WO2002023207A1

WO2002023207A1 - A method and a device for checking a condition of an electric switching device

Info

Publication number: WO2002023207A1
Application number: PCT/SE2001/001932
Authority: WO
Inventors: Magnus Backman; Lars Jonsson; Per Larsson; Lennart BALGÅRD; Jan-Anders Nolemo; Stefan Halén
Original assignee: Abb T&D Technology Ltd.
Priority date: 2000-09-18
Filing date: 2001-09-11
Publication date: 2002-03-21
Also published as: AU2001282841A1; SE0003304D0

Abstract

In a method for checking the condition of an electric switchingdevice (1) arranged in a current path of an electric plant, plant, said device being controllable between positions for closing and breaking the current path (5), the check, is carried out during operation of the electric plant by carrying out a measurement of at least one parameter associated with the condition of the device without influencing the operation.

Description

A method and an apparatus for checking a condition of an electric switching device

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method for checking a condi- tion of an electric switching device arranged in a current path of an electric plant, said switching device being controllable between locations for closing and breaking the current path, as well as an apparatus for carrying out such a condition check.

"Electric plant" is here to be given a very broad sense and principally comprises all types of electric circuits, such as transmission and distribution systems for electric power, electric plants within the industry and the like.

The invention is directed to all types of electric switching devices, which however have in common that they normally stays in a waiting position and carry out the task they are assigned to only at long time intervals. Such electric switching devices may for example be of the type arranged in a current path for break- ing thereof upon occurrence of fault currents, such as for example in a switch gear for electricity supply within the industry or in distribution or transmission networks. "Electric switching device" comprises also so-called transfer switches through which a current in a current path may be broken upon occurrence of a fault current in the current path for switching in another current path to a load or the like instead. This electric switching device may also be of the type that in normal operation of the electric plant is open, i.e. keeps the current path therethrough broken, and only closes when anything particular happens. This "particular" is neither to be anything unpredictable, such as a sudden fault of the electric plant, but the electric switching device could also carry out the task thereof at a predetermined point of time, such as change the position once a month or the like. It may be a question about alternating current as well as direct current.

In common for such electric switching devices normally staying in a waiting position during long periods of time is the need of checking that the state of the electric switching device is satisfying during these times of waiting, i.e. that it really functions as it is intended to when it is time to work. It is for example ex- tremely important that an electric switching device functioning as any type of protective breaker really carries out the breaking function sufficiently when this is to be done, which could take place at an interval of several years. With respect to especially such electric switching devices the checking of the state has hitherto been carried out by maintenance staff visiting the place for the electric switching device at regular time intervals for taking the electric switching device out of operation for testing the operation thereof between said two closing and breaking positions. Some type of bypassing of the electric switching device is then normally carried out and it is taken out for carrying out the checking of the state, and it is sometimes even taken out physically. A number of time consuming switchings have in any case to take place, and it gets sometimes even necessary to make the electric plant currentless during a short period of time. Thus, considerable costs are associated with carrying out this condition check, which of course means that there is a high risk that it will not be carried out as often as desired. SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and a device for cheeking the state of an electric switching device of the type defined in the introduction, which enable a state checking of such electric switching devices being at least in some regard improved.

This object is according to the invention obtained by carrying out the checking in such a method during operation of the electric plant by carrying out the measurement of at least one parameter associated with the condition of the electric switching device without influencing the operation of the electric switching device.

That the electric switching device does not influence the operation means here that the electric switching device is switched in the current path as before in its waiting position and is able to carry out the task thereof. Carrying out for example any by- passing of the electric switching device or in another way remove the electric switching device from the current path for making the condition check is to be considered as if the operation of the electric plant is influenced. Thus, the operation of the electric plant is to be considered to be uninfluenced if the elec- trie switching device is still switched in so that it may carry out its task.

