EP1197936B2 - Alarm system - Google Patents

Abstract

The system has warning devices and possibly other line elements that respond to at least one hazard criterion connected to a two-wire line and a central station with stored warning device addresses and a program for monitoring warning device states. A test circuit checks the operating states of the network using a test unit. A test processor has evaluation software and a switch arrangement for selective connection of the test unit(s) to the line. The system has a number of warning devices (M1-Mn) and possibly other line elements that respond to at least one hazard criterion and are connected to a two-wire line (Linie A) and a central station connected to the line with stored warning device addresses and a program for monitoring warning device states. A test circuit (10) in the central station checks the operating states of the network using a test unit and contains a test processor (20,22) with evaluation software and a switch arrangement (30,33) for selective connection of the test unit(s) to the line. Independent claims are also included for the following: a method of measuring the resistance of sections or lengths of line in hazard warning systems and a method detecting a short circuit between the line and a screen.

Description

The invention relates to a hazard alarm system according to the preamble of patent claim 1.

Danger alarm systems, for example fire alarm systems, usually have a larger number of Gefaluenmeldern, which are connected to a two-wire signal line. This can be designed as a branch line or as a loop, via which the individual detectors communicate with a control center. Each detector has a sensor or the like which produces measured values in dependence on parameters of its environment. The measured values are transmitted via the line to the control center, which usually polls the individual detectors cyclically. In order to be able to assign the measured values to the individual detectors, it is necessary to assign an identifier or an address to each detector. The address is stored in a non-volatile memory of the detector. The message addresses are stored in the processor of the control center so that the control center can monitor the individual detectors with the aid of a suitable program.

The installation and commissioning of such a hazard alarm system is associated with a considerable effort. Frequently, the installation work is transferred to companies that can not be described as specialist companies for such systems. The commissioning of such a signaling system is, however, usually by specially trained personnel.

For the reasons mentioned, there is the necessity of detecting and identifying faults and malfunctions which occur as a result of faulty installation as soon as possible before commissioning, but at the latest during commissioning.

It is known to provide separate test circuit arrangements which are connected to the signaling line, for example for checking short-circuit faults or reverse polarity of lines. It is off DE 29611600 U1 an arrangement for testing an alarm system is known in which a plurality of speakers is connected to a central control device. Via a third line, a test unit provided with an address for each loudspeaker is connected to the control device for checking the loudspeakers and their lines.

The invention has for its object to provide a hazard alarm system in which a variety of errors can be detected and located in a simple manner, the cost of the test circuit and the cost of measurement are minimal.

This object is solved by the features of patent claim 1.

In the hazard alarm system according to the invention, a test circuit arrangement is provided which is part of the control center and, for example, checks the operational state of the network of the hazard alarm system on a special command of the central control processor. This is done with the help of at least one test unit that contains its own test processor in which a test program is stored. In addition, a controlled by the test processor switch assembly is provided for selectively connecting the at least one test unit with the reporting line.

In the hazard alarm system according to the invention, the measuring means for checking the operable state of the alarm system are integrated into the central office, so that in connection with an intelligent evaluation software installation errors can be detected quickly and effectively.

Frequent faults in alarm systems include polarity reversal of the wires, exceeding of permissible cable lengths, short-circuiting or touching of wires or shields as well as permutation of detector types and deviations from the installation plan as well as changes in contact resistance.

For such errors, in each case a special test unit can be provided in the test circuit arrangement, wherein all test units are connected to a test processor. However, this can be provided redundantly.

According to one embodiment of the invention, the test circuit arrangement is designed as a module, for example in the form of a plug-in card, on which all components of the test circuit arrangement are arranged.

According to one embodiment of the invention, the test circuit arrangement has a modem connection for checking the network via a remote connection. This can take place over the telephone network, for example. With the help of such a possibility, the check can be started from a remote location, for example the place of manufacture of the hazard alarm system. The results obtained during the check, in particular the errors found, can then be read out and transmitted via the remote connection to the remote location. In this way, for example, installation errors can be detected and remedied even before the final commissioning or acceptance of the hazard alarm system.

