[0001] The present invention relates to a high performance miniaturised magneto-thermic
automatic switch (or breaker).
[0002] It is known that magneto-thermic switches are automatic switching devices in which
two contacts are closed by a manual arming or setting device and must open in an automatic
manner, by the effect of excess current which traverses the switch, in order to ensure
protection of electrical systems against overloads and short circuits.
[0003] To this end two types of protective action are in general envisaged:
- protection against overloads,
- protection against short circuits.
[0004] Protection against overloads is ensured by a bimetallic element which, when heated
by the current which flows through it, deforms until it acts on a release mechanism
of the arming device and thus causes the contacts to open.
[0005] Since the bimetallic element forms a cantilever beam having a certain flexibility,
the end of which acts on the release device, the deformation induced in given overload
conditions depends on the resisting forces exerted by the release device, which can
vary over time and as a function of the working conditions.
[0006] In order to ensure repeatability of intervention it is therefore necessary that the
opposing resistant forces are minimised so that their variations will be negligible
both in absolute value and in relation to the mechanical forces which can be exerted
by the bimetallic element.
[0007] The action of the thermal overload protection does not have to be particularly rapid
but must be repeatable.
[0008] The problem of protection against short circuits is very different.
[0009] In this case the current flowing through the contacts can rapidly increase in the
space of only a millisecond to levels of the order of thousands of amperes with catastrophic
effects on the system and on the switch if allowed to continue for even only a few
milliseconds.
[0010] The first consequence, due to the contact resistance between the contacts, which
however small is not zero, is the local heating and possible welding together of the
contacts which prejudices the operation of the switch and its ability to act as protection
to avoid further consequences.
[0011] It is therefore essential that immediately a short circuit occurs, and, that is,
as soon as the current reaches levels greater than normal, the contacts are opened
with extreme rapidity.
[0012] For this purpose a plunger type electromagnet energised by the current flowing through
the switch projects a striker against the release mechanism causing release of the
arming device.
[0013] Simultaneously the striker acts, in general through an intermediate element, on one
of the contacts moving it away from the other.
[0014] The speed imparted to the striker by the magnetic field generated by the current,
however high, is not infinite, and the energy transferred by the striker to the release
device and the speed imparted to it depend on its inertia and on the resisting forces
which oppose the release operation.
[0015] It is therefore necessary that the release devices have a minimum inertia and the
lowest possible resisting forces.
[0016] At the same time the devices must be constructionally simple, must allow easy operations
of assembly and setting up and must be small and compact in order to be able to be
housed in miniaturised containers with dimensions which are in fact standardised,
in which a plurality of other components must be housed such as terminals, rapid acting
electromagnets, and arc quenching chamber.
[0017] In order to satisfy these requirements simultaneously a wide variety of release devices
have been proposed, among which a particularly effective one as far as the speed of
operation is concerned appears to be the toggle-break release devices with inclined
plane and retaining hook pawl, one example of which is described in French Patent
publication No. 2 674 679.
[0018] In these devices an arming link is pressed against an inclined plane formed in a
support lever for a movable contact arm and holds the support lever and the movable
contact arm in the armed position (with the contacts closed) against the action of
a spring biasing the support lever and the contact arm to a rest position in which
the contacts are open.
[0019] The thrust exerted by the arming link on the inclined plane is not perpendicular
to the plane so that the link tends to slide along the plane, but is held in place
by a retaining hook pawl which absorbs that component of the thrust exerted by the
link tangential to the inclined plane.
[0020] This component, which is conveniently less than the thrust of the link, is transferred
to a pawl pivot pin.
[0021] Upon turning the retaining hook pawl about its pin the arming link, no longer restrained,
can slide along the inclined plane and release the support lever from the previously-imposed
constraint, thus allowing it to assume the rest position urged by the spring.
[0022] The inclined plane formed by the support lever and the tooth of the retaining hook
pawl form an articulation capable of opening, then breaking, the toggle constituted
by the support lever and the arming link.
[0023] Although release devices of this type are effective there remains the requirement
of even better performance and of providing miniaturized automatic switches with even
higher switching power and even shorter response times.
[0024] The miniaturised automatic switch which forms the subject of the present invention
satisfies this requirement in that an arming device of conventional type is associated
with a release device of the type just described in which the retaining hook pawl
is replaced by a latch pawl controlled by an electromagnetic striker via the intermediary
of a trip lever or member.
