GB2245775A - Electrical connector short circuiting arrangements - Google Patents

Abstract

A bi-partite electrical connector (26) comprises a pin header part (30) and a receptacle part (32) comprising a receptacle housing (62) having rows of cavities therein each receiving an electrical receptacle terminal (64) crimped to an electrical lead (66). Pairs of the terminals (64) located in respective spaced pairs of the cavities have their leads (66) connected to electrically fired actuators of air bags of an automotive air bag system. The terminals (64) of each of these pairs of cavities are normally short circuited by means of a resilient short circuiting link (70) secured in a socket (82) between the cavities of each pair. This protects the air bag actuators against accidental firing as a result of spurious energisation of their leads (66), until the pin header part (30) and the receptacle part (32) have been substantially fully mated. As the pin header part (30) and the receptacle part (32) are moved together proximate to their fully mated position, actuator pins (38) projecting from the pin header part (30) depress the short circuiting links (70) to disengage them from the terminals engaged thereby, whereby the air bag actuators are connected in circuit. <IMAGE>

Description

ELECTRICAL CONNECTOR SH.ORT CIRCUITING ARRANGEMENTS This invention relates in general to short circuiting arrangements for electrical connectors, especially such arrangements for the protection of the electric actuators, generally known as squibs, of automative air bags. The invention relates in particular to a bi-partite electrical connector having protective shortcircuiting means, to a mating part of such a connector and to a short circuiting link, which is suitable for use therein.

In an automotive air bag deployment system, crash sensors disposed in the forward part of a vehicle, may be connected to the squibs of air bags in the front seats of the vehicle, by way of a diagnostic connector and a bi-partite electrical connector the parts of which can be unmated to allow the air bags and/or the squibs thereof to be removed for replacement or testing, for example. In the unmated, or indeed in the partially unmated, condition of the bi-partite connector, there is the risk of the squibs, which are necessarily very sensitive, being accidentally triggered off, so that the air bags are accidentally inflated, by, for example, leakage, or other stray, or accidentally applied currents, energizing the firing current supply leads of the squibs, for example by induction.The bi-partite connector should, therefore, be provided with means for short circuiting such ignition leads of the squibs as the parts of the bi-partite connector are being unmated. There is described in GB-A-2,025,711 and GB-B-2,025,711, an electrical connector comprising an insulating socket containing two ignition pins projecting from an air bag actuator shell, the socket having therein springs, projecting from a metal shell mounted within the socket. When a plug, which is connected by means of ignition leads of the actuator to a crash sensor, is mated with the socket the insulating housing of the plug depresses the spring against the pins thereby short circuiting them.

Such a connector is, however, unsuitable for use as a bi-partite connector for connecting further leads to other devices, for example to fault readout devices of an air bag deployment system.

According to one aspect of the invention, therefore, a bi-partite electrical connector comprises a receptacle part and a pin carrying part for mating therewith, the receptacle part comprising an insulating housing defining first and second, speced, parallel, through cavities each receiving an electrical pin receptacle therein, and intermediate said cavities, a short circuiting link having a normal first position in which contact surfaces thereof engage against the terminals to provide a short electrical path therebetween and a displaced second position in which said contact surfaces are out of engagement with said terminals, the pin carrying part comprising an insulating member from which project active pins for mating with said receptacle terminals as said parts are mated, and at least one abutment pin for engaging said short circuiting link to displace it from its first to its second position when said parts have been substantially fully mated, said short circuiting link being automatically returned to its first position upon withdrawal of said abutment pin from said short circuiting link.

The connector may have any desired number of further terminals and active pins, the first and second cavities being located in any convenient positions in the housing. Also, more than one pair of first and second cavities may be provided, where more than one air bag squib is to be connected to actuating current by means of the bi-partite connector. The squibs are effectively protected from spurious actuation,-almost as soon as the two parts of the connector start to be unmated.

The bi-partite electrical connector could, of course, be used, for example, for the protection of electrically actuable devices other than the actuating means of air bags.

The short circuiting link conveniently comprises a retention portion, for example, a serrated retention portion which is secured in a socket defined by the housing and a resilient, cantilever beam, which is preferably planar, projecting from the retention portion and carrying said contact surfaces which are, for example, formed on upstanding lugs projecting from a cross piece at one end of the beam, and ears projecting from the cross piece, for abutment by said engagement pin.

