WO2011000838A1

WO2011000838A1 - Motor control center for powering and controlling electric motors

Info

Publication number: WO2011000838A1
Application number: PCT/EP2010/059220
Authority: WO
Inventors: Giampaolo Gonella
Original assignee: Lafer S.R.L.
Priority date: 2009-06-29
Filing date: 2010-06-29
Publication date: 2011-01-06
Also published as: ITVI20090155A1; IT1395971B1

Abstract

The invention is a motor control centre (1) for remotely powering and controlling electric motors, comprising a frame (2) that can be inserted in a supporting structure and is associated with the following: an isolator provided with clamps (4) for connection to a power supply network; a first rod (6) for operating the isolator (3), associated with a first bushing (7) suitable for coupling with means for operating the first rod (6); a circuit breaker (5) electrically connected with the isolator (3) and the respective electric motor; a second rod (9) for operating the circuit breaker (5), associated with a second bushing (10) suitable for coupling with means for operating the second rod (9). It further comprises blocking members (12) for blocking the access of the operating means to the bushings (7, 10) and actuating means (11) cooperating with the blocking members (12) and designed to bring the blocking members (12) into different positions to enable or disable coupling between the operating means (8) and the bushings (7, 10). The operating means comprise a single removable operating key (8) that is at the disposal of the operator and can be coupled with the bushings (7, 10).

Description

MOTOR CONTROL CENTER FOR POWERING AND CONTROLLING ELECTRIC MOTORS

DESCRIPTION

The invention concerns a motor control centre for remotely powering and controlling electric motors applied to machines and/or plants.

It is known that centralized remote systems are used for powering and controlling the motors that operate machines and plants, said systems being usually indicated by the English acronym MCC (Motor Control Centre).

Said centralized systems comprise a plurality of motor control centres for powering and controlling motors, suited to be slidingly inserted in and extracted from a supporting structure that is substantially constituted by a cutout box. Each motor control centre comprises a frame that supports the following: an isolator provided with clamps for its connection to busbars connected to the power supply network, called "vertical omnibus bars" and arranged inside the supporting structure; a circuit breaker electrically connected to the isolator, with the cables supplying power to the motor and to the auxiliary electric devices for controlling the motor branching off the terminals of said isolator.

The front of the frame of each motor control centre gives access to the operating rods of the isolator and of the circuit breaker, said rods being manoeuvred by an operator with manual operating means.

The supporting structure and thus all the motor control centres associated with it are installed in a remote position with respect to the motors that they power and control, and this advantageously allows the maintenance staff to localize immediately the area where intervention is required in case of fault or malfunction of the motors.

In fact, any fault or operating malfunction of a motor is displayed on the corresponding motor control centre, on which the operator intervenes carrying out the following operations:

- opening the circuit breaker;

- opening the isolator;

- extracting the faulty motor control centre from the supporting structure;

- replacing the faulty motor control centre with a spare one;

- closing the isolator;

- closing the circuit breaker.

In this way the operation of the motor and therefore of the plant or machine to which the motor belongs is restored almost immediately and takes place after a very short interruption that substantially corresponds to the time needed to replace the faulty motor control centre.

The faulty motor control centre can be checked at a laboratory and if possible repaired at a later moment.

The market offers motor control centres of the type described above that are made according to various construction types.

A known embodiment of said motor control centres comprises operating members consisting of keys or levers manoeuvred by an operator that are always coupled with the corresponding isolator and circuit breaker, and also comprises mechanical blocks that allow them to be operated only according to the sequence described above.

A first drawback posed by said known motor control centres is constituted by the fact that since the operating members can be always accessed for manoeuvring, if for any reason the mechanical blocks should break down, there is the risk that the circuit breakers and the isolators may be manoeuvred with wrong sequences, causing serious damage to the system and the operators.

Several systems are known which prevent the simultaneous actuation of different functional modules in switch gears, as for example the one described in DE 10 2006 012 056 B3. But they do not provide for systems which in case of the breakdown of the mechanical blocks avoid the actuation of circuit breakers and isolators with a wrong sequence.

