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This invention relates to an electrical plug with built-in safety
switch adapted to be used in electrical apparatuses.
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More particularly, this invention relates to a plug of the above
said kind which enables in simple, reliable and inexpensive way to switch
off the power supply to a load connected thereto in all cases of current
leakage and which is highly ergonomic.
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It is known that many accidents occurring at home or on the
working sites are due to electrical apparatuses and machines. Such
accidents are usually caused by accidental contacts with electrically
active items that causes a dangerous current flow usually supplied by the
mains voltage.
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The above mentioned electrically active items can be either
cables or electrical devices amenable to be improperly contacted, or also
electrically conductive components that, due to leakages caused by
failures or short circuits, are driven to dangerous voltages.
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Even if they do not result into accidents, all leakages due to
improper operation of any electrical apparatus are harmful to the
apparatuses themselves.
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Aiming at limiting all risks for the involved people and all
damages to the concerned apparatuses many solutions have been
proposed; the most popular solution provides for inserting a differential
switch upstream of all terminals or current sockets of the electrical circuit
connected to a user apparatus. By way of illustration, in the case involving
an electrical circuit of a dwelling place, said differential switch is located
downstream of the current meter and upstream of all current sockets
where electrical appliances and any other electrical apparatus are
connected.
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A differential switch senses the behaviour of the current
supplied by the power supply mains to the user and responds to any
sudden increase thereof higher than a pre-established threshold value by
switching off the power supply to all terminals of the user electrical circuit.
As it is known to those skilled in the art, such possible sudden increase in
the supplied current can be caused either by accidental contact of a body
or by a current leakage due to a short circuit condition. In detail, the
above said threshold value is designed so as to account both for the
amount and for the increase speed of the supplied current, it can be
defined as a current differential threshold and it is quantitatively identified
as an intervention or trigger current value.
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Anyway, all safety devices based upon utilisation of differential
switches have some drawbacks.
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Since the differential switches have been found to be rather
expensive, it is not convenient to couple a differential switch to each
apparatus connected to the current socket. In this way, when a current
leakage occurs, the current supply to all current sockets is switched off,
so that it is no more possible to easily and immediately locate the single
apparatus wherein the current leakage occurred. This situation can be
particularly critic in dwelling places during the evening hours, since it
entails the interruption of the current supply to all illumination devices.
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Furthermore, due to the fact that the differential switch is
inserted upstream of all terminals, the differential threshold value beyond
which the current supply is switched off is rather high, since it should
account for a number of apparatuses that can be contemporaneously
connected to the current sockets coupled downstream of said switch. As a
consequence of this, some electrical user apparatuses can be anyway
even partially damaged. By way of illustration, it can be mentioned that
the plug associated to the defective apparatus generally burns out.
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It is an object of this invention, therefore, to provide in simple,
reliable and not expensive way a safety device that can be built in any
apparatus connected to a current socket and is adapted to adjust the
differential current threshold to any specific apparatus and to switch off
the current supply to it without affecting the remaining apparatuses
connected to the user electrical circuit, that enables any defective
apparatus to be easily located and that, regardless of the cause itself of
the failure, avoids any further damage thereof.
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It is a further object of this invention is to provide a highly
ergonomic electrical plug adapted to be optimally integrated with the
apparatus coupled thereto and with the environment in which it is
arranged.
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It is a specific object of this invention to provide an electrical
plug with built-in safety switch for use with an electrical apparatus
comprising a casing, at least two terminals and a corresponding number
of power supply lines for said electrical apparatus, an electrically
controlled switch that couples two terminals to said power supply lines, a
sensor to sense the behaviour of the current supplied to said apparatus,
inserted between said switch and said apparatus, as well as an electronic
control unit which receives power supply from said terminals and a signal
corresponding to said sensed behaviour from said sensor and which
generates control signals for said switch, said electronic control unit
operating in differential mode, according to which it closes said switch at
power-up and opens it when the current sensed by said sensor is higher
than a differential threshold value.
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According to this invention, said switch can include an
electromagnetic device adapted to impart a translatory movement to a
movable member and mechanical means for interaction between said
movable member and two metal contacts connected to power supply lines
for said electrical apparatus, each of said metal contacts being adapted to
move between a first position in which it is insulated with respect to said
terminals and a second position in which it is in contact with a
corresponding terminal.
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Further according to this invention, each of said two metal
contacts can be provided with a tablet facing a tablet of a corresponding
terminal, said facing tablets being adapted to be in contact with one
another in said second position in which said metal contacts and said
terminals are in contact with one another.