According to another aspect of the invention, a short circuiting link which has been stamped and formed from a single piece of sheet metal stock, comprises a resilient planar beam, a cross piece at one end of the beam and a retention portion at the other end thereof for retention in a socket in an insulating housing, the cross piece having at each end thereof an upstanding lug surmounted by a contact surface and intermediate each lug and said beam, an ear projecting toward said retention portion and being inclined with respect to the plane of said beam.

According to a further aspect of the invention, a mating part for a bi-partite electrical connector for an air bag deployment system, comprises an insulating housing defining a plurality of rows of parallel, spaced through cavities each receiving an electrical terminal therein and including first and second cavities each receiving a terminal for connection to an automotive air bag actuating element, the housing further defining a socket arranged intermediate said first and second cavities and having anchored therein, a short circuiting link having a resilient beam on which are provided contact surfaces normally engaging the terminals in said first and second cavities to produce a short electrical path therebetween and abutment means for engagement by an abutment element inserted into said housing longitudinally of said cavities, resiliently to deflect said beam to displace said contact surfaces from the terminals in the first and second cavities.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made by way of example to the accompanying drawings in which Figure 1 is a schematic diagram of an automotive air bag deployment system in situ, comprising a bi-partite electrical connector providing an interface between a diagnostic unit and a pair of air bags of the system; Figure 2 is an enlarged isometric view of an insulating housing of a receptacle part of the bi-partite connector; Figure 3 is a fragmentary plan view of the housing shown in Figure 2; Figure 4 is an exploded longitudinal sectional view of said receptacle part; Figure 5 is a similar view to that of Figure 4 but showing one of a pair of short circuiting links assembled to the housing; Figure 6 is an enlarged front view of the housing as shown in Figure 5, and showing both of said links;; Figure 7 is a longitudinal sectional view showing the receptacle part in a fully assembled condition and about to be mated with a pin header part of the bipartite connector; Figure 8 is a longitudinal sectional view showing the receptacle part mated with the header part; Figure 8A is an enlarged, fragmentary, longitudinal sectional view showing details of Figure 8; Figure 9 is an enlarged isometric view of one of the short circuiting links; Figure 9A is an end view of Figure 9; Figure 10 is an enlarged isometric view of the pin header part of the connector; Figure 10A is a diagramatic front view of the pin header part illustrating the arrangement of pins therein; and Figure 11 is an exploded, isometric view of the bi-partite connector when provided with bung seals for sealing electrical terminals of the receptacle part in cavities therein.

As shown in Figure 1, an air bag deployment system assembled to the forward part 2 of an automotive vehicle, comprises a pair of crash sensors 4, disposed forwardly of the fire wall 6 of the vehicle, a diagnostic unit 8 disposed rearwardly of the wall 6 and two air bags 10 arranged before the drivers seat 12 and the front passenger seat 14, respectively, of the vehicle. The sensors 4 are connected to the diagnostic unit 8 by way of lines 16 each comprising a ground lead and a pair of signal leads. The battery 18 of the vehicle is also connected to the unit 8 by way of a line 20. Each air bag 10 has an electric actuator in the form of a squib 22 which is actuable to inflate the air bag 10, each squib 22 being connected to the diagnostic unit 8 by way of a line 24 and a bi-partite interface connector 26 which is described in detail below.Each line 24 comprises two signal leads for transmitting ignition current generated by the unit 8 in response to actuation of a sensor 4, to the respective squib 22, to activate it and a test lead. In practice, the connector 26 will be incorporated in the diagnostic unit 8. Where the parts of the connector 26 are in an unmated condition, for example, where one or other of the squibs 22 is to be removed, for example for testing purposes, a warning lamp 28 of the unit 8 is automatically illuminated to indicate that the air bags 10 are no longer in circuit.

It is a disadvantage of air bag deployment systems, that as explained in the foregoing, the squibs, which are necessarily very sensitive, can be triggered off when they have been disconnected from the diagnostic unit, for example, by leakage or other fortuitous, or accidentally applied currents. The connector 26 is, as explained in detail below, constructed for the avoidance of this disadvantage.

