Technological background

The technical solution relates to a device which increases the operating reliability of voltage limiters containing a protection element. The voltage limiter is used to eliminate impermissible high contact voltages on non-live metal parts of railroad devices in both AC and DC traction systems by creating a temporary or permanent connection of conductive parts with a rail over the time the permissible value of the contact voltage is exceeded. It is also used in protection systems of photovoltaic sources.

Current state of technology

Document US3755715“LINE PROTECTOR HAVING ARRESTER AND FAIL-SAFE CIRCUIT BYPASSING THE ARRESTER” describes the solution of a surge arrester used in telecommunications which has a gas discharge tube as a primary protection element, and an air spark gap as a secondary protection element, with the arc voltage being higher than that of the gas discharge tube. The secondary protection element provides the grounding in case of a failure of the primary protection element. A sheet metal shell, a eutectic tin alloy pellet which represents the security part, a pre compressed spring and a moving short-circuiting shell telescopically inserted into the metal shell, are associated in series to both protection elements connected in parallel. The excessive current melts the pellet via the secondary spark gap and the short- circuiting shell moves by spring force so that it connects the opposing conductors. If the surge arrester subject to this document is loaded with a pulse current, the current will divide into two paths in the ratio of their impedance. One part of the current passes through the metal shell, movable connection and the short-circuiting shell into the ground. The other part of the pulse current passes through the spring and the short- circuiting shell also into the ground. The inductance of the spring negatively influences the division of the pulse current to both current paths. A considerable part of the pulse current also passes through the moving connection of both shells and can cause welding of one part to another, which results in a loss of protection in case of surge current overload states. Such a condition cannot be reasonably established or measured. Another disadvantage of this solution lies in tough requirements for production of both shells, which must necessarily have narrow tolerances.

Another document, US4002952 "ELECTRIC OVERVOLTAGE ARRESTER WITH CARBON AIR GAP AND GAS TUBE", describes a device containing a protection element connected in parallel, i.e., a carbon spark gap and coaxial gas discharge tube, with a fusible security element made of eutectic tin alloy, a pre-stressed spring acting on the moving short-circuiting element telescopically inserted into the metal housing, being associated in series, whereas the parts of the short-circuiting element are flexible and touch both the inner side of the metal housing as well as the outer housing of the coaxial gas discharge tube. If the protection elements are overloaded, the fusible element melts and by the influence of the spring the short-circuiting device moves to create a conductive path between both poles of the device. This solution partially eliminates the disadvantages of the solution above, because the flexible short- circuiting element provides better contact with the metal housing and the protection element, however, the fact that part of the pulse current also passes through the movable connection of both housings and can cause welding of one part to another, which means the loss of protection effects in the event of overloading by excessive current, i.e., low resistance of the telescopic interconnection of both housings against pulse and discharge currents still remains.

There are several further documents, e.g., document US4158869 “LINE PROTECTOR”, US4208694 “LINE PROTECTOR", US4132915 “SPARK GAP PROTECTOR”, US4321649“SURGE VOLTAGE ARRESTER VENTSAFE FEATURE” or US4340923“ELECTRICAL CIRCUIT PROTECTOR” and others which, as in the first two quoted documents, use the same solution of a telescopically-inserted fixed and movable short-circuiting housing, temperature-dependent fusible element and a compressed spring as the energy source for the movement of the short-circuiting housing to form a parallel conductive path for bypassing protection elements. All the quoted documents feature the same less than perfect solution, i.e., the pulse current partially passes through the telescopic interconnection of both housings, which can result in the loss of protection effects due to the welding of both parts together.

The invention according to documents DE102008027589A1 , resp. W02009049940A1 “SPARK GAP ARRANGEMENT WITH A SHORT-CIRCUITING DEVICE” deals with a spark gap arrangement with a short-circuiting device containing two opposing main electrodes arranged in a pressure-resistant housing, the said main electrodes being electrically connected to an external connection. A cavity which houses a movable, conductive short-circuiting element, is provided in at least one of the main electrodes, the short-circuiting element being loaded by a pre-stressed spring. The short-circuiting element is held in the cavity by means of a temperature-sensitive substance or a similar element. In the event of thermal or surge current overload, the temperature around the short-circuiting element increases up to the melting point of the temperature-sensitive substance and by force of the spring, the short-circuiting element moves and connects both main electrodes. This solution is closely connected to the use of a spark gap as a protection element and cannot be used for other types of protection elements such as gas discharge tubes or metal oxide varistors or their combination.

Another document, WO2016045982A1 OVERVOLTAGE PROTECTION ASSEMBLY HAVING A SHORT-CIRCUITING DEVICE”, relates to an overvoltage protection assembly containing a two-part metal housing, a surge arrester and a short-circuiting device consisting of two opposing contact electrodes which are under mechanical spring force and are allowed to move relatively one towards the other by force of the spring in case of a short circuit. The contact electrodes are separated by means of a lost element which, if the surge arrester heats up, allows the contact electrodes to make a relative motion. The surge arrester contains a varistor or a varistor designed in series and a gas discharge tube. The disadvantage of this solution is that the short- circuit current passes through the spring, which limits its size.

