be selectively electrically connected to earth by being engaged by one end of an electrically conductive elongate single earth conductor (33) having contacts (34) at each end. The single earth conductor is movable between two terminal positions, namely one in which the one surge arrestor is connected into the operative condition by being connected to earth and one in which it is open circuit so that the bifurcated cradle is not connected to earth. The surge arrestor earth contacts are insulated from direct contact with earth at least sufficiently to isolate them from earth to a substantial extent with a single earth connection being made to the single earth conductor that connects only one of the earth contacts of a surge arrestor to earth at any one time. The insulation used is merely sufficient to create sufficient dielectric strength that the two are not at a potential, common enough for the two surge arrestors to operate in unison. Should that one surge arrestor drop out, it will be retained by the cradle but its weight under the influence of gravity is employed to engage and move the movable single earth conductor to its other terminal position in which the earth terminal of the other surge arrestor is connected to the single earth conductor and thus to earth.

A bi-stable retainer device (35) of any suitable type that typically includes a spring catch may be used to hold the movable single earth conductor in either of its two terminal positions at any one time as may be required. When a failed dropped out surge arrestor bottom is to be replaced this can be replaced live with a new surge arrestor installed that can be installed whilst the other surge arrestor stays in circuit. Given that the older surge arrestor being the only one in circuit means that will ensure that the newer surge arrestor will serve as back up protection. The design offers advantageous functionality in that the failed surge arrestor can be identified at all times.

Figures 14 and 15 are schematic side views of an embodiment of a third variation of a surge arrestor and fuse assembly according to the invention. In this third variation a combination surge arrestor and dropout fuse assembly (40) comprises an electrically conductive supply bus-bar (41 ) carried by an insulator (42) supported on a mounting base (43). In this third variation, the insulator (42) has a first insulator part (42A) and a second insulator part (42B) and the bus-bar (41 ) is carried by an end of the first insulator part. The first and second insulator parts may be provided by a single insulator.

The insulator (42) has a connector for connecting to the mounting base (43) which is intermediate the two insulator parts (42A, 42B). The bus-bar (41 ) supplies an electrically conductive supply contact in a resilient leaf, similar to the variations described above. The resilient leaf and supply contact are arranged to supply an operatively upper cooperating releasable terminal of a dropout fuse holder. The other end of the fuse holder is carried by an articulated contact assembly (45) supported by the second part (42B) of the insulator. The insulator parts (42A, 42B) are arranged to support the dropout fuse holder in a generally upright orientation and parallel to the insulator (42) when it is in its operative condition. The insulator (42) may accordingly provide a so-called straight cut-out insulator.

The bus-bar (41 ) also has two electrically conductive surge arrestor contacts (47) for cooperation with releasable terminals of two dropout surge arrestors (51 ) arranged electrically in parallel with each other between the surge arrestor contacts connected to the bus-bar and articulated earth contacts (53) carried by the mounting base (43). The dropout surge arrestors are arranged to move, when they drop out, in planes that are generally upright and parallel to the plane of movement of the dropout fuse holder. The dropout surge arrestors are orientated, in their operative positions, in a generally inclined orientation relative to the dropout fuse.

The two dropout surge arrestors may be arranged one on each side of the dropout fuse holder. Similarly to the variations described above, the surge arrestor contacts thus form releasable terminal assemblies that each include an electrically conductive leaf arranged to cooperate with a contact surface provided on an input contact at one end of each surge arrestor. The articulated earth contacts (53) may include a bifurcated cradle for releasably receiving, in use, a relatively rotatable articulated contact assembly associated with a grounded end of the relevant surge arrestor.

Figures 16 and 17 are schematic diagrams which illustrate an example releasable terminal assembly (50) that may be used in aspects of this disclosure. The releasable terminal assembly (50) includes an electrically conductive resilient leaf (51 ) which is arranged to engage a cooperating end (53) of a surge arrestor or fuse holder, as the case may be. As will be described in greater detail below, in the illustration of Figure 16, a gap is shown between the end (53) of the surge arrestor or fuse holder and the leaf (51 ) to illustrate the effect of another component (e.g. surge arrestor operating). The leaf (51 ) may provide a protuberance (as illustrated) or a recess for cooperating with and engaging the end (53) of the surge arrestor or fuse holder and releasably to hold captive the surge arrestor or fuse holder in the assembly (50). A retaining member (55) is fixed to the leaf (51 ) and configured operatively to engage the end (53) of the surge arrestor or fuse holder.