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Preferably, according to this invention, said two metal contacts
are spring contacts fixed to said casing, said electromagnetic device is a
coil energised by said electronic control unit, said movable member is a
plunger coaxially movable within said coil and said interaction mechanical
means comprise a shaped rod rotatably coupled to said casing by means
of a rotation pivot and having, in correspondence to said rotation pivot, a
cam adapted to cooperate with said contacts and a pin rigidly connected
to the plunger of said coil and cooperating with a corresponding slot
provided in said shaped rod.
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Further according to this invention, the current plug can include
an optical and/or acoustical indication device coupled to said electronic
control unit and comprises at least a light emitting diode or LED and/or a
buzzer.
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Again according to this invention, said casing has, for each of
said terminals rigidly connected with one another, a slot and means for
rotation of said terminals within said slots and on an axis orthogonal to the
terminals, between two limit positions angularly spaced apart from one
another by no more than 180°, said rotation means occluding said slots in
any position of said terminals.
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Preferably, according to this invention, the rotation axis of said
terminals passes through their tablets.
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Further according to this invention, the casing of said plug
comprises a central head having a right half-shell and a left half-shell
hingedly connected thereto and also having a first open configuration and
a second closed configuration, coupling means being provided between
said central head, said right half-shell and said left half-shell.
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Again according to this invention, said slots have a width equal
to the diameter of said terminals in at least two points and a smaller width
at any other point.
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Further features of this invention will be apparent from the
dependant claims.
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This invention will be now described, by way of illustration, not
by way of limitation, according to its preferred embodiment, by particularly
referring to the Figures of the enclosed drawings, in which:
- Figure 1 shows a circuit diagram of the preferred embodiment
of the electrical plug according to this invention,
- Figure 2 shows a top plan view of some components of the
preferred embodiment of the plug according to this invention,
- Figure 3 shows a side elevation view of some components of a
second embodiment of the plug according to this invention,
- Figure 4 shows a front elevation view of the casing of the plug
of Figure 3, in open configuration,
- Figure 5 shows a side elevation view of the casing of Figure 3,
- Figure 6 shows a top plan view of the clamp set of the plug of
Figure 3,
- Figure 7 shows a side elevation view of the clamp set of the
plug of Figure 3,
- Figure 8 shows a front elevation view of a second embodiment
of the casing of the plug, in open configuration,
- Figure 9 shows a side elevation view of the casing of Figure 8.
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In all Figures, similar items are designated by similar reference
numerals.
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By referring now to Figure 1, it can be observed that a circuit
according to the preferred embodiment of the plug of this invention
comprises an electrically controlled switch 1 that connects the terminals of
the plug coupled to the mains voltage 2 to an electrical apparatus 3. A
sensor 4 is inserted between said switch 1 and said apparatus 3 in order
to sense the current supplied by the mains 2 to said apparatus 3 and
furnish a corresponding signal to a control electronic unit 5. Electronic unit
5, which is directly powered from said mains 2, transmits control signals to
said switch 1 and it is also connected to an indication device, that
preferably includes a light emitting diode or LED.
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By referring now to Figure 2, some electrical and mechanical
components of the preferred embodiment of the electrical plug can be
observed. Switch 1 of Figure 1 comprises, housed within a casing 7
having two terminals 8 rigidly connected thereto, a coil 9, having a
coaxially movable pin 10, which is driven by said electronic control unit,
such that when said coil 9 is energised, switch 1 is closed, and when said
coil 9 is de-energised, switch 1 is open. Switch 1 further comprises a
shaped rod 11 rotatably connected to said casing 7 by means of a rotation
pivot 12, as well as two spring metal contacts 13 fixed to said casing 7,
each of which has a tablet 14 correspondingly facing a tablet 15 of said
terminals 8. Preferably, such tablets 14 and 15 are silvered. In particular,
said two contacts 13 are coupled to the supply lines, not shown, of the
concerned electrical apparatus and such supply lines are passed through
the hole of a toroidal sensor designed to sense the behaviour of the
current as supplied to the apparatus. Shaped rod 11 is provided with a
cam 16 corresponding to said pin 12 and adapted to cooperate with
contacts 13. Plunger 10 of said coil 9 has a pin 17 that cooperates with a
corresponding slot 18 of said shaped rod 11.
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When coil 9 is de-energised, plunger 10 is in rest position, as it
is shown in Figure 2, in which cam 16 does not interacts with contacts 13,
so that contacts 13 are also in rest position and tablets 14 are spaced
from the corresponding tablets 15 of terminals 8.
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On the other hand, when coil 9 is energised, plunger 10 is
attracted within coil 9 and pin 17 cooperating with slot 18 transmits a
clockwise rotatory movement to said shaped rod 11 and consequently to
cam 16; this causes cam 16 to interact with said two contacts 13 in such a
way that tablets 14 contact tablets 15 of said terminals 8, thereby
establishing an electrical connection between said terminals and the
supply lines for the concerned electrical apparatus.
