Electromagnetically operable switch

An electrical switch having first and second sets of contacts and an operating member which is associated with both sets of contacts such that both sets of contacts can be moved from one of their operative states to the other operative state by a single movement of the operating member. A first electromagnetic arrangement is provided whereby the first set of contacts can be returned from the closed state to the open state by the application to the first electromagnetic arrangement of an electrical signal, and there is a second electromagnetic arrangement whereby the second set of contacts can be returned from the closed state to the open state by the application to the second electromagnetic arrangement of the electrical signal. The switch is so arranged that the first set of electrical contacts is returned by the electrical signal to its open state in advance of the return of the second set of electrical contacts to its open state.

This invention relates to an electrical switch. 
A switch according to the invention includes a first set of electrical 
contacts having open and closed operative states, a second set of 
electrical contacts having open and closed operative states, an operating 
member associated with both said first and second sets of contacts such 
that said first and second contacts can be moved from one of their 
operative states to the other operative state by a single movement of the 
operating member, first electromagnetic means whereby the first set of 
contacts can be returned from said other operative state to said one 
operative state by the application to said first electromagnetic means of 
an electrical signal, and, second electromagnetic means whereby said 
second set of contacts can be returned from said other operative state to 
said one operative state by the application to said second electromagnetic 
means of said electrical signal, the arrangement being such that said 
first set of electrical contacts is returned by said electrical signal 
from said other operative state to said one operative state in advance of 
the return of said second set of contacts from said other operative state 
to said one operative state. 
Preferably said switch has associated therewith a signal delay means 
whereby said electrical signal is applied to said second electromagnetic 
means at a point in time after its application to said first 
electromagnetic means to achieve the return of the second set of contacts 
after the return of the first set of contacts. 
Alternatively, said electrical signal is applied simultaneously to said 
first and second electromagnetic means and said first and second sets of 
contacts are mechanically dissimilar such that the second set of contacts 
is returned to said one operative state after the return of said first set 
of contacts to said one operative state. 
Preferably, said switch includes electrical signal delay means whereby said 
electrical signal is applied to said second electromagnet means after its 
application to said first electromagnetic means and in addition said first 
and second sets of electrical contacts are mechanically dis-similar so as 
to aid said delay means in ensuring return movement of said first set of 
contacts to said one operative state in advance of return movement of the 
second set of contacts to said one operative state. 
Desirably, the first and second sets of contacts are mechanically 
dis-similar in that the moving components of the first set of contacts 
have a lower inertia than the moving components of the second set of 
contacts and thus are moved more rapidly than the movable components of 
the second set of contacts. 
Preferably, said first set of contacts includes a first fixed contact, a 
second contact movable by said first electromagnetic means, and an 
electrically conductive bridging member movable with said operating 
member, said first fixed contact being engaged with the movable bridging 
member and said second contact being engageable with said bridging member 
in response to movement of said operating member while said first 
electomagnetic means is inoperative, said second contact being movable out 
of engagement with said bridging member by operation of said first 
electromagnetic means. 
Preferably, the second set of contacts includes a fixed contact, and a 
movable contact, the movable contact being movable into and out of 
engagement with said fixed contact by movement of said operating member, 
and being also movable either into, or alternatively out of, engagement 
with said fixed contact by operation of said second electromagnetic means. 
Desirably, said operating member is rotatable, and said bridging member of 
said first set of contacts is an arcuate conductive strip and said first 
fixed contact and said second contact are wiping contacts engageable 
respectively with the strip. 
Conveniently, said operating member is rotatable and said movable contact 
of said second set of contacts is movable relative to the fixed contact of 
the second set of contacts by rotation of the operating member through the 
intermediary of a cam carried by the operating member, said movable 
contact being resiliently biassed in one direction relative to said fixed 
contact and being movable against said resilient bias by rotation of the 
cam, said cam being movable axially by said second electomagnetic means to 
permit movement of the movable contacts under the action of said resilient 
means. 
Conveniently, said cam carries first and second cam forms each of which is 
capable, upon rotation of the cam, of moving said movable contact into 
engagement with said fixed contact, said second cam form commencing on 
said cam adjacent the termination of said first cam form and the 
co-operation of the cam and the movable contact being such that after 
movement of the movable contact against said resilient bias by the first 
cam form, and subsequent return movement of said movable contact under its 
resilient bias as a result of axial movement of the cam, the movable 
contact co-operates with the second cam form of the cam so that the 
movable contact can again be moved against its resilient bias by further 
rotation, in the same direction, of the operating member and the cam. 
