WO1997007564A1

WO1997007564A1 - Junction box for multiple shielded cables

Info

Publication number: WO1997007564A1
Application number: PCT/FR1995/001094
Authority: WO
Inventors: Michel Mehez; Bernard Warusfel
Original assignee: Cegelec
Priority date: 1994-02-18
Filing date: 1995-08-17
Publication date: 1997-02-27

Abstract

A junction box for connecting together the shields (4, 29) of all the shielded cables includes: a conductive enclosure (15, 19) forming a Faraday cage; a metal plate (8) with holes (7) for fixing each cable (1, 22, 18) by means of a self-locking clamping collar (6, 20); and tabs (25) cut out in the plate and folded away from the plane thereof, forming crimpable or weldable connection points for interconnecting the cable shields together and to ground, either by a filter (11) connected to the conductive enclosure (15, 18), for high frequency currents, or directly by an earthing wire. The device is particularly useful for industrial computer networks.

Description

Connection box for a plurality of shielded cables

The invention relates to a connection box for a plurality of shielded cables such as, for example, shielded cables intended to ensure the connection of industrial equipment transmitting or receiving data. Such a shielded cable generally comprises several insulated conductors, surrounded by an electromagnetic screen constituted by a conductive braid. A connection box is used, for example, to connect a drop cable to a main cable, for each device to be connected to the main cable.

For a good protection of the transmitted data, it is necessary to connect together the screens of all shielded cables connected by a connection box. On the other hand, it must include a conductive enclosure forming a Faraday cage around the devices connecting the conductors transmitting the data. For example, for the field bus known as FIP, and for the field bus known as WORLDFIP, standards NF C46 604 and IEC 1158-2 set physical interconnection rules allowing, on the one hand, to keep the adaptation of the data transmission lines, and on the other hand, to properly interconnect the screens of the cables, and to connect them to the ground. French patent applications Nos. 2,666,692 and 2,666,695 describe connection boxes which make it possible to comply with these standards, and which also make it possible to connect a drop cable to a main cable without interrupting data traffic on this main cable, although it is necessary to cut the conductors of the pair of this main cable to divert the data traffic to the switchgear by means of a first pair, then to bring the data traffic to the main cable to the by means of a second pair. These known connection boxes include: - a cut-off device to cut the pair of the main cable; insulation displacement contacts for connecting a first section of the pair of the main cable to a first pair of the drop cable;

- insulation displacement contacts to connect a second section of the pair of the main cable to a second pair of the drop cable;

- insulation displacement contacts to connect a drain which runs along the screen of the main cable, to a

Faraday consisting of a metal case or a metallized insulating case surrounding all of the connection devices;

- three cable glands for fixing the two sections of the main cable and the branch cable to the connection box; and allowing the screens of the two sections of the main cable and the screen of the drop cable to be connected to the Faraday cage.

Such a connection box has the advantage of allowing the connection of additional equipment in an already existing network, without interrupting data traffic for a significant period. However, this type of connection box has the disadvantages: a high cost price, large dimensions and the obligation to use a specific tool to operate the cutting device and operate the insulation displacement contacts. For certain applications, where it is immaterial to have to interrupt the data traffic to connect additional devices, or for applications where it is almost certain that there will be no need to add additional devices, such junction boxes are unnecessarily expensive.

The object of the invention is to provide a much cheaper connection box, of reduced bulk, and which can be installed without using specific tools. Naturally, this junction box must meet the physical interconnection rules defined by the standards mentioned above. In particular, it must allow the screens of the two sections of a main cable and of a drop cable to be connected reliably to each other and possibly to earth.

- on the other hand protect the connections against electromagnetic interference by forming a Faraday cage and protect people from contact with the connections and the cable screens. The object of the invention is a connection box for a plurality of shielded cables, each cable comprising an electromagnetic screen; this box comprising: means for fixing the cables and for connecting together the screens of all the cables; and a conductive enclosure forming a Faraday cage, and comprising a passage orifice for each cable; characterized in that the means for fixing all the cables and for connecting together the screens for all the cables comprise: - a metal plate isolated from the conductive enclosure forming a Faraday cage and comprising:

- two holes for each cable, these two holes having a spacing close to the diameter of this cable.

- And a tab for each cable, this tab being cut out of this plate and being folded out of the plane of this plate, to constitute a connection point for the screen of this cable;

- a self-locking clamp for each cable, this collar passing successively through the two holes corresponding to this cable; surrounding this cable; and pressing it against the plate to make it integral with the plate.