[0025] It is noted here that a hook pawl is a stop element stressed in tension whilst a
latch pawl is a stop element stressed in compression.
[0026] With this simple substitution it is possible not only to form a simple and particularly
compact release device which leaves more space for other components within a casing
of standardised volume, in particular rapid response electromagnet and arc quenching
chamber, but also allows the achievement of shorter release device response times
than are obtainable with hook pawl devices.
[0027] A theoretical explanation is proposed for this fact which in many ways is unexpected
and surprising given that hook pawls and latch pawls appear to be functionally equivalent.
[0028] The advantages achieved in performance compensate largely for the relatively greater
constructional complexity of the device which requires an intermediate actuating element
for the latch pawl.
[0029] The characteristics and advantages of the invention will become clearer from the
following description of a preferred embodiment of the invention given with reference
to the attached drawings, in which:
Figure 1 is a general view of the whole of the structure of an automatic switch formed
in accordance with the present invention;
Figure 2 is an exploded perspective view of a preferred embodiment of the release
device of the switch of Figure 1;
Figure 3 is a front view of the release device of Figure 2 in a rest position;
Figure 4 is a front view of the release device of Figure 2 in an armed position;
Figure 5 is a front view of the release device of Figure 2 in an armed position with
the electromagnetic protection acting;
Figure 6 is a vector diagram showing the forces acting on some of the elements of
the trip device of Figure 2;
Figure 7 is a qualitative energy diagram relating to a latch pawl; and
Figure 8 is a qualitative energy diagram relating to a hook pawl.
[0030] With reference to Figure 1, a miniaturised automatic switch formed according to the
present invention comprises a modular casing 1 (constituted by two coupled half shells
one of which is removed to allow the interior to be seen) in which is housed a plurality
of components, and in particular:
- first and second terminals 2, 3 for external electrical connections;
- a bimetallic strip 4;
- a pull-in electromagnet 5;
- a stack of arc quenching plates 6;
- a manual arming lever 7;
- a fixed contact 8;
- a movable contact 9 at the end of a contact arm 10 of conductive material;
- an arming link 11;
- a rapid release device formed by a support lever 12 for the contact arm 10, a biasing
spring 13 for the contact arm 10 urging it to an open position, a latch pawl 14 and
a trip lever or member 15;
- a mechanical unidirectional coupling guide for coupling the bimetallic strip 4 and
the release device.
[0031] The two terminals 2, 3 are electrically connected together via the series connection
of the bimetallic strip 4, a flexible conductive braid 16 connecting the strip 4 to
the contact arm 10, the contacts 8 and 9, when closed, and the electromagnet winding
5.
[0032] The electromagnet winding is obviously connected at its ends to the contact 8, via
a rigid copper projection 18 of the electromagnet, which supports the contact 8, and
to the terminal 3 via a soldering projection 19 of the terminal 3.
[0033] Upon opening of the contacts 8 and 9 the electrical connection is interrupted with
the development, between the contacts, of an electric arc which commutes, when the
two contacts 8, 9 are conveniently spaced from one another, on to a commutation electrode
20 connected to the terminal 2 and which conveys it towards the stack of quenching
plates 6.
[0034] In order for the automatic switch to be effective it is necessary not only that the
contacts 8 and 9 are opened, but also that they are separated from one another as
rapidly as possible in such a way that the commutation of the arc onto the electrode
20 takes place as soon as possible so as to reduce the duration of the arc to the
minimum.
[0035] Figure 2 shows the structure of the rapid release device in exploded perspective
view for greater clarity.
[0036] The support lever 12 for the contact arm 10 is provided at one end with a cylindrical
bearing 21 to receive a pin 22 which acts as a support pivot for the lever 12 which
is thus articulated to turn about the pin 22.
[0037] The pin 22 is engaged at its ends in a fixed position in the casing 1.
[0038] At the opposite end of the lever 12 two cylindrical pins 23, 24 are formed on opposite
faces of the lever and have axes parallel to that of the pin 22, which pins act as
pivot pins for the arm 10 and for the latch pawl 14 respectively.
[0039] The arm 10 is provided at its end opposite the contacts 9 with a bearing 25 for receiving
the pin 23.