Preferably, said movable contact of said second set of contacts is 
resiliently supported on said contact carrier and is movable by said cam 
through the intermediary of said contact carrier, the switch further 
including a slide member carried by a housing of the switch and movable 
relative thereto, and a lever pivoted on the housing, the lever coupling 
the slide member and the contact carrier whereby the slide member is moved 
relative to the housing by movement of the contact carrier relative to the 
housing, the position of the slide member relative to the housing 
providing an indication of the open or closed condition of the fixed and 
movable contacts of the second set of contacts and there being a degree of 
lost motion in the coupling between the slide and the contact carrier such 
that the slide is only moved to indicate the contacts closed operative 
state during movement of the contact carrier relative to the movable 
contact to apply contact pressure after the movable contact has engaged 
the fixed contact. 
Conveniently the slide member and the housing are so arranged that in one 
position of the slide member a region of the slide member is obscured by 
the housing and in the second position of the slide member a region of the 
housing is obscured by the slide member, said regions of the slide member 
and the housing carrying respective indicia one of which is indicative of 
the contacts open operative state and the other of which is indicative of 
the contacts closed operative state, the slide member being in a position 
wherein the contacts open indicium is visible and the contacts closed 
indicium is obscured when the movable contact is spaced from the fixed 
contact, and being in a position where the contacts open indicium is 
obscured, and the contacts closed indicium is visible when the movable 
contact is engaged with the fixed contact.

Referring to the drawings, the electrical switch is intended as a battery 
master switch for use in a road vehicle. A battery master switch is 
intended to provide control primarily over the electrical connections 
which are made to the battery, and is capable of isolating the vehicle 
battery from the remainder of the vehicle electrical circuit. In addition 
to containing heavy duty contacts whereby connections are made directly to 
the battery the switch also includes further sets of contacts controlling 
auxiliary circuits. 
The switch includes a moulded synthetic resin housing 11 closed at one end 
by a metal panel 12. 
The housing 11 is sealed and at its end remote from the panel 12 carries a 
boss 13 having an operating handle 14 journalled for rotation therein. The 
handle 14 includes a hollow spigot 15 within which is slidably received a 
spindle 16. A transversely extending pin 17 carried by the spindle 16 
engages elongate slots in the spigot 15 to rotatably couple the spigot 15 
and the spindle 16. 
Mounted on the spindle 16 for rotation therewith is an electrically 
insulating drum 18 carrying a conductive band 19. The conductive band 19 
extends circumferentially of the drum and includes an axially extending 
extension 21. The boss 13 carries internally a contact assembly comprising 
a pair of wiping contacts 22 is disposed axially of the drum so as to be 
able to engage the extension 21 in one predetermined angular position of 
the drum relative to the boss 13. The other of the contacts 23 is disposed 
axially of the drum so as to engage the continuous band 19 and thus will 
make electrical contact therewith irrespective of the angular position of 
the drum. However the contact 23 is carried by the movable armature of an 
electromagnetic relay 24 which, when energised lifts the contact 23 out of 
engagement with the band 19. Thus in an angular position of the drum 18 
where the contact 22 engages the extension 21 then the contacts 22, 23 are 
electrically interconnected by the band 19 provided that the 
electromagnetic relay is not energised. 
The spindle 16 extends through a partition wall 25 separating the interior 
of the boss 13 from the interior of the housing 11. The spindle 16 is 
ferromagnetic, and extends co-axially within a solenoid winding 26 
disposed within the housing 11. A ferromagnetic plunger 27 also extends 
co-axially into the solenoid winding 26 and is coupled to the spindle 16 
for axial movement therewith relative to the solenoid winding 26. 
Energisation of the solenoid winding 26 thus results in axial movement of 
the plunger 27 and the spindle 16 relative to the solenoid winding 26 and 
the housing 11. A spring 16a acts between the spigot 15 and the drum 18 to 
urge the spindle 16 and plunger 27 towards a rest position as shown in 
FIG. 1. Thus rotation of the handle 14 rotates the spindle 16 and the 
plunger 27 relative to the housing. Secured to the plunger 27 is a moulded 
synthetic resin cam 28, the cam 28 having a pair of circumferentially 
extending cam forms 29, 31. With the exception of a single internal key 32 
the cam 28 is symmetrical about a diameter thereof. Each of the cam forms 
29, 31 extends through approximately 160.degree. of the circumference of 
the cam and each of the cam forms spirals outwardly from a low point 
indicated by the suffix a to a high point indicated by the suffix b (FIG. 
