Note: Descriptions are shown in the official language in which they were submitted.
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HI 149-124
Thermally-Sensitive Controls
This inven~ion relates to thermally-sellsitive
controls for electric heaters of containers s-lch
as electric kettles, ~ugs, urns, pans, coffee perco-
lators, laboratory equipment and the like (hereinafter
referred to as "containers of the kind described'l).
The electric heaters of such con~ainers are typica~ly
either immersion heaters, or similarly constructed
heaters mounted externally to the container in
good thermal contact wi~h a wall thereof, for example
by being brazed to the underside of the container
base. Traditionally the heating elements of such
heaters terminate in so-called "cold leads" by
means of which electrical connections are made
to the heater.
Thermally-sensitive controls for heaters
of this t~rpe are well known which include electrical
switch means operable to interrupt the power supply
and thus deenergise the heater upon its element
overheating, after for example having been switched
on with insufficient liquid in the container to
cover the element or having boiled dry. Such arrange-
ments conventionally include a thermally responsive
actuating means, typically a snap-acting bimetallic
actuator, coupled mechanically to a set of breakable
switch contacts wired in series with the element
and operable to open the contacts upon the element
overheating. Examples of such controls are descrîbed
in British patent specifications Nos. 1,470,366
and 2,042,26g.
In such arrangements failure of the thermally
responsive actuating means can represent a serious
problem since if, for example, the container boils
dry and the element overheats, the power supply
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will not be automatically interrupted, and in the
absence of manual intervention the element will
become hotter and hotter until it eventually burns
out. This not only leads to the expense of a replace-
ment element but may also constitute a serioussafety hazard in the case o~ plastic containers
or containers having plastic parts, since the over-
heating can cause meltir.g and in the extreme burning
of all or part of the container. The problem is
of particular significance where the control is
used with an appliance which is likely to be unattended,
for example an automatic electric kettle or an
early morning tea maker.
In an attemp~ to overcome this problem it
has been proposed to provide back-up protection
including means operable to interupt the power
supply upon failure of the primary actuating means
when the element overheats. One such arrangement
is described in British patent specification No.
2,149,217, wherein back-up protection is provided
by a further thermally responsive actuating means
operable independently of the primary actuating
means to open a set of switch contacts and thus
deenergise the heater in the event of serious over-
heating consequent upon failure of the primaryactuating means. In the arrangement described
in British patent specification No. 2,149,217 the
primary actuating means comprises a snap-acting
bimetallic actuator whilst the back-up protection
is provided by a slow acting creep bimetal, although
it has also been proposed to include two snap-acting
bimetallic actuators coupled to respective sets
of switch contacts and calibrated to provide primary
and back-up protection.
whilst in many applications the use of a
thermally responsive actuating means such as a
bimetallic actuator coupled to a suitable switch
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means provides adequate back-up protection, in
certain circumstances, particularly in the case
of high wat~ge density heaters, experience has
shown that reliable operation may not be achieved.
For example, problems can be caused by the effects
of the ~over-shoot" heating which results from
the residual heat in the element after the heater
is deenergised. Such heating can cause melting
and deformation of plastic parts of the control
and this in turn can render the known type of back-
up protection means ineffective, and in certain
circumstances a secondary bimetallic actuator may
operate initially to open the swi~ch contacts but
shortly thereafter distortion of the control consequent
upon the over-shoot heating is such that the contacts
reclose. Under such circumstances current is thus
allowed to flow once more to the element and this
clearly represents a serious hazard.
An alternative approach to providing back-
up protection is ~he provision of an electricalfuse wired in series with the element. Whilst
a suitably callibrated fuse can provide adequate
protection and avoid problems discussed above concerning
possible re-closing of switch contacts, the provision
of a fusable link can undesirably increase manufacturing
costs.
Viewed ~rom one aspect the invention provides
a thermally-sensitive control for an electric heater
of a container of the kind described, said control
including a thermally-sensitive actuating means
arrange~ in use in good thermal contact with a
part of the element and coupled to a set of switch
contacts of electrical switch means of the control
so that the actuating means is operable to open
the contacts and deenergise the heater upon over
heatinq of the element, wherein back-up protection
for the control is provided by a pre-loaded spring
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mechanism associated with the same or a further
set of swi~ch contacts of the control and comprising
an actuating sprin~ which in normal use of the
control is held in a pretensioned condition by
a retaining means and an opera~ing member coupled
~o the spring, said re~aining means being adapted
and arranged ~o release the spring in the event
of serious over-heating of the control consequent
upon failure of said thermally-sensitive.actuating
means, whereupon the operating member is moved
under the force of the spring and co-operates
with the switch contacts so as ~o open the contacts
and deener~ise the heater.