Document US2295320 “ELECTRIC DISCHARGE DEVICE” describes a technical solution of a gas discharge tube containing an insulating housing, two electrodes and an internal short-circuiting element in the shape of a cover loaded with a spring element, fixed to one of the electrodes by means of a fusible connection. In the event of overloading by surge current, the generated heat melts the fusible connection and the cover moves, as a result of which the two electrodes connect. The disadvantage of this solution is the requirement for manufacturing accuracy, as the size of the gap between the electrodes directly influences the ignition voltage of the gas discharge tube; another disadvantage is that it is difficult to produce it from a discharge-resistant material, i.e., it features low parameters and a short service life. The non-patented documentation describes technical solutions of voltage limiters comprising an efficient gas discharge tube positioned in a metallic, or non-metallic, housing in the shape of a hollow cylinder with a terminal situated at the bottom and one electrode of the gas discharge tube touching the terminal, while the second terminal provided with a stud touches the second electrode. The housing is closed with a pressed-in cover, or both the housing and cover are provided with a thread. These solutions show a crucial fault i.e. , after overloading of the protection element it is not ensured that a continuous short-circuit occurs and the limiter will lose its protection function.

Basis of the technical solution

The above-stated disadvantages are eliminated by the voltage limiter with a short- circuiting device according to the submitted solution. The voltage limiter consists of a housing shaped as a cylinder with the inner cavity provided with terminals on the opposing sides, protruding from the housing and electrically connected to the protection element. The short-circuiting device comprises fixed and movable parts connected via a fusible element. The limiter features a pre-stressed spring. The principle of the new solution is that the fixed part of the short-circuiting device consists of a cylinder-shaped, electrically-conductive contact situated in the cavity of the housing. The contact touches one terminal with one of its surfaces, while its second surface touches one surface of the protection element. The other surface of the protection element contacts the second terminal. A movable part is slipped onto the contact, the part consisting of an electrically-conductive short-circuiting element which is connected via the fusible element to the contact. This short-circuiting element is provided with a stopper at the upper end loaded with the upper end of the pre-stressed spring. The spring slips onto the short-circuiting element with its bottom end leaning against the terminal. The short-circuiting element is geometrically adjusted to bridge the distance between the short-circuiting element and the terminal in case of thermal surge current overload.

In one possible embodiment, the short-circuiting element forms an electrically- conductive movable part of a short-circuiting device, and it partially embraces the protection element, i.e. , 50 % up to 90 % of its length.

If the housing is electrically conductive, at least one terminal is insulated from the housing by an insulating pad. Another embodiment is also possible, in which one of the terminals is insulated from the electrically-conductive housing by an insulating pad and the second terminal is connected to the housing to form one unit. In an electrically non-conductive housing, the protruding terminals are not insulated.

In the advantageous embodiment, the fusible element is made of a tin alloy with a melting temperature of between 140 °C and 240 °C.

The contact which forms the fixed part of the short-circuiting device can consist of a solid or hollow cylinder.

In the design with a hollow cylinder, the cavity inner diameter ranges from 50% to 75% of the external diameter of the contact and the protection element sits on the annulus surface. This embodiment can be modified in such a manner that the inner shell of the hollow cylinder is extended at its upper end with a recess to plug in the bottom end of the protection element.

The protection element comprises any of the elements from the below-stated group - at least one varistor, at least one varistor connected in series with a gas discharge tube, at least one gas discharge tube.

The spring can be a pre-stressed compressive spring or it can consist of a wire with shape memory, or it can contain at least one flat spring.

Another embodiment is also possible with the terminal being connected to the contact to form one unit.

The advantage of this new arrangement of the voltage limiter is that it is electrically and mechanically resistant to the influence of pulse overvoltage and short-circuit currents, as no electromagnetic forces which would load the parts specified to form the short circuit arise from the passage of the current. The solution does not generate any circuit loops which would worsen mechanical strength and increase the protection level. The behaviour of the voltage limiter can be optimized by the selection of a suitable spring and the melting temperature of the fusible element for the passage of ordinary and failure current and can ensure a reliable continuous short circuit after loading with excess failure current.

Drawing explanation

The voltage limiter with a short-circuiting device is further shown in the attached drawings. Fig. 1 shows the arrangement of the voltage limiter with its fixed part consisting of a contact in the form of a solid cylinder and a short-circuiting device in the open condition. Fig. 2 shows the same arrangement; however, the contact consists of a hollow cylinder and, at the same time, the short-circuiting device is in the closed condition. A modification of this arrangement with a recessed extension of the cavity is shown in Fig. 3.