A number of advantages of carrying out the condition check through such a parameter measurement without influencing the operation through the electric switching device are obtained. An important advantage is that the operation of the electric plant is not disturbed in any way. Another advantage is that time consuming switchings or even movements of the electric switching device are avoided, so that a checking gets considerably simpler to carry out. Furthermore, there is a possibility that the measurement is carried without any need of having any maintenance staff to get to the electric switching device, so that the condition checks get considerably less expensive than before and may in principle be carried out as often as desired. Accordingly, it gets possible to for' example automatically carry out said parameter measurement for condition checks at regular time intervals, such as once a month, once a week or even daily. This means in its turn that possibly degeneration of the condition of the electric switching device may be discovered shortly after they have occurred, so that the probability of a good function of the electric switching device when it has to fulfil its object gets very high.

According to a preferred embodiment of the invention at least one parameter describing the state of any part of the electric switching device is calculated on the basis of the result of said measurement. It is advantageous to calculate such a parameter so as to directly get a value of the condition of said part of the electric switching device, even if measured parameters already mentioned may give an indication thereof.

According to another preferred embodiment of the invention the condition check is carried out for an electric switching device comprising an electric actuating member such as an electric motor, arranged to control two contacts movable with respect to each other between said two positions. It is advantageous to carry out the condition check according to the invention of such an electric switching device, since it is very important that it is known that when a determined relative movement of the two contacts is ordered this also will be carried out by the electric actuating member. It is then a need of checking different pa- rameters associated with the function of the electric actuating member, such as the friction between contacts, the energy supply of the actuating member and so on.

According to a preferred embodiment of the invention the condi- tion check of the electric switching device is carried out by checking the function of the electric actuating member by or- dering a determined relative movement of the two contacts and measuring the result of this order and comparing it with an expected result. It may in this way be ensured that the electric switching device really functions as it shall. It is then possible to discover different defects thereof. The distance for the relative movement may for example be measured through a position detector and a comparison of this distance with an expected distance may be carried out. Would there be a deviation this may for example depend upon a too high friction between the two contacts or a non-satisfying functioning of the energy supply to the electric actuating member. By measuring the distance of a relative movement ordered and the energy consumed by the electric actuating member for achieving this and comparing these values with an expected energy consumption for the movement, it may for example be determined whether the friction against the movement is to high. This may then for example be a result of oxide coatings on the contact surfaces.

The electric actuating member is according to a preferred em- bodiment of the invention adapted to receive energy stored in a capacitor, and the voltage drop across the capacitor is measured and is taken as a value of the energy consumption when carrying out said relative movement. By combining this with a measurement of the current to the electric actuating member the condition of the capacitor may also be tested, i.e. that it can provide the energy required for the control of the electric switching device through the electric actuating member.

According to another preferred embodiment of the invention a detector of the position of the mutual position of the contacts is checked by checking that the position detector delivers a reading when simultaneously a movement of a moveable part of the electric actuating member is detected. The good function of the position detector may be directly decisive for a functioning on the hole of electric switching devices of some types, since very accurate co-ordination of movement they have to carry out with for example the phase position of an alternating current may be required for the correct function of the electric switching device.

According to another preferred embodiment of the invention ap- plied to en electric switching device adapted to be closed and conduct a current therethrough during normal operation of the electric plant, the temperature of at least one of two contacts of the electric switching device and the current through the electric switching device are measured, and these values are compared with expected values therefor so as to determine the condition of the electric switching device with respect to the contact resistance thereof. It may by this be determined whether the contact resistance of the switching device has become to high, and suitable measures have to be taken for reducing this. Such a too high contact resistance may cause unnecessary power losses and heating of the electric switching device and a possibly damage thereof may occur if the temperatures will be too high. Another way to determine the contact resistance is to measure the voltage between the two contacts of the electric switching de- vice and the current through the electric switching device and compare these values with expected values therefor.

According to another preferred embodiment of the invention applied to an electric switching device adapted to be closed and conduct a current therethrough during normal operation of the electric plant at least one part connected to one of two contacts of the switching device moveable with respect to each other is brought to move while the two contacts maintain their mutual contact. By carrying out such a "small" movement of such a part a very good condition check of the entire electric switching device may be made, without influencing the very operation position thereof in any way. This may particularly easily be done in the case of an electric actuating member for controlling the movement of this part. Thus, it gets possible to check the posi- tion detector, the contact resistance and the friction of the contacts, the energy storage for the actuating member and the good function thereof through a small movement without the contacts leaving their mutual contact.

