CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from South African patent application number 2022/10396 filed on 20 September 2022, which is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a surge arrestor and adapter that provides a dropout type of surge arrestor for the convenience of replacement or other servicing. In some embodiments, a conventional fixed type of surge arrestor can be modified using an adapter to provide the dropout surge arrestor.

The invention also relates to a dropout surge arrestor that embodies a ground lead dis-connector forming a part of the surge arrestor.

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 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 avoiding to a large extent consequential fires; and also avoiding 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 electrical surges to ground. Such surges may otherwise damage an associated transformer that would typically be caused by a breakdown in the insulating properties of the usual insulating oil in the relevant transformer housing. A surge arrestor is thus connected between supply to ground and functions by routing energy from an transient over-voltage to ground if one occurs, while isolating the conductor from ground at normal operating voltages.

Commonly, a surge arrestor is of a 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 an expulsion device or ground lead dis-connector is to disconnect the surge arrestor from ground 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 may be designed to be disconnected by an expulsion device or ground lead dis-connector comprising an explosive powder to forcefully and automatically disconnect a failed surge arrestor from the network to prevent a direct short circuit to earth.

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 conventional 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 conventional fixed surge arrestor generally requires the electrical supply network to be isolated during the replacement procedure 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 a Combi Unit a modification comprises an additional busbar 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. However, the arrangement of the components, the applicant understands, is susceptible to some of the expulsion force of the expulsion fuse being transferred to the upper and lower end of the surge arrestor with a consequent possibility of the fuse or cutout becoming disconnected or damaged. Also, the ground lead disconnector is not designed for mechanical connection, as it has a deliberate weak point, to facilitate a freely unobstructed opening of the circuit as with fixed surge arrestors.

Conventional surge arrestors of the more common fixed type generally have an integral transverse mounting bracket at one end, typically a lower end, whereby it can be mounted to a supporting insulator or other support therefor. Such surge arrestors are commonly available whereas the more sophisticated drop out surge arrestors are rather costly and less freely available.

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 one aspect of this invention there is provided a method of modifying a surge arrestor (optionally of the type that is designed to be a fixed surge arrestor in use) and that has a mounting bracket at one end thereof, the method comprising permanently attaching an adapter to the surge arrestor wherein the adapter has an articulated electric contact assembly at or towards an end of the surge arrestor that is to be grounded in use, the articulated electric contact assembly being configured for releasable co-operation with a co-operant electric contact carried by a support therefor such that the surge arrestor is either operatively bridging between a live terminal and a grounded co-operant electric contact or is suspended by the co-operant electric contact in a dropped out condition, and attaching an electrical contact at an operatively live opposite end of the surge arrestor for releasable engagement with a co-operating leaf contact such that the modified surge arrestor may serve as a drop out surge arrestor.

Further features of the first aspect of the invention provide for the mounting bracket to be integral with a part of an electrically insulating body or collar of the surge arrestor with the mounting bracket extending radially from the surge arrestor in which instance the mounting bracket may be severed from the electrically insulating body or collar and the operatively grounded end of the surge arrestor permanently bonded into a socket in an electrically insulating part of the adapter with an earth contact terminal, conveniently in the form of a rod passing through the socket; for the adapter to have an electrically insulating body with the articulated electric contact assembly secured to a radially outer region thereof wherein a ground lead is connected to a terminal forming part of the articulated electric contact assembly and to the grounded end of the surge arrestor; for the electrical contact at the operatively live end of the surge arrestor to be a smooth tipped male type of contact for co-operation with a female recess in a co-operating leaf contact which may be a resilient leaf spring contact; and for the electrical contact at the operatively live end of the surge arrestor to have an integral loop whereby it can be manipulated by a manually operated hook on an electrically insulated handle during replacement activities.

The articulated electric contact assembly includes a toggle or catch that is spring loaded and held in position by a ground lead disconnector or resilient member such that upon expulsion of the ground lead from the failed surge arrestor, collapse of the articulated electric contact assembly is permitted. This isolates and prevents live current from passing to ground through a defective surge arrestor of which the ground lead has been expelled when the surge arrestor fails to operate properly to inhibit electrical flow at normal voltages. The articulated electric contact assembly may have oppositely directed operatively horizontal terminal pins that rest in an electrically conductive bifurcated cradle of the co-operant electric contact carried by a supporting insulator to thereby be relatively rotatable but capable of being lifted out of the cradle remotely using a hook on an elongate electrically insulating handle.

