Fire alarm system testing

Abstract

A method of determining crosstalk between a pair of installed two-wire fire alarm loops of an addressable fire alarm system, comprises: generating a test signal which includes a string of test bytes; applying the test signal to a first of the pair of loops; detecting any bits or bytes on the second of the pair of loops; comparing the detected bits or bytes with the generated test bits or bytes to determine how many bits or bytes are matching; and determining the extent of crosstalk based on the number of matching test bits and / or bytes detected on the second of the pair of loops.

Description

[0001] Fire Alarm System Testing

[0002] Field of Invention

[0003] The present invention relates to the installation and commissioning of fire alarm systems on a premises.

[0004] Backg round

[0005] Fire alarm systems are installed in many premises, such as office buildings, factories, homes, and the like, and typically include a fire alarm control panel (often known as control and indicating equipment, CIE), a number of detectors and sounders, and wiring connecting the detectors and sounders to the fire alarm control panel. The system might also include call points and a range of other ancillary modules. In many cases, the wiring which is installed is a 2 wire addressable loop, and the detectors, sounders, call points and other ancillary modules (hereafter called "the devices") are arranged on the loop connected across its 2 wires. Multiple loops are generally installed from the control panel, with each loop typically carrying the devices to a different part of the premises. The loops typically provide power to the devices on the loop, and convey instructions and data from the control panel to the individual devices, such as configuration data to all devices or an alarm signal to the sounders, and convey data from the devices to the control panel, such as an indication that a fire has been detected. The looped arrangement means that there is some resilience to a break in an individual loop occurring during operation and that the voltage level between the wires in the loop are maintained sufficiently high to sustain all of the devices around the complete loop. In one known addressable fire alarm system, a fire alarm control panel can support up to 16 loops, and each loop can support up to 250 devices. In it, communication on the loop uses frequency shift keying (FSK) with sinusoidal signals on a DC pedestal of 37.6V.

[0006] The installation of an addressable fire alarm system begins with determining where each device in the system is to be located, and laying 2-wire cables in loops from the intended position of the control panel to the location of each intended device on a loop, with the loop ending back at the intended position of the control panel. The devices can then be attached to the cables and secured in position.

[0007] Once the system has been installed, it must be commissioned, which involves testing that everything is operating correctly and configuring all of the devices. This often takes place before the control panel has even been installed. Each loop can be as long as 2000 m, and the cable used to form it might be supplied by different manufacturers, so they have different electrical properties, and this can negatively affect communication between the control panel and the devices, potentially resulting in false alarms being triggered, or even no alarm being signalled when a fire condition is present. One detrimental characteristic that can occur in non-addressable systems, where cables are installed close by other cables, is capacitive coupling between them, causing a voltage to be induced in a cable when a high voltage is applied to another of the cables even when there is no electrical connection between them. To test the loops, it is known to apply a high voltage to one of the cables for a few milliseconds at a time, and detecting any increase in voltage in the cable being tested to see if there is capacitive coupling.

[0008] In addressable systems, coupling between loops can still be a problem, although it tends to manifest itself in the form of crosstalk, where data being communicated in one loop can be picked up in a different loop as a result of coupling. While the known testing for capacitive coupling can help, it isn't very accurate for addressable loops.

[0009] A portable commissioning tool can be arranged to conduct tests on the loops, such as ensuring that each wire has continuity, and by applying a DC voltage across one end of the loop and testing that the voltage at the opposite end of the loop is maintained at a sufficiently high voltage that it is able to operate all of the devices around the loop. The portable commissioning tool also communicates with each device on the loop once they have been installed in order to configure them, as appropriate.

[0010] An aim of the present invention is to improve the testing of the system during commissioning in order to reduce faults which arise later and to identify how prone the system is to crosstalk.