Examples of the technical solution ^' s implementation

The voltage limiter with a short-circuiting device according to Fig. 1 , consists of the cylinder-shaped housing 1 which features an inner cavity, the housing is provided with terminals 2 on opposing sides, protruding from the housing 1 and electrically connected to the protection element 3. The short-circuiting device comprises a fixed and a movable part. The fixed part of the short-circuiting device features a cylinder- shaped electrically-conductive contact 4, situated in the cavity of the housing 1 One surface of the contact 4 adjoins one terminal 2_and its second surface adjoins one surface of the protection element 3. The second surface of the protection element 3 is in contact with the second terminal 2. The protection element 3 can be implemented in various ways, for example, with at least one varistor, or at least one varistor designed in series with the gas discharge tube, or at least one gas discharge tube. A moving part is slipped over the contact 4 which comprises an electrically-conductive short-circuiting element 5, which is connected to the fusible element 7_at the connection 9, for example, with a solder consisting of a tin alloy with its melting temperature being between 140 °C and 240 °C, with the contact 4. The short-circuiting element 5 can partially embrace the protection element 3, i.e. , in the scope of 50 % to 90 % of its length. The short- circuiting element 5 comprises a stopper 5A at its upper end, loaded with the upper end of a pre-stressed spring 6. This spring 6 is slipped over the short-circuiting element 5 and fits against the terminal 2 at its bottom end. The short-circuiting element 5 is geometrically arranged in such a manner that in case of thermal or surge current overload it bridges the distance between the short-circuiting element 5 and the terminal 2.

The housing 1 can be electrically conductive or non-conductive. If it is electrically conductive, then at least one terminal 2 is insulated from the housing 1. with an insulating pad 8. The second, non-insulated terminal can be connected to the housing 1 in one unit. If the housing 1 is electrically non-conductive, the terminals 2 are not insulated.

The contact 4 can be formed in different manners. Fig. 1 shows a solution where the contact 4 is in the shape of a solid cylinder. Fig. 2 shows an example where the contact 4 consists of a hollow cylinder and, at the same time, the short-circuiting device is in the closed condition. The inner diameter of the cavity is from 50% to 75 % of the external diameter of the contact 4 and the protection element 3 sits on the annulus surface. This embodiment can be modified in such a manner that the inner shell of the hollow cylinder is at the upper end extended into a recess in order to tightly plug in the bottom end of the protection element 3, as shown in Fig. 3.

The spring 6 can consist of a pre-stressed compressive spring, a spring made from a shape memory wire, or at least one flat spring.

In the advantageous embodiment, the terminal 2 and contact 4 are connected into one unit.

Should impermissible high voltage occur on the voltage limiter terminals, the protection element 3 is activated to form a temporary conductive path featuring low impedance between the protected item and the earth. Dangerous voltage is eliminated and consequently, once the failure phenomenon disappears, the voltage limiter renews the state of high impedance. In case of dangerous failures, e.g., the traction line in contact with a protected item, a prolonged current or a high amplitude current can overload and consequently damage the protection element 3, which would result in the loss of the protective function. In this case, the flowing current heats up the path of the current, especially the protection element 3, and the contact surface 9, contact 4 and the short- circuiting element 5. If the temperature of the contact surface 9 exceeds the specified limit, the fusible element 7 will soften on the contact surface 9, as a result of which the contact surface 9 loses its strength. By the force of the compressed spring 6 the short- circuiting element 5 moves to create a continuous electrical connection with the terminal 2, as a result of which a parallel current path to the protection element 3 is formed. The short-circuiting element 5 is geometrically adjusted to bridge the distance between the short-circuiting element 5 and the terminal 2. The advantage of the submitted solution is that the contact surface 9 between the contact 4 and the short- circuiting element 5 is not loaded by the passing current in the functional condition.

The voltage limiter with the short-circuiting device is usually connected between a protected device and the earth via terminals 2, whereas the optimum possibility of the design can always be selected.

The housing 1 is, in the advantageous embodiment, electrically conductive and can be connected into one unit with one terminal 2. The second terminal 2 in this arrangement is insulated from the housing 1_by an insulating pad 8.

In another advantageous embodiment, the housing 1 is electrically non-conductive, so the electrically conductive terminal 2 protrudes non-insulated, without using the insulating pad 8.

In another advantageous embodiment, the short-circuiting element 5 and the stopper 5A are well conductive electrically and thermally, and they are made as one unit for manufacturing reasons.

Due to production reasons, the terminal 2 and the contact 4 are connected in one unit.

Application in industry

The design of the voltage limiter with a short-circuiting device according to this solution can be used where the protection of persons, instruments, machinery and metal constructions from dangerous contact voltage, overvoltage and stray currents should be provided. The voltage limiter with a short-circuiting device creates a transient or continuous connection of non-live conductive parts with a return circuit, or with the earth over the time the permissible value of the contact voltage is exceeded. It also eliminates high impulse overvoltage induced into the traction line or electrical and electronic equipment by lightning strikes.