TECHNICAL FIELD OF THE INVENTION

The present invention relates to a surge arrester, more specifically to a surge arrester for a domestic robot assembly.

BACKGROUND

In general, autonomous machines such as robots are increasingly becoming more and more common, often due to the fact that they are a significant labor-saving factor for many consumers. Repetitive and time- consuming tasks may nowadays be performed without any significant supervision or involvement by operators or users.

Robotic lawn mowers are an example of autonomous machines which have been very commercially successful and which, more or less, have revolutionized the lawn mowing industry. Robotic lawn mowers are often provided with a perimetral or boundary wire which is used to provide the robotic lawn mower with an enclosure within which it is permitted to operate. Though, some robotic lawn mowers are provided with a guiding wire which is used for a similar purpose as the perimetral wire. Such wires (perimetral or guiding) generally carry a low frequency, low-voltage signal which is detected by the robot so that it can determine its position relative to the wire.

However, robotic lawn mowers are generally very exposed to all kinds of hardships, e.g. rough weather, theft, vandalism, etc., in large part due to the fact that they operate outdoors, are fully autonomous and thus often unsupervised, which may make them a liability.

During thunderstorms, also known as lighting storms or electrical storms, electrical charges are built up within clouds. When the charge is large enough, a large discharge in the form of a lightning strike occurs. Relating back to the robotic lawn mowers, a lighting strike may cause a transient surge caught by the wire arrangements and led to the control station, where the risk of damage due to surge voltages is imminent. Moreover, if the control station further has docking or charging capabilities there is a chance of the robotic lawn mower also being damaged, and in some cases, when the control station is connected to a home grid, devices or appliances within the home grid may also suffer damages. Thus, for owners of robotic lawn mowers, thunderstorms may result in a very unpleasant situation involving, in a best case scenario, repairing or replacing the control station.

Today, the recommended course of action for owners of robotic lawn mowers, dictated by manufacturers and insurance companies, is to

disconnect the perimetral (and/or guiding) wires from the control station and to disconnect the control station from the home grid. This solution may however not always be possible as thunderstorms cannot always be anticipated, and even so, it is a very cumbersome approach. Hence, owners of robotic lawn mowers going on extended trips should always disconnect their robotic lawn mowers. However, this may of course defeat the purpose of investing in the robotic lawn mower in the first place as manual lawn mowing may be needed after an extended trip due to the height and/or thickness of the grass and vegetation.

There is therefore a need for a device which is capable of preventing these types of situations from occurring, protecting for example robotic lawn mower, and to minimize the damages resulting from lighting strikes in a simple, efficient and cost effective manner.

SUMMARY

In view of the above, an objective of the invention is to solve or at least reduce one or several of the drawbacks discussed above. Generally, the above objective is achieved by the attached independent patent claims.

According to a first aspect of the present invention, this and other objectives are achieved by a surge arrester for a domestic robot assembly including a perimetral wire connectable to a control station, such as robotic lawn mower assembly. The surge arrester comprises a ground rod adapted to be inserted into the ground, a set of control station connectors for electrical connection to the control station, a set of perimetral wire connectors for electrical connection to the perimetral wire, and a protection circuit arranged between the set of control station connectors and the set of perimetral wire connectors. The protection circuit comprises a first electrical path from a first perimetral wire connector to a first control station connector, a second electrical path from a second perimetral wire connector to a second control station connector, and at least two surge protecting elements connected to and along each electrical path, and wherein each surge protecting element is further connected to the ground rod, such that the protection circuit is able to discharge at least a portion of a transient surge, entering from the set of perimetral wire connectors, via the at least two surge protecting elements to the ground rod.

