[0001] '[he present invention relates to a residual-current device, such as for example
a residual-current circuit breaker, having improved functional characteristics.
[0002] In general, a residual-current device is a device which is suitable to directly or
indirectly interrupt the flow of current in an electrical circuit when a residual
current occurs. A typical example of residual-current device is constituted by a residual-current
circuit breaker, and particular reference is made hereinafter to this specific embodiment
without thereby intending in any way to restrict the scope of application of the device
according to the present invention.
[0003] It is known that a residual-current circuit breaker is an automatic circuit breaker
for AC circuits which opens when the vector sum of the currents in the conductors
of the circuit, which is zero in normal conditions, exceeds a preset value; the main
characteristic of said circuit breakers is that they achieve extremely short circuit
opening and closure times.
[0004] Residual-current circuit breakers are generally meant to prevent metallic parts,
such as for example the enclosures of electrical appliances and the metallic masses
of the protected area which are connected to an earth system of appropriate resistance,
from becoming live: indirect protection is thus achieved. Moreover, if said circuit
breakers are sufficiently sensitive and fast-acting, in certain conditions they can
also provide protection against contact with normally live parts, thus providing direct
protection.
[0005] A residual-current circuit breaker is shown schematically in Figure 1. Said circuit
breaker comprises a sensor 1 for detecting the residual fault current, generally constituted
by a magnetic core in which the neutral 2 and the phase 3 pass, said neutral and phase
being connected to a load (not shown), and by a secondary winding 4 across which a
voltage is generated when the fault current is present. If an earth fault current
occurs, the voltage generated across the secondary winding is supplied to an actuator
6 by means of a suitable electronic coupling system 5 which is interposed between
the sensor 1 and the actuator 6. In turn the actuator actuates a release or disengagement
mechanism 7 which opens the contacts 8 and 9 of the circuit and thus interrupts the
power supply.
[0006] One of the main drawbacks of conventional residual-current circuit breakers is the
fact that the actuators currently in use are of the electromagnetic type, typically
electromagnetic relays of the demagnetization type. These relays are in fact sensitive
to magnetic fields and this can cause several drawbacks: for example, the presence
of a magnetic field can cause unwanted and unwarranted actuation of the actuator,
or an external magnetic field might alter the magnetization of the electromagnetic
relay, consequently modifying the sensitivity of the residual-current circuit breaker
and negatively affecting its performance.
[0007] Another drawback is the fact that electromagnetic actuators have a mechanically complicated
structure which is sensitive to impacts and vibrations: this can cause untimely interventions
of the circuit breaker and negatively affects its overall reliability. The cost is
also high.
[0008] The aim of the present invention is to provide a residual-current device which is
insensitive to external magnetic fields, so as to avoid untimely interventions.
[0009] Within the scope of this aim, an object of the present invention is to provide a
residual-current device which comprises an actuator which has a simplified structure
and is insensitive to impacts and vibrations, so as to improve the performance and
overall reliability of said circuit breaker.
[0010] Another object of the present invention is to provide a residual-current device which
is highly reliable, relatively easy to manufacture and at competitive costs.
[0011] This aim, these objects and others which will become apparent hereinafter are achieved
by a residual-current device which comprises an actuator which is operatively connected
to kinematic means for opening contacts following the detection, by a sensor, of an
earth fault current, characterized in that said actuator comprises a bistable metallic
element which is operatively connected to said kinematic means and to which at least
one piezoelectric element is coupled, said piezoelectric element being excited by
an electrical signal as a consequence of the detection of said fault current and inducing
a movement of the metallic element from a first stable equilibrium position to a second
stable equilibrium position, the bistable metallic element actuating said kinematic
means by way of said movement; said device further comprising reset means which are
adapted to return the bistable metallic element to the first stable equilibrium position
before the contacts close again.
[0012] In this manner, by using a piezoelectric actuator, the device according to the invention
is insensitive to external magnetic fields and is therefore immune to untimely and
unwanted interventions.