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For a better comprehension of this invention, the operation
modes of the preferred embodiment of the electrical plug will be
hereinbelow described and it should be understood that similar operation
modes apply to other embodiments.
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When the plug is inserted into a current socket, control
electronic unit 5 is energised and carries out a self-control operation
aimed at ascertaining that all electrical and electronic components of the
plug are correctly operating. Upon completing such self-control operation
with positive outcome, said electronic unit 5 energises the LED of the
indication device 6, in order to indicate a correct operation of the plug,
and closes switch 1, so as to establish an electrical connection between
mains 2 and the supply lines of the electrical apparatus 3. The above
mentioned toroidal sensor 4 detects the behaviour of the current supplied
through the supply lines to said electrical apparatus 3 and, should the
maximum differential threshold value be exceeded, said electronic unit 5
de-energises said coil 9 and consequently switches off the current supply
to said apparatus 3 and de-energises the LED of said indication device 6.
Preferably, electronic unit 5, that operates in differential mode, namely in
response to an unbalance between the two phases, has an intervention or
trigger current value in the range of 8 to 10 mA, maximum current.
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It is immediately possible to those skilled in the art to modify
the plug according to this invention so as to make it fulfil specific
requirements.
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By way of illustration, the plug according to this invention can
be modified so as to substitute other kinds of electromagnetic devices for
said coil 9.
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Furthermore, it is possible to provide a third terminal
permanently connected to the ground line of the electrical apparatus 3,
for instance by means of a plaited wire.
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In particular, in respect both of their shape and of their
dimensions and applied voltage, terminals 8 can be of modified type with
respect to the ones shown in Figure 2, that are of typical Italian standard,
and they can be realised according to any other standard, such as U.S. or
U.K. standard.
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It should also be understood that the indication device can be
comprised of more than one optical and/or acoustical indicator; for
instance, it can comprise a first green LED to indicate a correct operation
of the plug, a second red LED to indicate the open condition of switch 1
and a buzzer designed to be energised when the electronic control unit 5
causes said switch 1 to be opened.
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The embodiment of Figure 2 minimises the volume of said
switch 1. However, the mechanical components of switch 1 by which the
double conversion of the movement is carried out, namely from a
translatory to a rotatory movement and from a rotatory to a translatory
movement, that is to say the shaped rod 11 and the spring contacts 13,
could also be replaced by different mechanical elements adapted to
directly utilise the translatory movement of said plunger 10 to establish or
to interrupt the connection between terminals 8 and the supply lines of
said apparatus 3.
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Furthermore, the mechanical components of said switch 1,
specifically the coil 9, the shaped rod 11 and the spring contacts 13 could
also be replaced by power logic gates operating under control of said
electronic unit 5.
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Furthermore, said toroidal sensor can be replaced by any
current sensor adapted to operate in differential mode.
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Said electronic control unit 5, that is realised by analogue
components, in the preferred embodiment, can also be realised by digital
components, and it can comprise a digital signal processor and/or a
microprocessor and/or a circuit programmed for this specific application or
ASIC (Application Specific Integrated Circuit).
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It is also possible to enable the differential threshold to be
programmed at which said electronic control unit 5 triggers the switch to
be opened; in particular, this can be realised in factory or directly by the
user, for instance by setting suitable microswitches or dip-switches.
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Additionally, said electronic control unit 5 can be operated so
as to permanently open said switch 1 after said differential threshold has
been exceeded a number of times.
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For instance, when switch 1 is maintained in open condition
after said differential threshold has been exceeded only once, electronic
control unit 5 can be arranged in such a way that coil 9 is enabled by the
charge stored in a capacitor that, by way of illustration, could drive the
gate terminal of a suitable power MOS transistor, charged at power up of
said electronic unit 5; in this case, the signal indicating that the differential
threshold has been exceeded as generated by sensor 4 could energise a
circuit for rapid discharge of said capacitor, so as to no more enable a
current flow to energise said coil 9. If it is desired to again close said
switch 1, it will be necessary to withdraw said plug from the socket,
thereby switching off power supply to said electronic unit 5, and reinsert it
into said socket, so as to re-charge said capacitor.
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Correspondingly, when said switch 1 is maintained in open
condition after said differential threshold has been exceeded two or three
times, said electronic unit 5 could be realised by means of digital
components. Should a short circuit condition be established in said
electrical apparatus 3, said electronic unit 5 would detect the failure
condition and would open said switch 1, thereby switching off any current
flow into said supply lines; consequently, sensor 4 would no more detect
any failure condition and said electronic unit 5 would no more recognise
any short circuit condition and would again close said switch by detecting
a failure condition a second time and would permanently open switch 1,
when the maximum allowed number of times the differential threshold can
be exceeded is two; when closure if switch 1 is to be again enabled, also
in this case, it will be necessary to withdraw the plug so as to switch off
current supply to electronic unit 5 and to reinsert it into the current socket.