It often happens that too long cable lengths are used when installing a hazard alarm system. This can result in the transmission of signals on the line being weakened or disturbed, so that proper operation is no longer guaranteed. The test unit according to the invention for the detection of impermissibly large cable lengths provides a constant current source, which is connected via a modulator and a controllable switch to the line. Using a data word generated by the test processor via a modulator and also containing the address of a detector, a detector can be controlled and a cross-switch can be made to connect the wires of the line. The constant current source limits the current on the line to a predetermined value, and a voltage measuring device can measure the total voltage drop across the shorted section of the line. As the voltage drops are known by the detectors located in the section , the voltage drop caused by the lines results from the difference of the measured voltage drop and the sum of the voltage drops at the detectors of the measured section and, if necessary, a measuring resistor over which the constant current flows to ground. If the voltage drop, which is determined solely by the line length, is known, the resistance of the line length can be determined, because the cross section of the line is known. From the determined in this way resistance for the lines of the measured section can therefore also determine the length of the measured section. In this way, the total length of a line can be determined. It is also possible in the manner described above to determine the length of line sections between selected detectors by sequentially closing the cross-switches in the detectors delimiting the line section.

The data word for controlling the individual detectors and for closing the cross-switches is preferably voltage-modulated according to an embodiment of the invention. In the detector is usually a logic circuit and a demodulator, so that the selected or addressed detector detects when it is given a command to close the cross switch. It can also be provided a timer, which opens after a predetermined time, the cross switch again, the line length can be established for another section between detectors.

Cables for the described networks often have a shield in the form of a braid or a conductive foil, which surrounds the wires of the lines.

Such a shield has a very low resistance. It is either grounded or at a predetermined potential. It can happen especially in the field of detectors during installation that a wire touches the shield and thereby causes a short circuit. With the help of the test unit for a so-called. Shield monitoring, such a short circuit can be determined. In a simple way, this is done according to the invention in that the potential of the shield is monitored by the test processor. If the potential deviates from a predetermined value, there is a contact between a wire and the shield.

The monitoring circuits described have spatially considerable dimensions. It is therefore advantageous if it is not only determined whether there is a short circuit, but also where it is located. Therefore, an embodiment of the invention provides that the shield is connected via a measuring resistor with a potential source. As already described, the test circuit arrangement has a constant voltage source. This ensures that in the case of the short circuit described limited in the amount of predetermined current flows through the line, via the short circuit point and the measuring resistor. The total voltage drop is essentially composed of the voltage drop from the line sections and the measuring resistor. As mentioned, the shield hardly contributes to a voltage reduction and can therefore be neglected. Since the voltage drop across the measuring resistor is known, can be calculated in this way, the voltage drop caused by the line. From the current and the line voltage drop can also determine the resistance of the line section to the short circuit point. Since the cross-section and the resistivity of the wires are known, can therefore be calculated from the resistance and the length of the line to the short-circuit point. These calculations can be done in the test processor.

The length of the line from the central office to the short circuit point is already an essential statement, which makes it easier to find a short circuit point. It is even easier if it can be determined between which adjacent detectors a short circuit has occurred. In the method described above, the length of the line sections between the detectors can be determined. If the individual cable lengths are therefore stored in the test processor, it can then be calculated between which detectors there is contact between the shield and the wire or the short circuit

In the alarm systems described ring lines are often used, the ends of which are each connected to symmetrical circuits of a central office. It is therefore possible to operate a loop from both ends, for example when it is interrupted in the region of the short circuit. In this case, z. B. from one central portion of a stub and operated by the other central portion of the other spur line. So that certain detectors can be taken out of the signaling system, an embodiment of the invention provides that the detectors have in series with lying on the wire circuit breaker for separating the line on both sides of a short circuit point. In normal operation, the disconnectors are closed, but are opened by command from the control panel. Since the control panel "knows" between which detectors there is a short circuit, the detectors adjacent to the short circuit can be activated in order to open the circuit breaker.

