The invention relates to an arrangement for protective grounding for providing a fixed, electricity conducting connection between a first part with a fixed position and a second part rotating restricted ly and/or moving a restricted distance with respect to the first part, for use in connection with electricity supply to provide a protective grounding connection, the arrangement comprising a spiral, helical or volute spring, or a combination of same, a first end of the spring being in galvanic contact with the first part arranged on the inside of the spring and a second end being in galvanic contact with the second part.

Preferably, the arrangement of the invention can be used, for example, in different cable drums or in conjunction with them, especially for providing protective grounding. In particular, the arrangement of the invention is suitable for use in a cable drum by which electric power is transferred from a grounded plug to e.g. a car, trailer or other machines and equipment comprising heaters or other such electrically driven devices. Protective grounding has here proved problematic, since a protective grounding connection must be very low-ohmic, whereas in a connection of a phase current and a zero conductor, more conventional sliding contact solutions with a higher resistance can be used.

A further object of the invention is to provide such an arrangement for providing protective grounding in a vehicle between an external electrical network and the chassis of the vehicle that when a protective grounding connection is broken, transfer of electric power through an equipment comprising an arrangement like this also becomes impossible.

The above objects are achieved by an arrangement according to the invention, which is characterized by comprising a spiral, helical or volute spring, or a combination of same, a first end of the spring being in galvanic contact with the first part arranged on the inside of the spring and a second end being in galvanic contact with the second part. The ends of a spring here mean those points of a spring which define a section at which the spring participates in establishing an electricity conducting connection.

Most preferably, the spring is a spiral spring made of tape-like material by winding, whereby the spring, because of its fairly high springback factor, can also function as a pullback spring e.g. in a cable drum.

Alternatively, the spring is a spiral, helical or volute spring made of yarn-like material by winding, or a combination of same. All these springs function in principle in the same way in the arrangement of the invention, except that whereas a spiral spring requires space in the diametral direction, a helical or volute spring requires space especially in the axial direction of the spring. In other respects, all the above springs function in the same way, i.e. when a point on the outer circle of the spring, attached to one end of the spring, is rotated to such a direction that the spring is tensioned, while the other end of the spring is in a fixed position, the spring, naturally depending on its size, can be tensioned to such an extent that the point on its outer circle can move, if desired, several metres.

If a spring according to the invention is used for transferring electric power, it has to be provided with an insulating coating, such as a plastic coating, to prevent sparking between the laps of the spring. When the arrangement according to the invention is used in conjunction with electricity supply to establish a protective grounding connection, a particularly preferred embodiment of the invention is characterized in that one end of the spring providing protective grounding comprises a contact part that is arranged to spring-loadedly short-circuit the poles of the electricity supply and thereby prevent the supply of electricity when a protective grounding connection is broken. This is an absolutely reliable way of ensuring that if a protective grounding connection fails because the spring breaks, other supply of electricity is no longer possible, either.

When the arrangement according to the invention is used in vehicles for establishing a protective grounding connection between an external electrical network and the chassis of a vehicle, there are preferably at least two springs with possible contact parts that can short-circuit the supply of electricity, and they are connected in parallel. This makes it possible to further improve the reliability and to accomplish low resistance for the protective grounding connection, even though the electrical conductivity of the spring material used would not be very good.

In the following the arrangement provided by the invention will be described in greater detail with reference to the attached drawing, in which fig. 1 is a scheme illustrating the basic idea of an arrangement according to the invention,

fig. 2 shows an exemplary embodiment of an arrangement according to the invention arranged in a cable drum, the cable drum being shown as a sectional view in the direction of its longitudinal axis, and fig. 3 shows a partly sectional view of a cable drum according to fig. 2 seen in the axial direction.

Fig. 1 is a scheme illustrating the basic idea of an arrangement according to the invention for providing a fixed, electricity conducting connection between a first part with a fixed position and a second part rotating restrictedly and/or moving a restricted distance with respect to the first part. In fig. 1 , conductors supplying electricity to the rotating or moving part are indicated by reference numbers 10 and 11 , conductor 10 being a protective grounding conductor, and conductors 11 usually being zero and phase conductors of an electrical network. Reference numbers 10" and 11 " stand for conductors which are located in the part with a fixed position and through which protective grounding and electric current, respectively, are forwarded to the desired customer device. In the arrangement of fig. 1 , the arrangement according to the invention is applied only as regards the protective grounding conductor 10. In conjunction with conductors 11 , it would also be possible to apply an electricity conducting connection provided by means of a spring, especially if the spring were provided with an insulating coating, but with these conductors, a sufficiently low transfer resistance can usually also be provided by conventional solutions based on a sliding contact (e.g. carbon brushes). A solution like this is described in greater detail in connection with figs. 2 and 3, and so they are not shown in fig. 1. With regard to the protective grounding conductor 10, the arrangement according to the invention comprises springs 5 and 6, to first ends 20 of which leads a protective grounding conductor and from second ends of which leads conductor 10" with a fixed position. Springs 5 and 6, which are electrically connected in parallel, are either spiral springs made of tape-like material by winding, or spiral, helical or volute springs made of yarn-like material by winding, or combinations of same. All these spring types can be tensioned such that one end (in a spiral spring, the outer end) can rotate and/or move in relation to the other end (in a spiral spring, the inner end), which is fixedly attached to conductor 10".