The invention extends to a surge arrestor modified or manufactured according to the method described.

In accordance with another aspect of the invention there is provided an adapter for use in modifying a surge arrestor as indicated above wherein the adapter has an electrically insulating body providing a socket into which an operatively grounded end of a surge arrestor may be bonded for use with a passage through a bottom of the socket for operatively receiving an earth contact terminal carried by a grounded end of a surge arrestor and wherein the electrically insulating body has an articulated electric contact assembly at a position radially outwards of an axis of the socket, the articulated electric contact assembly including a mechanism for holding it in an operative position with a ground lead or resilient member operatively connecting it to a surge arrestor earth contact terminal in a normal operative position and for releasing the articulated electric contact assembly to provide for drop out of a surge arrestor to which the adapter has been operatively fitted in response to activation of a ground lead dis-connector of the surge arrestor.

Further features of the second aspect of the invention provide for the adapter to be made of a moulded plastics material having a pair of collinear journals to which the articulated electric contact assembly is attached by a pivot; and for the mechanism for releasing the articulated electric contact assembly in response to a ground lead activation operation to be a toggle or catch that is spring loaded and held in position by a ground lead or resilient member such that upon expulsion of the ground lead or resilient member from the surge arrestor, collapse of the articulated electric contact assembly is permitted.

The drop out surge arrestor adapter is positioned to ensure that the discharge pressure is removed from the top and bottom contacts.

Additional features of the second aspect of the invention will become more apparent from what follows and also from the various possible aspects of the invention defined above.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 is a side view of a fixed type of surge arrestor having a radially extending mounting bracket integral with an operatively lower part of an electrically insulating body of the surge arrestor;

Figure 2 is a similar view showing the mounting arm removed and a plastic electrically insulating body of an adapter in exploded relationship relative to the surge arrestor;

Figure 3 is a three-dimensional view of a plastic moulding of the electrically insulating body part of an adapter showing the top thereof;

Figure 4 is a view similar to Figure 2 but showing the final dropout surge arrestor; Figure 5 is a three-dimensional underneath view of the adapter and the articulated electric contact assembly;

Figure 6 is a side view of the dropout surge arrestor operatively installed in position in one form of insulator assembly;

Figure 7 is the same as Figure 6 but showing the dropout surge arrestor in the process of dropping out;

Figure 8 is a side view of a dropout surge arrestor fitted to a different insulator assembly;

Figure 9 is a front view of a variation in which two dropout surge arrestors are mounted in parallel to a supporting insulator assembly;

Figure 10 is a front view of a simple parallel arrangement of two dropout surge arrestors;

Figure 11 is a side view of a cut out isolator attached to a part of a transformer tank;

Figure 12 illustrates a front view of a three-phase transformer with parallel dropout surge arrestors in each of the phases;

Figure 13 is similar to Figure 5 but includes an eject toggle arrangement which includes a resilient member as part thereof;

Figure 14 shows an adapter with a light provided therein; and

Figure 15 shows an arrangement of the dropout surge arrestor mounted in an arrangement where it provides over-voltage protection to a distribution transformer or other plant.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the embodiment of the invention illustrated in the drawings, a fixed type of surge arrestor (1 ) that is illustrated in Figure 1 has an axially extending ground lead terminal in the form of a screw threaded rod (2) at its normally lower end that is to be grounded during normal use of the surge arrestor. A similar live terminal in the form of a screw threaded rod (3) is provided at the operatively upper end that is to be the live end in use.