Fig. 1

zeigt schematisch eine Prüfschaltungsanordnung nach der Erfindung für eine Gefahrenmeldeanlage.

Fig. 2

zeigt schematisch einen Melder der Gefahrenmeldeanlage nach Fig. 1.

The invention will be described below with reference to circuit arrangements shown in drawings.

Fig. 1

schematically shows a test circuit arrangement according to the invention for a hazard alarm system.

Fig. 2

schematically shows a detector of the hazard alarm system Fig. 1 ,

In Fig. 1 a test circuit arrangement is shown, which is arranged within a dashed box 10 shown. The test circuit arrangement 10 is part of a not shown control center of a hazard detection system having a loop. In Fig. 1 only the line A of the loop is shown. The other end, which is also connected to the center and with a test circuit arrangement 10 symmetrical circuit arrangement is not shown for reasons of simplicity. The line A consists of the wires 12 and 14, and in the course of the line A a series of detectors M to M _{n is} connected. In Fig. 1 the detectors M1, M2 and M _n are shown. Part of the circuit of the detector M is in Fig. 2 played. One recognizes a cross-switch T3, which connects the wires 12, 14 in the closed state. It also recognizes the power supply U _STAB with a capacitor C and a diode D. As a result, the signaling circuit is also supplied with voltage when the voltage of the line A briefly decreases or goes to zero. The detector M further comprises a modulator / demodulator 16, which converts a voltage pulse on the line line into logic signals for a logic circuit 18. The logic circuit 18 includes an address memory and a plurality of input / output lines. It receives a serial data signal (eg, an address or an instruction) and executes an instruction when a received address matches the address stored in the logic circuit 18. This can be z. B. be the case to actuate the cross switch T3 and thus short the wires 12, 14.

Each detector M has on both sides of the cross switch T3 in the wire 14 disconnectors T1, T2, which are normally closed during operation of the detectors. In addition, the wires 12, 14 connected via unspecified Zener diodes, so that when a polarity reversal of the detector during installation, a short circuit, which in turn can be determined by a Kurzschlußprüfschaltung, which will be discussed below.

The test circuit arrangement 10 has a first test processor 20 and a second test processor 22 (CPU1 or CPU2). The test processor 20 is connected via an interface 24 (COM1) to the central processor, not shown, of the alarm system alarm center. The test processor 22 is provided redundantly.

A constant voltage source 26 (I _KA ) is connected to the core 12 via a modulator 28 (MA) and a switch 30 (S _1A ). The constant voltage source 26 is connected to a power supply 32 (U _STABA ). The test processor 20 controls the modulator 28 and the switch 30 to operate z. B. to give a voltage modulated data word on the line when the switch 30 is closed. Another switch 33, also controlled by the test processor 20 (S _2A ), connects the wire 12 to ground when closed.

A voltage measuring device 36 (A / D1 _A ) is connected to the wire 12 and its output is connected to the test processor 20. The same is true for a voltage measuring device 38 (A / D2 _A ) connected to the wire 14.

The wires 12, 14 are surrounded by a shield 40, which in Fig. 1 indicated by dashed lines. The shield 40 is connected to a shield check unit 42 whose output is connected to the test processor 20. It contains a test resistor 44 (R _A ), which is connected to the shield 40 and with the other terminal to the potential U _s . In addition, the shield is connected to the positive input of an operational amplifier 46 whose output is connected to the test processor 20.

The wire 14 is connected to ground (R _MA ) via a measuring resistor 46a, the same pole of the resistor 46a connected to the wire 14 being connected to the positive input of an operational amplifier 48 whose output is connected to the test processor 20.