The present invention is based on the realization that a domestic robot assembly used outdoors may be protected by a surge arrester arranged between the control station and a perimetral wire, and that in order for the transient surge to be reduced or minimized at least two surge protecting element where each protective element is connected to a ground rod should be utilized. In use, the surge arrester is installed between the perimetral wire and the control station and prevents a transient surge from reaching the control station by dissipating the transient surge through the surge protecting elements to the ground rod. The present invention thereby allows owners of domestic robot assemblies to leave their domestic robot assemblies

connected during thunderstorms as there is a reduced risk that the control station is harmed when a lighting strike hits the ground close to the perimetrial wire. Hence, another advantage is that the owner of a domestic robot assembly may leave his domestic robot connected while leaving his domicile, e.g. going on an extended trip as the risk for any surges harming his robot or home is reduced. Further, if the surge arrester is damaged by a transient surge the protection circuit may be replaced while the ground rod is reused with a new protection circuit. Hence, components of the surge arrester are replaceable in order to even further lower the cost of a lighting strike for the consumer. A surge arrester according to the present invention is also easy and cheap to produce, and may use several off the shelf components. A transient surge may also be known as a transient voltage surge, or transient voltage spike. The surge arrester is as understood suitable for a domestic robot assembly such as a domestic automatic lawn mower assembly or system. However, it is of course possible to use a surge arrester according to the present invention for protecting other types of electrical circuits or devices arranged outdoors. For example, the surge arrester may also be used to protect an alarm system for a fence. Such an alarm system may comprise, equivalent of the perimetral wire, an alarm wire in and/or along a fence which detects a movement of the fence, e.g. generated from an intruder climbing the fence.

In at least one exemplary embodiment, the at least two surge protecting elements for each electrical path are connected in parallel with each other so that each surge protecting element of the at least two surge protecting elements form a protective stage, wherein each protective stage is configured to discharge a portion of the transient surge to the ground rod. This means that there is less chance that the protection circuit needs to be replaced as the energy in the transient surge is divided over the protection stages.

In at least one exemplary embodiment, each protective stage is configured to discharge the transient surge at different voltage magnitudes to the ground rod, wherein the protective stages are arranged in a decremental manner from the perimetral wire connectors to the control station connectors along each electrical path, with respect to voltage magnitude discharge capability. Hence, the different protective stages may comprise different surge protecting elements adapted for the expected magnitude of the voltage discharger. For example, a gas tube may be used for as the first surge protecting element, and in a decremental manner a varistor and then a TVS (transil) diode.

In at least one exemplary embodiment of the invention, each protective stage is configured to trigger to discharge the transient surge at different speed to the ground rod, the protective stages may be arranged in a decremental manner from the perimetral wire connectors to the control station connectors along each electrical path, with respect to time for triggering for each surge protecting element. This means that surge protecting elements which have a longer time for triggering, i.e. to become conductive, and discharge a portion of the transient surge may be arranged closer to the perimetral wire connectors. For example, surge protecting elements which can handle large amounts of voltage and/or current may but trigger more slowly and may be arranged closer to the perimetral wire connector from which a transient surge may enter.

In at least one exemplary embodiment, the protection circuit further comprises at least one inductor connected in series along the electrical path between each surge protecting element. The inductor(s) may slow down the transient surge along each electrical path in order to enable each surge protecting element to direct a portion of the transient surge to the ground rod. In other words, the inductor may slow down the transient surge such that first and subsequent protective stages has time enough to trigger and become conductive to direct the transient surge to the ground rod.

In at least one exemplary embodiment, the protection circuit is arranged on a printed circuit board (PCB) being connected to the ground rod. Thereby, each surge protecting element and the ground rod is connected via the PCB.

In at least one exemplary embodiment, the printed circuit board is in direct contact with the ground rod. The printed circuit board may comprise and opening through which the ground round is electrically and mechanically connected. The opening is preferably provided in the centre of the PCB in order to provide a short connection path to the ground rod for the surge protecting elements. Alternatively, the printed circuit board may be connected to the ground rod via a connection wire. The connection wire may have a cross-sectional area of at least 50 mm ² in order to provide a sufficiently large electrical conductivity.