[0011] Summary of Invention

[0012] According to a first aspect of the invention, a method of determining crosstalk between a pair of installed two-wire fire alarm loops of an addressable fire alarm system, comprises: generating a test signal which includes a string of test bytes, each test byte containing test bits; applying the test signal to a first of the pair of loops; detecting any bits or bytes on the second of the pair of loops; comparing the detected bits or bytes with the test bits and / or test bytes to identify test bits and test bytes on the second of the pair of loops; and determining the extent of crosstalk based on the number of test bits and / or bytes detected on the second of the pair of loops.

[0013] The present invention is intended to reduce the number of faults which are found later in the commissioning process and after commissioning of a fire alarm system is complete, especially where the cables of several loops are installed in close proximity to each other. This close proximity can cause crosstalk between the cables. If crosstalk is too high, it can negatively affect communication between the control panel and the devices, potentially resulting in false alarms being triggered, or even no alarm being signalled when a fire condition is present.

[0014] In one arrangement, the determining step includes comparing the number of test bits detected on the second of the pair of loops with a series of ranges to determine in which range the crosstalk falls, each one of the ranges representing a different rating of a second measure of the crosstalk. Additionally or alternatively, the determining step may include comparing the number of test bytes detected on the second of the pair of loops with a series of ranges to determine which range the crosstalk falls, each one of the ranges representing a different rating of a first measure of the crosstalk.

[0015] The determining step may include determining the proportion of the test bits that are detected on the second of the pair of loops with the proportion giving a value representing a second measure of the crosstalk. Additionally or alternatively, the determining step may include determining the proportion of the test bytes that are detected on the second of the pair of loops with the proportion giving a value representing a first measure of the crosstalk.

[0016] In one arrangement, one end of the first of the loops is connected across a pair of terminals, and one end of the second of the pair of loops is connected across another pair of terminals. In this case, the other ends of the pair of loops may be connected as spurs. Both ends of the first of the loops may be connected in a loop to the same pair of terminals, and both ends of the second loop may be connected in a loop to the same pair of terminals.

[0017] According to a second aspect of the invention, a fire alarm loop tester arranged to test the crosstalk between a pair of installed two-wire addressable fire alarm loops of a fire alarm system, the tester comprises: a signal generator arranged to generate a test signal encoded with a string of test bytes, each test byte containing test bits; a signal terminal arranged for connection across a first of the pair of loops and arranged to apply the test signal across that loop; a decoder circuit arranged for decoding any bits or bytes on the second one of the pair of loops; and a processor arranged to compare the decoded bits or bytes from the decoder circuit with the test bits or bytes and to produce a comparison signal indicative of matching bits and / or bytes; wherein the processor is further arranged to determine the extent of the crosstalk between the pair of loops based on the number of test bits and / or test bytes in the detected bits and / or bytes. In one arrangement, the processor is arranged to compare the number of test bits decoded from the second of the pair of loops with a series of ranges to determine in which range the crosstalk falls, each one of the ranges representing a different rating of a second measure of the crosstalk. Additionally or alternatively, the processor may be arranged to compare the number of test bytes decoded from the second of the pair of loops with a series of ranges to determine which range the crosstalk falls, each one of the ranges representing a different rating of a first measure of the crosstalk.

[0018] The processor may be arranged to determine the proportion of the test bits that are decoded from the second of the pair of loops with the proportion giving a value representing a second measure of the crosstalk. Additionally or alternatively, the processor may be arranged to determine the proportion of the test bytes that are decoded from the second of the pair of loops with the proportion giving a value representing a first measure of the crosstalk.

[0019] In one arrangement, the signal terminal is a pair of terminals for connection across the first of the pair of loops.

[0020] A second pair of terminals may be included for connection across the second of the pair of loops.

