EP2017803B1 - Active function maintenance and safety system for warning loudspeaker networks in double-wire loop system - Google Patents

Abstract

The system has a central module for feeding and operational mode switching of a double-wire loop. A central feeding and operational mode switching module registers an individual line and loudspeaker fault using a function maintaining and securing module installed at an alarming loudspeaker. The switching module switches to a two-way operation in response to registration for function maintenance. A faulty transmission chain with exception of defective parts functions again within seconds. Ground fault control is present for securing protection against accidental contact.

Description

The invention relates to an active function maintenance and security system for alarming loudspeaker networks.

It works with electronic modules in a serial two-wire loop loudspeaker network. If anything, loops for speaker networks have been formed so that you connected the speakers in parallel. Function maintenance was not given. The vast majority of loudspeaker networks are realized in star cabling. At alarm networks high demands are made in terms of reliability and operate a correspondingly required installation effort. Often, redundancy of two installations is required.

The need for the invention results inter alia from excerpts from the European standard EN 60849 for electro-acoustic emergency warning systems with respect to demands placed there:

The wiring must comply with the standards for emergency and / or alarm systems (point 6 in EN 60849).

State of the art:

This has been achieved so far by laying special cables with 30 minutes of passive functional integrity by fire retardation up to a certain temperature and special separate cable routing. The on-site effort for this is much higher than with conventional cabling.

A failure of a loudspeaker circuit must not lead to a complete failure of the supply of the loudspeaker area (point 4.1-g in EN 60 849).

State of the art:

This is achieved by dividing the circles into individually secured and monitored fire alarm sections with a limited number of speakers per cable, in order to keep the unreachable areas low in the event of a failure.

The following must be displayed centrally: Failure of a loudspeaker circuit due to interruption or short-circuit fault
(Point 5.3-j in EN 60 849).

The monitoring should indicate the failure ... of a loudspeaker circuit.
(Point 4.1 in EN 60 849: note 1)

State of the art:

This is so far achieved by the individual circles associated central monitoring devices. These work in their majority for physical reasons with detection tolerances of 5 to 30% and are based on impedance measurement (disadvantage: no measurement during a sound transmission possible and false triggering by noise level in front of the speakers) or DC resistance measurement (disadvantage: high detection inaccuracies in networks with many speakers).

Especially in smaller buildings, it is not absolutely necessary to set up two separate loudspeaker circuits in one loudspeaker area. A decision on this may depend on local regulations.
(Point 4.1 in EN 60 849: Note 2)

State of the art:

The loop principle for maintaining functionality has been used for quite some time in alarm and alarm siren technology.

For two-wire 100V speaker networks, however, there has been no function preservation solution according to the principle of the loop principle. As a redundancy solution, double cable routing with double loudspeaker equipment is recommended for this and, in the case of too much effort, the redundancy is omitted in individual cases.

• Lautsprechernetze sind meistens als 100V-Systeme aufgebaut. Laut VDE sind aber 100V-Wechselspannung, auch wenn sie bei Tonübertragung zwischen Null und 100V im Rhythmus der Modulation schwanken, eine Hochspannung und dürfen nur dann als, wie und mit Niederspannungskabel verlegt werden, wenn sie symmetrisch und erdfrei aufgebaut sind. Diese Erdfreiheit muss aber ständig überwacht und das Unterschreiten eines bestimmten Wertes (= höherer Fehlerstrom!) gemeldet und beseitigt werden. Ein automatisches Abschalten des Netzes würde eine höhere Priorität, in diesem Fall die Alarmierungssicherheit unterbinden.

Touch safety, isolation, VDE:

• Speaker networks are mostly built as 100V systems. According to VDE, however, 100V AC, even if they fluctuate with sound transmission between zero and 100V in the rhythm of the modulation, a high voltage and may only be laid as, as and with low-voltage cable, if they are symmetrical and floating. However, this ground clearance must be constantly monitored and the falling below a certain value (= higher fault current!) Reported and eliminated. An automatic shutdown of the network would prevent a higher priority, in this case the alarming security.

In the Laid-open specification DE 33 47 609 A1 a communication and monitoring system for traffic engineering is proposed in which a loop connects a control unit with extensions. The extensions contain amplifiers for amplifying the signals circulating in the loop. The extensions can imprint monitoring signals on the loop, for example, to report a fault to the control unit. If a fault occurs in a line section of the loop, then the unidirectional signal flow remains obtained until the last extension before the faulty line section. The branches behind the faulty line section send fault messages to the control unit in the specified unidirectional direction. Since the control unit can transmit signals via the loop to the branches only in the predetermined direction, the extensions located behind the faulty line section receive no signals from the control unit.