In at least one exemplary embodiment, the surge arrester further comprises a protective mat arranged around the ground rod, such that vegetative growth around the ground rod is at least partially hindered. In order to reduce the risk for any harm to perimetral wires or control station wires the protective mat may be arranged to prevent vegetative growth around the ground rod such that there is no need to cut the vegetation there. The protective mat may comprise openings for the ground rod, the perimetral wires and/or guiding wires. The protective mat may have radial extension, as seen from the ground rod of at least 10 cm.

In at least one exemplary embodiment, the protection circuit further comprises at least one equalizing component connected between the first and second electrical path at a portion of the first and second electrical path located between the first and second control station connector and the at least two surge protecting elements. The at least one equalizing component further reduces the risk for any transient surge to harm the control station by at least partly equalizing a residual voltage left after the surge protecting elements, between the two control station connectors for the perimetral wire.

In at least one exemplary embodiment, the domestic robot assembly further comprises a guiding wire, the surge arrester further comprising a set of guiding wire connectors for electrical connection to a guiding wire, wherein the protection circuit is a perimetral protection circuit and the at least two surge protecting elements are surge protecting elements for the perimetral protection circuit, and wherein the surge arrester further comprises a guide protection circuit, which guide protection circuit comprises, a first electrical path from a first guiding wire connector to a third control station connector, a second electrical path from a second guiding wire connector to a fourth control station connector, at least two surge protecting elements for the guide protection circuit connected to and along each electrical path of the guide protection circuit, and wherein each surge protecting element of the at least two surge protecting elements for the guide protection circuit is further connected to the ground rod, such that the guide protection circuit is able to discharge at least a portion of a transient surge, entering from the set of guiding wire connectors, via the at least two surge protecting elements for the guide protection circuit to the ground rod. As mentioned above, a domestic robot assembly or system may also comprise a guiding wire which the robot uses for guidance and/or navigation. The guiding wire may be connected in the same manner as the perimeter wire, and thus protect the control station station or the robot through the surge arrester. The at least two surge protecting elements for the guide protection circuit may be arranged in the same manner as described above in exemplary embodiments for the at least two surge protecting elements for the perimetral protection circuit.

In at least one exemplary embodiment, the guide protection circuit further comprises at least one equalizing component connected between the first and second electrical path of the guide protection circuit at a portion of the first and second electrical path of the guide protection circuit located between the third and fourth control station connector and the at least two surge protecting elements for the guide protection circuit. The at least one equalizing component further reduces the risk for any transient surge to harm the control station by at least partly equalizing a residual voltage left after the surge protecting elements, between the two control station connectors for the guiding wire.

In at least one exemplary embodiment, the perimetral protection circuit is arranged on a first side of a printed circuit board, and the guide protection circuit is arranged on a second side of the printed circuit board. This means that there is little space required for the PCB in the surge arrester, and that only one PCB may be needed.

In at least one exemplary embodiment, both the first and second side of the printed circuit board is connected to the ground rod. Hereby, both the perimetral protection circuit and the guide protection circuit have a short connection path to the ground rod and thereby may dissipate a transient surge efficiently.

In at least one exemplary embodiment, the surge protecting elements are selected from the group comprising: gas discharge tubes, varistors, TVS diodes, trisils, avalanche diodes and zener diodes. The listed electrical component may all be used to advantage with the present invention for connecting an electrical path to the ground rod. In at least one exemplary embodiment, the perimetral protection circuit and/or the guide protection circuit comprises at least three surge protecting elements. Thereby, three separate protective steps which couple different magnitudes of the transient surge to the ground rod may be provided.

According to a second aspect of the present invention, the objectives are also at least partly achieved by a domestic robot assembly, such as a robotic lawn mower assembly. The domestic robot assembly comprising a control station for the domestic robot, the control station being connectable to an electric outlet, wherein the control station comprises a control unit configured to transmit control signals through a perimetral wire connectable to the control station. The perimetral wire is connected to the control station via a surge arrester according to the first aspect of the invention.

This second aspect may generally have the same features and advantages as the first aspect. For example, the domestic robot assembly may further comprise a guiding wire and a guiding wire protection circuit as described above in connection with the first aspect.