According to another preferred embodiment of the invention ap- plied to an electric switching device adapted to keep the current path therethrough broken during normal operation of the electric plant said checking may in a corresponding way take place by carrying out a relative movement of two contacts of the electric switching device while keeping these contacts out of contact with each other and by that preventing current from flowing through the switching device.

According to a very preferred embodiment of the invention a condition check is intended to take place for an electric switch- ing device for alternating current comprising two branches connected in parallel in the current path therethrough, in which a first one of them comprises a first contact member having two movable contacts movable with respect to each other for opening and closing and the second one comprises an element with ability to block current therethrough in at least one blocking direction and conduct current therethrough in at least one direction, in which a second contact member having two contacts movable with respect to each other for opening and closing is connected in series with said element, in which the switching device is adapted to break the current in the current path by the fact that the first contact member is controlled to open for transferring the current to said element when this is in or going into a conducting state and then the second contact member to open when said element is in a state of blocking current there- through after a zero-crossing of the current, and in which the electric switching device is associated with an electric actuating member adapted to cause a relative movement of the movable contacts, in which the checking is carried out by controlling the electric actuating member to move said contacts with respect to each other without breaking the current through the electric switching device. A function checking of all parameters impor- tant for the good function of the electric switching device may take place by this without influencing the operation of the electric plant in any way. This has a great importance for this type of electric switching devices, since it is important to carry out the breaking in a correct way when fault currents occur so as to keep damages of the surrounding equipment and of parts included in the electric switching device on a level being as low as possible.

According to a preferred further developments of the embodiment last mentioned the moveable contacts of the first contact member may be moved with respect to each other while maintaining a contact therebetween in said checking, but it would also be possible to control the electric actuating member to move the moveable contacts of the first contact member out of engagement with each other in said checking for transferring the current to said member and then move them back for establishing contact therebetween again before said element arrives to the blocking state. However, a very short relative movement between the two contacts is probably enough for checking the condition of the electric switching device entirely, but it would also be possible to use the embodiment last mentioned when particular needs are there.

According to another very preferred embodiment of the invention applied on an electric switching device co-ordinated with an electronic unit for control of an electric actuating member arranged for mutual movement of moveable contacts of the electric switching device for bringing this between said two posi- tions, said condition check is controlled by the electronic unit. Exactly the combination of an electric actuating member for movement of contacts of the electπc switching device and an electronic unit for control of the electric actuating member results in extraordinary possibilities to easily carry out condition checks of an electric switching device nearly as often as desired. The electronic unit may be "programmed" to carry out these checkings at regular time intervals or be controlled to do this through signals which may be sent out for example from an operation central being remotely located. The parameters used when carrying out the checking may then be collected from sig- nals to or in the electronic unit and accordingly be such that the electric unit in any case knows all the time for deciding whether the electric switching device shall be controlled or not.

It is then possible in the case that the condition check results in a determined condition of the electric switching device deviating in any way from the expected condition to call for attention by triggering an alarm or the like, which is then controlled by the electronic unit.

The invention also relates to an apparatus, a computer program and a computer program product according to the corresponding appended claims. It is easily understood that the method according to the invention defined in the appended set of method claims is well suited to be carried out through program instruc- tions from a processor which may be influenced by a computer program provided with the program steps in question. This is particularly the case in the embodiments with an electronic unit included in the electric switching device, since it is very suitable to program this electronic unit to carry out the condition check. Although not explicitly expressed in the claims the invention comprises such apparatuses, computer programs and computer program products combined with a method according to any of the appended method claims.

The invention also relates to a device for checking the condition of an electric switching device arranged in the current path of an electric plant, said switching device being controllable between positions for closing and breaking the current path, according to the appended device claims. The function and the advantages of such a device appear without any doubt from the discussion above of corresponding embodiments of the method according to the invention.