The provision in accordance with the invention
of a pre-loaded spri.ng mechanism adap~ed and arranged
to be released in the event of serious over-heating
consequent upon failure of the actuating means
providing primary protection is advantageous to
the extent that reliable operation may be achieved
despite deformation of plastic parts of the control
due ~o over-shoot heating. Thus, a spring loaded
mechanism can provide a stronger, more positive
action for opening the contacts than is possible
with bimetallic actuators used in the prior art
proposals, and the possibility in extreme conditions
of the contacts reclosing may be avoi~ed~
In the design of thermally-sensitive controls
of this type there are generally size constraints
or the various components having regard to the
desired overall compactness of the control~ and
a preloaded sprîng mechanism is also advantageous
in this respect since such a mechanism can be designed
relatively com~actly. Moreover~ such a spring
mechanism can be incorporated in a control without
substantially increasing the manufacturing costs.
The preloaded spring mechanism make take
any convenient form, and it could for example comprise
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a tension spring normally held in an extended condition
by the retaining means and adapted to be released
in the event of serious over-heating so that the
switch contacts are opened. However, since it
is desirable to provide a relatively strong biasing
force generally within relatively narrow size constraints
the preferred preloa~ed spring mechanism comprises
a compression spring which in normal use is held
in a compressed state by the retaining means.
The form of the retaining means may vary
depending on the general configuration of the control
and on the nature of the actuating spring provided.
For example, however, it is en~isaged that such
means ma~ include a retaining element fixedly secured
lS to or moulded integrally with a body portion of
the control which is adapted to mount the spring,
the element being formed of an appropriate material
and being so arranged as to deform, melt or break
so as to release the spring when hea~ing of the
control has reached a predetermined level consequent
upon ~ailure o~ the primary protector. In the
case o~ a compression spring, such may be mounted
during assembly of the control within a suitable
recess in the body portion and held under tension
therein by means of the retaining element located
at one end of the recess from which end the spring
thus projects when released upon deformation of
the retaining element.
In a preferred embodiment, however, the retaining
menas comprises a displaceable means cou~led to
or carried by the spring and arranged to co-operate
with part of an external element defining the mounting
location for the control so as to be displaced
in such a way as to pre-load the ac~ua~ing spring
when the control is secured in place. Thus, the
retaining element is resiliently urged bv the actuating
spring against part of the mounting location for
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the control, and such part is arranged in suitable
thermal contact with the element so that upon serious
over-heating consequent upon failure of the primary
protector the end of the retaining element remote
S from the sprin~ melts or deforms sufficiently
to release the spring mechanism and open the contacts.
In a particularly preferred such embodiment where
the spring mechanism includes a compression spring,
~he retaining elemen~ may conveniently comprise
a slidable push rod which at one end is coupled
to the spring and which at the other end pro~ects
externally of the control for engagement with part
of the mounting location therefor.
~he mounting location for the control will
vary depending on the nature of the heater and
the type of container. For example, the control
may be adap~ed for use with an immersion heater
having a hea~ing element carried by a head which
is securable in a water tigh~ manner in or adjacent
an opening in the container wall. In such an
embodiment, the heater head may define the mounting
location for the control, and in the arrangement
discusse~ above wherein the retaining element comprises
a slidable push rod, the push rod may convenien~ly
be arranged for engagement with part of the head
when the control is mounted thereto. Thus, the
push rod an~ spring mechanism are advantageously
arranged so that upon mounting of the control to
the heater head the push rod engages the head and
is displaced axiall~ so as to compress and pre-
load the spring. In the even~ of failure of the
primary protector, when the head reaches a certain
temperature the end of the push rod softens sufficiently
to be distorted un~er the force o~ the spring which
is thus release~ to open the contacts.
The operating member which is adapted to
be moved under the force of the spring into engagement
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with the contacts may take any convenient form,
and in a preferred embodiment where the spring
mechanism comprises a compression spring such member
make conveniently comprise an arm which extends
laterally from one end of the spring~ In the
case of a coil spring, such arm may conveniently
~e formed integrally therewith. The manner in
which the operating member co-operates wi~h the
contacts of such a control may vary depending on
the configuration o~ the swi~ch means. Typically
however, the switch contacts include interengaging
contact members, at least one of which is carried
by a resilient conducting strip or leaf spring
50 as to be displaceable away from the other contact
member. In such an arrangement, the operating
member may conveniently be arranged to engage the
leaf spring carrying the movable contact member.