According to a further aspect of the present invention, the objectives are also at least partly achieved by a surge arrester for a fence alarm system including an alarm wire connectable to an alarm control station. The surge arrester comprising a ground rod adapted to be inserted into the ground, a set of control station connectors for electrical connection to the control station, a set of alarm wire connectors for electrical connection to the alarm wire, a protection circuit arranged between said set of control station connectors and said set of alarm wire connectors. The protection circuit comprising a first electrical path from a first alarm wire connector to a first control station connector, a second electrical path from a second alarm wire connector to a second control station connector, at least two surge protecting elements connected to and along each electrical path, and wherein each surge protecting element is further connected to said ground rod, such that said protection circuit is able to discharge at least a portion of a transient surge, entering from said set of alarm wire connectors, via each surge protecting element of said at least two surge protecting elements to the ground rod. This further aspect may generally have the same features and advantages as the first aspect or second aspect. For example, it is

understood that there may be several alarm wires connected by using a more protection circuit such as the protection circuit described above to be used with a guiding wire but instead being used for an additional alarm wire.

Hence, there may be a plurality of protection circuits each connected to an alarm wire.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. Brief description of the drawings

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings. The same reference numerals will be used for similar elements.

Figure 1 shows a domestic robot assembly including a perimetral wire connectable to a control station, such as robotic lawn mower system and a surge arrester according to at least one exemplary embodiments of the invention.

Figure 2 shows a domestic robot assembly further including a guiding wire connectable to a control station, such as robotic lawn mower system and a surge arrester according to at least one exemplary embodiments of the invention.

Figure 3 shows an exploded perspective view of a surge arrester according to at least one exemplary embodiment of the invention.

Figure 4 shows a bottom-up cross sectional view of a protection circuit according to at least one exemplary embodiment of the invention. Figure 5 is a schematic view of an electric circuit diagram for a protection circuit in surge arrester in accordance with different embodiments of the invention.

Figure 6 shows a surge arrester according to the present invention being used with an alarm system for a fence.

Detailed description of embodiments

In the present detailed description, embodiments of a method and system according to the present invention are mainly discussed with reference to schematic views showing a domestic robot assembly and a surge arrester according to various embodiments of the invention. It should be noted that this by no means limits the scope of the invention, which is also applicable in other circumstances for instance with other types or variants of systems or devices than the embodiments shown in the appended drawings. Further, that specific components are mentioned in connection to an embodiment of the invention does not mean that those components cannot be used to an advantage together with other embodiments of the invention. The invention will now be described with reference to the enclosed drawings where first attention will be drawn to the structure, and secondly to the function.

Figure 1 show a surge arrester 1 for a domestic robot assembly according to at least one embodiment of the invention. The surge arrester 1 is also shown in an enlarged view. The surge arrester 1 is further illustrated in connection with a domestic robot assembly which includes a perimetral wire 2 which is connected to a control station 3 via the surge arrester 1 . The control station 3 in turn is connected to a house 9. The control station 3 is thus provided electrical power from the grid of the house 9. The domestic robot assembly illustrated herein is a robotic lawn mower assembly. However, it is of course also possible to user the inventive surge arrester 1 with other types of domestic robot assemblies or other types of equipment which are typically placed outdoors. The surge arrester 1 comprises a ground rod 4, a set of control station connectors 5, herein a pair of control station connectors, for electrical connection to the control station 3, a set of perimetral wire connectors 6, herein a pair of perimetral wire connectors, for electrical connection to the perimetral wire, and a protection circuit 7a arranged between the set of control station connectors 5 and the set of perimetral wire connectors 6. The protection circuit 7a is arranged within an enclosure 8.

The enclosure 8 may be made of plastic, or any other suitable insulating material. The enclosure 8 protects the protection circuit 7a arranged therein from rain and other environmental hazards. The enclosure 8 is therefore preferably at least partly watertight. The enclosure 8 has feedthroughs for the ground rod 4, the control station connectors 5 and the perimetral (and/or guiding) wire connectors 6.