[0021] Brief Description of the Drawings

[0022] The present invention will now be described by way of example only with reference to the accompanying drawings in which:

[0023] Figure 1 is a schematic diagram of a fire alarm system and a portable commissioning tool which forms part of the present application;

[0024] Figure 2 is a schematic diagram of a fire alarm system and a portable commissioning tool according to the present invention with a part of the commissioning tool being shown in more detail with a decoder circuit;

[0025] Figure 3 is a diagram showing a test byte of a loop signal applied to one end of a loop;

[0026] Figure 4 is a schematic diagram of the decoder circuit of the commissioning tool of Figure 2;

[0027] Figure 5 is a flow diagram showing a method of determining the quality of signal communication of a two-wire addressable fire alarm loop according to the present invention; Figures 6A & 6B are graphs showing the loop signal applied to a first loop 8, and the loop signal on the second loop 9 where there is very little crosstalk and where there is a lot of crosstalk, respectively; and

[0028] Figures 7A and 7B are graphs showing the loop signal applied to a first loop and the loop signal on the second loop 9 where there is some crosstalk, and showing the loop signal applied to the second loop and the loop signal on the first loop 9 where there is some crosstalk.

[0029] Detailed Description

[0030] Once a fire alarm system has been installed, it needs to be commissioned as described above, and in this case, a portable commissioning tool according to the present invention is used, and is connected to the ends of two loops, as shown in Figure 1. While the portable commissioning tool according to the invention is novel compared with known ones, it is intended that it would retain the functionality to conduct known system testing too. Figure 1 shows a fire alarm system 1 having 2-wire addressable network wiring in the arrangement of a first loop 8 with two ends, and a second loop 9, also with two ends. Both loops 8 & 9 include a number of addressable networked devices 4 attached to them. A portable commissioning tool 5 is shown having four terminals 3 to which an end of the two loops 8, 9 are connected. Although the loops 8 & 9 are shown connected as spurs with the opposite ends open in this embodiment, they can be connected in closed loop configuration with the ends of the individual wires of a loop connected together.

[0031] The addressable networked devices 4 can be any of a range of different fire alarm system devices, including : sensors 4a such as smoke detectors, heat detectors, fire detectors and the like; notification devices 4b such as sounders and strobes; and other ancillary modules such as call points 4c which are typically found on the loop of a fire alarm system. The devices 4 are connected across the wires of the loop such that they are powered from the loop and are able to transmit and receive data to and from other devices on the loop, such as the portable commissioning tool 5. Base units are often connected directly to the wires of the loop, and the devices may then be attached to the base units for easy connection to the network loop at a location defined by the position of the base unit (not shown).

[0032] The portable commissioning tool 5 is able to test for crosstalk from the first loop 8 to the other loop 9 and to determine the extent of the crosstalk.

[0033] It is advantageous to install the addressable network loops and the devices 4 and test them using the portable commissioning tool 5 before the control panel is fitted. Then the loops are connected to the panel, and the ends are connected together to make a closed loop. The closed loop connection is an established way to ensure resilience in the event of the continuity of a loop being broken because the devices on both sides of the break remain connected to the panel.

[0034] Figure 2 shows the fire alarm system of Figure 1 in which a part of the portable commissioning tool 5 is shown in more detail with its electronic components arranged in a block diagram. The tool 5 is connected to the first and second loops 8, 9 with the networked devices 23, 25 located on the loops 8, 9. The first loop 8 is a two wire loop having a first wire 21 and a second wire 22, and the loop has a first end 27, 28 and a second end 31, 32. The first end 27, 28 of the wires 21, 22 are each connected to a terminal 3 of the portable commissioning tool 5. The second loop 9 is a two wire loop having a first wire 35 and a second wire 36, and the loop has a first end 29, 30 and a second end 33, 34. The first end 29, 30 of the wires 35, 36 are each connected to the other of the terminals 3 of the portable commissioning tool 5.

[0035] The portable commissioning tool 5 includes a signal generator 11 which, during a test, generates a loop signal encoded with a string of test bytes, and that string of test bytes is supplied to the terminals 3 which apply the loop signal to the first end 27, 28 of the first loop 8. In this embodiment, the encoding used to modulate the loop signal is frequency shift keying (FSK) using sinusoidal signals on a DC pedestal of 37.6V. The general structure of a data packet under this encoding method is shown in Figure 3 in which the packet comprises: a header; a byte comprising an 8 bit data packet; and a stop bit. The data is encoded such that the frequency of a "0" is much higher than the frequency of a "1". In this case, the frequency of a "0" is 6667Hz, and that of a "1" is 3030Hz, and it is clearly seen from Figure 3 that, when converted to a pulse width signal, a "0" is represented with a much narrower pulse width than a "1". Of course, it is to be understood that FSK encoding could use different frequencies from this embodiment, and could use a different pedestal voltage. Indeed, different encoding systems altogether could be used.