With the help of the circuit arrangement shown can be z. B. determine the line length of the line A or the wires 12, 14 and also the line lengths between desired detectors M, z. B. between adjacent detectors M. To this end, the description of an embodiment.

• $${\mathrm{U}}_{\mathrm{RMA}}={\mathrm{I}}_{\mathrm{IKA}}\times {\mathrm{R}}_{\mathrm{MA}}$$
  
• $${\mathrm{U}}_{\mathrm{TX}}={\mathrm{I}}_{\mathrm{KA}}\times \left({\mathrm{M}}_{\mathrm{N}}\times 2\times {\mathrm{R}}_{\mathrm{TX}}\right)$$
  
• $${\mathrm{U}}_{\mathrm{RL}}={\mathrm{I}}_{\mathrm{KA}}\times {\mathrm{R}}_{\mathrm{L}}$$
  
• $${\mathrm{U}}_{\mathrm{LT}}={\mathrm{U}}_{\mathrm{RMA}}+{\mathrm{U}}_{\mathrm{RL}}+{\mathrm{U}}_{\mathrm{TX}}$$
  

wobei

U_RMA

der Spannungsabfall über Widerstand R_MA,

U_TX

der Spannungsabfall über T1, T2 eines jeden Melders vor M_n,

U_LT

der Spannungsabfall am Linienanschluß A,

R_TX

der Gesamtwiderstand aller Schalter T1, T2 der Melder M1 bis M_n und

R_MA

der Meßwiderstand vor dem Anschluß Minuslinie A ist.

It should be z. B. the line length between the detectors M2 and M _{n are} measured. It is assumed that a normal operating state in which the switch 30 is closed and the switch 33 is opened. The switches T1 and T2 in the detectors M1 ... M _n are closed. The switch T3 in the detectors M1 ... M _n are open. Thus the line A line is energized (operating voltage). By driving the modulator MA is a voltage modulated signal on the line, z. B. a loop, sent. The data word includes the address of the detector or its communication address and a command to close the switch T3, such as M _n . After M _{n has received} the command, its switch T3 is closed. Now flows a constant current I _A , caused by the constant current source 26. The current flows through the switches T3 and T1 of M _n , and via the resistor R _MA . With the aid of the voltage measuring device 36, the voltage drop at the terminal plus line A is measured and fed to the test processor 20. The measured voltage drop is composed as follows:

• $${\mathrm{U}}_{\mathrm{RMA}}={\mathrm{I}}_{\mathrm{IKA}}\times {\mathrm{R}}_{\mathrm{MA}}$$
  
• $${\mathrm{U}}_{\mathrm{TX}}={\mathrm{I}}_{\mathrm{KA}}\times \left({\mathrm{M}}_{\mathrm{N}}\times 2\times {\mathrm{R}}_{\mathrm{TX}}\right)$$
  
• $${\mathrm{U}}_{\mathrm{RL}}={\mathrm{I}}_{\mathrm{KA}}\times {\mathrm{R}}_{\mathrm{L}}$$
  
• $${\mathrm{U}}_{\mathrm{LT}}={\mathrm{U}}_{\mathrm{RMA}}+{\mathrm{U}}_{\mathrm{RL}}+{\mathrm{U}}_{\mathrm{TX}}$$
  

in which

U _RMA

the voltage drop across resistor R _MA ,

U _TX

the voltage drop across T1, T2 of each detector before M _n ,

U _LT

the voltage drop at the line terminal A,

R _TX

the total resistance of all switches T1, T2 of the detectors M1 to M _n and

_RMA

the measuring resistor before the connection minus line A is.

$$\mathrm{RL}\left({\mathrm{M}}_{\mathrm{n}}\right)=\frac{{\mathrm{U}}_{\mathrm{LT}}-{\mathrm{U}}_{\mathrm{RMA}}-{\mathrm{U}}_{\mathrm{TX}}\left({\mathrm{M}}_{\mathrm{n}}\right)}{{\mathrm{I}}_{\mathrm{KA}}}$$