Another drawback is represented by the fact that the operating members are frequently provided with operating mechanisms with lever systems that are complex to carry out.

A further drawback is also represented by the fact that said lever mechanisms, in general, comprise mechanical elements made of stamped and sheared sheet and/or processed with a machine tool, and consequently involve high construction and assembly costs.

Furthermore, another recognized and non-negligible drawback posed by the known motor control centres is represented by the lack of safety devices suited to prevent the unauthorized operation of the isolator and circuit breaker after they have been intentionally disconnected, for example in order to carry out maintenance operations. The present invention intends to overcome the limitations and drawbacks described above.

In particular, it is a first object of the invention to provide a motor control centre for remotely powering and controlling electric motors that compared to the motor control centres of known type guarantees more safety in the operations carried out to open and close the circuit breaker and the isolator.

It is another object of the invention to provide a motor control centre that also offers a higher degree of safety against possible mechanical faults that may affect its correct operation,

It is a further object of the invention to provide a motor control centre that is equipped with safety devices capable of preventing the unauthorized connection of the motor control centre to the power supply network after it has been disconnected.

It is another, yet not the least object of the invention to provide a motor control centre that is less expensive to produce compared to the motor control centres of known type.

The objects listed above are achieved by a motor control centre for remotely powering and controlling electric motors whose characteristics are described in the main claim, to which the reader should refer for the sake of brevity.

Other details of the invention are described in the dependent claims.

The motor control centre that is the subject of the invention is provided with a single removable operating key that is alternatively coupled with the circuit breaker and the isolator according to sequences that are imposed by different positions assumed by blocking members.

The motor control centre comprises also a safety unit that can be operated by an operator and prevents the unauthorized closing of the isolator and the circuit breaker after they have been opened, for example in order to carry out maintenance operations.

Advantageously, the motor control centre of the invention guarantees that the manoeuvres for connecting and disconnecting the motor power supply are carried out in safer conditions than in the motor control centres of known type.

Still advantageously, the motor control centre of the invention guarantees safer operating conditions also during maintenance operations.

The objects and advantages described above will be highlighted in greater detail in the description of an embodiment of the invention that is supplied as an indicative, non-limiting example, with reference to the enclosed drawings, wherein:

- Figure 1 shows an axonomethc view of the motor control centre that is the subject of the invention;

- Figure 2 shows the motor control centre of the invention shown in Figure 1 without some structural elements;

- Figure 3 shows the motor control centre of the invention inserted in the supporting structure that accommodates it;

- Figure 4 shows another axonomethc view of the motor control centre of Figure 3 without some of its constructive parts;

- Figures 5 and 6 show some details of Figure 4;

- Figure 7 shows another axonomethc view of the motor control centre of Figure 4 in a different operating position;

- Figures 8 and 9 show some details of Figure 7;

- Figure 10 shows another axonomethc view of the motor control centre of Figure 2 in a further operating position;

- Figure 1 1 shows an axonomethc view of a detail of Figure 10;

- Figure 12 shows another axonomethc view of the motor control centre of Figure 10 with the operating means associated therewith;

- Figure 13 shows a detail of the operating means of Figure 12;

- Figures from 14 to 16 show some details of Figure 12;

- Figure 17 shows another axonomethc view of the motor control centre of Figure 12 arranged in a different operating position;

- Figure 18 shows another axonomethc view of the motor control centre of Figure 17;

- Figure 19 shows another axonomethc view of the motor control centre of Figures 17 and 18 arranged in a different operating position;

- Figure 20 shows another axonomethc view of the motor control centre of Figure 19 arranged in a further different operating position;

- Figure 21 shows another axonomethc view of the motor control centre of Figure 20;

- Figure 22 shows a detail of Figure 21 ;

- Figure 23 shows a detail of the motor control centre of Figure 2;

- Figure 24 shows another axonomethc view of the motor control centre of Figure 2 in a different operating position; - Figure 25 shows a detail of Figure 24;

- Figure 26 shows another axonometric view of the motor control centre of Figure 24 provided with the front cover.

The motor control centre for remotely powering and controlling electric motors that is the subject of the invention is represented in different axonometric views in the figures from 1 to 3 and the following, where it is indicated as a whole by 1.