In order to be able to supply the extensions behind the disturbed line section with signals when a fault occurs, a second ring line is provided, the signal flow direction is opposite to the first ring line. In this way, extensions can be supplied with signals before the first faulty line section as well as behind the last faulty line section. To avoid excessive line lengths, the transmission path between the last extension and the controller can be replaced by looping.

The aim of the invention is to significantly reduce and simplify their use both the previously required installation effort, as well as to ensure a timely unrestricted and independent of the fire temperature and the installation material used functional integrity of the alarm.

This object is achieved with the features of claim 1. The invention is further developed in its dependent claims.

All examples given below, not falling within the scope of the invention as defined in the appended claims as well as the necessary configuration loud FIG. 1 are not to be regarded as embodiments or examples of the invention.

Required when using the invention for the cable network only the formation of a ring line with locally different round-trip of the 2-wire cable, a a suitable interface in the central device and per alerting loudspeaker an intelligent function maintenance security module which divides the loop by serial insertion into separable sections.

For this, no special functional cable and special cable routing system need be used during the installation. The division into individually sonicated fire alarm sections and a limitation of the number of speakers can be omitted.

In building renovation work, an existing cable network can continue to be used, if you realize a loop construction later.

The tolerance-sensitive loudspeaker circuit monitoring in the central device can be omitted. When using the invention, each error is registered and reacted to it at the same time.

In the event of a short-circuit failure, the affected loudspeaker or cable section is isolated from the unaffected parts of the system and a two-way supply of the faulty network is established, as in the case of a failure due to interruption, and a fault is registered centrally. Within a few seconds, the malfunctioning alarming loudspeaker network automatically re-operates with the exception of the defective part.

To monitor the freedom from earth, there are already numerous circuit options.

If, however, on the speaker line in addition to a maximum of 100V audio frequency and a significant DC voltage is conveyed as in this two-wire loop system, all known known primarily on DC resistance measurement methods fail because of the influence of external voltages. Therefore, the method developed for this application is based on a continuous natural frequency identification of a special PLL circuit which, in addition to self-monitoring, has high interference immunity and can actually be influenced not by possible voltages but only by a DC or equivalent AC resistance. By cable damage to metal structures without cable interruption or short circuit such a large cores capacity may have arisen against earth, that this would allow a high fault current at 100V audio frequency in case of contact. Therefore, the inclusion of AC resistors in the monitoring for the overall concept of the invention.

The invention will be apparent from four drawings FIG. 1 to FIG. 4 described in more detail.

FIG. 1 shows the basic structure for the alarming loudspeaker network with active functional maintenance system on a two-wire loop basis.

An operating voltage (for example 24V) is applied to 1-2, the amplifier output with potential-free output transformer to 3-4 and the forward line 5-6 and the return line 7-8 to the infeed and mode switching module FIG. 2 connected. The required function maintenance and security modules FIG. 3 are connected in series and each have a speaker output 55-56 to connect the speaker. In practice, the module FIG. 3 each housed in the speaker cabinet or a separate installation junction box in the speaker room. The last module FIG. 3 becomes on another, spatially not the same route back to the feed and mode switching module FIG. 2 recycled. In this central module, electronics with fault alarm contact outputs 34 are also installed for error message forwarding.

FIG. 2 shows the principle of the central interface as a feed and mode switching module for loop alarming loudspeaker networks.

Basic condition connections:

At 1-2 is the operating voltage (usually 24V). At 3-4 is the amplifier output (usually 100V). At 5-6 the outgoing line and at 7-8 the return line of the loudspeaker loop is connected. There are still various connections for the fault message output and a reset input for resetting correctly latched, stored fault messages.

The function is explained on the basis of some possible operating states.

