Abstract:
A method and apparatus for detecting a fault in a joint connecting sections of an electrical transmission line together are disclosed. Previously known methods for detecting joint faults require a visual inspection of the joint or testing the transmission line using sophisticated, expensive equipment. This manual testing is expensive and inefficient. In the proposed method, a fault in a joint ( 301 ) connecting sections of an electrical transmission line ( 107 ) together is detected by measuring the resistance to current flowing through the joint ( 301 ) in one and the other directions along said electrical transmission line ( 107 ) and detecting a fault in the joint ( 301 ) if the measured resistance differs substantially in said one and the other directions. The method has particular utility in relation to low power transmission lines such as telephone lines.

Description:
This application is the US national phase of international application PCT/GB2005/000900 filed 9 Mar. 2005 which designated the U.S. and claims benefit of GB 0407198.1, dated 30 Mar. 2004, the entire content of which is hereby incorporated by reference. 
   FIELD OF TECHNOLOGY 
   This invention relates to a method and apparatus for detecting a fault in a joint connecting sections of an electrical transmission line together. It has particular utility in relation to low-power transmission lines such as telephone lines. 
   BACKGROUND 
   The “access network” is that part of a telecommunications network between customers&#39; premises and a local exchange (end office in the United States). Pairs of copper or aluminium wires provide the signal transmission links used in connecting the local exchange to the customer&#39;s premises. The pairs of wires are twisted and often buried underground. Usually, several twisted pairs leave the local exchange bundled together in cables that run to customers&#39; premises in the geographic area served by the local exchange. Each twisted pair is usually split into a plurality of sections, with sections being connected together by a joint. After sometime, the condition of the joint can deteriorate (e.g. by corrosion) which leads to a deterioration in the quality of the telephony service provided to the customer but often does not lead to a total loss of service. For voice communications, the customer experiences this deterioration in the form of a noisy, crackling or faint line. For data communications, the customer experiences this deterioration in the form of dropped connections and reduced data transfer rates. 
   Existing techniques of fault finding in a telecommunications network, such as those disclosed in U.S. Pat. Nos. 4,139,745 and 4,113,998, cannot detect these types of joint faults and when used to test a customer&#39;s telephone line report that the line is fault free even when such joint faults are present. In certain circumstances, the current flowing in the telephone lines (sometimes called a ‘wetting current’) can ‘blow away’ some of the corrosion in a corroded joint (this is sometimes known as the current having a “sealing effect) and thus can even improve the quality of the line. However, this is not a satisfactory solution since it will not permanently correct the fault. Moreover, faults in joints that are not caused by joint corrosion will remain undetected. 
   As mentioned above, a customer complaint about the quality of their telephone line (which, unbeknownst to them or the telephone company is owing to a joint fault) will pass a conventional line test. Until the advent of the present invention, an engineer had to be dispatched in order to either visually inspect joints for signs of deterioration or to test the telephone line between the exchange and the customer&#39;s premises using sophisticated, expensive testing equipment. This manual testing is both expensive (since an engineer has to be dispatched leading to increased manpower costs) and also inefficient. 
   SUMMARY 
   According to a first aspect of the present invention there is provided a method of detecting a fault in a joint connecting sections of an electrical transmission line together, said method comprising: measuring the resistance to current flowing through said joint in one and the other directions along said electrical transmission line and detecting a fault in said joint if the measured resistance differs substantially in said one and the other directions. 
   The inventors found that a deteriorating joint has an effect on the loop resistance of a telephone line that causes the resistance of the line, when current flows in the line in one direction, to differ from the resistance of the line when current flows in the other direction. Hence by detecting dependence of resistance to current flowing through a joint upon the direction of the current flow, a fault in the joint can be detected more reliably, efficiently and economically than has hitherto been the case. 
   Preferably the method further comprises applying the current to the electrical transmission line. Thus joint faults can be detected in transmission lines which are not constantly connected to a current supply. 
   In preferred embodiments, the electrical transmission line comprises a pair of electrical conductors extending between a telephone exchange and a customer&#39;s premises and the method further comprises connecting the electrical conductors together by applying a resistive load between the conductors, applying the current at the telephone exchange and remotely controlling a termination device to apply the resistive load between the conductors. Preferably the remote controlled termination device is situated in the customer&#39;s premises. Thus a fault in a joint connecting sections of a transmission line in a telecommunications network can be detected without the need to: a) dispatch an engineer to visually inspect one or more joints in the line; b) test the line with sophisticated test equipment; and c) install additional equipment in the customer&#39;s premises at the time of testing the line. 