[0036] The first end 29, 30 of the second loop 9 is connected to the other of the terminals 3 so that the first end 29, 30 of the second loop 9 is connected to a voltage decoder circuit 15.

[0037] The voltage decoder circuit 15 comprises a discriminator circuit 16 arranged to demodulate any received loop signal and an overcurrent sense circuit 17. This enables the detection of any bits or bytes on the second loop 9 by decoding any signal on the second loop 9, and for any received bits or bytes to be compared with the test bits or test bytes generated by the signal generator 11. The decoder circuit receives the attenuated received loop signal in this embodiment. The discriminator circuit 16 filters FSK AC signal from DC pedestal and converts the FSK frequencies 6667 Hz and 3030 Hz into a decoded byte signal in the form of a PWM signal representing "0" and "1" respectively as can be seen the "Discriminator Output" square wave in Figure 3.

[0038] The decoded byte signal and the decoded bit signal are directed to a processor 18.

[0039] The processor 18 compares the string of test bytes of the decoded loop signal with the string of test bytes of the loop signal generated by the signal generator 11.

[0040] Ideally, when there is no crosstalk, there should not be any test byte decoded by the voltage decoder circuit 15

[0041] The processor 18 compares the test bits of the decoded loop signal with the test bits of the loop signal generated by the signal generator 11.

[0042] Ideally, when there is no crosstalk, there should not be any test bit decoded by the voltage decoder circuit 15.

[0043] There is also an overcurrent sense circuit and return path switching circuits receiving any received loop signal.

[0044] The processor uses the ranges and thresholds from the following table to determine the extent of Crosstalk between the two Loops 8 & 9.

[0045] The portable commissioning tool also includes a main MCU, a main circuit breaker control, a voltage discriminator, a current loop amplifier and overcurrent sense circuit, and a control circuit for loop isolator. However, the components described above give a skilled person sufficient information about how to carry out the invention without the need to describe these features in detail.

[0046] Figure 6A is a graph showing the loop signal applied to the first loop 8, and the loop signal on the second loop 9 where there is very little crosstalk from the first loop 8 to the second loop 9. It will be seen that the loop signal applied to the first loop 8 is a sine wave using FSK, as described above, with the individual waves varying in frequency depending on the value of the bit being transmitted. The signal on the second loop shows no corresponding waves, only a small amount of noise.

[0047] Figure 6B is a graph showing the loop signal applied to the first loop 8 and the loop signal on the second loop 9 where there is a large amount of crosstalk from the first loop 8 to the second loop 9. As in Figure 6A, the loop signal applied to the first loop 8 is a sine wave using FSK. The signal on the second loop shows a similar sine wave to that for the first loop, except that the amplitude of the wider sine waves is lower, which might make bits corresponding to those waves harder to identify. In any event, the high level of crosstalk means that a large number of bits and of bytes can be counted on loop 2, and a large number of the bits and bytes match between the signal on the first loop 8 and the signal on the second loop 9. In this example, the high level of crosstalk would represent a failure of the installation of the fire alarm system, and rectification needs to be done before the commissioning process is complete.

[0048] It will be appreciated that testing for crosstalk is unidirectional. In the examples given above, crosstalk is determined for a signal in the first loop 8 being induced in the second loop 9 by coupling. Once this is complete, crosstalk should be tested for the other way around, where a signal in the second loop 9 is induced in the first loop 8. The crosstalk can be found to be different simply by reversing the loops on the portable commissioning tool 5. To carry out the reverse test, the first ends of the first loop 8 and the second loop 9 are disconnected from the terminals 3, and are reconnected to the terminals 3, but with the first end 27, 28 of the first loop 8 connected to the terminals which are connected to the voltage decoder circuit 15, and with the first end 29, 30 of the second loop 9 connected to the terminals which are connected to the signal generator 11.