[0040] The bearing 25 is preferably a saddle-like cylindrical seating because this allows
the centre of rotation of the arm 11 to be as close as possible to its end and therefore
increases the lever arm within the same dimensions.
[0041] A slot 26 formed in an intermediate position in the arm 10 allows the free passage
of the pin 22 in the arm, prevents the escape of the pin 23 from the saddle-like bearing
25, and allows a predetermined and limited relative rotation between the arm 10 and
the lever 12 on the pivot formed by the pin 23.
[0042] A tooth 27 formed on the arm 10 in an intermediate position between the slot 26 and
the saddle-like bearing 25 provides a positional engagement for the compression spring
13 which, reacting against the casing, urges the contact arm 10 (and consequently
the lever 12) to a rest position, with the contacts open, defined by a convenient
abutment of the casing 1.
[0043] Integrally with the support lever 12 there is formed a tooth 30 provided with an
engagement plane 31 cooperating with a tooth 29 of the latch pawl to form a dihedral
which receives one end 32 of the arming link 11 pivoted at the other end on a drum
33 of a manual arming lever 7.
[0044] The tooth 30 extends beyond the plane 31 to form a throat for housing the end of
the link 32 when the actuation of the latch pawl allows the end 32 to slide on the
plane 31 and to penetrate into the housing throat by the effect of relative movement
between the end 32 and the support arm 12 caused by the spring 13 or, as will be seen
further below, by the electromagnetic actuator 5.
[0045] The latch pawl 14 is essentially constituted by a small lever provided at one end
with a lug 28 for receiving the pin 24 and at the opposite end with the previously-mentioned
retaining tooth 29.
[0046] Conveniently the latch pawl 14 extends as a U-shape arm 34 terminating in an actuation
tooth 35 for the said latch pawl, bent towards the lug 28 and at a radial distance
from the lug equal to or less than the radial distance of the retaining tooth 29 from
the lug 28.
[0047] The U-shape arm 34 forms, with the tooth 30 of the support arm, a closed containment
slot in which the end 32 of the link can slide.
[0048] The latch pawl 14 is biased towards a rest and abutment position by a spring 36 having
two ends respectively interfering with the latch pawl 14 and the body of the arm 12.
[0049] A conveniently shaped plastics cap 37 is snap engaged over the top of the support
lever 12 for the contact arm 10 and prevents the latch pawl 14 from separating from
the pin 24.
[0050] Advantageously the cap 37 can be made of coloured plastics or provided with coloured
adhesive labels in such a way as to provide, via an open window 38 (Figure 1) in the
casing 1, a visual indication of the position of the cap in the casing and therefore
of the position of the support lever of the movable contact arm.
[0051] The release device also includes a trip lever or member 39 operating as an actuation
intermediary between the electromagnet striker and the latch pawl 14.
[0052] In Figure 2 the trip member 39 is shown as being divided into two parts 39A and 39B
for greater clarity.
[0053] The trip member, which is conveniently made of insulating material, is formed by
two parallel juxtaposed cheeks joined by a plate 42 to form a saddle partially housing
the lever 12 and the arm 10.
[0054] The two cheeks are provided with axially aligned apertures 40, 41 through which the
pin 22 passes freely.
[0055] It is clear that the assembly comprising the trip member 39, the arm 10, the lever
12, the latch pawl 14 and associated spring 36, the cap 37 and the pin 22 can easily
be assembled as a unitary assembly to form a trigger device which can be installed
with a simple operation into the casing 1.
[0056] The trip member can turn relative to the support lever 12 and the arm 10 through
a predetermined angle defined by the interference of a projection 43 of the support
lever 12 with a tooth 44 of the cheek 39A and by the interference between a projection
45 of the plate 42 with the movable contact arm 10.
[0057] A second tooth 46 of the trip member interferes with a tooth 47 of the slide 48 when
the slide is displaced in the sense indicated by the arrow A of Figure 2 by the flexing
of the bimetallic strip 4 (Figure 1) which acts on a tooth 49 of the slide.
[0058] In order to actuate the latch pawl 14 the trip member is provided with a trip lever
50 terminating in a tooth 51. When the trip member is caused to turn clockwise (as
viewed in Figures 1 and 2) about the pin 22 by the effect of a force applied to the
plate 42 by the electromagnet striker 52, the tooth 51 contacts the actuation tooth
35 of the latch pawl and causes it to turn in an anticlockwise sense about the pin
24.