The bi-partite interface connector 26 comprises a pin header part 30 which, as shown in Figures 7 and 8 is surface mounted to a printed circuit board PCB of the diagnostic unit 8, and a receptacle part 32 for mating with the part 30 and having at two spaced pairs of positions therein, electrical terminals connected to the signals leads of the squibs 10, and at other positions therein, terminals connected to the test leads of the squibs 10 and terminals connected by leads (not shown in Figure 1) to fault read out means for the diagnostic unit 8. The connector 26 serves to connect twelve leads in all to the diagnostic unit 8, three leads for each squib 22 and six other leads.

The pin header part 30, which is of rectangular cross section, comprises a one piece, moulded, insulating block 34 in which are mounted twelve active electrical pins 36 and two pairs of actuator pins 38, all of the pins being of identical dimensions and each having a rectilinear mating section 40 having a chamfered free end 42 projecting into, and being surrounded by, a rectangular cross section peripheral hood 44 defining a socket and extending forwardly from the block 34 from a mating face 35 thereof and being formed integrally therewith. Each pin 36 and 38 further comprises a transition section 46 extending rearwardly through the block 34 and a tail 48 bent down at right angles to the section 46, to project below, and behind, the block 34.The tail 48 of each pin 36 extends through a hole in the circuit board PCB and is soldered to a respective printed conductor on the underside thereof, whereas the tail 48 of each pin 38, which also extends through a hole in the board PCB is not connected to any circuitry thereon, or, indeed, to any other circuitry, as shown in Figures 7 and 8.

As best shown in Figure 10A, the pins 36 and 38 are arranged in three parallel superposed rows, there being 6 pins 36 in the top row, 3 pins 36 in the middle row, 3 pins 36 in the bottom row, 2 pins 38 in the middle row and 2 pins 38 in the bottom row. There are two redundant pins 50, in the middle and bottom rows. The pins 38 are arranged in two pairs, one pair being disposed on each side of the pins 50.

As best seen in Figure 10, there project from the upper wall 45 of the hood 44, two laterally spaced pairs of longitudinally aligned locking ribs 52 and 54, the rib 52 of each pair being substantially longer than the rib 54 thereof and the ribs 52 and 54 of each pair being spaced from one another longitudinally of the pin sections 40 and 46, that is to say in their longitudinal direction. The ribs 52 and 54 of each pair define a gap 53 between them. On the lower face of the hood 44 and proximate to its free edge 56, is a polarizing key 58. Between the ribs 52 is a pair of laterally aligned latching lugs 60. The hood 44 is formed at its forward end with an internal peripheral, seal receiving recess 61.

The receptacle part 32 comprises a one piece, moulded insulating housing 62, twelve electrical receptacle terminals 64, only some of which are shown, crimped to insulated electrical leads 76, an annular peripheral seal 68, two short circuiting links 70, and an anti back-out insert 72. As shown in Figure 11, the terminals, all of which are referenced 64/, maybe provided with bung seals 74 and flanges 76 for crimping about the seals 74 to secure them to the terminals 64/, and a blanking plug 75-may also be provided.

As shown in Figures 2 and 6, the housing 62 has, opening into a mating face 77 thereof, an upper row of through cavities 1/ to 6/, a middle row through cavities 7/to 9/ and a low row of through cavities 10/ to 12/, each of the cavities 1 to 12/ being dimensioned to receive one of the terminals 64. To the right, as seen in Figures 2 and 6, of the cavity 9/ of the middle row, is a cavity 79 for receiving one of the pins 50 of the pin header part 30, and to the right of the cavity 79, is a blind socket 78, best seen in Figure 4, comprising an enlarged forward portion 80 and a smaller cross section rear portion 82, for receiving one of the short circuiting links 70.A similar blind socket 78/, having an enlarged forward part and and a smaller cross section rear part 8 is formed adjacent to a cavity 8 of the bottom row for receiving the other pin 50, but on the opposite side of the socket 79 to the socket 78. The socket 78 / is for receiving the other short circuiting link 70. Around the mating face 77 thereof, the housing 62 has a recess for receiving the seal 68, as well as the hood 44 of the pin header part 30.

Each cavity 1/ to 12/, comprises a rear portion 86 for receiving part of a respective lead 66, a bung seal 74, where this is provided, and a crimped insulation barrel 88 of a terminal 64. Each cavity 1/ to 12/ has a forward portion 90 for receiving the receptacle 92 of the terminal 64. A latch arm 94 for retaining a terminal 64 in the cavity, projects from an internal wall of the housing 62 at the forward end of the cavity portion 86 of each cavity. The bottom of each of the cavities 4/, 6/, 7/ and 9/ is formed with a through slot 96. Each latch arm 94 defines, in co-operation with an adjacent wall of the housing 62, a recess 97.