In figure 1 the ground rod 4 is shown inserted into the ground 40. The length of the ground rod 4 is in the range of 1 .5 m to 2.5 m. The ground rod 4 is typically inserted for about 1 to 1 .5 m into the ground 40 to provide a sufficient electrical and mechanical connection to the ground 40. The ground rod 4 may be a steel rod or cylinder coated with copper or a copper alloy. The copper or copper alloy provides a better electrical connection to the ground 40. A first end of the ground rod 4 may be tapered in order to facilitate insertion into the ground 40. The second end of the ground rod, opposite the first end, may have an external thread for connecting the ground rod 4 to the enclosure 8 and protection circuit 7a. The ground rod 4 may further comprise a nut 41 (see figure 3) which may be tightened against the outside of the enclosure in order to secure the ground rod 4 to the enclosure 8 and protection circuit 7a.The enclosure 8 may have an internal thread for securing the ground rod 4 to the enclosure 8. It should be noted that is important that the ground rod 4 has a good electrical connection to the protection circuit 7a. For example, the protection circuit 7a may be directly contacted with the second end of the ground rod 4. This may be provided by many different known means, e.g. nuts, bolts and threading as described or other means such as snap connections and the like. The protection circuit 7a comprises a first electrical path 70a from a first perimetral wire connector 61 to a first control station connector 51 , and a second electrical path 70b from a second perimetral wire connector 62 to a second control station connector 52. The protection circuit 7a is preferably arranged on a printed circuit board 7' (PCB). The PCB 7' may be directly connected to the ground rod 4. Therefore, the PCB may comprise an opening 7" in the centre of the PCB for receiving the second end of the ground rod 4. The protection circuit 7a further comprises at least two surge protecting elements 75 connected to and along each electrical path 70a, 70b. Each surge protecting element 75 is further connected to the ground rod 4.

Thereby, the protection circuit 7a is able to discharge at least a portion of a transient surge, entering from the set of perimetral wire connectors 6, via each surge protecting element 75 of the set of surge protecting elements 75 to the ground rod 4.

In use, the control station 3 sends low frequency low voltage, e.g.

below 100 Hz and 48 V, perimetral control signals along the perimetral wire 2. This enables the autonomous lawn mower 31 to navigate the area contained within the perimetral wire 2 and cut the vegetation therein. The control signals transmitted on the perimetral wire 2 are largely unimpeded by the protection circuit 7a. The lawn mower 31 may perform the lawn mowing according to a pre-set schedule with regular intervals, or with intervals being influenced based on weather data or in any other known manner. The control station 3 may be a combined control and charging station in the case that the lawn mower 31 is a rechargeable electric lawn mower. If there is a lighting strike within a close distance, e.g. 0-100 m, of the domestic robot assembly a transient surge may be generated in the perimetral wire 2. Without the surge arrester 1 , such a transient surge reaches the control station 3 and possibly also the house 9. Hence, there is a large risk that the control station 3 and possibly the lawn mower 31 are harmed or even destroyed by such a transient surge. Further, since the control station 3 is connected to the grid of the house 9, the transient surge may also reach the house 9 and cause damage. However, by the inclusion of the surge arrester 1 between the perimetral wire 2 and the control station 3, the transient surge generated in the perimetral wire 2 first reaches the surge arrester 1 where it is impeded. The transient surge travelling along the electrical paths 70a, 70b triggers the surge protecting elements 75 to become conductive and redirect the transient surge to the ground rod 4 which dissipates the transient surge into the ground 40.

A transient surge sufficiently large will destroy the protection circuit 7a. In order to replace the protection circuit 7a, the owner of the domestic robot assembly may connect each end of the perimetral wire 2 to each other while ordering or installing a replacement for the protection circuit 7a. Then, the owner may simply detach, e.g. unscrew, the enclosure 8 and thus protection circuit 7a from the ground rod 4. Hence, there is no need to remove the ground rod 4. A replacement protection circuit 7a may then be installed upon the second end of the ground rod 4, and the perimetral wire connected to the perimetral wire connectors 6. The protection circuit 7a and enclosure 8 may thus be sold as a separate replacement unit.