[0059] Consequently the end of the link 32, no longer restrained on the inclined plane 31,
lifts so that the entire release device becomes free to turn clockwise.
[0060] If the striker 52 acts, the rotation is caused by the spring 13 acting on the contact
arm 10 together with the action exerted by the striker 52 on the trip member and transferred
from this to the contact arm 10 via the tooth 45. Opening is particularly rapid because
no resisting couple is applied to the release device except for the moment of inertia
of the device itself.
[0061] If the thermal overload protection operates the displacement of the slide 48 causes
a rotation of the trip member and the lever 50 about the pin 22 which causes release
of the latch pawl 14. Rotation of the contact arm 10 and of the entire release device
is caused solely by the action of the spring 13.
[0062] In both cases the rotation of the lever 12 caused by the spring is transferred, by
the interference of the projection 43 with the tooth 44, to the trip member 39, two
projections 39C and 39D of which can be mechanically coupled in a known way to corresponding
projections of the trip members of juxtaposed switch modules to cause them to trip
as well.
[0063] For an easier understanding of the operation of the trip release device described,
Figures 3, 4 and 5 schematically show the device in different operating states.
[0064] In Figure 3 the trip release device is in the rest position. In this condition the
spring 13 presses the upper part of the contact arm 10 and the support lever 12 against
an abutment stop 53 formed by the casing 1.
[0065] The contact arm 10 is maintained stably in the open position.
[0066] The arming lever 7 is in the disarmed position and the end of the link is held without
force in the dihedral formed by the inclined plane 31 and the tooth of the latch pawl
14.
[0067] The latch pawl is maintained in the stop position by its biasing spring and the tooth
for actuation of the latch pawl, pressing against the tooth 51 of the trip member,
tends to cause it to turn in an anticlockwise sense about the pin 22 maintaining it
in the rest position defined by contact of the tooth 44 against the projection 43.
[0068] By turning the arming lever 7 in the direction indicated by the arrow 54, the end
of the link 11 is pushed against the inclined plane and, since the latch pawl stops
it from sliding, the support lever is turned, together with the contact arm into the
position indicated in Figure 4.
[0069] In this condition the arming lever 7 contacts an end-of-stroke abutment and the end
of the link 11, pivoted on the lever 7, passes beyond the radial line extending from
the fulcrum of the lever 7 to the end 32 of the link 11 and the spring 13 stably maintains
the release device in the armed position.
[0070] This also ensures a predetermined contact pressure between the contacts 8 and 9 closed
by the effect of the pivoting of the contact arm on the pin 23 and thanks to the freedom
of rotation ensured by the slot 26.
[0071] If L1 is the length of the lever arm between contacts 8 and 9 and the axis of rotation
of the contact arm, with L2 being the force-application arm exerted by the spring
with respect to the axis of rotation of the contact arm and FM is the thrust exerted
by the spring perpendicular to the contact arm, the force P exerted between the contacts
is given by
.
[0072] Figure 5 shows the trip device when activated due to an excess current.
[0073] The striker 52 is projected against the trip member 39 which turns about the pin
22 together with the trip lever 50.
[0074] The tooth of the trip lever acts on the latch pawl 14 causing its release.
[0075] From this moment the end of the link 11, no longer retained by the pawl, is free
to slide on the inclined plane and allows the release device to rotate by the combined
effect of the thrust exerted by the striker 52 and the spring 13.
[0076] The release time of the latch pawl is of extreme importance to ensure the speed of
response of the protection which must be as short as possible.
[0077] Since the mechanical force which can be developed by an electromagnet is limited,
it is necessary that the work required to effect release be as small as possible in
order to minimise the time necessary to perform the work.
[0078] Figure 6 is a schematic diagram showing the forces on the different components which
define the work to be done to release the latch pawl.
[0079] The numerals 31, 32 and 56 respectively indicate the contact plane of the support
lever, the end of the link 11, which in general has a circular section of non-zero
diameter, and the retention plane of the latch pawl 14.
[0080] If F is the reaction exerted by the link 11 to oppose and balance the thrust of the
spring, the force F can be split up into two components, FT perpendicular to the plane
31, and FR perpendicular to the plane 56.
[0081] The component FT is entirely absorbed by the support arm and the biasing spring.