It can be observed, also with reference to Figures 4 and 7, that it comprises a frame 2 with an isolator 3 and a circuit breaker 5 associated therewith, said frame being suited to be removably and slidingly connected to a supporting structure, not illustrated in the figures, only one of the vertical uprights M of which is visible in Figure 3.

The isolator 3 is provided with connection clamps or busbars 4, not illustrated herein, connected to a power supply network, while the circuit breaker 5 is electrically connected, via conductors that are not illustrated in the figures, both to the isolator 3 and to the corresponding electric motor, arranged in a remote position.

The isolator 3 is provided with a first operating rod 6, visible in greater detail in

Figure 4, which is associated with a first bushing 7 suitable for coupling with operating means of said first operating rod 6.

Analogously, the circuit breaker 5 is provided with a second operating rod 9 that is associated with a second bushing 10 suitable for coupling with operating means suited to operate the second operating rod 9.

According to the invention, the motor control centre 1 comprises blocking members 12 for blocking the access of said operating means to the bushings 7, 10 and actuating means 11 designed to bring said blocking members 12 into different positions to enable or disable coupling between the operating means 8 and the bushings 7, 10, said operating means comprising a single removable operating key 8 at the disposal of the operator, provided with a shaped profile 8a matching a shaped counter profile 6a, 9a present in both the bushings 7, 10.

The shaped profile 8a of the operating key 8 can be seen in Figures 15 and

16, and the shaped counter profile 6a, 9a of the bushings 7, 10 can be seen in

Figures 14, 17 and 19.

Regarding in particular the actuating means 11 , with reference to Figures 2, 3, 10 and 12 it is possible to observe that they comprise the following:

- cam means 13 suited to define different positions of the blocking means 12, so as to allow different coupling conditions between the operating key 8 and one or both of the bushings 7, 10;

- lever mechanisms 14 suited to define a position in which the blocking members 12 prevent the coupling between the operating key 8 and both of the bushings 6, 9.

Specifically, the cam means 13 define the following positions:

a first position A, illustrated in Figures 10 and 12, in which the blocking members 12 allow the coupling of the operating key 8 only with the first bushing 7 so as to operate only the isolator 3;

- a second position B, illustrated in Figures 17, 18 and 19, in which the blocking members 12 allow the coupling of the operating key 8 with both of the bushings 7, 10 so as to operate the isolator 3 or the circuit breaker s;

- a third position C, illustrated in Figures 20 and 21 , where for the sake of clarity only one of the blocking members 12 is shown, in which the latter allow the coupling of the operating key 8 only with the second bushing 10 so as to operate only the circuit breaker 5.

In particular, it can be observed in Figures 12, 13 and 14 that the cam means 13 comprise a first cam 7a associated with the first bushing 7 and placed in contact with a first blocking member 17 suited to block access of the operating key 8 to the second bushing 10 and a second cam 10a associated with the second bushing 10 and placed in contact with a second blocking member 20 suited to block access of the operating key 8 to the first bushing 7. Each one of said first and second blocking members 17, 20 comprises a shaped rod that, as shown also in Figures 10, 12, 17 and 19, is provided with the following:

in an intermediate position, with a pivotal pin, not illustrated herein and defining a rotation axis, respectively X and Y;

at one end, with a shutter, respectively 17a, 20a, suited to cooperate with a corresponding bushing 7, 10;

at the opposite end, with a contact surface, respectively 17b and 20b, suited to cooperate with a corresponding cam 7a, 10a.

Regarding the lever mechanisms 14, it can be observed that they comprise the following:

an arming lever 40 , visible in particular in the Figures from 4 to 7 and provided with a pivotal pin 40a;

a sliding rod 50 that can be mechanically coupled, via engaging/releasing means 60, to the arming lever 40 provided with a projection 51 that cooperates with the second member 20 for blocking access of the operating key 8 to the first bushing 7;

a safety unit 70, visible in particular in Figure 20 and in Figures from 23 to

25, for locking the second blocking member 20 in the fourth position D shown in Figures 3 and 24, in which the blocking members 12 prevent the operating key 8 from being coupled with both of the bushings 7, 10.