The junction box thus characterized has a reduced cost price owing to the fact that it does not include cable glands for fixing the cables and ensuring the interconnection of their screens, but that it comprises for the same functions a metal plate comprising some easy cutouts to machine; and self-locking cable ties which can be conventional polyamide ties, usually used to fix cables on cable trays, and which are very inexpensive. According to a preferred embodiment, each tab has a curved hook-shaped end intended to be crimped or welded on stranded conductors extracted from the screen of a cable. It is therefore easy to connect the cable screens to each other, using ordinary pliers or a soldering iron. The insulated conductors that the cables comprise can be interconnected by known means, any one and inexpensive, requiring only conventional tools. For example: small insulation displacement jaws or screw terminals. The elimination of cable glands leads to a significant reduction in the size of the connection box. This reduction is further accentuated by the elimination of the cut-off device, which is useless in all applications where it is not necessary to minimize the duration of interruption of data traffic.

The invention will be better understood and other details will appear with the aid of the description below of an exemplary embodiment, and of the accompanying figures:

- Figure 1 shows an overview of an embodiment of the connection box according

1'invention, part of its housing being removed to allow the interior to be seen;

- Figure 2 shows a top view of the metal plate alone, in this embodiment; - Figure 3 shows a side view of this metal plate and two spacers of insulating material intended to support it and keep it isolated from the conductive enclosure forming the Faraday cage;

- Figure 4 shows one of the tabs cut from this plate. The embodiment shown in Figure 1 includes:

- a first section 1 of a main cable;

- a second section 18 of this main cable; - a branch cable 22;

- A metal plate 8, having the main functions of fixing the cables 1, 18, 22, and of interconnecting their screens;

- Three self-tightening collars, only two of which, collars 6 and 20, are visible in Figure 1;

- four connection devices with self-stripping jaws, 16, each making it possible to interconnect two insulated conductors;

- a parallelepipedal metal housing made up of two parts, 15 and 19, contiguous along a line 17 which passes through three cable passage openings 1, 18, 22; only two orifices, 5 and 21, being visible in FIG. 1; this housing constituting a Faraday cage;

- A filter 11, which can consist for example of a capacitor and a varistor wired in parallel, connecting the plate 8 to the housing 15, 19 for high frequency currents;

- an earth connection, 14, comprising a lug 13 at its end, intended for possibly short-circuiting the filter 11 to directly connect the plate 8 with the conductive housing 15 and 19.

The housing 15, 19 is kept closed by four screws 12. It is fixed to any support by means not shown, which are screws for example. It is normally connected to a local earth connection by the fixing means on this support.

The operations for fixing and interconnecting the cables are carried out on the plate 8 outside the housing 15, 19. The housing therefore presents no discomfort for these operations. A notch, not shown, allows the passage of the connection 14 through the wall of the lower part 15 of the housing.

Each section 1 and 18 of the main cable comprises one or more pairs of insulated conductors, 2 and 3. These pairs are shielded by a screen 4 made up of a braid of fine metallic conductors. The branch cable 22 generally comprises at least two pairs of insulated conductors generally referenced 28, shielded by a screen 29 consisting of a braid of fine metallic conductors. Two conductors of a first pair of the drop cable 22 are connected to a pair 2,3 of the first section 1 of the main cable, by means of two connection devices 16. A second pair of the drop cable 22 is connected to a pair of the second section 18 of the main cable by two other connection devices 16. In the case of cables comprising unused pairs, the unused pairs are short-circuited at each end. The four connection devices 16 can be of various known types, inexpensive and requiring no specific tools. In this example they are self-stripping jaws.

Some conductors of the braid constituting the screen 4 are extracted from this braid and are combined into a strand 23 which is used to connect this screen to the metal plate 8. The plate 8 has a tab 25 whose end is folded back to constitute a hook 24 in which the end of the strand 23 is crimped or welded. The screen 29 of the branch cable 22 is connected to the metal plate 8 in a similar manner. The screen of the second section 18 of the main cable is not shown in FIG. 1, but it is connected to the metal plate 8 in a similar manner.

The first section 1 of the main cable is made integral with the metal plate 8, and therefore with the connection box, by means of a self-locking clamping collar 6. This collar surrounds section 1, in an area where the screen 4 is not stripped, and passes through two holes drilled in the plate 8, only the hole 7 being visible in FIG. 1. The self-tightening collar 6 is a conventional polyamide collar, comprising a passage 27 provided with a ratchet making it possible to pull on the end 26 of the collar but preventing return back of this end. This collar is tightened by pulling on the end 26 using flat pliers. He presses section 1 of the main cable against the plate 8, in order to make it integral with this plate.

The second section 18 of the main cable is fixed similarly to the plate 8, by a clamp not visible in the figure. The branch cable 22 is fixed analogously to the plate 8, by means of a clamp 20.