Operating state One-time commissioning of the module:

The operating voltage is applied. The DC-DC converters 9 and 10 each generate a 24V output voltage. Relays 11 and 12 energize and switch over with their contacts. This puts each converter on its normal workstation. Via the diodes 15 and 16, the two voltage outputs are combined to form an operating voltage for the floating control functions of the electronics 17 and 18. About the switched contacts 11 and 12 and the 5W resistors (22 ohms) 19 and 20 are the two Electrolytic capacitors (10,000 uF) 21 and 22 pre-charged to avoid a transformer short-circuit. In parallel pull on the control electronics 17 and 18, the relays 25 and 26, so that in the external modules, the loop voltage of 5-6 does not arrive creeping and in the fault message 34, the reset relay 33 is set. Thus, relay 30 can not be energized, although opto-coupler relay 29 could not attract across 7-8 due to lack of voltage feedback, and contact 29 is therefore closed.

After about 5 seconds, the control electronics 17 and 18 pull the relays 23 and 24 and bridge the resistors 19 and 20, since now the capacitors 21 and 22 are pre-charged. At the same time, the relays 25 and 26 drop off again. The full voltage of converter 9 is thus present across the closed contact at the forward output 5-6 and the loop is energized. This builds up the error-free loop through the external speaker modules and must eventually arrive at the return line input 7-8 again. After about 60 seconds (variable), the fault signaling logic 30 switches off, and a few seconds later the fault message readiness switches on. This would initially allow the two-way function as needed and then issued from this point any errors as a message.

In the meantime, in parallel to these DC voltage functions, the sound transmission from the amplifier output 3-4 via the capacitor 21, contact 24 and 25 to the forward output 5-6 could reach the ring line and thus the loudspeakers.

With the capacitor 21 at the same time a Toneinpeisung is prevented in the converter 9, since 21 for AC voltage has almost zero ohms. At the return line entrance is parallel to the optocoupler relay 29 of the electrolytic capacitor 35, which also eliminates the AC voltage for 29 via the series resistor 31 (6.8 kohm) and thus only DC voltage can switch the optocoupler relay 29.

In the commissioning phase of the module, the start functions in converter 9 and 10 run in parallel, except that converter 10 is in the standby position and is not output at 7-8 because relay 30 has not switched.

Operation Normal state:

At the forwarding output 5-6, both the loop supply voltage 24V and the 100V audio frequency voltage are available. The return line 7-8 is connected as input to optocoupler relay 29, which has attracted.

Operating state line interruption:

There is no voltage at return input 7-8. Thus, 29 no longer receives tension and contact 29 closes. This pulls relay 30 and switches the return line output 7-8 with its contacts 30 as Zweitwegausgang over the contacts 26-24-12 on the converter 10 and the capacitor 22 to the amplifier output 3-4. Thus, the loop is fed from both ends and an existing break is compensated.

An error is reported via 34.

Operating state Line interruption between amplifier and connection 3-4:

Since the repeater output transformer is in series with the converter minus, a line interruption fault with over-reporting occurs. Of course, no sound transmission is possible in this case.

Operating state Short circuit somewhere on the line:

The external loudspeaker modules react and switch off the short-circuited section by disconnecting the line. Miscellaneous Behavior of the central module as in a normal interruption.

Operating state Short circuit immediately before the central module on the outgoing line 5-6:

There is no voltage at return input 7-8. Thus, 29 no longer receives tension and contact 29 closes. This pulls relay 30 and switches the return line output 7-8 with its contacts 30 as Zweitwegausgang over the contacts 26-24-12 on the converter 10 and the capacitor 22 to the amplifier output 3-4. Thus, the loop is fed from both ends.

During this short switching phase, the external loudspeaker modules briefly disconnected the loop and then rebuilt the loop. Only the first module before the short circuit separates the loop from the short circuit.

Simultaneously with the switching phase to two-way supply, the electronics 17 registers a voltage dip by the short circuit on the outgoing line 5-6 and immediately switches relay 25. This opens contact 25 and the voltage of the converter 9 is through the resistor 31 (4.7 kohm / 5W) protected against short circuit at output 5-6. Likewise, at the same time the amplifier output is protected by the resistor 31 from shorting to 5-6. The electronics 17 only switches the relay 25 off again when it receives a voltage again via the resistor 31, that is to say after the short circuit has been eliminated. The module remains in the two-way state until the relay 30 drops again when the reset 33 is triggered after short-circuit removal.

Operating state Short circuit immediately before the central module on the return line 7-8:

There is no voltage at return input 7-8. Thus, 29 no longer receives tension and contact 29 closes. This pulls relay 30 and switches the return line output 7-8 with its contacts 30 as Zweitwegausgang over the contacts 26-24-12 on the converter 10 and the capacitor 22 to the amplifier output 3-4.