[0049] If there are other loops in the system, the crosstalk between all of them should be tested in turn using the portable commissioning tool 5. Figure 7A shows the loop signal applied to a first loop (Loop 1), the demodulated loop signal applied to the Loop 1, the loop signal received from a fourth Loop (Loop 4) where there is some crosstalk, and the demodulated loop signal received on Loop 4 in which several false bits are identified, representing crosstalk. Loops 1 and 4 are then swapped on the terminals, and Figure 7B shows the loop signal applied to the Loop 4, the demodulated loop signal applied to the Loop 4, the loop signal received from Loop 1 where there is some crosstalk, and the demodulated loop signal received on Loop 1 in which several false bits are identified, representing crosstalk. It will be noted that there is much more crosstalk from Loop 4 to Loop 1 than from Loop 1 to Loop 4. The number of bits and / or bytes present in the decoded loop signal is proportional to the extent of crosstalk from the first loop 8 to the second loop 9. By measuring and counting the number of bits and the number of bytes in the loop signal, it is possible to make an assessment of the extent of crosstalk. The worse the crosstalk, the greater the number of bits and / or bytes detected in the decoded loop signal. The system is able to operate within crosstalk quality range, but once it exceeds a certain level, which corresponds to the number of detected bits and / or bytes exceeding a certain number, the system may not function properly. By measuring the number of bits and / or bytes, it is possible to assess whether the crosstalk is lower than a threshold for reliable operation.

[0050] The extent to which the test bytes of the decoded loop signal match the test bytes applied to the loop depends on the extent of crosstalk from the first loop 8 to the second loop 9. The worse the crosstalk, the greater the number of matches. The system is able to operate within crosstalk quality range, but once it exceeds a certain level, which corresponds to the number of matches exceeding a certain number, the system may not function properly. By measuring the number of matches, it is possible to assess whether the crosstalk is lower than a threshold for reliable operation. This might be done, for example, by comparing the number of matches between the test bytes of the decoded loop signal and the test bytes of the loop signal applied to the loop. The extent to which the test bytes of the decoded loop signal match the test bytes applied to the loop depends on the extent of crosstalk. The worse the crosstalk, the greater the number of matches.

[0051] The extent to which the test bits of the decoded loop signal match the test bits applied to the loop depends on the extent of crosstalk from the first loop 8 to the second loop 9. The worse the crosstalk, the greater the number of matches. The system is able to operate within crosstalk quality range, but once it exceeds a certain level, which corresponds to the number of matches exceeding a certain number, the system may not function properly. By measuring the number of matches, it is possible to assess whether the crosstalk is lower than a threshold for reliable operation. This might be done, for example, by comparing the number of matches between the test bits of the decoded loop signal and the test bits of the loop signal applied to the loop. The extent to which the test bits of the decoded loop signal match the test bits applied to the loop depends on the extent of crosstalk. The worse the crosstalk, the greater the number of matches.

[0052] The portable commissioning tool also has a number of other functions which are not described in detail here because they don't impact on the present invention. For example, the tool is able to carry out continuity tests on the individual wires 21 and 22 of the first loop 2, and is able to communicate with the networked devices 4 on the loop 2 during configuration, for example to configure and set up the networked devices, and to set their system addresses. These other tests are carried out on a single loop, in this example, the first loop 8, which means that the first end 27, 28 of the loop 8 and the second end 31,31 are connected to the terminals, and since there are four ends, all four of the terminals are used for a single loop. Since these different processes require different kinds of connections to the ends of the wires, the portable commissioning tool 5 also includes a switch array 11 containing five controllable switches which can be opened and closed on the signal of a switch controller 13.

[0053] The process of determining the extent of crosstalk of the two-wire addressable fire alarm loop 2 will now be described with reference to the flow chart in Figure 4.