Further advantages as well as advantageous features of the in- vention appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a very schematic view illustrating the construction of a device for condition check of an electric switching device and explains a method for such condition check according to a preferred embodiment of the invention,

Fig 2 is a simplified view illustrating the construction of an electric switching device for alternating current with a device for condition check connected thereto,

Fig 3 and 4 are views corresponding to fig 2 illustrating methods according to two preferred embodiments of the invention for condition check of the electric switching device,

Fig 5 and 6 are very simplified views illustrating a method according to the invention for condition check of an electric switching device of another type than the one illustrated in fig 2, and Fig 7-10 are also very simplified views illustrating a method according to the invention for condition check of an electric switching device of conventional type.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An electric switching device is indicated at 1 in fig 1 . An electric actuating member 2 in the form of an electric motor schemati- cally indicated is co-ordinated with this electric switching device so as to control two contacts 3, 4 of the electric switching device moveable with respect to each other between a position closing and breaking the current path 5, in which it is arranged. It is then advantageous that the electric switching device is adapted to be closed and conduct current therethrough during normal operation of the electric plant, but it is also possible that it may instead be arranged to keep the current path broken during normal operation. The electric motor 2 receives electric energy for actuating the electric switching device from an energy storing member 6, which here is constituted by a capacitor. The direct voltage from the capacitor is converted by an inverter 7 to an alternating voltage for operation of the motor 2. An electronic unit 8 is also co-ordinated with the electric switching device for controlling the electric motor 2. A position detector 9 is adapted to detect the position of a moveable contact 4 of the electric switching device and send information thereabout to the electronic unit 8. Furthermore, a member 10 is adapted to measure the current from the inverter 7 to the electric motor 2. A member 11 is adapted to measure the voltage across the capacitor 6 so as to in that way be able to judge how much energy has been tapped therefrom when controlling the electric actuating member 2. Information from the means 9-1 1 is sent to the electronic unit 8. It is also indicated that a member 12 is adapted to detect the magnitude of the current in the current path 5 and send informa- tion thereabout to the electronic unit 8. The electronic unit 8 is adapted to take decisions with respect to whether the electric switching device shall be controlled to go from the normal position thereof, i.e. a position closing or breaking the current path, to the other of these two positions, on the basis of information received from the detecting member 12. However, there is also a possibility to control the switching device by hand to change position by pressing a button not shown. Signals from the button may then go to the electronic unit through the detecting member.

The electronic unit is adapted to carry out a condition check of the electric switching device 1 at regular time intervals. This may be done by the fact that it controls the inverter 7 to deliver electric energy from the capacitor 6 to the electric actuating member 2 so that the contacts 3, 4 of the electric switching device move with respect to each other without changing the influence of the electric switching device upon the electric plant to which it belongs. This means that if the electric switching device normally is adapted to let the current in the current path flow therethrough, this will continue to do so also during the condition check. It is in this way possible to check the condition of the electric switching device through the electronic unit 8, i.e. that it and the actuating circuit thereof function in such a way that the function it has to carry out, breaking or closing, when this is re- quired will function entirely according to the desires. The electronic unit receives through the member 10 information about the current required by the electric motor for carrying out the movement, and the torque required for the movement of the contacts may be calculated through this information. In the case that the contacts bear against each other during said movement this is then a value of the friction between the contacts. Would this be to high for a good function of the electric switching device, which may depend upon a heavy oxide formation on the surfaces of the contacts, the electronic unit may through an alarm unit 13 tell that maintenance of the electric switching device is necessary. However, exactly by carrying out this condi- tion test at regular time intervals the friction between the contacts may be kept down by the fact that oxide layers possibly formed thereon may be scraped off during such a check, so that such a maintenance may be avoided.