During this short switching phase, the external loudspeaker modules briefly disconnected the loop and then rebuilt the loop. Only the last module before the short circuit separates the loop from the short circuit.

Simultaneously with the switching phase to two-way supply, the electronics 18 registers a voltage dip through the short circuit at the return line 7-8 and immediately switches relay 26. This opens contact 26 and the voltage of the converter 10 is through the resistor 32 (4.7 kohm / 5W) protected against short circuit at output 7-8.

Likewise, at the same time, the amplifier output is protected by resistor 32 from shorting at 7-8. The electronics 18 switches off the relay 26 again only when it receives a voltage again via the resistor 32, ie after Elimination of the short circuit. The module remains in the two-way state until the relay 30 drops again when the reset 33 is triggered after short-circuit removal.

Operating state DC-DC converter failure:

The converters are permanently short-circuit proof and generously designed according to the expected current drain (up to 1A, depending on the number of external speaker modules). Before feeding the amplifier output voltage (up to 100V - audio frequency AC voltage) protect the two transducers 9-10, the capacitors 21 and 22. About the diodes 15-16, a permanent operating voltage for working in the potential-free electronics is created. Parallel to the converter 9 is the relay 11 and parallel to the converter 10, the relay 12. If a transducer voltage is present, then pull these relays and the output voltage of converter 9 is connected via the contact 11 to Hinleitungsausgang from converter 10 via the contact 12 for provided the second channel. If one of the two transformers fails, its relay (11 or 12) drops out and switches the respective channel to the still functioning converter with the aid of the contacts (11 or 12).

On the module is still a fault detection 34 with reporting disturbance signal outputs as required in the current regulations.

FIG. 3 shows the principle of the function preservation backup module, which is in practice at the respective alarm loudspeaker and divided the two-wire loop into serial sections, which are either turned on or locked.

The function is explained below on the basis of individual operating states.

Loop building phase to normal operation:

At the forwarding input 51-52 come 24V. Through the speaker at 55-56, the voltage across the fuse 57, the choke 58, the diode 59 to the capacitor 60 (220uF) and builds up slowly. At the same time the voltage across the resistor 61 (5 W-4.7 kOhm) arrives at the return line output 53-54. From 53 via the throttle 62, the diode 63 also to the capacitor 60th

Throttle 58, in conjunction with capacitor 64 (220uF), cuts off an additional 100V AC incoming voltage on transmission 51-52, as well as choke 62 and capacitor 65 (220uF). The diodes 66 and 67 connected in parallel to the capacitors 64 and 65 serve as polarity protection for these capacitors. Since the series components are relatively high impedance (chokes about 300 ohms, 100V speakers 50 ... 500 ohms), the internal operating voltage 68 builds up slowly (in about 1 second) until it reaches its full height. Therefore, a threshold circuit 69 is connected to the operating voltage, which only turns on its output 71 when it reaches a voltage sufficient for the 24V relay 70 with sufficient certainty of over 20V. Before, however, a voltage of at least 5V must still be present at the inputs 73 and 74 at the NAND gate 72, before the arrival of the voltage 71 can cause the NAND element 72 to attract the relay 70.

The operating voltage 68 drops due to the load from the relay 70 to a safe for the relay holding voltage of about 12V. The threshold switch 69 is designed to be at turns on over 20V and then holds itself until it is reset by another input 73 from the NAND gate 72 when one of the voltages 73 or 74 after the relay suit was no longer present.

In operation, the relay now pulls and remains energized. The two contacts 75 and 76 switch over. At 75, the speaker 55-56 is switched to 52-54 via the fuse 57 and the bipolar electrolytic capacitor 77 (10 μF), thus obtaining the audio frequency voltage of 51-52. Contact 76 bypasses resistor 61 and turns the loop on by connecting 51 and 53 (to the next module or as return). Now normal operation is on.

Loop interruption:

The loop break causes no change from the normal operating state on this module. It is only registered in the central module, as there at the return line input nothing arrives.

Loop Short circuit:

If either 51-52 or 53-54 is short-circuited, the operating voltage for all modules will first break down briefly. Since now all relays 70 fall off, the loop will then rebuild immediately and only the maximum two immediately before and after the short circuit modules would not turn their relay 70 through, since the NAND gate on one of the two inputs 73-74 no Voltage is received and thus the switching condition is missing.