After conversion of equation 4, the result is: $${\mathrm{U}}_{\mathrm{RL}}={\mathrm{U}}_{\mathrm{LT}}-{\mathrm{U}}_{\mathrm{RMA}}-{\mathrm{U}}_{\mathrm{TX}}$$

$$\mathrm{RL}\left({\mathrm{M}}_{\mathrm{n}}\right)=\frac{{\mathrm{U}}_{\mathrm{LT}}-{\mathrm{U}}_{\mathrm{RMA}}-{\mathrm{U}}_{\mathrm{TX}}\left({\mathrm{M}}_{\mathrm{n}}\right)}{{\mathrm{I}}_{\mathrm{KA}}}$$

Equation 2 is calculated in the test processor 20 and the result R _L (M _n ) is stored. This value includes the line resistance between the connection of the line A and the detector M _n .

After a certain time t _M , the switch T3 is opened again in the detector M _n . This is done with the help of a suitable timer, which is housed in the detector, for example in the logic module 18. The line voltage returns to operating potential.

Subsequently, the above steps are performed for the detector M2. The result R _L (M2) is also stored in the memory by the test processor 20. Now the difference between the two measurements is formed: $${\mathrm{.DELTA.R}}_{\mathrm{L}}={\mathrm{R}}_{\mathrm{L}}\left({\mathrm{M}}_{\mathrm{n}}\right)-{\mathrm{R}}_{\mathrm{L}}\left({\mathrm{M}}_{\mathrm{n}}\right)$$

wobei A der Querschnitt der Leitung und p der spezifische Widerstand ist. Die einfache Länge einer Ader bzw. eines Aderabschnittes ergibt sich aus $$\mathrm{I}=\frac{{\mathrm{I}}_{\mathrm{G}}}{2}$$

For a given conductor diameter (cross-section), the cable length between detector M2 and M _n can be determined: $${\mathrm{I}}_{\mathrm{G}}=\frac{\mathrm{A}\times {\mathrm{R}}_{\mathrm{L}}}{\mathrm{\rho }}$$

where A is the cross section of the line and p is the resistivity. The simple length of a wire or a wire section results from $$\mathrm{I}=\frac{{\mathrm{I}}_{\mathrm{G}}}{2}$$

$$\mathrm{I}=\frac{\mathrm{A}\times {\mathrm{R}}_{\mathrm{L}}}{\mathrm{\rho }}$$

The same procedure can be used to determine the total length of the pipe. Is z. B. provided a loop, the switch is closed at the other end of the switch 33 after Fig. 1 equivalent. As a result, a constant current flows via the wire 12 to ground. About the voltage measuring device 36, the voltage at the terminal of the wire 12 is now measured. The measured voltage can be converted directly into the length of the line: $${\mathrm{R}}_{\mathrm{L}}=\frac{{\mathrm{U}}_{\mathrm{LT}}}{{\mathrm{I}}_{\mathrm{K}}}$$

$$\mathrm{I}=\frac{\mathrm{A}\times {\mathrm{R}}_{\mathrm{L}}}{\mathrm{\rho }}$$

The measured values for the line sections and the entire line can be stored in the test processor 20.

With the help of the circuit arrangement shown, a short circuit between the shield 40 and one of the wires can be determined as well as the location of the short circuit.

As already mentioned, the shield 40 consists of a wire mesh or a foil and has a low impedance and is neglected in the following calculations. The starting point is again a normal operating state, i. H. Switch 30 is closed and switch 33 is open. Now, the short circuit K1 should be detected and the short circuit location determined.

wobei

A

der Querschnitt der Ader,

R_LK

der gemessene Widerstandswert und

ρ

der spezifische Widerstand ist.