The position detector 9 is adapted to send information about the position of the moveable contact 4 to the electronic unit 8 when checking the condition. It may by this be determined whether the position detector functions as it shall. Furthermore, the elec- tronic unit 8 may through information received from the voltage measuring member 1 1 check that there is sufficient energy in the capacitor 6 for controlling the electric switching device by measuring the voltage drop across the capacitor when actuating the device for said condition check. Thus, it may in this way be checked that when a certain movement is ordered through the electronic unit 8 this is also obtained, i.e. that the actuation of the electric switching device functions as it shall. A measure of the overall condition of the electric switching device may also be obtained by a possibility to determine the contact friction and testing the position detector and the energy storage.

An electric switching device in the form of a so-called hybrid breaker for which the method for condition check according to the invention is particularly well suited is schematically illus- trated in fig 2. This electric switching device 1 is switched in in a current path 5 for being able to rapidly break a current therethrough by opening it. The electric switching device is intended for alternating current and one such switching device is arranged per phase, so that a three phase network has three such switching devices on one and the same location. The switching device has an inner cylinder 14, which is rotatably arranged around an axle 15 and has a moveable contact 4. A second cylinder 16 is arranged outside the cylinder 14 and has four contacts 3, 3' , 3" and 3'" arranged along the movement path of the moveable contact 4 and to form good electric contacts while bearing against the moveable contact 4. The switching device is switched in in the current path through the two outer contacts 3 and 3'", respectively.

A semiconductor device in the form of a diode 17, 18 is con- nected between the two outer contacts and the adjacent inner contact with the conducting direction from the outer to the adjacent contact. The diodes could just as well be both directed with the conducting direction towards the outer contact.

The switching device has also an electric actuating member 19 adapted to drive the inner cylinder 14 to rotate for movement of the moveable contact 4 with respect to the other contacts 3-3'".

Members 12 are arranged to measure the current in the current path 5 and send information thereabout to the electronic unit 8 for controlling the electric motor 19 on the basis thereof. The device has also a position detector 20 adapted to detect the actual position of the cylinder 14 and by that also of the move- able contact 4.

The function of a switching device of this type appear more in detail from the Swedish patent application 9904164-2 of the applicant but will here be briefly summarised: when there is a desire of breaking the current in the current path 5, for example since the detecting member 12 detects a very high current in the current path 5, which may be caused by a short-circuit there- along, it would then be possible for obtaining the quickest possible breaking to detect the direction of the alternating current and make the rotation direction of the cylinder 14 and by that of the moveable contact 4 depending thereupon. In the closed position according to fig 2 the entire current flows through the switching device between the two outer contacts 3, 3' through the moveable contact 4 connecting the former galvanically to each other. We do now assume that a decision is taken to carry out the breaking through rotating the inner cylinder 14 clockwise as seen in fig 2. This shall then preferably be made so that an opening of the contact member formed by the contacts 3 and 3' is carried out through a zero-crossing of the currents, so that this may take place without the formation of any noticeable arc. This shall take' place when the diode 17 is going to be forward biased, so that the current then is switched over to this diode instead. When then the voltage across the switching device changes direction no current will flow therethrough, but a voltage will be built up across the diode 17 then reverse biased and the rotation movement of the moveable contact 4 is now contin- ued in the same direction as before, so that the galvanic connection between the contact 3' and the contact 3'" is broken and the breaking of the current is by that made permanent. This breaking may take place arc-free, since no current flow through the contact place.

However, the electronic unit 8 may be adapted to achieve a condition check of this electric switching device during operation of the electric plant in which the current path 5 is a part by controlling the electric motor 19 and then carry out measurements of at least one parameter associated with the condition of the electric switching device. It is illustrated in fig 3 how this electric motor 19 for such a condition check controls the moveable contact 4 to carry out a sliding movement along the fixed contact 3 while maintaining the contact therewith. The function of the electric switching device and the influence upon the current in the current path 5 is by that not changed at all, but important information about the condition of the electric switching device may be obtained. The position detector 20 may send information to the electric unit 8 about the size of the actual movement taken place. This information may be compared with the ordered movement for checking that the actuation of the electric switching device functions as it shall. Other condition parameters of the electric switching device and the actuating circuit thereof may at the same time be calculated in a way described with ref- erence to fig 1 . It is illustrated in fig 4 how it principally would be possible to continue the movement of the moveable contact 4 for said condition check of the electric switching device so that the contacts 3, 4 of the first contact member are moved out of engagement with each other, so that the current is transferred to the member 17, but it would then be necessary that the electric motor 19 is controlled to move the contact 4 back for establishing contact with the contact 3 again before the member 17 arrives to the blocking state.