The speaker 55-56 is above 57 and 77 via the closed contacts 75-76 parallel to 61 in series between 51 and 53. It does not matter if the short circuit takes place at 51-52 or 53-54. The loop is interrupted for the central module.

Speaker interruption:

If there is an interruption between 55-56, the voltage 73 is missing and thus a turn-on condition. The loop is interrupted for the central module.

Speaker short circuit:

If 55-56 are short-circuited, then the fuse 57 is triggered by the audio frequency 100V (distance from 51 via 55-56, capacitor 77, contact 75 to 52). In this case, then a fuse break is generated again with fuse melting.

FIG. 4 shows the earth fault monitoring for the central feed and mode switching module FIG. 2 , The monitoring may not detect ground fault resistors by DC voltage, since no fixed potential reference can be assumed due to the potential-free 24 V DC voltage on the loop and potential-free. For frequencies in the 100V audio transmission range, the circuit must also be immune, so that no false triggering can take place. Higher measuring frequencies could emit interfering radiation or be influenced by the inaudible pilot tone frequency (20 to 25 kHz) of the monitored 100V amplifiers, because unbalance is measured against earth. There are only measurement frequencies 1 to 10 Hz, which are far below the listening range. But measurements usually require one in this area longer period of time for safe evaluation. However, the speaker wires are sampled only briefly for measurement for several reasons.

Solution: A PLL circuit consists of a generator whose frequency is equal to the evaluation frequency of its detector circuit. If the generator frequency is then fed back to the detector input via a defined adjustable threshold circuit, the PLL circuit recognizes its own frequency and switches its evaluation output. If the level of the detector input drops below its threshold, the evaluation input responds immediately to an interruption. This happens much faster than if the PLL had to catch an offered frequency first. At the same time, in principle, it does not matter which PLL frequency it is, it just has to be its own.

It is advantageous to have a low frequency outside of the hearing range, since the filtering of up to 100V level of audio frequency transmission is simply possible with an RC low-pass filter. Because only transmitted low-frequency switching noise could interfere, the evaluation output is blocked against short pulses.

Since the measurements can not be made simultaneously on both wires of a loudspeaker line, the measurements must be made one after the other. According to the regulations, a measuring process must be carried out on every wire every 100 seconds at the latest. As measuring time, 2 seconds have proven sufficient. In order not to burden the wires with earth due to the measuring process and at the same time to protect the measuring circuit from possible overvoltages up to 100V ~ in the event of a fault, the control circuit is connected to high impedance.

In detail: The PLL generator output 81 is fed via a level regulator 82 to an output amplifier and impedance converter 83, which are strongly counter-balanced for harmonics. The output capacitor 84 is followed by a high-impedance low pass 85 (47 kOhm) for further harmonic removal. This is followed by another high-impedance low pass 86 (130 kOhm) for the detector input 89, which is preceded by the DC voltage cutoff capacitor 87. Before that, there is an overvoltage limitation 88, consisting of two Zener diodes 10V connected in opposition to each other. At the detector output 90, the evaluation result appears, which normally indicates a detected frequency by outputting a voltage. This voltage is passed through a delay RC element 91, which bridges momentary power interruptions (up to 1/2 second).

Longer interruptions are passed on via the inverting switching stage 92 as an impulse to an RC element 94, which must store this pulse for at least 120 seconds (ie, certainly longer than one measurement cycle) and output via a further switching amplifier 95. The symbolic diode 93 prevents a discharge of 94 over 92.

Between low pass 85 and 86 is the entrance to the fair. This is via two reed relay contacts 96-97 (because of 2 x 100V wires) to the wires 5 and 6 of the feed and mode switching module FIG. 2 in the measuring cycle one after the other. With the level controller 82 can be set the desired threshold of the PLL detector and thus the residual current sensitivity.

If there are 5 kOhms parallel to the measuring input, a fault current of 20 mA fault message would be triggered at 100V (UxU: R). With an alternating voltage one would have to adjust it possibly to peak value, what a value tripling (300Vss / 15 kOhm). This also applies to capacitive resistors, their impedance changing with the reference frequency, so you can assume only from a middle Tonfrequenzhöhe for the threshold.

There are no valid specifications for public address systems in 2007.

The measuring cycle is generated via a slow-running ring counter 98. In order to be able to interrogate up to four feed-in and mode changeover modules with a single earth fault control circuit, a 10-stage ring counter would make sense. It advances every 9 seconds and switches the connected relay (here only 99-100) for 2 seconds each. With four modules FIG. 2 That would be eight relays plus the two remaining free steps of the ring counter, which could be used to check the operation.