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A second embodiment of the plug according to this invention is
illustrated in Figures 3 to 7, in which only those mechanical components
have been shown that are different with respect to those provided in the
embodiment of Figures 1 and 2.
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In the arrangement of this second embodiment, terminals 8 also
comprising a third central ground terminal coupled by a plaited wire to the
ground frame of the electrical apparatus 3 are rigidly coupled to one
another, by means of a base member 19, and can rotate on an axis
orthogonal thereto passing through said tablets 15, between two limit
positions spaced apart by 90° from one another.
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As it is particularly shown in Figures 6 and 7, base member 19
of said terminal set 8 has two laterally arranged shaped plates 20, whose
cross section shape orthogonal to the rotation axis of said terminals 8 has
a symmetry axis parallel to the axes of terminals 8 and can be obtained
from a circular shape from which substantially an angular sector of 90°
has been removed. Each of said shaped plates is externally provided with
a protruding disc member 21.
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As it has been particularly shown in Figures 4 and 5, the casing
7 of the concerned plug comprises a central head 22 having a right half-shell
23 and a left half-shell 24 hinged thereto. Each of the two end walls
of head 22 has a semicircular outline 25 ending with two side pawls 26
that, when casing 7 is in closed configuration shown in Figure 3, are
inserted into corresponding recesses 27 of the symmetrical outlines of the
side walls of said right 23 and left 24 half-shells. In particular, the side
walls of each of said half- shells 23 and 24 have a quarter of
circumference outline 28 adjacent to the corresponding recess 27, so that,
when casing 7 is in closed configuration incorporating said terminal set 8,
they form together with the semicircular outline 25 of the wall of head 22
the edge of a circular hole which is received in a corresponding groove 29
of the associated protruding disc member 21.
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Furthermore, for each terminal 8, casing 7 is provided with a
slot 30 partially extending along head 22 and along said right half-shell
23.
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Casing 7 is additionally provided with means for coupling the
right half-shell 23 and the left half-shell 24 in their closed configuration,
for instance screws inserted into corresponding holes 31 of the left half-shell
24 and cooperating with the right half-shell 23.
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As it is particularly shown in Figure 3, in both limit positions that
can be reached by said terminal set 8 in view of its rotation, said shaped
plates 20 assure the occlusion of the slots 30, so as to guarantee non-accessibility
to the inner room of the plug.
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This can be realised in view of the fact that said slots 30 have
vertical walls, not shown, protruding inwardly of casing 7 and having a
quarter of circumference outline complementary to the outline of said
shaped plates 20 of terminal set 8.
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In a different embodiment, base member 19 of said terminal set
8 can also have an internally hollow, cylindrical side surface, that couples
together said two shaped plates 20, so that said slots 30 enable access to
an internal chamber not communicating with the internal room of the plug.
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In particular, said two limit positions can be made stable by
shaping the slots 30 in such a way that they have a width smaller than the
internal diameter of terminals 8, wit exception for the two ends, where the
width of said slots 30 is equal to the diameter of terminals 8.
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Casing 7 that, in the embodiments shown in Figures 3 - 7, can
be repeatedly opened and closed a number of times, also for
maintenance purposes, could also be realised by only two half-shells
each of which incorporates one half of head 22: said half-shells could be
permanently closed after having been firstly assembled, for instance by
welding together the contact surfaces of said half-shells.
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By referring to Figures 8 and 9, it can be observed that, in a
further embodiment according to this invention, said casing 7, that can be
repeatedly opened and closed, comprises a first half-shell 32, that
incorporates the head of the casing, and a second half-shell 33. Said two
half- shells 32 and 33 could be hingedly connected with one another
and/or could be provided with coupling means, for instance screws
inserted into corresponding holes, not shown, in one of the two half-shells
and cooperating with the other half-shell. Said two half-shells could also
be permanently closed after having been firstly assembled, for instance
by welding together the contact surfaces of the half-shells.
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Furthermore, said terminal set 8 as shown in Figures 3 -7 can
be provided with only two rigidly interconnected terminals 8. In this case,
said two terminals 8 can be rotated between two limit positions, angularly
spaced apart by 180° from one another (and, if desired, three stable
positions), when said shaped plates 20 have a circular cross-section
orthogonal to the rotation axis of terminals 8.
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This invention has been described by way of illustration, but not
by way of limitation, according to its preferred embodiment, but it should
be understood that variations and/or changes can be made by those
skilled in the art without departing from the scope of this invention as
defined by the attached claims.
