Method for securing a network, a security system, transmitters and receivers as part of such a system and a lightning protection provided with such a system

The invention is concerned with a method for securing a network of conductors, like a lightning protection, in which in at least a first conductor a first alternating current is induced and in which in at least a second conductor a second alternating current, which is derived form this first alternating current, is measured. The method is based upon the fact that if in a network a branch is removed, the current distribution will change. More in particular this is true for a protection against lightning, in which the several current conductors form a network together with the earth contacts. When part of the conductor is removed, this can be easily detected using the invention and theft of larger parts of the protection system against lightning can be prevented.

A favourable embodiment of the method according to the invention is characterised in that the second alternating current is measured by using a phase sensitive detector, as a result of which not only the amplitude of the measured second alternating current is taken into account when detecting but also the phase of the second alternating current.

A further very favourable embodiment of the method according to the invention, is characterised in that the first alternating current and a reference signal for the phase sensitive detector are derived from a locally available source signal, like the radio signal of a broadcasting transmitter. The advantage of this method is the fact that no connection needs to exist between the component parts of the security system.

The invention is also concerned with a security system for securing a network of conductors, like a lightning protection. In such a network of conductors a large quantity of metal has been included. Besides that the network has been placed on the outside of for instance a building, as a result of which it can be stolen easily. The security system according to the invention allows to check in an easy way for the absence of a part of the conductor and is characterised in that the system comprises at least a transmitter for the induction of an alternating current in a first branch of the network, as well as at least

a receiver for the measurement of an alternating current in a second branch of the network. Herein it is assumed that an alternating current in a branch is distributed over the entire network in a manner which can be determined in advance and that this distribution changes when a part of the conductor is removed.

A favourable and cost effective embodiment of the system is characterised in that the transmitter comprises an oscillator, which is connected to a coil, which coil is wound around a ring core which is made out of a ferromagnetic material, as well as synchronising means for the synchronisation of the oscillator with a locally available source signal, like the radio signal of a broadcasting transmitter. The first branch can then be led through the ring core, in which way a good magnetic coupling can be achieved. Moreover the ferromagnetic material can be chosen such that it can be easily saturated, so the coupling of a current induced by a lightning strike with the transmitter will remain limited.

A very favourable embodiment of the system is characterised in that the receiver comprises a phase sensitive detector, which is connected to a coil, which coil is wound around a ring core, which is made out of a ferromagnetic material, as well as synchronising means for the synchronisation of a reference signal for the phase sensitive detector with a locally available source signal, like the radio signal of a broadcasting transmitter. The second branch can then be led through the ring core, because of which a good magnetic coupling is achieved. Besides this, the ferromagnetic material can be chosen such that it will be saturated easily, because of which the coupling of a current induced by a lightning strike with the transmitter will remain limited.

A further favourable embodiment is characterised in that a receiver is provided with signalling means, for signalling a change of an output signal of the phase sensitive detector. The signalling means can for instance consist of a relay contact which is connected to the existing alarm system.

A favourable alternative embodiment is characterised in that the system has also been provided with a central unit to which two or more receivers can be connected. By means of measuring the current in a number of branches with two or more receivers, it can be established fairly precise where a part of a conductor has disappeared. Also small changes in the magnetic field can be found, which changes have been caused by the movement of large objects in the space which is surrounded by the network of conductors, such as containers or pallets carrying for instance domestic appliances. In that case too an alarm can be produced.

The invention is also concerned with a transmitter or receiver, as a part of a security system of the kind which has been described in the preceding paragraphs.

The invention is also concerned with a lightning protection, provided with a security system of the kind which has been described in the preceding paragraphs.

The invention will here be explained further using the following drawings, in which:

Figure 1 a possible embodiment of a lightning protection provided with a security system against theft is shown;

Figure 2 a possible embodiment of a transmitter is shown;

Figure 3 a possible embodiment of a receiver is shown;

Figure 4 a possible embodiment of a security system provided with a central unit is shown.

In figure 1 is shown a possible embodiment of a protection against lightning 1, which exists of a network 2 of copper conductors which is applied to a flat roof of a building 3 and which is connected with earth plates 4 which are placed in the earth. Such a network 2 comprises a large quantity of copper that can be easily cut loose and taken away by thieves. For that reason, the lightning protection 1 is provided with a security

system against theft according to the invention, in this case existing of a transmitter 5 for the induction of an alternating current in a branch of network 2, as well as a receiver 6 for the measurement of an alternating current in an other branch of the network 2. The alternating current as induced by the transmitter 5 reaches receiver 6 via the several branches and via the several earth connections. The resulting input signal of receiver 6 is a superposition of these several alternating currents, a signal which is substantially constant at least for low frequencies. When one of the branches is cut, then the phase and / or amplitude of the input signal of receiver 6 changes, which according to the invention results in a signal which can for instance be connected to a burglar alarm.