5). The two cam forms 29, 31 are co-planar, and are disposed towards one 
axial end of the cam 28. Each of the cam forms is defined by the base of a 
channel and thus is bounded on either side by a side wall The high end 31b 
of the cam form 31 terminates circumferentially adjacent the low end of 
the cam form 31. At the high end of the cam form 31 the innermost side 
wall is cut away and the side walls are inclined, the cam form terminating 
in a radial step 31c. A plateau 31d is defined below the step 31c and is 
bounded in a circumferential direction by a radial axially extending wall 
31e. A further radial step 31f descends from the plateau 31d to a plateau 
32 which is at the height of the low end 29a of the cam form 29. An 
axially extending channel 33 co-planar with the plateau 32 and the low end 
29a of the cam form 29 interconnects the plateau 32 and the commencement 
of the cam form 29 interconnects the plateau 32 and the commencement of 
the cam form 29. A similar sequence of steps walls and plateaux is defined 
between the high end of the cam form 29 and the commencement of the cam 
form 31. The cam 28 is mounted on the plunger 27 with its axial end region 
carrying the cam forms closest to the solenoid winding 26. 
Within the housing 11, diametrically opposite one another and on opposite 
sides of the plunger 27 are first and second battery contact assemblies 34 
the majority of the second contact assemblies being omitted from FIG. 1 
for clarity. 
Each contact assembly 34 includes a pair of copper contact posts 35 which 
are secured to the housing 11 by having the housing 11 moulded therearound 
during manufacture of the housing. Each of the contact posts 35 includes 
an integral screw threaded terminal pillar 36 accessible at the exterior 
of the housing 11. Additionally, each contact arrangement 34 includes a 
moving contact assembly 37 comprising a moulded synthetic resin contact 
carrier 38, and a copper bridging plate 39. 
Each contact carrier 38 is mounted for sliding movement in the housing in a 
direction radially of the cam 28 and each contact carrier 38 carries a cam 
follower 41 which engages a respective cam form of the cam 28. The 
bridging contact 39 of each assembly 37 is resiliently mounted on its 
carrier 38 and is urged by a compression spring 42 to a limit position at 
a maximum spacing from the carrier 38. In addition, a further compression 
spring 43 acts between each moving contact assembly and the housing to 
urge the assembly radially towards the cam 28. The moving contact 
assemblies are disposed between the fixed contacts 35 and the cam and in 
operation rotation of the cam causes the cam followers 41 to ride up their 
respective cam forms so that the moving contact assemblies are moved 
radially outwardly to engage their bridging contacts 39 with the fixed 
contacts 35 to complete electrical circuits between their respective fixed 
contacts 35. The throw of each of the cam forms is greater than the 
maximum spacing between the bridging contacts and their respective fixed 
contacts, so that not only are the bridging contacts engaged with their 
fixed contacts during rotation of the cam but additionally the contact 
carriers are moved further compressing the springs 42 to apply contact 
pressure to the contacting faces of the bridging contacts 39 and their 
fixed contacts 35. 
Although only one of the contact arrangements 34 is shown in FIG. 1 it is 
to be understood that both contact arrangements operate simultaneously and 
identically, the only point of difference being that while one of the 
content arrangements is being operated by the cam form 29, the other will 
be operated simultaneously by the cam form 31. 