It is illustrated in fig 5 and 6 how a method according to the invention may be applied on another type of electric switching device 1 for condition check thereof. Parts having correspondence in the device according to fig 1 have here been provided with the same reference numerals. The two contacts 3, 4 of the electric switching device are adapted to bear under pretension against each other through a spring member 21 schematically indicated in normal operation of the electric switching device. The electric motor 2 is adapted to control the moveable contact 4 to such a movement for said condition check that the spring member is allowed to release a determined amount of potential energy being below the level required for a separation of the contacts, so that for example the position according to fig 6 is obtained. It may through the position detector 9 be checked that the movement ordered by the electronic unit has really taken place.

It is illustrated in fig 7-10 how a method according to the invention may be applied to an electric switching device 1 of conven- tional type with a rectilinear relative movement of the two contacts. Parts having correspondence in the device according to fig 1 have here been provided with the same reference numerals. The case in which the electric switching device is adapted to be closed and conduct current therethrough during normal operation (fig 7) of the electric plant is illustrated in fig 7 and 8. The electric motor 2 is for said condition check adapted to con- trol the moveable contact 4 to such a movement that the electric switching device remains closed, such as to the position according to fig 8. Any of the parameters discussed above is simultaneously-measured for the condition check. The case of the arrangement of the electric switching device to keep the current path therethrough broken during normal operation (fig 9) of the electric plant is illustrated in fig 9 and 10. The condition check here takes place by moving the moveable contact 4 towards the contact 3 without closing the breaker, such as to the position shown in fig 10.

The electronic unit 8 is preferably through a suitable programming adapted to entirely autonomously control the condition check of the electric switching device 1 . It may be designed to do this automatically at predetermined points of time, so that possible faults of the function of the electric switching device, i.e. actually the actuating circuit thereof, may be discovered before the electric switching device shall carry out the task it is intended for, which may be expected to take place at very long time intervals. When then any maintenance really has to be carried out on the electric switching device and the actuating circuit thereof the electric unit sends information thereabout to for example an operation central through an alarm system. However, this means that no maintenance has to be carried out before this is stated by the electronic unit, so that considerable maintenance time may be saved and at the same time the degree of utilisation of the electric switching device may be increased, since it has not to be taken out of operation for the checks.

The invention also relates to such a condition check of an electric switching device which may take place without any movement between the two contacts moveable with respect to each other. It is then for example possible to measure the contact resistance between two contacts of the switching device by meas- uring the current through the switching device and the voltage between the contacts. Another possibility is to measure the tern- perature of the contacts and the current therethrough. The temperature at a given current magnitude is also a measure of the contact resistance.

The invention is of course not in any way restricted to the preferred embodiments described above, but an amount of possibilities to modifications thereof would be apparent to a person with skill in the art, without departing from the basic idea of the invention as defined in the appended claims.

The invention is applicable to electric switching devices intended for all conceivable levels of voltages and currents, even if it is particularly well suited for condition check of electric switching devices in a current path in switch gears for electricity supply within industry, distribution or transmission networks having a voltage in the current path on intermediate and high voltage level, i.e. between 1 and 800 kV. Furthermore, the invention is particularly applicable to electric switching devices adapted to be able to take an operation current of 1 kA, prefer- able 4 kA.

That the contacts of the electric switching device are moveable with respect to each other is in this disclosure to be interpreted in its broadest sense, i.e. one of them may be stationary while the other is moveable or both may be moveable. They could even move in the same direction but with different speeds.