Figure 2 gives a possible embodiment of a transmitter 5. The transmitter 5 consists of an oscillator 7 which generates the actual sending signal and which is connected via a protection network 8 to a coil 9 which is wound on a ferrite core 10 through which a branch of network 2 is led. In the embodiment which is shown here, the transmitter 5 also comprises a supporting receiver 11, which receiver is provided with a receiving antenna 12 and which is tuned in to the DCF77 broadcasting transmitter which is known in the professional field, which broadcasting transmitter transmits on the frequency of 77,5 kHz. The received signal is fed, if so desired via a frequency multiplier or a frequency divider, to a phase-locked-loop 13 which is known in the professional field, in which it is compared to the output signal of the oscillator 7 and in which the phase lock loop 12 generates a regulating signal which is fed to the oscillator 7, in such a way that the frequency of the oscillator 7 will always be equal to a multitude or a partial frequency of the frequency of the broadcasting transmitter DCF77. Because during a lightning strike a very high current may flow through the branch of the network 2, the ferrite core 10 is chosen such that it will be saturated soon, as a result of which the induction voltage induced by the coil 9 remains limited. To protect transmitter 5 against the induction voltage which is induced nevertheless, the network 8 comprises for instance a circuit known as such provided with a spark-gap and some VDR resistors or zener diodes. Finally, transmitter 5 comprises a power supply 14 which can be connected to the mains of the building 3.

It is obvious that supporting receiver 11 can be tuned in to a different sending signal. Also the frequency of the signal which is incited by oscillator 7 can be a multitude of the signal which is received by supporting receiver 11.

In figure 3 a possible embodiment is shown of the receiver 6. Receiver 6 consists of a pre-amplifier 15 which is, via a protection network 8, connected to a coil 9 which has been wound upon a ferrite core 10 through which a branch of the network 2 is lead. The pre-amplifier 15 is connected to a phase-sensitive detector 16, which receives a reference signal from oscillator 7. In the embodiment which is shown here, the receiver 6 comprises a supporting receiver 11 which has been provided with a receiving antenna 12 and which is tuned in to the broadcasting transmitter DCF77 which is known in the professional field, and which broadcasting transmitter transmits on the frequency of 77,5 kHz. The signal which is received will be fed, if so desired through a frequency multiplier or a frequency divider, to a phase-locked-loop 13, which is known in the professional field, where it is compared with the output signal of the oscillator 7 and whereby the phase-locked-loop 12 generates a regulating signal which is fed to the oscillator 7, such that the frequency of the oscillator 7 will at every moment be equal to a multitude or a partial frequency of the frequency of the broadcasting transmitter DCF77. The signal which is produced by the phase sensitive detector 16 is fed to a differential network 17 which will, when a change in the phase and / or amplitude of the produced signal is detected, produce a signal which can be connected to e.g. a burglar alarm. Because a very high current may flow through the branch of network 2 during a lightning strike, the ferrite core 10 is chosen such that it will soon be saturated, as a result of which the induction voltage as created by the coil 9 will remain limited. To protect the receiver 6 against the induction voltage which is created nevertheless, the protection network 8 comprises e.g. a circuit which is known as such, which circuit is provided with a spark-gap and a number of VDR resistors or zener diodes. Finally, the receiver 6 comprises a power supply 14, which can be connected to the mains of the building 3.

In figure 4 is shown a possible embodiment of a security system which has been provided with a central unit 18, which is connected to a monitor 19. In the embodiment

which is shown here, the system comprises a transmitter 5 and four receivers, 6a, 6b, 6c, 6d, which are connected to the network 2 in several locations. When the network 2 is disturbed by the interruption of a branch at a certain location, the central unit 18 is usually able on the basis of the phase differences and amplitude differences as measured by the receivers 6a, 6b, 6c, 6d, to precisely detect which branch has been interrupted and it will be possible to have this information displayed at monitor 19. To calibrate the security system, one could for instance, at the moment of installation, interrupt or seemingly interrupt the several branches by placing a ferrite core around them and record the phase and amplitude differences which are measured. It is also possible of course to simulate this when the dimensions of network 2 and the frequency of transmitter 5 are known. Besides this the central unit 18 can feed a switching signal 20 to a phase-locked-loop 13, as a result of which the frequency of transmitter 5 will double. In this case also the phase and amplitude difference can be measured, so the detection of the location of the interruption can be enhanced further.

When large metal objects are stored in the building 3, these objects will influence the impedance of the several branches of the network 2. This means that receivers 6a, 6b, 6c, 6d can be used for the detection of movements of these objects, as a movement of these objects will result in a phase change of the output signal. The security system could therefore be used with advantage for the guarding the contents of the building 3, especially when the contents consists of large metal objects such as containers or e.g. pallets loaded with household appliances or consumer electronics.