In operation, assuming that the cam followers 41 are engaged at the low 
points of their respective cam forms then the bridging contacts 39 will be 
spaced from their fixed contacts 35 and the main battery connections will 
thus be in an open-circuit condition with the battery isolated. Rotation 
of the handle 14 through 160.degree. rotates the cam 28 through 
160.degree. that is to say rotates the cam 28 through the angular extent 
of the cam forms 29, 31. During this rotation movement the contact 
assemblies 37 are moved sufficiently far firstly to engage their bridging 
contacts 39 with the respective fixed contacts 35 to complete the battery 
circuits through the fixed contacts 35, and thereafter after the move the 
contact carriers relative to the bridging contacts to apply the contact 
pressure by way of the spring 42. Thus after 160.degree. rotation the 
followers 41 are at the high ends of their respective cam forms. It will 
be noted that at the high end of each of the cam forms the side walls are 
inclined towards the end of the cam 28 remote from the solenoid. The last 
few degrees of rotation of the cam form thus causes the cam 28, the 
plunger 27 and the spindle 16 to be moved axially against the action of 
the spring 16a and at the 160.degree. point the cam followers 41 ride over 
the step 31c and the equivalent step on the cam form 29. The drop of the 
step is insufficient to release the movable contacts 39 from the fixed 
contacts 35 and the height of the plateau 31d and the equivalent plateau 
29d is sufficient to maintain the correct contact pressure. It will be 
noted also that the step 31c and the equivalent step 29c are inclined, and 
in the contacts closed position reverse rotation of the handle 14 thus, by 
virtue of co-operation between the inclined steps 31c, 29c and the cam 
followers 41 causes further axial movement of the cam 28 plunger 27 and 
spindle 16 against the action of the spring 16a. The plateaux 31d, 29d 
extend circumferentially around the cam at a constant height to terminate 
ultimately in steps 31g, 29g which drop to the plateaux 32. Thus reverse 
rotation of the handle 14 from the contacts closed position causes the cam 
followers to ride on the plateaux 29d, 31d maintaining the contacts closed 
throughout virtually the whole of the return movement, until the cam 
followers drop over the steps 29g, 31g and onto the plateaux 32. 
Thereafter of course the cam followers are aligned with the passages 33 so 
that the cam 28, the plunger 27 and the spindle 16 can be returned under 
the action of the spring 16a in an axial direction to re-engage the cam 
followers 41 with the commencement of their respective cam forms 29, 31. 
It will be appreciated that reverse rotation of the handle 14 may prove too 
slow to effect open circuit of the battery in an emergency situation. 
Furthermore, the switch may be remote from the driver of the vehicle so 
that the handle 14 is not conveniently accessible. For this reason a push 
button switch is provided adjacent the driver location and connected to 
the solenoid winding 26. Closure of the push button switch energises the 
solenoid winding 26 in the closed condition of the main battery contacts 
and thus causes instantaneous movement of the spindle 16, the plunger 27 
and the cam 28 against the action of the spring 16a. It will be understood 
at this point in time the cam followers are engaged on the plateaux 29d, 
31d adjacent the high ends of their respective cam forms and so the axial 
movement of the cam 28 will cause the cam followers 41 to drop over the 
shoulders 31f, 29f to immediately engage the plateaux 32 permitting the 
movable contact assemblies to return to their rest positions under the 
action of the springs 43 opening the main battery contacts. Return of the 
contacts de-energises the solenoid but the cam cannot return in the axial 
direction under the action of spring 16a until the cam has been rotated 
through a small angular distance to align the cam followers with the 
passage 33 so that the cam followers can engage once again with the 
commencement of the cam form 29. It follows therefore that after emergency 
operation of the switch to open circuit the battery by means of the push 
button, the contacts can then be re-closed merely by further rotation of 
the handle in the first mentioned direction, that is to say without 
reverse rotation. 
Within the housing adjacent the panel 12 is a printed circuit board 40 and 
supported by the printed circuit board inwardly of the housing is a 
moulded synthetic resin shoe 46. The shoe 46 is pivotally engaged by a 
pair of levers 44 which extend towards the boss 13 and have their ends 
opposite the shoe 46 received in respective slide blocks 45, slidable 
relative to the housing. Intermediate their ends each of the levers 44 is 
coupled to the contact carrier 38 of a respective contact arrangement 34. 
The slide blocks 45 when abutting one another include regions visible 
through a window 11a of the housing, said regions of the slide blocks 
carrying the legend OFF. In this position of the slide blocks the slide 
blocks obscure a legend plate 47 of the housing which carries the legend 
ON. The slide blocks 45 can be moved away from one another by means of the 
levers 44 so that the legend on the plate 47 is visible through the window 
of the housing and the two parts of the legend carried by the slide blocks 
are obscured by the housing. 