As appears from fig. 1 , the protective grounding passes through springs 5 and 6, and if one or the other of these springs breaks, contact part 20 is brought into contact with opposite contacts 11a and 11b by the action of

spring 7, which is practice can be either an extension spring or a pressure spring, whereby it short-circuits them. A typical consequence is that a fuse melts in a plug or fuse box. This is a reliable way of preventing supply of electricity when one of the springs 5 and 6 is broken. The reason for the parallel connection between springs 5 and 6 is not so much that it improves operational reliability of the device, since breaking of either one of the springs will cause the supply conductors to short-circuit, but - above all - that such doubling is required in vehicle use and, on the other hand, that such a parallel connection arrangement helps to reduce resistance of a protective grounding conductor. With some spring materials, this may be necessary to bring the total resistance of the protective grounding to an acceptable level. According to the requirements set, the resistance on the current path used for protective grounding between a power supply and a consumer device must not exceed 0.1 ohm when a load current of 25 A is supplied through a protective grounding conductor and the voltage is 12 V. The total resistance concerned can naturally also be achieved by a single spring, if the spring material is selected such that it has good electricity conductivity. Typically, however, such materials with good electricity conductivity do not have ideal spring characteristics. In a cable drum, where the springs are required to have a certain kind of springback factor, it is also possible to use a parallel connection as shown in fig. 1 , because it enables the use of metals with better spring characteristics, even though their resistance is higher than that of metals with better conductivity. The conductivity of a metal used in a spring can naturally also be improved, for example, by coating with copper. In figures 2 and 3, the structure of a cable drum in which an arrangement according to the invention is used is shown both as a cross- sectional view in the axial direction and as an axial view. The cable drum comprises a body 1 with a fixed position and connected to an electric cable 9 that leads to a consumer device, the cable comprising zero and phase conductors 11" and a protective grounding conductor 10", which in turn is connected to a protective grounding coupling bar 10". A drum arranged to rotate about the body 1 with a fixed position comprises an outer part 3 on which a cable 8 to be connected to the electric network will be coiled, and an inner drum body 2 leaning against this outer drum 3 via intermediate sleeves 14. The protective grounding conductor 10 and zero and phase conductors 11 of the cable 8 extend through the connecting space provided by the

intermediate sleeves 14 to connecting points located on the drum body 2. From the connecting points, the zero conductor and phase conductor extend to lead-in pins attached to the drum body 2 and to annular contact parts 17 located on the inner surface of the cylindrical drum body. From these contact parts 17, an electricity conducting connection is implemented through carbons 4 spring-loaded against the contacts 17 to conductors 11" of the cable running to a consumer device 9.

A protective grounding conductor 10, in turn, is connected to the spring ends on the outer circles of spiral springs 5 and 6 made of tape-like material by winding. The spring ends rotate with the drum body 2. The inner ends of the springs 5 and 6, in turn, are connected to a protective grounding bus bar 10', and the ends are fixedly attached to the body 1.

As shown in fig. 3, the outermost lap of the spiral spring comprises an aperture or hole 19, which is gripped by a thorn 18 formed on the outermost but one lap of the spiral spring. This helps to prevent the spring from being released if it breaks at a point on an inner lap, and thereby to maintain the outer diameter of the spring as desired. The restriction to a certain outer diameter is important, since otherwise it would be impossible to implement the protective grounding backup system described in fig. 1. The spring could then be released, and its outermost lap would be tightly wedged inside the drum body 2. As the release of the spring is prevented by the aperture 19 and thorn 18, the outermost lap of the spring can move by the action of spring 7 if the spring breaks at a point on an inner lap, such that the contact part 20 formed in conjunction with the end of the spring can short- circuit conductors 11. This can be clearly seen, for example, in the somewhat schematic 3 presentation shown in the figure. In this way, as shown in fig. 1 , the zero conductor and phase conductor can thus be short-circuited and the supply of electricity can be prevented if there is something wrong with the protective grounding. Fig. 3 further shows how the end of the spring located at the midpoint of the spring is connected to the protective grounding bus bar 10', while the end on the outer circle of the spring is connected to the protective grounding conductor 10 of cable 8.

As shown in fig. 2 and partly in fig. 3, the cable drum also comprises an outer body 13, with respect to which the cable drum can be locked by locking means12. As appears particularly from fig. 3, the operation of the locking means 12 is based on a tooth system on the circumference of the

lateral flanges of the drum 3 and on a load spring in the locking means 12, which presses the locking pin to the locking holes of the lateral flanges of the drum 3 when the yarn associated with the locking pin is not pulled. As also shown in fig. 3, the drum locks only in one direction thanks to the slanting tooth system of the lateral flanges, thereby preventing the cable 8 from coiling back on the drum when the locking system is used.

The arrangement according to the invention for providing a fixed, electricity conducting connection between a part with a fixed position and a part rotating restrictedly and/or moving a restricted distance with respect to that part is described above only by a single embodiment based on a spiral spring. As stated above, a similar operation could also be achieved by a helical spring and/or a volute spring or a combination of same. With a helical spring, in particular, it would be possible to use a method in which helical springs electrically connected in parallel would be positioned co-axially within each other. The innermost spring would be first placed on a suitable axle whose diameter would be suitably smaller than that of the helical spring, and on top of this innermost helical spring one would place a second tubular axle, around which a second helical spring would be placed, the diameter of the second helical spring, in turn, being suitably larger than that of the second tubular axle. One could continue to add new layers in this way for some time without that the outer diameter of the arrangement would grow notably. The pullback force could thus also be improved significantly. In conjunction with this kind of arrangement, which would have some kind of a length in the axial direction of the springs, a cord yarn or some other such cord-like conductor that endures repeated winding and unwinding even better than a spring could form a part of protective grounding. One end of the cord yarn would be attached to a fixed position, and the other end would rotate with the rotating drum body. Likewise, unlike in the embodiment described, it would also be possible to place a spiral spring at the end of the drum either as a pullback spring or also as a part of protective grounding. The outer diameter of the spiral spring would then not be restricted to the inner diameter of the drum body 2 like in the embodiment described above.