Figure 2 show a surge arrester 21 according to at least one

embodiment of the invention. The surge arrester 21 is illustrated in connection with a domestic robot assembly which in addition to the features shown and discussed in connection with figure 1 also includes a guiding wire 22 which is connected to a control station 3 via the surge arrester 21 .

The surge arrester 21 is further illustrated in figure 3 and 4 where the protection circuit 7a previously discussed is a perimetral protection circuit arranged on one side of the PCB 7' and a guide protection circuit 7b is arranged on the other side of the PCB 7'. In figure 3 the top side of the PCB 7' is the perimetral protection circuit 7a with the corresponding surge protecting elements 75. The bottom side of the PCB, i.e. the side facing the ground 40, is the guide protection circuit 7b with the corresponding surge protecting element 75'.

The surge arrester 21 thus further comprises a third 53 and fourth 54 control station connector, and a first 63 and second 64 guiding wire connector for connecting to the guiding wire 22. Hence, the set of control station connectors now comprises four control station connectors. Note that the guiding wire connectors 63, 44 are arranged as the inner pair of connectors. This provides a larger distance between the perimetral wire connectors in order to reduce the likelihood of a spark forming. Further, if only the

perimetral wire connectors are to be connected, this may also adhere to industry standards for autonomous lawn mowers.

Note that the connection between the perimetral wire connectors and guiding wire connectors 61 , 62, 63, 64 and the wires 2, 22 is smooth and preferably does not have sharp angles in order to reduce the likelihood of sparks forming between the connectors 61 , 62, 63, 64 during a transient surge.

The surge arrester 21 further comprises a protective mat 42 arranged around the ground rod 4. The purpose of the protective mat 42 is to hinder vegetative growth around the surge arrester 21 . Thereby, there is less risk for any harm to perimetral wires or control station wires as there is no need to cut the vegetation around the ground rod 4 and accidentally cut either the perimetral wires 2 or control station wires 22. The protective mat 42 has openings for the ground rod 4, the perimetral wires 2 and/or guiding wires 22. The protective mat may have radial extension, as seen from the ground rod 4 of at least 10 cm. The protective mat 42 may be shaped either circular or square.

Referring to figure 4 which is a bottom-up cross sectional view of the surge arrester 21 , note that the guide protection circuit 7b is substantially the same as the perimetral protection circuit 7a (referred to as protection circuit 7a in figure 1 ). In other words, the guide protection circuit 7b also comprises a first and second electrical path 70a', 70b' which have surge protecting elements 75' connected in parallel directly to the ground rod 4.

During use the control station 3 sends out guide control signals on the guiding wires 22, which are low frequency and low voltage distinguishable from the perimetral control signals. The guide control signal in similarity to the perimetral control signals are also substantially unaffected by the guide protection circuit 7b. Hence, another advantage is that the same circuit and design, e.g. the same surge arrester 21 may be used for different makes and models of domestic robot assemblies as it is simply up to the user to connect only the perimetral wire if his or her robot assembly only has a perimetral wire, whereas a domestic robot assembly comprising also a guiding wire may also connect the guiding wire to the surge arrester. It may of course be possible that certain robot assemblies only utilizes a guiding wire, in that case those may be connected either the guiding wire connected or to the perimetral wire connectors.

Figure 5 is an electric circuit diagram for either the perimetral protection circuit 7a or the guide protection circuit 7b. As noted above, the same reference numerals refer to the same elements.