The general configuration of the switch means
of the control may take any convenient form, and
~0 the pre-loaded sprin~ mechanism may be associatefl
either with the switch con~acts which are adapted
to be opened in response to op~ration of the thermally-
responsive actuating means, or alternatively the
control may comprise a further set of contacts
arranged to be acted on by the spring mechanism.
In one embodiment, the control additionally comprises
means whereby the heater is disabled upon boiling
of li~uid within an associated container, such
means including a further thermally-responsive
actuating means exposed to steam or vapour emitting
from the container and coupled to a set of switch
contacts via an over centre spring mechanism.
In such an embodiment, the Pre-loaded spring mechanism
can alternatively be associated with the contacts
formins part of the steam sensitive switch mechanism.
The primary thermally-responsive actuatinq
means and, if provided, the actuating means for
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the steam sensitive switch mechanism, may both
comprise snap actln~ bimetallic actuators of any
convenient form, for example a stressed sheet of
bimetal having a generally domed configuration
and a central tounge such as that described in
British patent 1,542,257. The control may be fabri-
cated from the suitable thermoplastics material,
and in a preferred embodiment wherein the retaining
element comprises a push rod which is arranged
~o be engaged and axially displaced by ~he heater
head, the body part of the con~rol which confronts
the head and moun~s the push rod may if necessary
be formed a material having a higher heat resistance
than other plastic parts of the control and than
the push rod itself. This avoids the possibility
of this body portion being distorted in the event
of serious overheating before releas~ of the spring
mechanism in such a way as to impair the movement
of the push rod.
An embodiment of the present invention will
now be described, by way of example only, with
reference to the accompanying drawings, wherein:
Figure 1 is a partlY schematic vertical cross-
sectional illustrating a thermally-sensit;ve control
secured to the head o~ an immersion heater; and
Fi~ures 2a and 2b are respectively diagrammatic
cross-sectional eleva~ional views illustrating
in more detail a pre-loade~ spring mechanism.
Referring firstly to Figure 1, a therma]ly-
sensitive control 1 is mounted to the head 2 ofan electric immersion heater, the control and heater
head being interconnected by means of bolts (not
shown) and thus being clamped within an aperture
formed in the wall 3 of an associated container
35 such as a kettle or hot water ~ug. ~he general
configuration of the heater will be well known
to those skilled in the art, and as is conventional
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g
the heater includes a hea~ing element terminating
in a pair of cold leads 4 which extend through
the plate-like head 2 and having a so-called hot
return part which is braze~ to the head in good
thermal contact therewith. A resilient annular
sealing ring 6 is tightly engaged between the head
an~ the control so as to provide a waker-ti~ht
seal with the container wall 3.
The control 1 includes first and second body
portions 7,8 which interengage axially and are
so shaped to define cavities therebetween within
whlch electrical switch means of the control are
mounted. The second body por~ion 8 include~ three
terminal pins 9 arranged in a ~riangular configuration
and adapted to be engaged with a separate female
socket CQnneCtor for the supply of electrical power
to the heater, as is conventional wi~h this type
of control. The live and neutral ones of the terminal
pins 9 are connected via electrical switch means
to the respective cold leads 4 of the element,
the free end portions of which extend through apertures
10 formed in the first body portion 7 of the control.
Those skilled in this art will be familiar
wi~h the basic components of the illustrated control
and will recognise that the general layout could
be varied. Similar types of control are known,
for example from British Patent Specifications
Nos. 1470366, 2149217 and from our co-pending
British Patent Applica~ion No.8502170. The improvement
of the presen~ invention is applicable ~o all of
these and other known controls.
The control illustrated in Figure 1 further
comprises a primary overheat protector including
a snap-acting bimetallic actuator 11 mounted in
gOOa thermal contact with a dimple 12 ~ormed ;n
the head ad;acent the hot return 5 of the element.