   In an alternative embodiment, the method comprises applying the resistive load at the telephone exchange and applying the current at a point between the telephone exchange and the customer&#39;s premises. Thus a transmission line can be tested section by section for the presence of joint faults. 
   Preferably, the method comprises one of a sequence of tests carried out in order to test an electrical transmission line. Thus the method can improve the efficiency of previous testing methods in diagnosing faults. 
   Preferably, the sequence of tests is arranged such that the test to detect a fault in a joint is carried out after a) a test to check for connectivity between end points of said electrical transmission line indicates that said connectivity exists; and b) a test to check whether said electrical transmission line is in contact with earth and/or another electrical transmission line indicates that no such contact exists. Thus joint faults that would otherwise go undetected can be detected. 
   Preferably, the end points are a telephone exchange and a customer&#39;s premises. Thus faults in a telecommunications network that would otherwise go undetected but which lead to a deterioration in the quality of the telephony service provided to a customer can be detected. 
   According to a second aspect of the present invention there is provided apparatus for detecting a fault in a joint connecting sections of an electrical transmission line together, said apparatus comprising means for measuring the resistance to current flowing through said joint in one and the other directions along said electrical transmission line; and means for detecting a fault in said joint if the measured resistance differs substantially in said one and the other directions. 
   According to a third aspect of the present invention there is provided apparatus for detecting a fault in a joint connecting sections of an electrical transmission line together, said apparatus comprising a measurer operable to measure the resistance to current flowing through said joint in one and the other directions along said electrical transmission line; and a detector operable to detect a fault in said joint if the measured resistance differs substantially in said one and the other directions. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein like reference numbers refer to like parts, and in which: 
       FIG. 1  is a schematic illustration of a telephone network; 
       FIGS. 2   a - 2   c  show different operating modes of a network terminating device in the network of  FIG. 1 ; 
       FIG. 3  shows a circuit model of one customer line in the network of  FIG. 1 ; 
       FIGS. 4   a - 4   b  show a first embodiment of a loop line test; 
       FIG. 5  is a flow chart showing the interpretation applied to the results of the loop line test of  FIGS. 4   a  &amp;  4   b;    
       FIG. 6  shows a second embodiment of a loop line test; 
       FIGS. 7   a - 7   b  show examples of the test results obtained by the loop line test of  FIG. 6 ; 
       FIG. 8  shows a third embodiment of a loop line test; 
       FIG. 9  shows the method of performing a loop line test; 
       FIG. 10  shows the method of automatically performing a sequence of line tests; 
       FIG. 11  is a flow chart showing the interpretation applied to the results of the automatic line test of  FIG. 10 ; 
       FIG. 12  shows an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   With reference the  FIG. 1 , exchange  101  receives a trunk line  103  which connects exchange  101  to other exchanges in the telecommunications network. Trunk line  103  carries signals multiplexing many telephone calls and other communications made using the network. 
   Inside exchange  101 , trunk line  103  connects to switch  105 . To establish a connection for a telephone call, switch  105  connects trunk line  103  to a customer line  107 . Each customer line  107  comprises a twisted pair of copper wires. 
   Inside exchange  101 , many customer lines  107  are grouped together and leave the exchange bundled in cables  109  that are often buried underground. Cables  109  run near customer premises  111  throughout the geographic area served by the exchange  101 . Each customer premises  111  is connected to a cable through a drop wire  108 , which carries customer line  107  to the customer premises  111  either aerially or buried underground. Usually, cables  109  are split into a plurality of sections with sections being connected together by a joint  110 , also called a distribution point or a cross connect point. At the joint  110 , a cable  109  can be divided into two or more cables in order to serve different sub-areas within the geographic area served by exchange  101 . 
   Inside customer premises  111 , telephone signals are carried on customer wiring  115 . At the entry to customer premises  111 , the customer line  107  connects to a network terminating device (NTD)  117  which is the demarcation point between the access network (i.e. cables  109  including customer lines  107 ) and the customer wiring  115 . A suitable NTD is the NT Elite available from Spescom Limited UK, Spescom House, 53/55 Uxbridge Road, Ealing, London, W5 5SA, United Kingdom. Further functional details of NTD  117  will be given below. NTD  117  is connected to customer premises equipment (CPE)  119 , which is shown as comprising a telephone although it may alternatively comprise a facsimile machine or a modem, for example. 
   Exchange  101  includes a line test system (LTS) testhead  121  connected to switch  105 . LTS Testhead  121  could be part of a commercially available line test system, such as one available from Teradyne Inc., Broadband Test Division, 1405 Lake Cook Rd, Deerfield, Ill. 60015, USA. LTS Testhead  121  can be operated to perform various line tests on one or more customer lines  107  in order to detect and locate any faults on the customer line  107  under test. During a line test, switch  105  connects LTS Testhead  121  to the customer line  107  under test. LTS Testhead  121  is operable to generate or receive various test signals and measure various electrical properties including voltage, current, resistance, capacitance, inductance, charge stored on the line and impedance. A more detailed description of the line tests will be given below. 