[0013] Further characteristics and advantages of the device according to the invention will
become apparent from the following detailed description of preferred but not exclusive
embodiments thereof, illustrated only by way of non-limitative example in the accompanying
drawings, wherein:
Figure 1 is a block diagram of a residual-current circuit breaker;
Figure 2 is a schematic view of a first embodiment of an actuator which can be applied
in the residual-current device according to the present invention;
Figure 3 is a schematic view of a second embodiment of the actuator that can be used
in the residual-current device according to the present invention;
Figure 4 is a schematic view of a third embodiment of the actuator that can be used
in the residual-current device according to the present invention;
Figure 5 is a view of a detail of the actuator of Figure 4:
Figure 6 is a schematic view of a fourth embodiment of the actuator that can be used
in the residual-current device according to the present invention;
Figure 7 is a schematic view of a fifth embodiment of the actuator that can be used
in the residual-current device according to the present invention:
Figure 8 is a schematic view of a sixth and preferred embodiment of the actuator,
operatively associated with means suitable to reset said actuator:
Figure 9 is a side view of the actuator of Figure 8, in which the bistable metallic
element has reduced transverse cross-sections:
Figure 10 is a plan view of the actuator of Figure 8, in which the bistable metallic
element has thinner transverse cross-sections produced by means of notches;
Figure 11 is a view of a second embodiment of the reset means of the actuator used
in the device according to the present invention:
Figure 12 is a view of a third embodiment of the reset means of the actuator used
in the device according to the present invention.
[0014] With reference to Figure 1, the residual-current circuit breaker comprises an actuator
6 which is operatively connected to kinematic means 7 for opening the contacts 8 and
9 of the circuit breaker following the detection of an earth fault current by a sensor
1.
[0015] Advantageously, in the device according to the invention the actuator 6 comprises
a bistable metallic element which is operatively connected to the kinematic means
7 and with which at least one piezoelectric element is operatively associated. If
an earth fault current occurs, the voltage generated across the secondary winding
4 of the sensor 1 is supplied to the piezoelectric element, either directly or preferably
by means of a suitable electronic coupling system 5 interposed between the sensor
1 and the actuator 6. The electrically activated piezoelectric element undergoes deformation,
inducing a movement of the bistable metallic element from a first stable equilibrium
position to a second stable equilibrium position. By way of this movement, the bistable
metallic element transmits a movement to the kinematic means 7, which open the contacts
and interrupt the power supply.
[0016] In a first embodiment, shown in Figure 2, the actuator 6 comprises a bistable element
which is constituted by a metallic lamina 11 in which a first end is connected to
a supporting body 20, for example the case of the circuit breaker: suitable compression
means act on a second end of the lamina 11 which is opposite and substantially parallel
to the first one so as to keep the lamina loaded in a first stable equilibrium position.
Said compression means comprise, for example, an abutment element 13 which is fixed
to the second end of the lamina and a loading spring 12 which is interposed between
the supporting element 20 and the abutment element 13. Advantageously, at least one
piezoelectric element 10 is arranged in contact with the abutment element 13: in a
preferred embodiment, the piezoelectric element 10 is a bimorph element, i.e., it
is constituted by two mutually parallel layers of piezoelectric material. In this
manner, when an electrical stress is applied, one piezoelectric element layer deforms
by contracting while the other one deforms by elongating. As shown in Figure 2, an
element 30 which acts as an electrode is interposed between the two piezoelectric
layers.
[0017] In case of earth fault, the piezoelectric element 10 is supplied by the voltage generated
across the secondary winding 4 of the current sensor 1, either directly or preferably
by means of the electronic coupling system 5. The electric power supply induces a
deformation in the two layers that constitute the piezoelectric element 10: this allows
to reduce the loading force that retains the metallic element 11 below its critical
threshold, i.e., the threshold beyond which the metallic element passes from a first
stable equilibrium configuration, shown in solid lines, to a second stable equilibrium
position, shown in dashed lines.
[0018] The movement from the first position to the second position actuates an actuation
element 14. For example a pin, which is meant to actuate the kinematic means 7 for
opening the contacts of the circuit protected by the residual-current device.
[0019] As shown in Figure 3, in a second embodiment of the actuator used in the device according
to the invention the bistable metallic lamina, now designated by the reference numeral
111, is substantially M-shaped. In particular, the lamina 111 has two end portions,
which are substantially straight and mutually parallel and are fixed to a supporting
element 120, and an arc-shaped central portion which in practice reproduces the configuration
of the lamina 11 of Figure 1. In this manner, the lamina 111 is arranged in a first
stable equilibrium position and makes it unnecessary to use the spring 12 and the
abutment element 13. The arc-shaped central portion has, on one of its faces, two
mutually spaced piezoelectric elements 100, each element being constituted by one
or more layers of piezoelectric material. In this case, as a consequence of the supply
of the voltage due to a fault current, the piezoelectric elements 100 undergo deformation,
transmitting a bending moment to the metallic lamina 111, which snaps from the initial
stable equilibrium configuration to the second stable equilibrium position , shown
in dashed lines, and actuates an actuation element 14.
[0020] Also in this case, it is possible to arrange the layers of piezoelectric material
in a mutually bimorph configuration.