The arming lever 40, as shown in particular in Figures 3, 4, 5 and 7, comprises a first arm 41 that can be removably coupled with the sliding rod 50 via the engaging/releasing means 60, and a second arm 42, arranged in angled position with respect to the first arm 41 and counteracting the upright M of the supporting structure so as to set the arming lever 40 rotating around its pivotal pin 40a when the frame 2 is inserted in the supporting structure.

In the engaging/releasing means 60 it is possible to identify, in particular, with reference to the Figures from 7 to 1 1 :

- the engaging means 61 comprising a slot 62 provided in the second arm 42 of the arming lever40 and a pin 63 provided at one end of the sliding rod 50 and suited to be accommodated in the slot 62;

the releasing means 64 comprising a shaped body 65 provided with operating means 66 that can be accessed by an operator, in which it is possible to identify a sloping plane 67 designed to lift the sliding rod 50 in order to separate the pin 63 from the corresponding slot 62.

Regarding, finally, the safety unit 70, it can be noted (Figures 23 - 25) that it comprises the following:

a revolving pin 71 provided with a cut 72 for rotation by means of a screwdriver;

a counteracting rod 73 that projects radially from the revolving pin 71 to cooperate by contact with the second blocking member 20;

means 74 for locking the revolving pin 71 when the counteracting rod 73 cooperates with the second blocking member 20 arranged in the fourth position D. In particular, the locking means 74 comprise a lock 74a that is inserted in a hole 75 made in a projection 76 belonging to the revolving pin 71.

It is clear that the locking means may also be of another type, provided that they are suited to prevent the rotation of the revolving pin 71 and are equipped with a key or another removable operating member.

Finally, a front closing panel 80, visible in Figure 1 , protects the internal parts of the motor control centre 1 while the handles 81 applied to it make it possible to easily move the motor control centre for inserting and/or extracting it in/from the supporting structure that houses it.

In practice, when the motor control centre 1 is in the rest position, its configuration is as shown in Figure 2, with the connection clamps 4 of the isolator 3 arranged in a withdrawn position and the blocking members 12 arranged in the first position A.

This with reference to Figures 2 and 12, in which the position of the first blocking member 17 makes it possible to couple the operating key 8 only with the first bushing 7 in order to operate only the isolator 3.

When the motor control centre 1 is slidingly inserted in the supporting structure according to the direction shown by the arrow S1 visible in Figure 3, the first arm 41 of the arming lever 40 counteracts the upright M of the supporting structure and causes the counterclockwise rotation of the arming lever 40 around the pivotal pin 40a as indicated by the arrow R1 according to the vertical axis Z.

During rotation, the second arm 42 of the arming lever 40, being normally coupled with the sliding rod 50 via the engaging means 61 , as shown in Figure 5, causes the shift of the sliding rod 50 in the direction indicated by the arrow S2 in Figure 4, made elastic by the interposition of the spring 90 that can be observed in Figure 7 and in the detail of Figure 9.

During the shift in the direction S2, the projection 51 of the sliding rod 50 that can be observed in Figures 4, 6 and 7, cooperates with the second blocking member 20, as can be observed in Figure 3, and lifts it in the direction indicated by the arrow S3 until arranging it in the configuration in which the shutter 20a with which it is provided blocks access to the first bushing 7.

In this position access to both of the bushings 7, 10 is prevented and therefore neither the isolator 3 nor the circuit breaker 5 can be operated.

In this configuration, the electric/electronic system checks the good operating conditions of the equipment, the check being activated by the movement of the sliding rod 50 when the end projection 50a with which it is provided, and which is visible in Figures 4 and 6, cooperates with a microswitch, not shown herein.

The motor control centrei is thus arranged in the fourth position D shown in Figures 3 and 24, in which neither the isolator 3 nor the circuit breaker 5 can be operated.

When the sequence of checks has been completed, the motor control centre 1 can be connected to the motor.