Since fault storage 94 bridges a time of 120 seconds, the fault message would be present for at least this time if a single acquisition or an existing fault would stand still despite the cycle.

The operating voltage for the ground fault check is generated by a separate DC-DC converter from the operating voltage of FIG. 2 generated and is grounded.

Claims (10)

1. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology comprising a central module (Fig. 2) for the supply and operating modes switching-over of a two-wire loop and a number (depending on the demand) of function maintenance safety modules (Fig. 3) for serially dividing the loop into cable sections between the modules (Fig. 3), which can be automatically connected or disconnected in case of a malfunction,
   characterised in that
   the central supply and operating-mode-switchover module (Fig. 2), with the aid of the function maintenance and safety modules (Fig. 3) to be attached at each warning loudspeaker, registers each individual cable and loudspeaker malfunction and in response thereto switches over to two-way operation in order to maintain the function, so that within seconds the malfunctioning transfer chain is again operative except for the defective part, wherein earth-fault monitoring (Fig. 4) is provided as a safeguard against accidental contact,
   wherefore the function maintenance and safety modules each comprise an input line connection (51-52) and an output line connection (53-54) in order to be configured for connection with the central supply and operating-mode-switchover module via a forward and return line of a two-wire loop system, and
   wherefore the central supply and operating-mode-switchover module comprises a first (5-6) and a second (7-8) loop connection in order to be configured for connection with the function maintenance and safety modules via the forward and return line of the two-wire loop system, and the central supply and operating-mode-switchover module further comprises a signal input (3-4) for supplying electrical input signals at frequencies in the auditory range, a device for forwarding the input signals from the signal input (3-4) to the first loop connection (5-6), a measuring device (29) configured to output a measuring signal dependent upon a voltage at the second return line connection (7-8) and a switching device (30), which is configured to forward the input signals from the signal input (3-4) as a function of the measuring signal to the second loop connection (7-8).
2. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to claim 1, characterised in that in case a malfunction occurs within the loop loudspeaker network (X 1) the central supply and operating-mode-switchover module, logically acting, is switched over from a one-way to a two-way system and a fault message is output if the supply DC voltage output on the forward line (5-6) does not arrive at the input of the return line (7-8).
3. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to claim 1 or 2, characterised in that if one of the two potential-free supply DC voltages for the forward and return lines which are generated from the normal operating voltage (1-2) by separate DC voltage converters (9-10) fails, the still-functioning supply DC voltage automatically takes over the entire supply.
4. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to claim 1, 2 or 3 characterised in that the module comprises a sequence control electronics (17-18) and a fault reporting electronics (34) with permanent short-circuit protection for the two DC voltage converters (9-10) and the amplifier (3-4) to be connected for each forward line (5-6) and return line (7-8).
5. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to one of claims 1 to 4, characterised in that the connection (3-4) of the amplifier to be connected is co-monitored for interruption since it is connected in series with the loudspeaker circuit (5-6).
6. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to one of claims 1 to 5, characterised in that the module (Fig. 2) comprises an earth-fault monitoring circuit (Fig. 4), where a PLL circuit receives its own synchronising generator below-hearing-range frequency for continuous detection and issues a fault message if it drops below the level offered.
7. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to one of claims 1 to 6, characterised in that the earth-fault monitoring circuit (Fig. 4) operates extremely fault-free in relation to external DC voltages and sound AC voltages because due to its measuring frequency lying below the hearing range, simple filtering-out of faults is possible.
8. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to one of claims 1 to 7, characterised in that the module (Fig. 3) required for each warning loudspeaker comprises two inputs/outputs (51-52/53-54) of equal priority which in case of operation or line interruption are connected through and are interconnected, but in case of a short circuit this through connection is interrupted.
9. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to one of claims 1 to 8, characterised in that the output (55-56) of the function maintenance safety module (Fig. 3) to its own loudspeaker is included in the monitoring path in addition to the cable operating states.
10. Active function maintenance and safety system for warning loudspeaker networks in double-wire closed-loop technology according to one of claims 1 to 9, characterised in that a module electronics (61 to 77) of the function maintenance safety module (Fig. 3) provides for logically correct monitoring of the operations, so that when there is a fault in the cabling its own loudspeaker is always connected on the functioning side of the loop and able to transfer a warning.

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