[0021] Figure 4 is a view of a third embodiment of the actuator 6, in which the lamina made
of metallic material, now designated by the reference numeral 211, is kept in a first
stable equilibrium position by means of a compression spring 212 which is arranged
between two L-shaped elements of an external supporting body 220.
[0022] The piezoelectric elements are constituted, in this case, by two pairs of layers
of piezoelectric material, designated by the reference numerals 200a and 200b respectively,
which face each other and have the lamina 211 interposed between them, as shown in
detail in Figure 5.
[0023] Figure 6 illustrates a fourth embodiment of the actuator 6, which differs from the
embodiment of Figure 4 in that on the lamina made of metallic material, here designated
by the reference numeral 311, there are, on a same face of the lamina, a first piezoelectric
element and a second piezoelectric element, both designated by the reference numeral
300, which are mutually spaced on the face of said lamina.
[0024] Figure 7 is a view of a fifth embodiment of the actuator 6, which differs from the
embodiment shown in Figure 3 in that it has two piezoelectric elements 400 which are
arranged on the substantially straight portions of the M-shaped lamina 411.
[0025] Figure 8 illustrates a sixth particularly preferred embodiment of the actuator 6
used in the residual-current device according to the invention. In this case, the
lamina 511 has two substantially parallel ends which are fixed, preferably by interlocking
coupling, to a supporting element 520, for example the case of the circuit breaker:
two piezoelectric elements 500 are fixed to the two opposite faces of one of said
parallel ends, are arranged in a mutually bimorph configuration and are also at least
partially fixed to the support. In this manner, the lamina is kept in a first stable
equilibrium position, shown in solid lines in Figure 8: when an earth fault condition
occurs, the piezoelectric elements 500 are excited by a supply voltage, as described
earlier, and by undergoing deformation they generate a bending moment which induces
the movement of the lamina from the first stable equilibrium position in the second
stable equilibrium position, shown in dashed lines in Figure 8. By at least partially
fixing to the support the piezoelectric elements as well, in practice a rigid mutual
coupling of the piezoelectric elements and the lamina occurs in which the piezoelectric
elements behave like a pivot and allow the optimum transfer of the bending moment
only to the useful part of the free lamina.
[0026] The lamina, in moving from the first equilibrium position to the second equilibrium
position, actuates an actuation element 14, for example a pin, which is meant to actuate
the kinematic means 7 for opening the contacts of the circuit protected by the residual-current
device.
[0027] Advantageously, as shown in Figures 9 and 10, the lamina 511 includes one or more
functional hinges 530 in its structure which are obtained by means of a local reduction
in its transverse cross-section. In particular, the hinges can be provided either
by means of notches having a suitable shape, obtained by removing material as shown
in Figure 10, or by reducing the thickness of the lamina, for example by lamination
or electrical discharge machining, as shown in Figure 9. The hinges 530 can be formed
proximate to the section of the lamina that is coupled to the piezoelectric elements
500, proximate to the opposite end of the lamina 511 which is rigidly coupled to the
support, or in both of said regions, and have a dual function: proximate to the end
of the lamina that is not associated with the piezoelectric elements 500, they facilitate
the optimum movement of the lamina from the first equilibrium position to the second
one: proximate to the piezoelectric elements, they instead allow to stabilize the
lamina in the two chosen equilibrium positions. Moreover, the simultaneous presence
of the hinges in both regions allows their mutual cooperation in performing the above
described functions.
[0028] Clearly, the hinges, as well as the fixing of the lamina and of the piezoelectric
elements, can be used in all the embodiments previously described.
[0029] Another advantageous aspect of the invention is the fact that the two ends of the
lamina are fixed to the support 520 in two positions which are mutually staggered
with respect to a median plane 550 of the lamina in the non-deformed configuration.
This solution allows to minimize the amount of energy required for the lamina to snap
from the first equilibrium position to the second equilibrium position and allows
to utilize in an optimum manner the limited energy that can be obtained from the fault.
[0030] Also in this case, the movement of the metallic lamina 511 actuates a pin 14 for
transferring motion and force to the kinematic means 7 for opening the contacts. Advantageously,
the residual-current device according to the invention comprises reset means which
allow to reset the actuator and therefore to prepare said device for a new intervention
before the contacts are closed again.