   LTS Testhead  121  is controlled by a Test Controller  123  usually located remote to LTS Testhead  121 . Test controller  123  is operable to control LTS Testhead  121  to perform line tests on customer line  107 . Those skilled in the art will realise that Test Controller  123  can be operated to control additional LTS Testheads not shown in  FIG. 1 . Test Controller  123  also interprets the results of these line tests. 
   Test controller  123  is connected to a user interface  125  that is operable to output data to a user and accept data input by a user. From user interface  125 , a user can select a customer line  107  to be tested, select the line tests to be performed on that customer line  107  and view the results of the selected line tests. 
   The functionality of NTD  117  will be described in more detail with reference to  FIG. 2 . NTD  117  operates in two different modes. In a passive (PASS) mode, NTD  117  connects customer wire  107  to customer wiring  115  as illustrated schematically in  FIG. 2   a . NTD  117  operates in PASS mode when there is no line test being performed on the customer line and during a passive line test. In the simplest example of such a line test, a binary indication of whether or not there is a fault on the customer line  107  according to the passive line test can be obtained. An indication of ‘disconnection faults’ (where there is no connectivity between end points of a customer line  107 ) and ‘contact faults’ (where a customer line  107  is in contact with earth and/or another customer line) can also be obtained. The Teradyne line test system mentioned above is operable to perform such passive line tests and to interpret some of the test results. Those skilled in the art will already be aware of such line tests. Hence, they will not be described in any further detail. 
   NTD  117  can also operate in a loop (LOOP) mode, wherein NTD  117  is operable to isolate CPE  119  and customer wiring  115  from the network wiring. In LOOP mode, NTD  117  terminates the network wiring either in a closed circuit using a resistive load  201 , as illustrated schematically in  FIG. 2   b , or in a short circuit, as illustrated schematically in  FIG. 2   c . NTD  117  operates in LOOP mode during a loop line test, which will be described in more detail below. 
   A more detailed description of the various line tests that can be carried out by LTS Testhead  121  will now be given. 
   As has already been mentioned, each customer line  107  is usually split into a plurality of sections, with sections being connected together by a joint. After sometime, the condition of the joint can deteriorate (e.g. by corrosion). The inventors have found that joint deterioration causes the customer line  107  to exhibit ‘non-Ohmic’ behaviour, i.e. its current-voltage (I-V) characteristic does not conform to Ohm&#39;s Law (where current and voltage have a linear relationship). Thus, they found that a deteriorating joint can be modelled as a non-Ohmic resistance. With reference to  FIG. 3 , CPE  119  is connected to NTD  117  via customer wiring  115 . NTD  117  connects customer wiring  115  to customer line  107  which is connected to LTS Testhead  121 . Corrosion at a joint, which joins two sections of a customer line  107  together, is modelled as non-ohmic resistor  301 . Non-ohmic resistor  301  causes the resistance R AB  when current flows in one direction to differ from the resistance R BA  when current flows in the other direction. In the absence of joint deterioration, these two resistance values would be approximately the same. Hence joint deterioration can be detected by detecting a change in resistance of customer line  107  dependent upon the direction of current flow around the circuit. This is achieved by performing a loop line test. Owing to the effect joint deterioration has on the loop resistance, faults caused by joint deterioration are called unbalanced loop resistance (ULR) faults. 
   In a first embodiment of the loop line test, as shown in  FIGS. 4   a  and  4   b , NTD  117  operates in LOOP mode as described above, isolating CPE  119  and customer wiring  115  from the network wiring and terminating the customer line  107  in a resistive load  401 . A DC power source  403 , applied by LTS Testhead  121  as shown in  FIG. 4   a , drives a direct current around the loop. LTS Testhead measures the resistance R AB  around the loop. LTS Testhead  121  then reverses the polarity of the DC power source, as shown in  FIG. 4   b , and measures the resistance R BA  around the loop. Suitable values for the resistive load  401  and DC power source  403  are 600Ω and 50V respectively. However, the resistive load  401  could also be less than 600Ω, preferably even as small as 0Ω (short circuit) to 10Ω. 
   In order to determine whether there is a corroded joint and hence a ULR fault Test Controller  123  interprets the two resistance measurements according to the following rules ( FIG. 5 ):
     1. If R AB ≦R 1  and R BA ≦R 1 
       and |R AB −R BA |&gt;R 2  then a ULR exists.   