For this purpose, as first thing the operator intervenes on the means 61 for releasing the arming lever 40 from the sliding rod 50, and in particular by acting from the outside on the operating means 66 of the shaped body 65 pushes it towards the inside of the motor control centre, as indicated by the arrow S4 shown in Figure 4.

In this way the sloping plane 67 belonging to the shaped body 65 causes the lifting of the sliding rod 50 in the direction indicated by the arrow S5, as shown in Figure 7 and, in particular, in Figure 8.

In this way the pin 63 present at the end of the sliding rod 50 is separated from the slot 62 present in the second arm 42 of the arming lever 40, and they assume the configuration shown in Figures 7 and 8.

The elastic recovery of the spring 90 makes the sliding rod 50 return to its initial position, shifting it in the direction indicated by the arrow S6, opposite the direction S2, as shown in Figure 7.

The second blocking member 20, not supported any longer by the projection 51 of the sliding rod 50, moves downwards in the direction indicated by the arrow S7 in Figure 10, through rotation around the axis Y in the direction indicated by the arrow R2, allowing access to the first bushing 7, as shown in fact in Figure 10.

Substantially, therefore, after the electric check, the motor control centre 1 is inserted in the supporting structure and arranged in the configuration of Figure 10, equal to the initial rest configuration shown in Figure 2, which corresponds to the first position A in which it is possible to operate the isolator 3.

The operator inserts the only operating key 8 in the first bushing 7, as shown in

Figures 12 and 13, and rotates it clockwise as indicated by the arrow R3, making it perform a 180 " clockwise rotation.

During rotation, the first cam 7a associated with the first bushing 7 slides on the contact surface 17b of the first blocking member 17, as shown in Figure

14, until assuming the configuration of Figure 17, in which the first cam 7a does not touch the contact surface 17b any more.

The first blocking member 17 moves downwards in the direction indicated by the arrow S8 through the clockwise rotation indicated by the arrow R4 around the axis X so that the shutter 7a enables the access of the operating key 8 to the second bushing 10, as shown in Figure 17.

Said configuration shown in Figure 17 corresponds to the second position B in which access to both of the bushings 7 and 10 is possible,

The operator then extracts the operating key 8 of the first bushing 7 and inserts it, as shown in Figure 19, in the second bushing 10 in order to operate the circuit breaker s.

For this purpose the operator rotates the operating key 8 90° clockwise as indicated by the arrow R5, until arranging it in the configuration shown in Figure 20, in which the second cam 10a, associated with the second bushing 10, cooperates with the contact surface 20b of the second blocking member 20 and pushes it downwards in the direction indicated by the arrow S9.

The clockwise rotation R5 of the second blocking member 20 around the axis Y is thus obtained, so as to arrange the shutter 20a in front of the first bushing 7.

The third position C shown in Figures 20 and 21 is thus reached, in which when the circuit breaker 5 is closed the motor is powered and starts rotating.

In this position it is not possible to operate the isolator 3, since access to the first bushing 7 and therefore to the first operating rod 6 is prevented, as shown in fact in Figures 20 and 21.

In order to disconnect the motor from the power supply, the operator must insert the operating key 8 again in the second bushing 10, as shown in

Figures 20 and 21 , rotate it 90° anticlockwise, opposite the direction of the arrow R5 shown in Figure 19, so as to open the circuit breaker 5 and reposition the blocking members 12, and in particular the second blocking member 20, again in the configuration of Figure 19.

In this position access to the first bushing 7 is possible again and it is also possible to open the isolator 3.

For this purpose, the operator extracts the operating key 8 from the second bushing 10, inserts it in the first bushing 7 and by rotating it 180° counterclockwise opens the isolator 3 and arranges the first blocking member 17 with the shutter 17b at the level of the second bushing 10 that is thus kept closed, preventing access to the circuit breaker 5.

The initial configuration shown in Figure 10 is thus restored, in which access to the first bushing 7 is allowed in order to make it possible to carry out the successive closing of the isolator 3, according to the sequence already described above.

At this point it should be noted that the motor control centre that is the subject of the invention achieves the first of the set objects.

In fact, the use of a single removable operating key absolutely prevents any manoeuvre for opening or closing the isolator and the circuit breaker when the key is disconnected.