The recess 84 is defined between a central block 98 of the housing 62, in which block, the cavities and sockets just described are formed, and an outer wall 100 extending about the bottom wall of, and the two side walls of, the housing 62. The upper margins 101 of the wall 100 are each formed integrally with an outer side wall 103 of a respective channel member 102 having a roof 105 and an inner side wall 107. The side walls 103 and 107 and the roof 105 of each channel member 102 define a channel 104, the bottoms of the walls 103 and 107 being spaced from the block 84 in order to allow the upper wall 45 of the hood 44 of the pin header part 30 to pass between the walls 103 and 107 and the block 84.The rearward part of each roof 105 is formed with a longitudinal slot 108 which is blind at its forward end 109 and open at its rear end and which communicates with the respective channel 104. There extends from, and is formed integrally with, each upper marginal part of the outer wall 100, a resillient, cantilever locking arm 110 having at its free end an inwardly directed locking head 112 of arrow head - shaped cross section. Each arm 110 is depressible to engage its locking head 112 wedgingly in a reduced width portion 111 of the respective channel 104 by way of the slot 108, proximate to the blind end 109 of the slot 108, as best seen in Figure 8A.

The outer wall 103 of each channel member 102 is formed integrally with a side wall 114 of a gantry 116, the walls 114 being formed integrally with the rear of the block 84. The gantry 116 has a cross piece 118 from which projects forwardly, a resilient cantilever support bar 120 with the free end of which is integrally formed a yoke 122 extending between the channel members 102 and having limbs 124 cooperating with the bar 120 to define through guide slots 126 each of which is open at its rear end and is blind at its forward end, terminating in a latching shoulder 127. The slots 126 extend parallel to the channels 104. The yoke 122 is spaced above the block 84 to allow the passage of the upper wall 45 of the hood 44 of the pin header part 30, between the yoke 122 and the block 84. The bottom of the wall 100 is formed with a keyway 128, which is shown in Figures 2 and 6, for receiving the key 58 of the pin header part 30.

Each short circuiting link 70 which has been stamped and formed from a single piece of sheet metal stock, comprises, as best seen in Figures 9 and 9A, a hammer head 130 at its forward end and which is connected by means of flat, rearwardly tapered beam 132, to a retention portion 134. The hammer head comprises a cross piece 136 from which the beam 132 extends centrally, and which has at each end thereof an upstanding contact lug 138 terminating at its free end in a contact surface 140. There projects rearwardly from the cross piece 136, between the beam 132 and each lug 138, an upwardly inclined, rectangular abutment ear 142.

The retention portion 134 comprises a central part 144 which is coplanar with the beam 132 and from either side of which depends an anchoring flange 146 terminating in an outwardly projecting ear 148, the free lateral edge of which is formed with sharp ended, and forwardly directed, anchoring serations 150.

The anti-backout insert 72, which is moulded in one piece from an insulating material, comprises a rectangular base plate 152, from one side of which project normally thereof, as best seen in Figure 11, three rows of anti-backout spigots 154, each for reception in a respective recess 97 of the housing 62. Proximate to the root of each spigot 154 is a rectangular hole 156 in the plate 152, for the passage therethrough, of the mating section 40 of a respective pin 36 or 38 as the case may be, of the pin header part 30. The plate 152 also has a central rectangular hole (not shown) for receiving the plug 50.

The assembly of the receptacle part 32 will now be described with reference to Figures 4 to 7 and 11. Firstly, one of the short circuiting links 70 is inserted, in the direction of the arrow A in Figure 4 with its retention portion 134 leading, into the socket 78 of the housing 62, so that the portion 134 of the link 70 is received in the rear socket portion 82 of the socket 78, whereby the said one link 70 is anchored in the socket 78, by means of the serations 150 on the portion 134 which engage the side walls of the socket portion 82 and bite thereinto, the remainder of the link 70 being received in the enlarged portion 80 of the socket 78 (Figure 5), the other link 70 being similarly inserted into the socket 78/.When the links 70 have been so inserted, the contact surfaces 140 of the contact lugs 138 of said one link 70 are thus substantially aligned with the longitudinal axes of the respective cavities 4/ and 6/, the contact surfaces 140 of said other link 70 being substantially aligned with the longitudinal axes of the respective cavities 7/ and 9/, as shown in Figure 6.

