The present invention relates to a metal grate, usually made of steel, which contains a series of incorporates devices, which, in the case of tampering, will activate the alarm system being connected to. them. One is reminded that grates, usually built of steel and in different shapes, are barriers being secured in the openings of walls, such as windows, or transition areas between one room and the next, in order to permanently prevent people from entering into reserved or forbidden rooms, or, as in the case of jails, in order to prevent escapes through the openings being protected by these grates.
The grates are firmly secured in the walls by means of cementing or by means of similar systems and they constitute in themselves a barrier of the passive type, that is, they oppose by means of their structure and their type of anchorage possible tampering or attempted force.
Grates equipped with an electric alarm system which is triggered when the bars constituting the grate are sawn , are also known. For instance, US-A-2,374,139, describes a grate constituted by bars, each consisting of an internal and an external tube being insulated the one from the other and being part of an electric circuit. When the bars are sawn, the saw causes the electric contact between the internal tube and the external one, thereby triggering the alarm signal. In actuality this system is effective only when the bars are sawn, while the alarm may not be activated when the grate undergoes tamperings of a different nature, such as the spreading apart of the bars or the partial tearing of the grate off the wall.
Also, in DE-A-300 6999 a particular disposition of electric connections is described, which are suited for fencing safety systems, where the wires of the network belonging to a Wheatston bridge are connected to more copper wires, so as to avoid the possibility that the alarm may not be triggered if a bridge between the two wires is built.
The object of the present invention is that of realizing a grate, which, besides being a barrier of the passive type, is also an active means which prevents, becasuse of its constitution, any type of tampering, whether represented by the sawing of the bars, which constitute it, by their being forced apart, or by the tearing of the grate away from the anchoring wall.
This object is achieved by a grate having the features defined in claim 1.
Basically, there is provided a grate with an incorporated alarm system and it includes a metal structure consisting in hollow bars being welded to each other; inside said bars and even inside the anchoring of the grate there are electric conductors which run longitudinally and transversally in the structure and are connected to an alarm system. According to the invention, each bar and each anchoring unit is strung with one or more pairs of conductors, one end of which is connected to a load resistance placed inside the structure, while the other end of the conductors is connected to an electronic balancing circuit, which detects any variation of resistance in the circuit. Therefore, whenever there is tampering at any point of the grate - which may consist in the sawing of the bars, in the drilling, in flame cutting, in the tearing from the wall or from the anchoring frame, in the forcing apart - a variation of resistance occurs, which may be due, for example, to a short-circuit or to an interruption of the circuit: this variation, detected by a balancing circuit of the resistance being applied to it, will cause in the circuit itself the opening of a relay which is connected to the burglar alarm station and will, therefore, activate the alarm. In order to increase the reliability of the functioning of the grate circuits, it was deemed advisable to introduce into it two circuits at least, one independent from the other, with equivalent functions and based on the same functional principle and on the same construction method. Particular attention has been paid to the solution of the problem concerning the pulling off of the grate, that is, the complete removal of the same from the anchoring wall or frame. For this reason at least one pair of anchorages and a pivot, preferably in a diagonal position, which goes completely through the wall and is anchored by means of an iron loop or cross on the other side of the grate, have been introduced. The head of this pivot is provided with a loop, through which the wires of the electric circuit or circuits are strung, so that a possible removal from the wall of the anchorage in which the pivot is inserted will also cause the tearing of the electric circuit, with the ensuing activation of the alarm.
Another interesting aspect of the grate being discussed is that, because of the way in which it activates the alarm, false alarms due to accidental causes are practically impossible; in fact, if there is no alteration of the electric circuit in the grate, caused by the above-described causes, there will be no alarm.
These and other characteristics and construction details will be better illustrated in the description of a preferred form of execution of the grate, which is given by way of indication, but which is not limiting and which is illustrated in the enclosed drawings, in which:
With reference to Fig. 1, grate 1 is built of bars of structural steel, welded and hollow inside, so as to allow the passage of two pairs of electrical wires 2, 3 and 4, 5, each of which constitutes a closed circuit on a resistance, as shown in 6.
Fig. 1 shows the route of the wires, which can be strung in any direction, even going through the same section several times, although it should be kept in mind that no area, including the anchorage, should be left without wiring.
In the case being under examination there are 4 anchorages and each of them contains circuit structures and characteristics, which are different from one another.
With reference to anchorage 7 (Fig. 2), tube 8, made of plastic material, is placed inside of it; this tube goes all the way through the wall and serves as a channel for the introduction in the grate of the conductors being connected to the alarm station. It is blocked inside anchorage 7 by block 9, made of a gummy plastic material, which contacts the inside walls of the above-mentioned anchorage.
In Fig. 3, anchorage 10 holds in its interior pivot 11, made of iron, which goes all the way through the wall and which has a threaded end 12 protruding from the anchorage. Plate 13 is screwed to protruding end 12 and then cemented in the wall. Pivot 11 is provided at the other end with loop 14, through which the electric conductors are strung. Two bolts, 15 and 16, prevent the rotation of pivot 11 while plate 13 is being fixed with screws. The anti-tear function of the pivot is obvious: in fact, in the case of an attempt to tear anchorage 10 from the wall, one also tears the conductors which are strung through the loop.
In Fig. 4, inside anchorage 17, there is a block, made of plastic material, 18, which allows conductors 2, 3 and 4, 5 to wind around and continue to spread themselves in the horizontal and vertical sections of the grate. In Fig. 5, the inside of anchorage 19 also holds a block made of plastic material, for the reversing of the direction of the circuit wires, but inside the anchorage there is also the load resistance of one of the two circuits.
Conductors 2, 3 and 4, 5 are normal, insulated electric conductors, but with a very small section, so that, because of their length, they constitute in themselves a resistive load, in addition to the load resistance. Each circuit is fed with low tension (12 Volts), so that possible contacts between the grate and the conductors can not be dangerous to people.
Fig. 6 is a sketch of the circuit in blocks which connects the conductors contained in the grate to the alarm station. Resistances R1 and R2 represent respectively the resistive load of the two circuits 2, and 4, 5. Each of the two resistances is connected with a balancing circuit of a known type, which includes trimmer 20 having the task of balancing the connected load resistance, so that a transistorized circuit maintains relay 21 normally closed.
Each variation of the value of resistance R1 or R2 is felt by the balancing circuit which, consequently, causes the opening of relay 21, which is connected to alarm station 22. The electric opening of relay 21 will cause the activation of the alarm.
In the specific case being described, two circuits have been foreseen. They are introduced into the grate, are independent from one another and are . represented by resistances R1 and R2, but it is obvious that a single circuit is sufficient for the functioning of the alarm system; on the other hand, it is also possible, for security reasons, to increase the number of circuits beyond two.