The first electrical path 70a from the first perimetral wire connector 61 to the first control station connector 51 is connected via a set of surge protecting elements 75, herein three surge protecting elements 71 , 72, 73. Likewise, the second electrical path 70b from the second perimetral wire connector 62 to the first control station connector 52 is connected via a set of surge protecting elements 75, herein three surge protecting elements 71 , 72, 73. The surge protecting elements 75 are connected in parallel with each other to the ground rod 4. Each surge protecting element 75 thus forms a protective stage configured to discharge a portion of a transient surge to the ground rod 4. In the embodiment shown in figure 5, each protective stage is configured to discharge the transient surge at different voltage magnitudes to the ground rod 4. The protective stage, i.e. the surge protecting elements 75 are arranged in a decremental manner from the perimetral and guiding wire connectors 61 , 62 to the control station connectors 51 , 52 along each electrical path 70a, 70b, with respect to voltage magnitude discharge capability and time for triggering the conduction to the ground rod 4.

An inductor 700 is connected in series along the first and second electrical path 70a, 70b, between each surge protecting element 75 in order to slow down a transient surge. In this case, the protection circuit 7 comprises three surge protecting elements 75, and thus two inductors. The first surge protecting element 71 , as seen from the perimetral and guiding wire connectors 61 , 62 is a gas discharge tube. The gas discharge tube 71 can handle large amounts of current and voltage and activates and becomes conductive by high voltage spikes. However, the gas discharge tube may be slow to trigger. Therefore, the second surge protecting element 72 is a varistor which may also handle relatively large amount of current and voltage but triggers faster than the gas discharge tube 71 . The third surge protecting element 73 is a transil diode which has less capacity than either the gas discharge tube 71 or the varistor 72, but a very short triggering time. A capacitor 74 is arranged to redirect any remaining AC spikes to the ground rod 4. Downstream of the surge protecting elements 75 towards the control station connectors 51 , 52 three equalizing components are arranged which connects the first and second (referred to as a third and fourth in the case of a guide protection circuit 7b) electrical path 70a70b, in order to equalize any remaining voltage between the control station connectors 51 , 52, and thereby protect the control station 3 if there remains a voltage high enough to cause damage. As shown in figure 5 the equalizing component may comprise a varistor 77, a transil diode 78 and a capacitor 79.

Note that the ground rod 4 is directly connected to the PCB 7', and each one of the surge protecting element 71 , 72, 73are directly connected to the ground rod 4 with a short connection path.

The surge protecting elements 75 may of course comprise other types of electrical components for performing the same purpose, such as gas discharge tubes, varistors, TVS diodes, trisils, avalanche diodes and zener diodes.

Note that the electric circuit diagram for the perimetral protection circuit 7a shown in figure 5 and the guide protection circuit 7b are in principle identical and the components shown in figure 5 and explained above are the same components and are arranged similarly for the guide protection circuit 7b. The difference being that the perimetral wire connectors 61 , 62 would be the guiding wire connectors 63, 64 and the control station connectors 51 , 52 would be the control station connectors 53, 54. The reference numerals 75, 70a and 70b may equally well refer to the reference numerals 75', 70a' and 70b' of the guide protection circuit 7b.

In at least one exemplary embodiment not shown in the appended drawings, only two surge protecting elements, e.g. a gas discharge tube and a varistor, and an inductor is used for the protection circuit.

In other exemplary embodiments, more than three surge protecting elements and more than two inductors may be used.

Figure 6 shows a surge arrester 100 for a fence alarm system including an alarm wire 102 connectable to an alarm control station 106. The alarm wire 102 is arranged in, or on, and along a fence 104 in order to detect movement of the fence cause by e.g. an intruder which climbs the fence. The alarm wire may be threaded through a chain-link wire (not shown). The alarm control station 106 is connected to a house or building 9. The surge arrester 100 is thereby used to prevent a transient surge from e.g. a lighting strike which hits the fence 104 or close to the fence from damaging the alarm control station 106 and possibly the building 9. The above description of features, components and such of a surge arrester in connection with figures 1 to 5 is valid also for the surge arrester 100 shown in figure 6. The fence alarm system may of course utilize more than one alarm wire, in that case more than one protection circuit may be used as described for a guiding wire.

Additionally, even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.