Secondly, the terminals 64 are each inserted into a respective cavity 1/ to 12/, in the direction of the arrow B in Figure 5, by way of the rear face of the housing 62, so that, as shown in Figure 7, the barrel 88 of each terminal 64 is received in the cavity portion 86, the receptacle 92 of that terminal resiliently displacing the respective latch arm 94 until the latter snaps back to lodge against the rear of the receptacle 92 to latch the terminal in its cavity as shown in Figure 7. During the insertion of the terminals 64 into the cavities 4/, 6', 7/ and 9/, the receptacles 92 of these terminals engage the contact surfaces 140 of the respective links 70 thereby resiliently displacing the beams 132 thereof as shown in Figure 7 so that the surfaces 140 are engaged in firm electrical contact with the receptacles 92.

Lastly the anti-backout insert 72 is inserted into the housing 62 in the direction of the arrow C in Figure 5 so that, as will be apparent from Figure 7, each spigot 154 is received in a respective recess 97, thereby holding the respective latch arm 94 against the receptacle 92 of the respective terminal 64, so that it can, under no circumstances, back out from its cavity. Latch means 158 are provided for securing the insert 72 in its assembled postion in the housing 62.

The leads 66, of the terminals 64 in the cavities 4/and 6', are the said signal leads of one of the lines 24 connected to one of the squibs 22, the leads 66 of the terminals 64 in the cavities 7/ and 9/ being the said signal leads of the other line 24, when the leads in question hav been connected to the respective squibs 22.

The leads of the terminals 64 in two other of the cavities consitute the test leads of the squibs 22 and the leads 66 of the terminals 64 in the remaining cavities are connected to the said fault read-out means of the diagnostic system.

Since the leads 66 of the terminals 64 the cavities 4/and 6/ are short circuited by the upper link 70, the leads of the terminals 64 in the cavities 7'and 9/ being short circuited by the lower link 70, no accidental or fortuitous energisation of these leads can occur, so that neither of the squibs 22 can be fired.

When the part 32 of the bi-partite connector 26 is mated with the part 30 thereof, as shown in Figure 8, the mating sections 40 of the pins 36 enter the respective receptacles 92 by way of the holes 156 in the plate 152 of the insert 72, so that the leads 66 are connected to the diagnostic unit 8. The squibs 22 are not, however, connected in circuit until the parts 30 and 32 are closely proximate to their fully mated position which is shown in Figure 8, when the chamfered ends 42 of the mating sections 40 of each pair of inactive pins 38, each engage a respective abutment ear 142 of a respective link 70 so as further to resiliently depress the beam 132 thereof, whereby the contact surfaces 140 of the lugs 138 of that link 70 are disengaged from the receptacles 92. The squibs 22 are thereby armed and protected.Almost as soon as unmating of the parts 30 and 32 begins, the mating sections 40 of the pins 38 are withdrawn so that the beams 132 resile whereby, the contact surfaces 140 are again engaged with their respective terminals. Thus where the parts 30 and 32 are only partially mated, or unmated, the squibs 22 are still protected against accidental or fortuitous activation.

During the mating of the parts 30 and 32, the hood 44 enters the recess 84, the seal 68 being received in the recess 61 of the hood 44, the key 58 enters the keyway 128 and the ribs 52 and 54 enter the channels 104. In the fully mated condition of the parts 30 and 32, each gap 53 coincides with the reduced width portion 111 of the respective channel 104, and the lugs 60 having cammed the beam 120 away from the block 84, each lie in a respective slot 126, latching against its shoulder 127. In order to lock the parts 30 and 32 in their fully mated condition, each arm 110 is depressed, for example under finger pressure, or by means of a suitable tool, so that the locking head 112 of the arm 110 is firmly wedged in the respective channel portion 111. Since each rib 54 lies behind the respective locking head 112, as shown in Figures 8 and 8A, and since each arm 110 abuts the blind end 109 of the respective slot 108, the part 32 cannot be withdrawn from the part 30 unless the arms 110 are returned to their raised positions. Since the arms 110 can only be depressed when the parts 30 and 32 have been fully mated, the position of the arms 110 indicates, visually, whether or not the parts 30 and 32 have been fully mated, that is to say whether or not the squibs 22 are armed. If false latching of the parts 30 and 32 has occurred, the lugs 60 having jammed under the forward part of the yoke 22, the locking heads 122 cannot be inserted into the channel portions 111.