[0082] It is evident that the closer the plane 31 is to being perpendicular to the force
F the smaller is the angle α formed between F and FT and the smaller is the component
FR.
[0083] In practice the angle α cannot be less than a certain limit in order to ensure that
the link 11 is not stuck by the effect of friction on the plane 31, and in general
is not less than 20°.
[0084] It is also evident that, for the same angle α, the condition for which the component
FR is a minimum is that in which the plane 56 is perpendicular to the plane 31.
[0085] The force FR applied to the latch pawl at the point of contact between the plane
56 and the end of the link 32 is entirely balanced by the reaction exerted by the
latch pawl support pin 24 if the axis of the pin is aligned with the component FR.
[0086] Since the forces which act on the latch pawl are completely balanced a minimum couple
M, negligible to a first approximation, is sufficient to bias the latch pawl in the
locked condition.
[0087] Thus, to a first approximation, neglecting the couple M one can say that the virtual
work to be done to release the latch pawl is given by
where D is the movement of the plane 56 relative to the end of the link 32 necessary
to bring the corner 56 of the tooth of the latch pawl to the point of contact between
the plane 56 and the end 32 and where µ is the coefficient of friction.
[0088] By using material having a low coefficient of friction (bronze, steel) it is therefore
possible to make trip devices which require a minimum work for release of the order
of a few dynes (g · cm).
[0089] This conceptual arrangement is only valid in both the case where the plane 56 belongs
to the tooth of a latch pawl and where it belongs to the tooth of a hook pawl if the
rotation of the pawl (regardless of the type) necessary to release the tooth is negligible.
In practice this is not true and a predetermined rotation does have to take place,
necessitated by the fact that the centre of rotation of the pawl is relatively close
to the plane 56.
[0090] It is therefore evident that in the case of a latch pawl, a contact plane such as
56 forms an abutment with the end 32 of the link in conditions of stable equilibrium
and that angular displacements of the latch pawl from the equilibrium position are
obtained only by performing work which separates the end 32 from the centre of rotation
in some measure.
[0091] The diagram of Figure 7 qualitively represents the work to be done to rotate the
latch pawl through an angle from the stable position.
[0092] On the other hand in the case of a hook pawl a contact plane such as 56 forms an
unstable equilibrium condition for the end of the link represented by the energy diagram
A of Figure 8.
[0093] It is only by shaping the contact surface 56 as a cylindrical arc with a radius equal
to the distance between the contact surface and the axis of rotation of the pawl that
a condition of neutral equilibrium is obtained in both cases.
[0094] This however is not sufficient to render the behaviour of the two elements equivalent.
[0095] In fact, in the case of a latch pawl, the force FR is a simple compression force
which does not modify the orientation of the contact surface but only the distance
of the surface from the axis of rotation (by the effect of the elasticity, although
minimum, of the material).
[0096] On the other hand in the case of a hook pawl, the tension stress is inevitably associated
with flexing moments which tend to encourage the instability condition to avoid which
it is necessary to increase the biasing moment M to a considerable and non-negligible
degree, or to incline the surface/contact plane in such a way as to introduce a component
of thrust, which opposes the release of the latch pawl and increases the work to be
done in order to effect release.
[0097] For both arrangements the energy diagram is qualitatively modified as shown by diagram
B of Figure 8.
[0098] A comparison of Figures 7 and 8 will emphasise a further and not negligible difference
between the behaviour of a latch pawl and a hook pawl.
[0099] In the case of the latch pawl the work differential L/d needed to displace it from
the rest condition is initially zero and increases in proportion to the magnitude
of displacement.
[0100] On the other hand, in the case of a hook pawl the work differential is high in the
region of the rest condition and decreases with increasing distance there from.
[0101] Since, in the case of a trip device the angle of rotation of the latch pawl/hook
pawl is correlated to the time necessary for its actuation, although not necessarily
in a linear manner, the use of a latch pawl allows a greater efficiency and speed
of response because its dynamic characteristics are better adapted to those of an
electromagnetic actuator which is able to develop a thrust and force increasing gradually
with time as the magnitude of the air gap diminishes.
[0102] Thus the use of a latch pawl in substitution for a hook pawl, in a toggle-break trip
device of an automatic switch shows itself to be advantageous whether the tooth of
the latch pawl has cylindrical or flat contact surfaces.