On the contrary, in the known motor control centres having the operating means always coupled with the circuit breaker and/or the isolator, said operating means may be manoeuvred with wrong sequences in case of failure of the mechanical blocks that guarantee the correct operating sequence.

A further improvement in safety is due to the fact that it is the same single operating key that during rotation arranges the blocking members in the positions suited to guarantee the correct operating sequence of the circuit breaker and of the isolator.

Therefore, the correctness of the operating sequence is guaranteed by the proper coupling of the operating key with the circuit breaker or the isolator. The safety unit 70 with which the motor control centre 1 of the invention is provided protects the system and people from any accident that may occur when the motor control centre 1 is disconnected from the motor, for example to carry out maintenance operations.

In fact, when the operator intervenes in order to carry out maintenance operations on the motor, he/she often leaves the motor control centre inserted in the supporting structure and disconnects only the motor power supply, operating, in order to open them, first the circuit breaker 5 and then the isolator 3, and arranging the blocking members 12 in the configuration shown in Figure 10.

Accidentally, while the operator works on the motor from a remote position, another operator in charge with the system, noticing that the motor control centre is not connected to the motor, might carry out the connection operations closing, in sequence, first the isolator 3 and then the circuit breaker 5, with risk of electrocution for the maintenance man who is working on the motor.

In order to avoid the above, the safety unit 70 is structured so that after opening the circuit breaker 5 and the isolator 3 the maintenance man turns the revolving pin 71 with the screwdriver, so that the counteracting rod 73 passes from the position shown in Figure 23 to the position shown in Figure 24, in which it lifts the second blocking member 20 bringing the shutter 20a before the first bushing 7.

The final situation is the one shown in Figure 24, in which neither the isolator 3 nor the circuit breaker 5 can be operated.

In order to make sure that this position is maintained, the arc 74b of the lock 74a is inserted in the hole 75 provided in the projection 76 of the revolving pin 71 , so that it can be accessed from the slot 82 made in the front closing panel 80, as shown in Figure 26.

After carrying out these operations, the maintenance man removes the key 74c.

In this way, the impossibility to rotate the revolving pin 71 prevents the closing of the isolator 3 and the circuit breaker 5.

Only the maintenance man who is in possession of the key 74c, once having completed the operation and removed the lock, can restore the power supply to the motor.

It can be understood that this allows the motor control centre that is the subject of the invention to achieve also the object to increase safety during maintenance operations, which is not guaranteed by the known motor control centres.

Finally, as shown in the figures, all the elements that make up the motor control centre of the invention, and in particular the actuating means, the bushings and the blocking members, can be obtained from a plastic moulding process, with a mainly economical advantage compared to the metal parts of the known art.

On the basis of what has been described and illustrated herein, it is clear that the motor control centre that is the subject of the invention achieves all the set objects.

In the construction phase, changes or construction variants may be implemented which, although not represented or described herein, must all be considered protected by the present patent, provided that they fall within the scope of the following claims.

Where technical features mentioned in any claim are followed by reference signs, those reference sings have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the protection of each element identified by way of example by such reference signs.

Claims

1 ) A motor control centre (1 ) for remotely powering and controlling electric motors, comprising a frame (2) suitable for being inserted in a supporting structure, associated with:

- an isolator fitted with clamps (4) for its connection to busbars connected to a power supply network;

- a first rod (6) for operating the isolator (3), associated with a first bushing (7) suitable for coupling with operating means (8) suited to operate the first rod (6);

- a circuit breaker (5) electrically connected to the isolator (3) and to the corresponding electric motor;

- a second rod (9) for operating the circuit breaker (5), associated with a second bushing (10) suitable for coupling with operating means (8) suited to operate the second rod (9),

characterised in that it comprises blocking members (12) for blocking the access of said operating means to the bushings (7, 10) and actuating means (1 1 ) cooperating with said blocking members (12) and designed to bring said blocking members (12) into different positions to enable or disable coupling between the operating means (8) and the bushings (7, 10), said operating means comprising a single removable operating key (8) for use by the operator, with a shaped profile (8a) suitable for coupling with a shaped counter profile (6a, 9a) in both the bushings (7, 10).