Claims (13)

1. A bi-partite electrical connector comprising a receptacle part and a pin carrying part for mating therewith, the receptacle part comprising an insulating housing defining first and second, spaced, parallel, through cavities each receiving a pin receptacle terminal therein, and intermediate said cavities, a short circuiting link having a normal first position in which contact surfaces thereof engage against the terminals to provide a short electrical path therebetween and a displaced second position in which said contact surfaces are out of engagement with said terminals, the pin carrying part comprising an insulating member from which project active pins for mating with said receptacle terminals as said parts are being mated and at least one abutment pin for engaging said short circuiting link to displace it from its first to its second position when said parts have been substantially fully mated, said short circuiting link being automatically returned to its first position upon withdrawal of said abutment pin from said short circuiting link.

2. A connector as claimed in Claim 1, wherein said link comprises a retention portion secured in a socket defined by said housing and a resilient cantilever beam projecting from the retention portion and carrying said contact surfaces and means for engagement by said abutment pin.

3. A connector as claimed in Claim 2, wherein said contact surfaces are formed on upstanding contact lugs on opposite ends of a cross piece of said beam, said abutment means comprising at least one abutment lug projecting from said cross piece.

4. A connector as claimed in Claim 3, wherein said cross piece is planar and is provided on the free end of said beam, each abutment lug projecting towards the retention portion and being inclined with respect to the plane of the beam, for engagement by a chamfered end portion of said abutment pin.

5. A connector as claimed in Claim 2, 3 or 4, wherein the retention portion comprises a pair of anchoring flanges each depending from a respective edge of said beam and terminating in an outwardly projecting ear having a free lateral edge formed with anchoring serrations.

6. A connector as claimed in any one of the preceding claims, wherein said first and second cavities are disposed in a row of terminal receiving cavities defined by said housing, a further cavity of said row being disposed between said first and second cavities, said pin header part having two spaced abutment pins each for engaging a respective engagement means of said link, said respective engagement means being spaced from each other in the direction in which the row of cavities extends.

7. A connector as claimed in any one of the preceding claims, wherein said active pins are connected to circuitry on a printed circuit board, the, or each, abutment pin extending through an opening in said board, but being isolated from any circuitry thereon.

8. A mating part for a bi-partite electrical connector for an automotive air bag deployment system, said mating part comprising an insulating housing defining a plurality of rows of parallel, spaced, through cavities each receiving an electrical terminal therein and including first and second cavities, each receiving a terminal for connection to an automotive air bag actuating element, the housing further defining a socket arranged intermediate to said'first and second cavities and having anchored therein, a short circuiting link having a resilient beam on which are provided contact surfaces normally engaging the terminals in said first and second cavities to produce a short electrical path therebetween, and abutment means for engagement by an abutment element inserted into said housing longitudinally of said cavities, resiliently to deflect said beam to displace said contact surfaces from the terminals in the first and second cavities.

9. A short circuiting link which has been stamped and formed from a single piece of sheet metal stock, said link comprising a resilient, planar beam, a cross pieces at one end of the beam and a retention portion at the other end thereof for retention in a socket in an insulating housing, the cross piece having at each end thereof an upstanding lug surmounted by a contact surface, and intermediate each lug and said beam, an ear projecting towards said retention portion and being inclined with respect to the plane of said beam.

10. A short circuiting link as claimed in claim 9, wherein the cross piece is coplanar with the beam, the lugs extending in the same direction, normally of the plane of the cross piece and the retention portion comprising a pair of anchoring flanges each depending, in the opposite direction to that in which the ears project, from a respective edge of said beam, each anchoring flange terminating in an ear projecting outwardly of said beam and having a free lateral edge formed with anchoring serrations thereon.

11. A bi-partite electrical connector, substantially as hereinbefore described with reference to the accompanying drawings.

12. A mating part as claimed in Claim 8, substantially as hereinbefore described with reference to Figures 2 to 9a and 11 of the accompanying drawings.

13. A short circuiting link substantially as hereinbefore described with reference to Figures 4 to 9a and 11 of the accompanying drawings.