It flows the current I _A of the constant current source 26. It flows when the short circuit is K1, using the shield 40 and the resistor 44 to the potential V _s of the screen monitor 42. By means of the voltage measuring device 36, the voltage which is established can be measured, become. The voltage drop across resistor 44 is known. Consequently, the voltage drop caused by the line to the short-circuit point K1 can be calculated from this, ie, by the wire 12. This voltage drop U _LA and the current I _A allow the calculation of the resistance wire section, which is denoted by R _LK . The cable length to the short circuit point is therefore $$\mathrm{I}=\frac{\mathrm{A}\times {\mathrm{R}}_{\mathrm{LK}}}{\mathrm{\rho }}$$

in which

A

the cross section of the wire,

R _LK

the measured resistance value and

ρ

the specific resistance is.

In this way it can be determined in which line distance the short circuit has occurred. Since this line distance still says little about the actual location of the short circuit, this line length can also be related to the determined lengths of the line sections between the detectors M1 ... M _n . Therefore, it can be easily determined between which detectors the short circuit is located, so here between the detectors M1 and M2.

In a similar way, as described above, it can also be determined whether there is a reverse polarity. In a reverse polarity of the constant current I _{A flows} through the not-shown Zener diode and thus leads a limited by the constant current source 26 short-circuit current. By measuring the cable length can thus determine, at which point the short circuit is. Since in this case also the voltage drop across the measuring resistor 46a changes, it can be determined by the block D _A , whether a short circuit exists or the line is undisturbed, which then leads to a corresponding message to the test processor 20.

Claims (17)

1. A danger signalling system, comprising:

   - a multiplicity of detectors (M1 to M_n) and other line members, in case of need, which respond to at least one danger criterion and are connected to a two-wire line (line A),

   - a control centre connected to the line (line A), which has a voltage supply and a central processor in which the addresses of the detectors (M1 to M_n) are stored for individually addressing and polling the detectors (M1 to M_n) as well as a program for monitoring the status of the detectors (M1 to M_n),

   characterized in that a testing circuitry (10) is disposed in the control centre for checking the working order of the network formed from the line (line A) and the detectors (M1 to M_n) or line members by means of a testing unit wherein the testing circuitry (10) includes a testing processor (20, 22) which, in turn, has an evaluation software, and a switch assembly (30, 33) controlled by the testing processor (20, 22) is provided for selectively connecting the at least one testing unit to the line (line A), the testing unit for checking the line lengths has a stabilized-current source (26) which is adapted to be connected to the line (line A) via a modulator (28) and a controllable switch (30) wherein the testing processor (20, 22) and the modulator (28) help in generating a data word which contains the address of a detector (M1 to M_n) and a control signal for a cross-connection switch (T3) inside the detector interconnecting the wires (12, 14) and, further, a voltage measuring device (36) connected to the line (line A) is provided which is connected to the testing processor (20, 22).
2. The system according to claim 1, characterized in that the testing circuitry (10) is designed as a module, e.g. in the form of a p.c. plug-in card.
3. The system according to claim 1 or 2, characterized in that the testing circuitry (10) has a modem connection for checking the network via a trunk connection line.
4. The system according to any one of claims 1 to 3, characterized by a testing unit for checking any respective misplacement of poles of the detectors (M1 to M_n) and line members.
5. The system according to any one of claims 1 to 4, characterized by a testing unit for checking any respective short-circuits in the line and/or any contact of wires (12, 14) of the line (line A) and the shielding enclosure (40) of the line with a wire (12, 14).
6. The system according to any one of claims 1 to 5, characterized by a testing unit for checking the installed network with a predetermined installation scheme.
7. The system according to claim 1, characterized in that the data word is formed by modulating the voltage in the modulator (28).
8. The system according to claim 1, characterized in that a timing circuit is provided which causes the switch (T3) to open.
9. The system according to claim 1, characterized in that at least a second switch (33) is provided which connects a wire (14) of the line (line A) to ground for generating a stabilized current flowing in the line (line A).
10. The system according to any one of claims 5 to 9, characterized in that a shielding enclosure testing unit (42) monitors the potential of the shielding enclosure (40) by means of the testing processor (20, 22) and produces a signal if the potential deviates from a predetermined value.
11. The system according to claim 10, characterized in that the shielding enclosure (40) has connected thereto a precision resistor (44) the voltage drop of which is provided to the testing processor (20, 22) and the line resistance up to the short-circuit location is determined from the voltage level at the connection of the line (line A) and the stored voltage drop of the precision resistor (U_RA) and the line length up to the short-circuit location is determined from said resistance.
12. The system according to any one of claims 1 to 11, characterized in that the detectors (M1 to M_n) have disconnecting switches (T1, T2) located in series with a wire (14) for breaking up the line (line A) on either side of a short-circuit location (K1).
13. A method for measuring the resistance of line portions or line lengths in danger signalling systems according to one of the claims 1 to 12 having the following features:

    - a multiplicity of detectors (M1 to M_n) and other line members, in case of need, which respond to at least one danger criterion and are connected to a two-wire line (line A),

    - a control centre connected to the line (line A), which has a voltage supply and a central processor in which the addresses of the detectors (M1 to M_n) are stored for individually addressing and polling the detectors (M1 to M_n) as well as a program for monitoring the status of the detectors (M1 to M_n),

    characterized by the following process steps:

    - a testing unit (10) is connected to the line (12, 14),

    - a testing processor (20, 22) of the testing unit (10) in which the addresses of the detectors (M1 to M_n) are stored provides an instruction to a predetermined detector (M_n), via its address, to close a cross-connection switch (T3) interconnecting the wires of the line (12, 14) in the detector (M_n),

    - a stabilized-current source (I_KA) of the testing unit (10) generates a stabilized current (I_A) on the line (12, 14),

    - a voltage measuring device (36) measures the voltage drop at the connection of the line (12, 14) and provides the value measured to the testing processor (20, 22),

    - the testing processor (20, 22) calculates the resistance of the sum of line portions between the connection of the line (12, 14) and the detector (M_n) while subtracting the resistances of the detectors (M1 to M_n-1) and a limiting resistance (R_MA), if required.
14. The method according to claim 13, characterized in that the resistance or line length between adjoining detectors (M_n, M₂) is calculated by repeating the steps according to claim 15 for the adjoining detector (M2) and the minor resistance value is subtracted from the major one.
15. The method according to claim 13 or 14, characterized in that a timing circuit in the detectors opens the cross-connection switch (T3) after a predetermined time if it had been closed before.
16. The method according to any one of claims 13 to 15, characterized in that the resistances or line length of the single line portions between the detectors (M1 to M_n) and the predetermined resistance values of the single detectors (M1 to M_n) are stored in the testing processor (20, 22) and, when measurements are made in operation later, the resistances measured for the detectors are compared to the resistance values stored for the detectors.
17. A method for determining a short-circuit between the line of a danger signalling system according to one of the claims 1 to 12 and a shielding enclosure for the line, the danger signalling system comprising:

    - a multiplicity of detectors (M1 to M_n) and other line members, in case of need, which respond to at least one danger criterion and are connected to a two-wire line (line A),

    - a control centre connected to the line (line A), which has a voltage supply and a central processor in which the addresses of the detectors (M1 to M_n) are stored for individually addressing and polling the detectors (M1 to M_n) as well as a program for monitoring the status of the detectors (M1 to M_n),

    characterized by the following process steps:

    - the shielding enclosure (40) is connected to ground via a resistor (R_A) of the testing unit (10),

    - a stabilized-current source (I_KA) of the testing unit (10) generates a stabilized current (I_A) on the line (12, 14),

    - a voltage measuring device (36) measures the voltage drop at the connection of the line (12, 14) and provides the value measured to the testing processor (20, 22),

    - the testing processor (20, 22) calculates the resistance of the short-circuited line up to the short-circuit location (K1) and calculates the line length up to the short-circuit location (K1) from the parameters of the line (12, 14).

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