In this particular instance the surge arrestor has an electrically insulating body (4), the lower end of which has an insulating collar (5) from which extends a mounting bracket in the form of a radially extending arm (6) that is integral with the collar. In one embodiment of the invention the mounting bracket may be severed from the electrically insulating collar as indicated by numeral (7) and the operatively grounded end of the surge arrestor is permanently bonded into a socket (8) in an electrically insulating body of an adapter (9). The earth contact terminal in the form of the screw threaded rod (2) passes through a passage (10) in the bottom of the socket. An articulated electric contact assembly (11 ) is configured for releasable co-operation with a cooperant electric contact (12) carried by a supporting insulator (13). The articulated electric contact assembly includes a toggle or catch (14) that is spring loaded and held in position by a ground lead (15) such that upon expulsion of the ground lead from the surge arrestor, collapse of the articulated electric contact assembly is permitted. This prevents live current from passing to ground through a defective surge arrestor of which the ground lead has been expelled when the surge arrestor fails to operate properly to inhibit electrical flow at normal voltages.

The articulated electric contact assembly may have oppositely directed operatively horizontal collinear terminal pins or journals (17) to which the articulated electric contact assembly is attached by a pivot (18) with the pins or journals resting, in use, in an electrically conductive bifurcated cradle (19) of the co-operant electric contact carried by the supporting insulator (13) to thereby be relatively rotatable but capable of being lifted out of the cradle remotely using a hook on an elongate electrically insulating handle carried by a support therefor. The surge arrestor may either operatively bridge between a live terminal and a grounded co-operant electric contact as shown in Figure 6 or it may be suspended by the co-operant electric contact in a dropped-out condition, as shown in Figure 7.

The ground lead (15) is connected to a terminal (22) forming part of the articulated electric contact assembly and to the grounded end of the surge arrestor. This is done in a manner that ensures that the ground lead holds the spring biased toggle or catch (14) in an operative position with a ground lead operatively connecting it to the surge arrestor ground contact terminal in a normal operative position. However, the toggle or catch, when it is released consequent on the ground lead dis-connector of the surge arrestor operating to expel the ground lead connection, the toggle or catch moves under its own spring loading to release the articulated electric contact assembly and enable drop out of the surge arrestor (30).

A custom-made electrical contact (24) is provided for an operatively upper live end of the surge arrestor for releasable engagement with a co-operating leaf contact (25) such that the modified surge arrestor may serve as a drop out surge arrestor (30). The electrical contact at the operatively live end of the surge arrestor is a smooth tipped male type of contact for co-operation with a female recess in the co-operating leaf contact (25) which may be a resilient leaf spring contact. The custom-made electrical contact (24) at the operatively live end of the surge arrestor is preferably formed as an internally screw threaded electrically conductive sleeve having an integral loop (26) whereby it can be manipulated by a manually operated hook on an electrically insulated handle during replacement activities. It will be understood that the modified surge arrestor made according to this invention can be employed in substantially the same way as existing custom made drop out surge arrestors but that a number of advantages are achieved over that situation. In particular, it is highly advantageous from the point of view of utilising administratively approved items, especially the surge arrestor itself. Implementation of the invention enables advantage to be taken of the fact that certain fixed type of surge arrestors is accompanied by an administrative approval requirement whereas others are not. Also, it is not necessary to make an entire set of surge arrestors and supports therefor and an approved support structure can also be employed.

The ground lead dis-connector in use in implementing the present invention operates freely, with no obstruction. The arrangement makes use of a drop out mechanism which has a spring-loaded toggle, to assist the ground lead dis-connector action and direction. After operation of the ground lead dis-connector and surge arrestor drop out action, the spring-loaded toggle may keep the ground lead dis-connector separated from the surge arrestor, creating an additional isolation at the bottom of the surge arrestor, between the ground lead and the arrestor, after the ground lead dis-connector has operated and the arrestor is hanging in the isolated position. It should also be noted that the unit may be designed such that the surge arrestor hangs in a pre-determined position for ease of replacement.

It should be noted that the ground lead is pulled in the direction of a designed movement path of the ground lead dis-connector. This arrangement is significant in diverting any force away from the arrestor and any associated cut out. This complies with the same operational purpose of an approved fixed surge arrestor.

The added advantages, in conjunction with live replacement are that the surge arrestor is disconnected from both live and earth; there is no deviation by obstructing the ground lead disconnector; and, as mentioned, a dropped-out unit can be easily identified from a distance.