The coupling between each of the levers 44 and its respective contact 
carrier 38 is a lost motion coupling comprising a link 48 pivotally 
connected to the lever intermediate its ends and being connected to the 
contact carrier 38 by means of a headed pin 49. The headed pin 49 extends 
through the contact carrier 38 and is urged into abutment with one face of 
the contact carrier 38 by means of a light spring 50 acting between the 
head of the pin 49 and the contact carrier. The spacing between the head 
of the pin and the mutually presented face of the carrier 38 is equal to 
the spacing between the bridging contact 39 and the fixed contacts 35 of 
the contact arrangement 34 in the rest position of the moving contact 
assembly. Thus during operation of the moving contact assemblies the 
bridging members 39 engage their respective contacts 35 at the same time 
that the lost motion between the carriers 38 and the links 48 is absorbed. 
Thereafter, the further movement of the contact carrier, relative to its 
bridging contact to apply the contact pressure is transmitted to the link 
48 and through the link 48 to the respective lever 44. The positioning 
along the length of each lever 44 of the link 48 is so chosen that the 
degree of movement of the contact carrier 38 to apply contact pressure 
moves the appropriate slide block 45 sufficiently far for its legend to be 
obscured and the legend on the plate 47 to be revealed. Similarly during 
return movement of the contact carrier, the contact carrier initially 
moves relative to the link 48 to re-establish the full extent of the lost 
motion connection, and thereafter the link 48 is moved with the contact 
carrier thus moving the lever and the appropriate slide block 45 back to 
its original position. It will be appreciated that since the contact 
arrangements 34 move along in opposite directions to achieve their 
operative positions, then the slide blocks 45 are moved away from one 
another during this movement. The light spring 50 in the lost motion 
coupling moves its respective slide block rapidly and restores said lost 
motion once the block has started to move towards its ON position. 
However, it is to be understood that the force exerted by the spring 50 is 
not sufficient to overcome the inherent resistance to movement of the 
slide blocks and so the slide blocks are not moved to reveal the ON legend 
until the lost motion has been absorbed that is to say until the contacts 
35 have actually been engaged by their respective bridging members. 
As mentioned above, the contacts 35 and their associated bridging members 
39 control the direct connections to the battery. The previously mentioned 
contacts 19, 22, 23 serve in use to control the flow of current to the 
field winding of the alternator of the vehicle. The extension 21 of the 
contact band 19 is so arranged that the alternator field circuit is 
completed shortly after the bridging contacts 39 engage their fixed 
contacts 35 so that is to say after 160.degree. rotation of the handle 14. 
However it is important that the alternator field contacts are opened 
prior to the opening of the main contacts irrespective of whether or not 
the main contacts are opened manually or electrically, by means of the 
solenoid 26. It will be recalled that during reverse rotation of the 
handle 14 the co-operation of the cam followers 41 with the inclined 
radial steps 31c and 29c caused axial movement of the plunger 27 and the 
spindle 16. The drum 18 is carried by the spindle 16 and thus the drum 18 
and contact band 19 are moved axially relative to the contact 22, 23. The 
degree of axial movement is sufficient to break the electrical connection 
between the contacts 22 and 23 by way of the band 19 and thus while the 
main contacts are held closed during the return rotation of the handle 14 
by the cam followers 41 riding on the plateaux 29d, 31d by the cam field 
contacts are opened immediately the reverse rotation commences. The drum 
18 of course moves axially back to its original position with the spindle 
16 and the plunger 27 when the cam followers 41 engage the plateaux 32 but 
of course reverse rotation of the drum has also taken place and the 
extension 21 is thus angularly displaced by 160.degree. from the contacts 
22. Thus the return axial movement does not re-establish the connection 
between the contacts 22, 23. 
In the event that the main contacts are opened electrically by energisation 
of the solenoid 26 then the connection between the contacts 22, 23 is 
still broken first by virtue of the contacts 23 being mounted on the 
armature of the relay 24. The relay 24 is also energised by operation of 
the emergency push button mentioned above. The mass of the moving parts of 
the relay is considerably less than the moving parts of the solenoid 
arrangement and thus the relay will operate to open the contacts 22, 23 in 
advance of the main contacts opening. In order to increase the delay 
between opening of the alternator field contacts and opening of the main 
contacts, the solenoid winding 26 is arranged to be energised, upon 
closure of the push button, through an electrical delay unit. 