[0054] The first step 71 is to connect the first end 27, 28 of the wires 21, 22 of the first loop 8 to one pair of the terminals 3 of the portable commissioning tool 5 and to connect the first end 29, 30 of the wires 35, 36 of the second loop 9 to the other pair of terminals 3 of the portable commissioning tool 5. The words 'first end' and 'second end' in this context have no technical meaning, but simply indicate that the loops 8 & 9 have two ends. The loops could, for example, be connected by their second ends 31, 32 and 33, 34 to the terminals.

[0055] If there is a switch array 12, it is configured using the switch controller 13 such that the signal generator 11 is connected to the terminal 3 to which the first end 27, 28 of the first loop 8 is connected, and the voltage decoder circuit 15 is connected to the terminal 3 which is connected to the first end 29, 30 of the second loop 9. The switch controller 13 might be controlled by a main MCU.

[0056] In step 75, the signal generator 11 is activated to generate the loop signal encoded with the string of test bytes described above using FSK encoding. Each test byte includes 8 test bits, and 9 bytes are generated in a pulse lasting for 25ms. The pulse is paused for 15 ms to allow for the loop signal to be processed, and the pulse is repeated. In this example, the pulse is sent 100 times. Of course, the number of bytes transmitted in a pulse can be changed, as appropriate, and it is possible in some instances for the loop signal to have shorter pauses between pulses, and even no pause at all.

[0057] In step 77, the loop signal is applied to the first end 27, 28 of the first loop 8.

[0058] In step 79, any loop signal on the second loop 9 will pass from the first end 29, 30 of the second loop 9 to the terminal 3 to which the first end 29, 30 of the first loop 9 is connected.

[0059] In step 81, since the voltage decoder circuit 15 is connected to the terminal 3, any received loop signal is received by the discriminator circuit 16 which, in step 83, decodes the received loop signal to produce a decoded loop signal of bits and bytes which is directed to the processor 18.

[0060] Step 85 involves the processor 18 comparing the decoded bytes with the string of test bytes applied to the first loop. It identifies which of the bytes match, and which do not, and generates the byte comparison signal indicating the bytes which match and those which don't. In parallel, the step 85 involves the processor comparing the decoded string of test bits with the string of test bits applied to the first loop. It identifies which of the bits match, and which do not, and generates the bit comparison signal indicating the bits which match and those which don't.

[0061] It will be appreciated that comparing one of the bytes or the bits might be enough to identify enough matches, but it is advantageous to use both because it gives more data comparison points for a particular loop signal, either improving accuracy of the assessment of crosstalk, or permitting crosstalk to be assessed more quickly than if only one of the bytes and the bits were used.

[0062] In step 87, the processor receives the byte and the bit comparison signals and determines crosstalk from the byte and the bit comparison signals. The more bytes and bits that are matched, the higher the level of crosstalk. If the number of matches or the proportion of the bytes or bits which match is below a lower threshold, the crosstalk is rated 'low". If the number of matches or the proportion of the bytes or bits which match is above the lower threshold but below an upper threshold, the crosstalk is rated as 'medium'. If the number of matches or the proportion of the bytes or bits which match is above the upper threshold, the crosstalk is rated as 'high'. In this embodiment, there is high crosstalk when one or more bytes are detected. The lower bit threshold is 100, and the upper bit threshold is 1000. This gives ranges for the ratings:

[0063] In this embodiment, where the byte-matching and bit-matching have different ratings, the worst of the two ratings is used. However, in other embodiments, it might be appropriate to use the better of the two ratings, or to combine the different results in some way, perhaps also with some other measure of the signal quality to determine the rating.

[0064] It will be appreciated that the number of byte or bit matches which represent excellent, good or poor signal quality will depend on the particular system being tested, and on the number of test bytes and test bits applied to the first loop. Furthermore, different ratings might be used instead of 'Low', 'Medium' and 'High', such as a numerical rating.