The actuator 11 is of the general type descr ibe~
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in British Patent Specification No.1,542,257 and
thus includes a central ton~ue 13 which is secured
to the first body portion 7 of the control by means
such as a hammer drive screw~ and an upper peripheral
portion 14. The upper portion 14 is arranged for
engaqement with an axially slidable push rod tnot
shown) extending through an aperture in the first
body portion 7 and co-operating with a set of switch
contacts (not shown) wired in series between either
the live or neutral terminal pin 9 and a res~ective
cold leafl 4. The actuator 11 is calibrated with
a nominal operating temperature of 140DC so as
to undergo snap action when the element overheats,
for example having been switched on with insufficient
water in the container to cover the element or
in the event that the container boils dry. Upon
operation of the actuator 11 it reverses its curvature
from the configuration shown in Figure 1 and the
upper peripheral portion 14 thereof is moved right-
wardly ie, away from the heater head, so as toengage the push-rod which in turn opens ~he switch
contacts so as to interrupt the power supply to
the elemen~. This general arran~ement of primary
protector is well known, and the particular configura-
tion illustrated is similar to that described inmore detail in our co-pending application no. 850217n.
The control illustrated in Figure 1 further
comprises a second set of switch contac~s 15 arranc~ed
in series between the other cold lead 4 of the
element and a respective terminal pin 9. The contacts
15 include contact members 16 carried by conductive
leaf spring elements 17, 18, one of which 17 is
connected to the base portion of the respective
terminal pin 9 and the other of wh~ch 18 is connecte~
via a further conductive strip element to the respective
cold lead 4. Such a contact arrangement is of
the same general type as described in our co-pendinc~
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application no. 8502170, but many other similar
configurations are possible.
The second set of contacts 15 is arranged
to engaged by a slidable push-rod 20 which in turn
co-operates with a lever member 21 forming part
of a steam-sensitive over-centre spring mechanism
(not shown). The over-centre spring mechanism
may take any convenient form, and conventionally
comprises a second thermally-responsive ac~uating
means, for example a further snap-ac~ing bimetallic
actuator, which is exposed to steam or vapour emitting
~rom the container during boiling and which is
adapted to trip the over-centre spring mechanism
so as t~ de-energise the heater. Thus~ in the
arrangement illustrate~ in Figure 1, upon boiling
the lever 21 is tripped to a second position (not
shown) wherein the push-rod ~0 is urged leftwardly,
i.e. towards ~he head, and engages a free-end part
22 of the leaf sprin~ 18 whereby the contacts 15
~0 are broken. As is conventional means (not shown~
are provi~e~ whereby the over centre mechanism
may readily be manually reset.
The illustrated control further comprises
a pre-loade~ spring mechanism associated with the
second set of contacts 15 which is arranged to
provide back-up protection in the event that the
bimetallic actuator 11 should failO One form of
spring mechanism is shown in more detail ;n Figures
2a and 2b, from which it will be seen that the
mechanism comprises a coil spring 25 which is mounted
within a recess 26 formed in the second body portion
8 and which in normal use is held under tension
by means o~ a retainin~ element in the form of
a push-rod 27. Push-rod 27 is formed for example
from nylon and extends through and is axially moveable
within a bore 28 formed in the first body portion
7. Thus, when the control is clamped to the heater
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head, the push-rod 27 engages the head and is urged
rightwardly relative to the control so that the
spring 25 is placed under tension.
As shown in Figure 2b the spring 25 includes
an operating member in the form of an arm 30 formed
integrally therewith which extends laterally and
which is arranged for engagement with the leaf
spring 18 adjacent the free end portion 22 thereof.
~owever, as shown in Figure 1 in normal use when
the spring is pre-loaded by means of the push-rod
27, the operating member 30 of the spring is spaced
axially from the leaf spring 18 so that normally
the contacts 15 are free to open and close in response
to operation of the lever member 21 forming part
of the steam-sensitive over-cen~re spring mechanism.
~ owever, in event of failure of the actuator 11,
the arrangement is such that the consequent overheating
of the head causes the end of the push-rod 27 which
engages the head to soften and deform under the
~0 force of the spring whereby the spring is releasea
sufficiently to enable the operating arm 30 to
engage the leaf spring 18 and open the contacts.
The heater ;s thus disabled, and since the contacts
15 are open under the positive action of the coil
spring 25 the possibility of subsequent reclosing
of the contacts as a result for example of distortion
of part of the control caused by over-shoot heating
is avoided.
Modifications of the control described and
illustrated herein will be apparent to persons
skilled in this art and the disclosure hereof is
intended to encompass all such modifications.
