This invention relates to an electrical plug with built-in safety
switch adapted to be used in electrical apparatuses.
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.
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.
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.
Even if they do not result into accidents, all leakages due to improper
operation of any electrical apparatus are harmful to the apparatuses themselves.
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.
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.
Anyway, all safety devices based upon utilisation of differential
switches have some drawbacks.
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
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.
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.
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.
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
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
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.
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.
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.
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.
Preferably, according to this invention, the rotation axis of said
terminals passes through their tablets.
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.
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.
Further features of this invention will be apparent from the dependant
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
- 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.
In all Figures, similar items are designated by similar reference
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.
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.
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.
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.
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.
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
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.
By way of illustration, the plug according to this invention can
be modified so as to substitute other kinds of electromagnetic devices for said
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.
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.
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.
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
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.
Furthermore, said toroidal sensor can be replaced by any current
sensor adapted to operate in differential mode.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.