It will be understood that numerous variations may be made to the embodiment of the invention described above without departing from the scope hereof. In particular, the exact nature of the insulating support for the surge arrestor can be varied widely and could, for example, be of the general nature illustrated in Figure 8. Also, it is envisaged that surge arrestors modified as provided herein can be used in parallel with each other as indicated by numeral (31 ) in Figure 9.

Figure 10 illustrates a simple form of parallel arrestors that can in particular be used in conjunction with a cut out insulator (41 ) carried on a side wall of a transformer housing (42) as shown in Figure 1 1. Figure 12 illustrates a transformer housing (43) on which three parallel pairs of surge arrestors (44) have been mounted; one pair for each phase.

Figure 13 is similar to Figure 5 and illustrates an alternative embodiment of an adapter (50) and articulated electric contact assembly in which an eject toggle (51 ) is provided. The illustrated eject toggle (51 ) includes a first resilient member (52) which is connected to the screw threaded rod (2) of the ground lead by means of an additional bolt (53) at one end and rests against a lock in protrusion (58) at its other end. The eject toggle (51 ) also includes a second resilient member (56) which has a coil spring (57) at one end and is coupled to the first resilient member (52) by means of a bracket (55). Upon ejection of the ground lead (2), the tension in the first resilient member (52) is released which enables the coil spring (57) of the second resilient member (56) to move the trunnion in the direction shown by arrow 60 and cause the surge arrestor to drop out as previously described.

In this embodiment, rather than relying on the ground lead to hold the spring biased toggle or catch in an operative position, which may cause false ejection if a braided copper ground lead becomes stretched during normal operation or if it was not correctly tensioned, it relies on the resilient members of the eject toggle arrangement (51 ) to provide the necessary tension. This avoids a difficulty where stretching of a ground lead over time leads to drop out even where the ground lead terminal (2) has not been ejected. In this embodiment, the braided copper strap ground lead (15) has only one function, namely to carry discharge current.

The eject toggle (51 ) of the embodiment of Figure 13 thus has the function of:

• Locking the trunnion into a position whereby no holding in tension is lost,

• Taking any mechanical strain away from the flexible copper braided strap,

• By being spring loaded will assist in speeding up the ground lead dis-connector movement,

• Will create an isolation gap,

• Will release the holding in tension once the ground lead disconnect has actuated, and

• Will forcefully eject the trunnion into the open position allowing the entire surge arrestor to drop open into the identifiable and isolated mode.

A light that includes a gravitational or motion switch may be attached to a suitable part of the dropout surge arrestor (30), such as an upper portion of the insulated body (4) of the surge arrestor. The light may be off when a body of the light is in a generally vertical condition with the dropout surge arrestor in position as shown in Figure 8, and may be activated by the movement to a non-vertical position when the surge arrestor drops out, as shown in Figure 7. The light may flash, and functions as an additional visual indicator (which is especially apparent at night) that the surge arrestor needs replacement or maintenance. A light of this sort may be self-powered with a long-lasting battery and may be attached to the dropout surge arrestor (30) using any suitable means such as adhesive, ties or the like.

Figure 14 shows an embodiment of an adapter (100) which is similar to the previously illustrated adapter (9) except that it includes a light (102) fitted in a bore (104) formed in the body of the adapter (100). The light (102) may in some embodiments include a conductor (106) that is connected to the ground lead disconnector terminal (2), so that the light is switched on upon expulsion and disconnection of the ground lead as described above, with the conductor (106) thereby acting as a trigger wire. In other embodiments the light (102) may have a gravitational or motion switch which causes its actuation upon the dropping out of the surge arrestor as described above. The light may have an internal power source in the form of a battery (not shown).

Figure 15 illustrates an arrangement (150) in which the dropout surge arrestor (30) of the invention is mounted to a side wall of a transformer (152) or other plant requiring over-voltage protection. The surge arrestor is connected between the high-voltage live wire (152) and ground. It will be appreciated that once dropped out, the surge arrestor (30) can be replaced without requiring power to the transformer (150) to be switched off. Existing fixed surge arrestors on transformers can be replaced with the illustrated arrangement.

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.