[0103] The preceding description relates only to a preferred embodiment of the invention
and many variants can be introduced. For example the coupling between the latch pawl
and the trip member can be formed by means of a link on the arm of the latch pawl
and the trip lever rather than by means of interference teeth and the latch pawl biasing
spring can in this case be replaced by a biasing spring which acts on the trip member
which, in turn, imposes a predetermined rest position on the latch pawl.
1. A miniaturised automatic switch having a casing (1) housing a pair of contacts (8,9),
respectively a movable (9) and a stationary (8) contact, the said movable contact
(9) being disposed at a first end of a contact arm (10) movable between a closed and
an open position, comprising:
- a manual arming lever (7) coupled to a transmission link (11) to form a bistable
mechanical commutator,
- a striker (52) actuated by an electromagnet (5,17),
- a support lever (12) for the contact arm (10) pivoted on a first pin (22) carried
by the said casing (1), the said contact arm (10) being articulated at its second
end on a second pin (23) at the free end of the said support lever (12) and capable
of at least a predetermined rotation relative to the said support lever (12), the
said second pin (23) being radially opposite the said contacts (8,9) in relation to
the said first pin (22),
- a spring (13) acting on the said contact arm (10) in a position intermediate between
the first (22) and second pin (23), to bias the said support lever (12) and the said
contact arm (10) to the said open position,
- a tooth (30) formed on the said support lever (12) in an intermediate position between
the said first (22) and second (23) pin, the said tooth (30) forming a contact and
sliding plane (31) for a free end (32) of the said link (11), inclined and not perpendicular
to the direction of the thrust exerted by the said link (11), the said switch being
characterised in that it includes:
- a latch pawl (14) articulated on a third pin (24) at the free end of the said support
lever (12), the said latch pawl (14) having a retaining tooth (29) with a retaining
surface (56) for the said free end (32) of the link (11) which is orthogonal, at the
point of contact with the said end (32) of the link, to the said sliding plane (31),
the said third pin (24) being disposed with its axis in a plane parallel to the sliding
plane (31) and containing the point of contact of the said link (11) with the said
retaining tooth (29),
- a trip member (42) pivoted on the said first pin (22) and actuated by the said striker,
having an actuation arm (50,51) which causes rotation of the said latch pawl (14)
and disengagement of the said end (32) of the link (11) from the said retaining tooth
(29) and,
- means (36) for applying to the said latch pawl (14) a biasing couple towards the
retained position.
2. A switch as in Claim 1, in which the said means (36) for applying a biasing couple
to the said latch pawl include a second spring (36) around the said third pin (24),
the said second spring (36) biasing the said trip member (42) via the said latch pawl
(14) to a predetermined angular rest position defined by the interference of interference
means (44) of the said trip member with the said support lever (12).
3. A switch as in Claim 2, in which the said trip member (42) includes an opening tooth
(45) which interferes with the said contact arm (10) in its closed position, when
the said trip member (42) is displaced from its rest position by the said striker
(52).
4. A switch as in Claim 3, including a bimetallic element (4) in series between the said
movable contact (10) and an electric connector terminal (2) and a slide (49) actuated
by the said bimetallic element (10), the said slide (49) being provided with a tooth
(47) for interfering with a corresponding tooth (46) of the said trip member (42).
5. A switch as in Claim 4, in which the said third (24) and second (23) pins extend on
opposite sides of the said support lever (12), further including a resilient cap (37)
snap engaged on the said second and third pin to prevent separation of the said latch
pawl (14) from the said third pin.
6. A switch as in Claim 5, in which the said cap (37) faces an aperture (38) of the said
casing (1) and provides an indication of the position of the said contact arm (10).
7. A switch as in any preceding claim, in which the said latch pawl (14) includes a U-shape
arm (34), the said third pin (24) being fitted in a cylindrical bearing seat (28)
provided at one end of the said latch pawl (14), the other end of the said arm (34)
being bent towards the said cylindrical bearing seat (28) to form an actuation tooth
(35) contactable by the said trip member actuation arm (50,51) at a point of the said
actuation tooth (35) spaced from the said third pin (24) by a distance equal to or
less than that of the said retaining plane (31) from the said third pin (24).
8. A switch as in any preceding claim, in which the said movable contact arm (10) includes
a saddle type bearing (25) for receiving the said second pin (23).