2) A motor control centre (1 ) according to claim 1 ), characterised in that the actuating means (1 1 ) comprise cam means (13) defining:

- a first position (A) wherein the blocking members (12) only enable the coupling of the operating key (8) with the first bushing (7) in order to operate the isolator alone (3);

- a second position (B) wherein the blocking members (12) enable the coupling of the operating key (8) with both the bushings (7, 10) in order to operate the isolator (3) or the circuit breaker (5);

- a third position (C) wherein the blocking members (12) only enable the coupling of the operating key (8) with the second bushing (10) in order to operate the circuit breaker alone (5).

3) A motor control centre (1 ) according to claim 1 ) or 2), characterised in that the actuating means (1 1 ) comprise lever mechanisms (14) suitable for defining:

- a fourth position (D) wherein the blocking members (12) prevent the coupling of the operating key (8) with either of the bushings (7, 10).

4) A motor control centre (1 ) according to any of the claims 2) or 3) characterised in that the blocking members are a pair of a first blocking member and a second blocking member (17, 20) and in that the cam means (13) comprise:

- a first cam (7a) associated with the first bushing (7) and positioned in contact with the first blocking member (17) for enabling or disabling the access of the operating key (8) to the second bushing (10);

- a second cam (10a) associated with the second bushing (10) and positioned in contact with the second blocking member (20) for enabling or disabling the access of the operating key (8) to the first bushing (7), whereby coupling of one of the bushings (7, 10) with the operating key (8) and operating the corresponding rod (6, 9) activate the associated cam which, cooperating with the corresponding blocking member, changes the state of accessibility of the other bushing.

5) A motor control centre (1 ) according to claim 4), characterised in that each of the aforesaid blocking members (12; 17; 20) comprises a shaped rod with:

- a pivotal pin identifying an axis of rotation (X; Y) in an intermediate position;

- a shutter (17b, 20b) designed to cooperate with the corresponding bushing (7, 10) at one end;

- a contact surface (17b, 20b) designed to cooperate with the corresponding cam (7a, 10a) at the other end.

6) A motor control centre (1 ) according to claims 3) and 4) or 5), characterised in that the lever mechanisms (1 1 ) comprise:

- an arming lever (40) with a pivotal pin (40a) identifying an axis of rotation

(Z);

- a sliding rod (50) suitable for coupling mechanically, by means of engaging and releasing means (60; 61 ; 64), to the arming lever (40) that cooperates with the second blocking member for blocking the access of the operating key (8) to the first bushing (7);

- a safety unit (70) for stopping the second blocking member (20).

7) A motor control centre (1 ) according to claim 6), characterised in that the arming lever (40) comprises:

- a first arm (41 ) suitable for removably coupling with the sliding rod (50) by means of the engaging and releasing means (60);

- a second arm (42) that contrasts with the supporting structure to induce a rotation of the arming lever (40) when the frame (2) is inserted in the supporting structure.

8) A motor control centre (1 ) according to claim 7), characterised in that the engaging and releasing means (60) comprise engaging means (61 ) defined by:

- a slot (62) provided on the second arm (42) of the arming lever (40);

- a pin (63) provided at one end of the sliding rod (50) designed to be contained inside the slot (62).

9) A motor control centre (1 ) according to claim 7) or 8), characterised in that the engaging and releasing means (60) comprise releasing means (64) defined by:

- a shaped body (65) with operating means (66) accessible to the operator;

- a sloping plane (67) forming part of the shaped body (65) and designed to lift the sliding rod (50) in order to separate the pin (63) from the corresponding slot (62).

10) A motor control centre (1 ) according to claim 6), characterised in that the safety unit (70) comprises:

- a revolving pin (71 ) with a notch (72) to enable it to be turned with the aid of a screwdriver;

- a counteracting rod (73) extending radially from the revolving pin (71 ) to cooperate by contact with the second blocking member (20);

- means (74) for blocking the revolving pin (71 ) when the counteracting rod (73) cooperates with the second blocking member (20).