[0031] Said reset means can be obtained by using the pin 14, with which a shaped head 15
is associated, said head having a shape which is suitable to perform the correct resetting
of said actuator. In this case, resetting can be achieved directly after actuation,
following the rebound of said pin assisted by a return spring 16 which is controlled
for example by a lever for restoring electrical continuity, such as for example the
lever for closing the contacts of the circuit breaker again. When the circuit breaker
opens, the lever disengages the spring 16, which transfers its energy to the pin 14
either directly or by means of the interposition of a transmission mechanism suitable
for the purpose. As a consequence of the movement of the pin 14, the shape of the
head 15 couples geometrically to the surface of the lamina and induces its precise
movement to the first stable equilibrium position. In this position the actuator is
ready for a new intervention.
[0032] As shown schematically in Figure 11, in an alternative embodiment of the actuator
reset means the pin 14 only has the purpose of transferring motion and force to the
kinematic system for opening the contacts: in this case, the reset function is performed
by a cam 521 which is pivoted in the support 520 of the actuator. In practice, by
restoring the device by acting for example on the lever for closing the main contacts
of the circuit breaker, the rotary motion of the lever is transmitted to the cam 521,
for example by means of two pulleys, not shown, one pulley being keyed on the axis
of the closure lever, the other pulley being keyed on the rotation axis of the cam.
The cam 521, by rotating about its own axis in the direction indicated by the arrow
522, makes contact with the lamina 511, forcing the end section of the piezoelectric
elements 500 to bend downward; said bending is sufficient to exceed the value required
to restore the operating conditions for release. At the same time, the head of the
cam moves along the lamina, facilitating the reloading action by means of the tangential
component of the thrust force applied thereby, without forcing excessively the cross-section
at the piezoelectric elements and therefore avoiding possible breakage. Snap action
of the lamina is thus achieved while the cam is still pushing against it, maintaining
the end section of the piezoelectric elements beyond the bending required for correct
resetting. Once the snap action or resetting the lamina has occurred, the cam moves
away from contact with the lamina, thus allowing the piezoelectric elements to return
to the operating condition for intervention. At the end of the resetting operation,
the cam has turned through 360 degrees, so as to be ready for the subsequent resetting
of the lamina and so as to be in such a position that it does not interfere with a
subsequent snap action of said actuator. Said rotation can be achieved by correct
choosing the dimensions of the mechanism for transmitting motion from the circuit
breaker closure lever to said cam.
[0033] This solution ensures exact repeatability of the reloading action over time: moreover,
by means of an appropriate shaping of the cam head it is possible to achieve optimum
values of the force components applied by the cam to the lamina, avoiding impacts
with the piezoelectric elements which might damage the actuator and shorten its operating
life.
[0034] A further embodiment of the reset means is shown in Figure 12. In this case, during
the opening of the main contacts a pin 14 moves in a track 701 formed in the supporting
structure 520 and transmits motion to the kinematic means 7 for opening the contacts;
at the same time, suitable guides 702 associated with the pin move beyond a first
unidirectional spring 710 which prevents the pin from falling back along the same
section of the track 701.
[0035] By acting on the closure lever of the main contacts, the movement of the lever is
transmitted by means of appropriate kinematic systems to the guides 702, which slide
along a track 703 until the pin is placed at a section which is proximate to the piezoelectric
elements. In this position, the head of the pin begins to push against the lamina:
by following the rotation of the lever, the pin moves in the track 703 and its head
simultaneously slides along said lamina.
[0036] A position is thus reached in which the configuration is not stable and the lamina
snaps, returning into the initial stable equilibrium position. Once resetting has
been performed, the pin continues to slide along the track 703 and abandons contact
with the lamina, avoiding any interference during the subsequent snap action thereof.
At the end of the movement, the pin resumes the operating position for release, while
the guides 702 move beyond a second unidirectional spring 711 so that they cannot
return along the track 703 during release. The shape of the tracks can obviously be
configured differently according to specific requirements.
[0037] Clearly, the reset means for resetting the actuator can be used in an equivalent
manner in all the embodiments previously described.
[0038] In practice it has been observed that the device according to the invention fully
achieves the intended aim and objects, since it has an actuator which is fully insensitive
to external magnetic fields, thus allowing to avoid untimely interventions: moreover,
its structure is constructively simple and allows to considerably increase the overall
reliability of the device, allowing in particular to have high repeatability of the
opening interventions and correct and precise resetting of said device. Further, the
device can be constituted by a residual-current circuit breaker, as previously described,
or by a residual-current block, i.e. a block which is generally coupled to a thermomagnetic
circuit breaker.
[0039] Attention is also drawn to the fact that all the innovative functions and the inventive
aspects of the device can be achieved by using commonly commercially available elements
and materials with extremely low costs.