The metal plate 8 has another tab 10, similar to the tab 25, making it possible to crimp or weld the second end of the earth connection, 14, and a connection of the filter 11. The tabs, such as the tab 25, for connect the cable screens to the plate 8, are located on one side of the plate, while the tab 10 is located on the other side of the plate.

Figure 2 shows a top view of the metal plate 8 while Figure 3 shows a side view. These figures show that each of the legs 25, 46, 10 and 36 is manufactured by cutting a rectangular strip in the plate 8; by folding this strip perpendicular to the plate 8; then by bending the end of this strip so as to form a hook, respectively 24, 45, 40, 35.

Figure 4 shows the tab 46 seen from the side. The other legs are similar. The hook 45 comprises two half-punctures of triangular shape, 52 and 53, placed vis-à-vis inside the hook 45 to form two reliefs improving the crimping or welding of a connection.

Figure 2 shows that there are two fixing holes, 31 and 7 for section 1 of the main cable; and two similar holes 37 and 38 for section 18 of the cable main; and two similar holes 33 and 34 for the branch cable 22. The diameter of each of these holes is slightly greater than the width of the tongue of the cable ties, and the spacing of each pair of holes is close to the diameter of the cables to fix.

The plate 8 further comprises two holes 32 and 39 located approximately on a diagonal of this plate and which are intended to collaborate with two plastic spacers, 48 and 49, which are shown in side view in Figure 3. The spacer 48, for example, comprises: a flexible stud 50 having a diameter slightly greater than that of the hole 39, a shoulder 51 against which the plate 8 will rest, and an adhesive base 52, fixed to the bottom of the lower part 15 of the metal case , the function of these spacers being to keep the plate 8 isolated from the conductive housing 15 and 19.

When the operations for fixing and connecting the cables are finished, the plate 8 is plugged onto the pins 50 of the spacers 48 and 49 until it rests on the shoulders 51. The friction on the pins 50 is sufficient for the plate 8 is held in place against these shoulders 51.

The spacers 48 and 49 being insulating, the plate 8 is electrically insulated with respect to the housing 15, 19. This housing 15, 19 is connected to a local earth connection for the protection of persons. In a network, the plate 8 of all the connection boxes except one, is generally floating relative to the earth, but is connected to the housing 15, 19 by a filter 11 letting the high frequency currents pass. Only one of the network connection boxes is actually provided with the connection 14 for connecting the plate 8 directly to an earth connection. In the other boxes the connection 14 is deleted.

According to an alternative embodiment, two additional holes can be provided in the plate 8 to fix the earth connection 14, by means of a self-locking collar. tightening, so that traction on this connection is not fully supported by the connection lug 41, 42.

According to alternative embodiments, the metal housing 15, 19 can be replaced by a plastic housing, internally metallized to form a Faraday cage. This box can also be made up of more than two parts to facilitate its manufacture or assembly. The cables can be provided with grommets, adapted to the diameter of passage orifices, such as 5 and 20, to improve the tightness of the housing.

The scope of the invention is not limited to the type of cable considered in this example, it is applicable for the connection of any cable comprising a screen; and any number of cables.

Claims

1) Connection box for a plurality of shielded cables (1, 18, 22), each cable comprising an electromagnetic screen (4, 29); this box comprising: means for fixing the cables and for connecting together the screens of all the cables; and a conductive enclosure (15, 19) forming a Faraday cage and comprising a passage orifice (5, 21) for each cable; characterized in that the means for fixing all the cables and for connecting the screens (4, 29) of all the cables comprise:

- a metal plate (8) comprising:

- two holes (7, 31) for each cable, these two holes having a spacing close to the diameter of this cable; - And a tab (24, 25) for each cable, this tab being cut out of this plate and being folded out of the plane of this plate, to constitute a connection point for the screen (4, 29) of this cable;

- a self-locking clamp (6, 26, 27) for each cable, this collar passing successively through the two holes (7, 31) corresponding to this cable; surrounding this cable; and pressing it against the plate (8) to make it integral with the plate.

2) Junction box according to claim 1, characterized in that each tab (24, 25) has one end (24) curved in the shape of a hook and intended to be crimped or welded on stranded conductors (23) extracted from the screen (4) of a cable (1).

3) Junction box according to claim 1, characterized in that the metal plate (8) further comprises at least one additional tab (10), cut from this plate and folded out of the plane of this plate to form a connection point for a grounding conductor (14), and / or for a grounding filter (11). 4) junction box according to claim 1, characterized in that the enclosure (15, 19) forming a Faraday cage comprises at least two contiguous parts (15, 19); and in that these two parts have a junction line (17) which passes through all the holes (5,21) for cable passage.