Claims

1 . A method for checking a condition of an electric switching device (1 ) arranged in a current path (5) of an electric plant, said switching device being controllable between positions for closing and breaking the current path, characterized in that the checking is carried out during operation of the electric plant by carrying out a measurement of at least one parameter associated with the condition of the electric switching device without influencing the operation through the electric switching device.

2. A method according to claim 1 , characterized in that at least one parameter describing the condition of any part of the electric switching device is calculated on the basis of the result of said measurement.

3. A method according to claim 1 or 2, characterized in that the condition check is carried out for an electric switching device (1 ) comprising an electric actuating member (2, 19), such as an electric motor, arranged to control two contacts (3-3'", 4) movable with respect to each other between said two positions.

4. A method according to claim ³ , characterized in that a condition check is carried out for the electric switching device (1 ) by checking the function of the electric actuating member (2, 19).

5. A method according to claim 4, characterized in that the function check of the electric actuating member (2, 19) takes place by ordering a determined relative movement of the two contacts (3-3'", 4) and the result of this order is measured and compared with an expected result.

6. A method according to claim 5, characterized in that it is the distance for said relative movement that is measured.

7. A method according to claim 5 or 6, characterized in that the distance of the relative movement ordered as well as the energy consumed by the electric actuating member (2, 19) for obtaining this movement are measured and compared with an expected energy consumption for the movement.

8. A method according to claim 7, in which the electric actuating member is adapted to receive energy stored in a capacitor (6), characterized in that the voltage drop across the capacitor is measured and is considered as a value of the energy consumption when carrying out said relative movement.

9. A method according to any of claims 5-8, characterized in that a detector (9) of the position of the mutual position of the contacts is checked by checking that the position detector delivers a reading when a movement of a movable part of the electric actuating member is simultaneously detected.

10. A method according to any of the preceding claims, in which the electric switching device is adapted to be closed and conduct a current therethrough under normal operation of the electric plant, characterized in that the temperature of at least one of the two contacts of the electric switching de- vice and the current through the electric switching device are measured, and that these values are compared with expected values therefor so as to determine the condition of the electric switching device with respect to the contact resistance thereof.

1 1 . A method according to any of the preceding claims, in which the electric switching device is adapted to be closed and conduct a current therethrough under normal operation of the electric plant, characterized in that the voltage between two contacts of the electric switching device as well as the current through the electric switching device are measured, and that these values are compared with expected values therefor so as to determine the condition of the electric switching device with respect to the contact resistance thereof.

12. A method according to any of the other preceding claims, in which the electric switching device is adapted to be closed and conduct a current therethrough under normal operation of the electric plant, characterized in that at least a part

(14) connected to one of the two contacts of the switching device movable with respect to each other is during the checking brought to move without causing the two contacts to leave their mutual contact.

13. A method according to claim 12, in which the two contacts (3, 4) are adapted to bear under pretension against each other through a spring member (21 ) in the normal operation of the electric switching device, characterized in that said movement for the checking takes place by allowing the spring member to deliver a determined amount of potential energy being less than the level required for a separation of the contacts to take place.

14. A method according to claim 12, characterized in that the two contacts (3, 4) are brought to carry out a mutual sliding movement along each other while maintaining the contact to each other for said checking.

15. A method according to any of claims 1 -9, in which the electric switching device is adapted to keep the current path therethrough broken in normal operation of the electric plant, characterized in that said checking takes place by carrying out a relative movement of two contacts (3, 4) of the electric switching device without the result that these contacts enter into contact with each other and by that current flows through the switching device.

16. A method according to claim 3 for checking a condition of an electric switching device for alternating current compris- ing two branches connected in parallel in the current path, in which a first one of them comprises a first contact member having two movable contacts (3, 4) movable with respect to each other for opening and closing and the second one comprises an element (17, 18) with ability to block cur- rent therethrough in at least one blocking direction and conduct current therethrough in at least one direction, in which a second contact member having two contacts (3', 3) movable with respect to each other for opening and closing is connected in series with said element, in which the switching device is adapted to break the current in the current path by the fact that the first contact member is controlled to open for transferring the current to said element when this is in or going into a conducting state and then the second contact member to open when said element is in a state of blocking current therethrough after a zero-crossing of the current, and in which the electric switching device is associated with an electric actuating member adapted to cause a relative movement of the movable contacts, characterized in that the checking is carried out by controlling the electric actuating member (19) to move said contacts (3,

4) with respect to each other without breaking the current through the electric switching device.