[0040] The device thus conceived is susceptible of numerous modifications and variations,
all of which are within the scope of the inventive concept: all the details may also
be replaced with other technically equivalent elements.
[0041] In practice, the materials employed, so long as they are compatible with the specific
use, as well as the dimensions, may be any according to requirements and to the state
of the art.
1. A residual-current device comprising an actuator which is operatively connected to
kinematic means for opening contacts following the detection, by a sensor, of an earth
fault current, characterized in that said actuator comprises a bistable metallic element
which is operatively connected to said kinematic means and to which at least one piezoelectric
element is operatively coupled, said piezoelectric element being excited by an electrical
signal as a consequence of the detection of said fault current and inducing a movement
of the bistable metallic element from a first stable equilibrium position to a second
stable equilibrium position, the metallic element actuating said kinematic means by
way of said movement: said device further comprising reset means which are adapted
to return the bistable metallic element into the first stable equilibrium position
before the contacts close again.
2. The residual-current device according to claim 1, characterized in that said bistable
metallic element comprises a metallic lamina which has two opposite ends which are
fixed to a supporting structure so as to assume said first stable equilibrium position,
two piezoelectric elements arranged in a mutually bimorph configuration being fixed
correspondingly on the two opposite faces of one of said opposite ends.
3. The residual-current device according to claim 2, characterized in that said two opposite
ends are fixed to the supporting structure in mutually staggered positions with respect
to a median plane of the lamina in the non-deformed configuration.
4. The residual-current device according to claim 1, characterized in that said bistable
metallic element comprises a metallic lamina having a first end which is fixed to
a supporting structure and a second end which is opposite to the first end and on
which compression means act, said compression means being suitable to apply an axial
stress to the lamina and to keep it in said first stable equilibrium position.
5. The residual-current device according to claim 4, characterized in that said compression
means comprise an abutment element which is fixed to said second end of the lamina
and on which a loading spring acts, two piezoelectric elements arranged in a mutually
bimorph configuration further acting on said abutment element.
6. The residual-current device according to claim 1, characterized in that said bistable
metallic element comprises a metallic lamina having two opposite ends which are fixed
to a supporting structure which comprises two parts which are mutually connected by
way of the interposition of a loading spring, said loading spring being suitable to
apply an axial stress to the metallic lamina in order to keep it in said first stable
equilibrium position.
7. The residual-current device according to claim 6, characterized in that a first piezoelectric
element and a second piezoelectric element are arranged on a same face of said metallic
lamina and are mutually spaced.
8. The residual-current device according to claim 6, characterized in that two pairs
of piezoelectric elements are fixed on said metallic lamina, each one of said pairs
being constituted by at least two mutually facing and mutually parallel laminae made
of piezoelectric material, the metallic lamina being interposed between them.
9. The residual-current device according to claim 1, characterized in that said bistable
metallic element comprises a metallic lamina which is substantially M-shaped and has
two substantially straight and mutually parallel ends, which are fixed to a supporting
structure, and a curved central portion, on one face of which two piezoelectric elements
are fixed, each piezoelectric element being constituted by one or more laminae made
of piezoelectric material.
10. The residual-current device according to claim 1, characterized in that said bistable
metallic element comprises a substantially M-shaped metallic lamina which has a central
curved portion and two substantially straight and mutually parallel ends which are
fixed to a supporting structure, at least one piezoelectric element being associated
with each one of said straight ends.
11. The residual-current device according to one or more of claims 2 to 10, characterized
in that hinges are provided on the surface of the bistable metallic element.
12. The residual-current device according to claim 11, characterized in that said hinges
comprise shaped notches which are provided on the surface of the bistable metallic
lamina.
13. The residual-current device according to claim 11, characterized in that the bistable
metallic element has a transverse cross-section whose thickness varies along its longitudinal
extension in order to provide said hinges.
14. The residual-current device according to claim 1, characterized in that the reset
means comprise a pin which is operatively associated with a return spring, said pin
having a shaped head which is suitable to interact geometrically with the surface
of said bistable metallic element.
15. The residual-current device according to claim 1, characterized in that the reset
means comprise a cam which is pivoted to a supporting structure and is operatively
connected to a contact closure lever.
16. The residual-current device according to claim 1, characterized in that the reset
means comprise a pin which can slide in tracks formed in a supporting element of the
actuator and has a shaped head which is suitable to interact with the surface of the
bistable metallic element, said tracks containing unidirectional springs for guiding
said pin during the repositioning of the bistable metallic element in the first stable
equilibrium position.
17. The residual-current device according to one or more of the preceding claims characterized
in that it is a residual-current circuit breaker or by a residual-current block.