[0065] Alternatively, the crosstalk might be determined by the proportion of the received test bytes or bits which are matches.

[0066] Various further modifications to the above described examples, whether by way of addition, deletion or substitution, will be apparent to the skilled person to provide additional examples, any and all of which are intended to be encompassed by the appended claims.

Claims

Claims1. A method of determining crosstalk between a pair of installed two-wire fire alarm loops of an addressable fire alarm system, comprising : generating a test signal which includes a string of test bytes, each test byte containing test bits; applying the test signal to a first of the pair of loops; detecting any bits or bytes on the second of the pair of loops; comparing the detected bits or bytes with the test bits and / or test bytes to identify test bits and test bytes on the second of the pair of loops; and determining the extent of crosstalk based on the number of test bits and / or bytes detected on the second of the pair of loops.

2. The method of claim 1, wherein the determining step includes comparing the number of test bits detected on the second of the pair of loops with a series of ranges to determine in which range the crosstalk falls, each one of the ranges representing a different rating of a second measure of the crosstalk.

3. A method of any one of the preceding claims, wherein the determining step includes comparing the number of test bytes detected on the second of the pair of loops with a series of ranges to determine which range the crosstalk falls, each one of the ranges representing a different rating of a first measure of the crosstalk.

4. A method of claim 1, wherein the determining step includes determining the proportion of the test bits that are detected on the second of the pair of loops with the proportion giving a value representing a second measure of the crosstalk.

5. A method of claim 1 or 4, wherein the determining step includes determining the proportion of the test bytes that are detected on the second of the pair of loops with the proportion giving a value representing a first measure of the crosstalk.

6. A method of any one of the preceding claims, wherein one end of the first of the loops is connected across a pair of terminals, and one end of the second of the pair of loops is connected across another pair of terminals.

7. A method according to claim 6, wherein the other ends of the pair of loops are connected as spurs.

8. A method according to claim 6 wherein both ends of the first of the loops are connected in a loop to the same pair of terminals, and both ends of the second loop are connected in a loop to the same pair of terminals.

9. A fire alarm loop tester arranged to test the crosstalk between a pair of installed two-wire addressable fire alarm loops of a fire alarm system, the tester comprising : a signal generator (11) arranged to generate a test signal encoded with a string of test bytes, each test byte containing test bits; a signal terminal (3) arranged for connection across a first of the pair of loops and arranged to apply the test signal across that loop (2); a decoder circuit (15) arranged for decoding any bits or bytes on the second one of the pair of loops; and a processor (18) arranged to compare the decoded bits or bytes from the decoder circuit with the test bits or bytes and to produce a comparison signal indicative of matching bits and / or bytes; wherein the processor is further arranged to determine the extent of the crosstalk between the pair of loops based on the number of test bits and / or test bytes in the decoded bits and / or bytes.

10. The fire alarm loop tester according to claim 9, wherein the processor is arranged to compare the number of test bits decoded from the second of the pair of loops with a series of ranges to determine in which range the crosstalk falls, each one of the ranges representing a different rating of a second measure of the crosstalk.

11. The fire alarm loop tester according to claim 9 or 10, wherein the processor is arranged to compare the number of test bytes decoded from the second of the pair of loops with a series of ranges to determine which range the crosstalk falls, each one of the ranges representing a different rating of a first measure of the crosstalk.

12. The fire alarm loop tester according to claim 9, wherein the processor is arranged to determine the proportion of the test bits that are decoded from the second of the pair of loops with the proportion giving a value representing a second measure of the crosstalk.

13. The fire alarm loop tester according to claim 9 or 12, wherein the processor is arranged to determine the proportion of the test bytes that are decoded from the second of the pair of loops with the proportion giving a value representing a first measure of the crosstalk.

14. The fire alarm loop tester according to any one of claims 9 to 13, wherein the signal terminal (3) is a pair of terminals for connection across the first of the pair of loops.

15. The fire alarm loop tester according to any one of claims 9 to 14, further comprising a second pair of terminals for connection across the second of the pair of loops.

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