17. A method according to claim 16, characterized in that the contacts (3, 4) of the first contact member are moved with respect to each other while maintaining mutual contact at said checking.

18. A method according to claim 16, characterized in that the electric actuating member (19) is controlled to move the movable contact parts of the first contact member out of engagement with each other at said checking for transferring the current to said element (17) and after that to move back and establish mutual contact again before said ele- ment arrives to blocking state.

19. A method according to any of the preceding claims, in which the electric switching device is associated with an electronic unit (8) for controlling an electrical actuating member (2, 19) arranged for mutual movement of contacts (3, 4) of the electric switching device for bringing this between said two positions, characterized in that said condition check is controlled by the electronic unit.

20. A method according to claim 19, characterized in that parameters used for carrying out the checking are taken from signals to or in the electronic unit (8).

21 . A method according to claim 19 or 20, characterized in that condition check is carried out automatically at predetermined times and is initiated by the electronic unit (8).

22. A method according to any of the preceding claims, characterized in that the condition checks are carried out at least once every second week, advantageously at least once a week and preferably daily.

23. A method according to claim 21 or 22, characterized in that in the case that the condition check results in a discovered condition of the electric switching device deviating in any sense from the expected condition it is called for attention by starting an alarm (13) or the like.

24. An apparatus for checking a condition of an electric switch- ing device (1 ) arranged in a current path (5) of an electric plant, said switching device being controllable between positions for closing and breaking the current path, characterized in that it comprises means (9-1 1 , 20) adapted to carry out a measurement of at least one parameter associ- ated with the condition of the electric switching device during operation of the electric plant without influencing said operation through the electric switching device.

25. An apparatus according claim 24, characterized in that it comprises members (8) adapted to calculate at least one parameter describing the condition of any part of the electric switching device on the basis of results of said measurement.

26. An apparatus according claim 24 or 25, characterized in that it is designed to carry out the condition check on an electric switching device (1 ) comprising an electrical actuating member (2, 19), such as an electric motor, adapted to control two contacts (3, 4) moveable with respect to each other between said two positions.

27. An apparatus according to claim 26, characterized in that it comprises means (8, 20) adapted to carry out a condition check of the electric switching device by checking the func- tion of the electrical actuating member (2, 19).

28. An apparatus according to claim 26, in which the electric switching device is associated with an electronic unit (8) for controlling the electrical actuating member (2, 19) arranged for mutual movement of the moveable contacts of the electric switching device (1 ) for bringing this between said two positions, characterized in that the electronic unit is adapted to control said condition check.

29. An apparatus according to claim 28, characterized in that the electronic unit (8) is adapted to automatically initiate performing of the condition check at predetermined times.

30. An arrangement for checking a condition of an electric switching device (1 ) arranged in a current path (5) of an electric plant, said arrangement comprising a program modul containing at least one processor adapted to carry out program instructions to carry out said checking during operation of the electric plant by measuring at least one parameter associated with the state of the electric switching device without influencing the operation of the plant through the electric switching device.

31 . A computer program for checking a condition of an electric switching device (1 ) arranged in a current path (5) of an electric plant, in which the computer program comprises instructions for influencing a processor to cause a checking of the electric switching device during operation of the electric plant by measuring at least one parameter associated with the condition of the switching device without influencing the operation of the plant through the electric switching device.

32. A computer program according to claim 31 provided at least partially over a network as the Internet.

33. A computer program product loadable directly into the internal memory of a digital computer and comprising software code portions for carrying out the steps according to any of claims 1 -23 when the product is run on a computer.