The retaining member (55), a plan view of which is shown in Figure 18, may be provided by a flat steel spring which is bent back on itself to define a channel. Opposing sides of the spring may be bent inwards towards each other to narrow the channel near the open end thereof and to provide an engagement formation at the end of the retaining member (55) for operatively engaging the end (53) of the surge arrestor or fuse holder. The channel may be shaped and dimensioned to receive the end (53) of the surge arrestor or fuse holder. The retaining member (55) is supported on the leaf (51 ) in an orientation which is generally inclined relative to an axis of the surge arrestor or fuse holder which it engages, with side walls of the channel locating on either side of the dropout plane in which the surge arrestor or fuse holder moves when being expelled or installed. This provides a raised section which allows movement of the end of the surge arrestor or fuse holder past the retaining member. The retaining member may be located so as to allow a smaller gap associated with an adjacent component operation, which is not the same as the operation of the drop out travel distance of the drop out mechanism. The resilience of the steel spring may be selected such that the narrowing of the channel expands and allows passage of the end (53) of the surge arrestor or fuse holder during insertion or expulsion thereof with an adequate force. The releasable terminal assembly (50) with retaining member (55) is configured to prevent drop out of a non-operated surge arrestor or fuse holder.

Figures 19 to 21 are schematic diagrams which illustrate the releasable terminal assembly (50) in use. The respective Figures include a schematic side view of the assembly (50) and a schematic view illustrating the retaining member (55) interacting with the end (53) of the fuse holder (57).

With a fuse holder (57) in situ, as shown in Figure 19, the engagement formation of the retaining member (55) bears against the end (53) of the fuse holder. In the event of operation of another surge arrestor or fuse, an upward force can be expected to be applied to the releasable terminal assembly (50) which may temporarily reduce the downward force being exerted by the leaf (51 ), in some cases leading to a loss of contact as illustrated. Without the retaining member (55), this temporary reduction in downward force could allow the fuse holder to fall into the open position. However, the retaining member (55) bearing against the end (53) of the fuse holder may prevent the fuse holder from falling into the open position, i.e. drop out of a non-operated surge arrestor or fuse holder is prevented.

Should an operator need to remove the fuse holder (57), he or she may use an appropriate elongate tool to urge the end (53) of the fuse holder (57) out from the releasable terminal assembly (50). The operator may be required to apply sufficient force in an opening direction (59), which is parallel to the dropout plane and generally about a pivot point of the fuse holder, such that the side walls of the flat steel spring are urged apart, as shown in Figure 20, against their bias so as to permit passage of the end (53) of the fuse holder (57) along the channel. Continued movement of the fuse holder (57) in the opening direction (59) may free the end (53) of the fuse holder (57) from the retaining member (55), as is shown in Figure 21 , to allow removal thereof. Although Figures 19 to 21 illustrate lateral movement of a fuse holder, it should be appreciated that in a practical implementation urging the fuse holder in the opening direction would cause the fuse holder to rotate about the articulated contact assembly which supports the opposite end of the fuse holder.

The retaining member (55) described with reference to Figures 16 to 21 may be provided in any of the embodiments or variations described herein. A gap created by an adjacent component operation (e.g. another fuse or a surge arrestor blowing) may not be as extensive as the drop out mechanism of an operated or failed component and the retaining member may allow the non- operated component to move upwards and create a small gap while preventing the non-operated component from falling out. The retaining member prevents the fuse or other (non-operated) surge arrestor from falling into the open position thereby preventing unwanted disconnection of supply (in the case of the fuse opening) or loss of a back-up surge arrestor (in the case of the non-operated surge arrestor opening).

Aspects of this disclosure facilitate easy identification, and provide for live replacement, of operated or blown surge arrestors or fuses. It should be appreciated that various modifications or alterations may be made without departing from the scope of the invention. For example, the combination surge arrestor and dropout fuse assembly described herein may be configured for any suitable distribution voltage. Throughout the specification unless the contents requires otherwise the word 'comprise' or variations such as 'comprises' or 'comprising' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.