FIG. 2 illustrates the electrical circuit of the switch and its association 
with other parts of the vehicle electrical circuit. The two pairs of fixed 
contacts 35 with their associated bridging members 39 constitute a double 
pole switch, one pair of contacts 35 controlling the positive line and the 
other pair of contacts controlling the negative line. Thus a positive 
input contact 35 is connected to the positive pole of the battery 51 while 
the negative input contact 35 is connected to the negative pole of the 
battery 51. The negative output contact 35 is connected to the vehicle 
earth and the positive output contact 35 is connected to the vehicle 
auxiliary circuits to supply, for example, the windscreen wipers, the 
lights, the direction indicator system and the like of the vehicle. A 
further pair of contacts 52 not seen in the other drawings are bridged by 
a bridging member 53 at the same point that the bridging members 39 bridge 
the respective contacts 35. The bridging member 53 is in fact carried by 
one of the contact carriers 38 in exactly the same manner as its bridging 
member 39. The solenoid winding 26 consists of a pull in coil 26a and a 
hold on coil 26b. One end of the pull in coil is connected to one of the 
contacts 52 while the other end is connected through a thermal cut-out 
switch 54 to the negative input contact 35. A diode 55 has its anode 
connected to said one end of the coil 26b and its cathode connected to the 
anode of a second diode 56. The cathode of the second diode 56 is 
connected to the positive input contact 35 and thus when the battery 51 is 
correctly connected to the input contacts no current flows in the coils 
26a, 26b provided that the contacts 52 are open. In the event that the 
battery 51 is wrongly connected however, that is to say with its negative 
pole connected to the positive input terminal and its positive pole 
connected to the negative input terminal then the diodes 55, 56 conduct 
energising the solenoid 26 to move the cam 28 so that the contact assembly 
34 cannot be operated by rotation of the handle. Thus the switch cannot be 
closed with the battery wrongly connected. 
The positive output terminal is connected through the aforementioned push 
button switch 57 and the winding of the relay 24 in series to the negative 
output contact 35. Thus assuming that the contacts 35 are bridged by the 
members 39 then closure of the switch 57 energises the relay winding to 
move the contact 23 away from the drum. Since in the operative condition 
where the contacts 35 are bridged the contacts 23 will be connected to the 
contacts 22 through the band 19 then closure of the switch 57 disconnects 
the contact 23 from the contacts 22. The contact 22 is connected through 
the voltage regulator 58 of the vehicle and the field winding 59 of the 
alternator of the vehicle to earth. It will be recalled that operation of 
the push button 57 must not only open the alternator field contacts, but 
must also energise the solenoid 26. Thus a connection is taken from the 
negative side of the switch 57 through a delay unit 61 to the other of 
said contacts 52. Since the contacts 52 are bridged by the member 53 in 
the closed position of the main battery contacts then in fact the 
connection is through the delay unit and the two coils of the solenoid to 
the negative input contact. 
The delay unit 61 includes a pair of normally closed relay contacts in 
series with the contacts 52 and an operating winding having one end 
connected to earth. The other end of the operating winding is connected to 
the cathode of a diode the anode of which is connected to the contact 22 
of the alternator field winding contacts. A capacitor bridges the 
operating winding, and thus while the contact 23 engages the contact 22 
the operating winding of the delay unit 61 will be energised and the relay 
contacts thereof will be held open. However, when the push button 57 is 
depressed the contacts 23, 22 are immediately opened and the supply 
through the diode to the operating winding of the delay unit is broken. 
However the normally closed contacts are not immediately permitted to 
return to their closed condition since the capacitor discharges through 
the operating winding holding the relay contacts open for a predetermined 
length of time, conveniently a quarter of a second. Thereafter, the 
normally closed contacts close and the solenoid coils are energised from 
the positive output terminal through the switch 57, the normally closed 
contacts of the delay unit and the contacts 52. 
When the coils 26a, 26b are maintained energised for any significant length 
of time, the thermal cut-out 54 in series with the pull in coil 26a will 
operate to open circuit the pull in coil so that only the hold on coil 26b 
remains energised. The force generated by the hold on coil is of course 
sufficient to hold the plunger 27 and spindle 16 in their axially moved 
position. The thermal cut-out 54 is re-set automatically when its 
temperature drops below a predetermined value. The ignition switch of the 
vehicle is connected between the alternator field contact 23 and the 
positive output terminal 35 and thus the alternator field cannot be 
energised until the ignition switch is closed.