       2. If R AB &gt;R 1  or R BA &gt;R 1 
       and |R AB −R BA |&gt;R 3  then a ULR exists   
       3. In all other cases, no ULR exists.
 
where R 1 , R 2  and R 3  are threshold values. Examples of suitable threshold values are 892Ω, 5Ω and 10Ω respectively. If a perfectly accurate testing apparatus were achievable, it would be possible to use the single rule: “If |R AB −R BA |&gt;5Ω then a ULR exists”. In practice, however, testing apparatus is only accurate to different degrees for different ranges of measurements. Consequently, R 1  and R 3  have to be introduced to account for the inaccuracy of the apparatus. R 1 =892Ω and R 3 =10Ω are used in this example because the Teradyne line test system, of which LTS Testhead  121  can form a part, is accurate to 1Ω for 0&lt;R 1 ≦892Ω and accurate to 10Ω for R 1 &gt;892Ω. Those skilled in the art will realise, therefore, that R and R 3  can vary from these values depending on the accuracy of the line test system apparatus.
   

   In an alternative embodiment of the loop line test, as shown in  FIG. 6 , the DC power source  403  used in the first embodiment is replaced by an AC power source  601  producing a sinusoidal AC voltage as shown in  FIG. 7   a . A suitable value for the AC voltage is 50V supplied at a frequency of 8 Hz. As will be clear to a skilled person, the polarity of the voltage from AC power source  601  will periodically change direction and non-Ohmic resistor  301  will have a rectifying effect on the AC waveform, as shown in  FIG. 7   b . Rectification of the alternating current is therefore indicative of the change in loop resistance around the customer line in dependence upon the direction in which current flows around that loop and hence can be used to detect ULR faults caused by joint corrosion. One way that this can be achieved is to measure the DC component of the AC signal using a digital meter. The meter samples the current voltage waveform as shown in  FIG. 7   b  and if the sampling frequency is at least 4 times higher than the frequency of the sampled signal (i.e. at least 32 Hz in this case), averaging the samples should provide the DC component of the AC signal. This measurement will be more accurate for higher sampling frequencies. If there is no rectification, the measured DC component should be zero or very small. A large rectification results in a larger measured DC component. 
   Until the advent of the present invention, line test systems have not been able to detect ULR faults and the tests performed have all reported that the line having only a ULR fault is fault free. In a preferred embodiment of the invention, an automatic line test is selected by a user of user interface  125 . With reference to  FIG. 11 , the automatic line test consists of first performing a passive test, like the one described above, which can give a binary indication of whether or not there is, for example, a disconnection or contact fault on the customer line. If this passive test does not report ‘Test OK’, then further tests are needed to determine the exact nature of the fault. This is not relevant to the present invention and will not be described in any more detail. If, however, the passive test reports ‘Test OK’ then a loop line test is performed to check for the presence of ULR faults. 
   With reference to  FIG. 9 , the process of testing a line will now be described. In order to carry out a loop line test, a user  900  inputs (step  901 ), via user interface  125 , the customer line  107  that is to be tested and the nature of the line test that is to be carried out; in this case, a loop line test: User interface  125  forms a message containing the user inputted information and forwards the message (step  903 ) to Test Controller  123  which then sends a message (step  905 ) to LTS Testhead  121  requesting access to the specified customer line  107  in order to carry out the specified line test. LTS Testhead  121  then accesses (step  906 ) the specified customer line  107  via switch  105  and sends a message (step  907 ) to Test Controller  123  informing it that the specified customer line  107  has been accessed. Test Controller  123  then sends a message (step  909 ) to LTS Testhead  121  requesting that the specified test (in this case a loop line test) be carried out on the specified customer line  107 . LTS Testhead  121  then sends a signal (step  911 ) to the NTD  117  that terminates the specified customer line  107 , which signal causes NTD  117  to enter its loop mode of operation (step  912 ). NTD  117  sends a signal (step  913 ) to LTS Testhead  121  in order to acknowledge that it has entered loop mode at which time LTS Testhead  121  can take the relevant loop line test measurements (step  914 ) and send the results (step  915 ) of the test measurements to Test Controller  123 . Test Controller  123  then interprets the test results (step  916 ) (in this case, according to the rules described above in relation to the loop line test) and also sends a message (step  917 ) to LTS Testhead  121  informing it that access to the specified customer line  107  can be terminated. LTS Testhead  121 , via switch  105 , drops access (step  918 ) to the customer line  107 . Having interpreted the test results, Test Controller  123  sends a message (step  919 ) to User Interface  125  containing the interpreted test results and User Interface  125  displays the test result to the user (step  921 ) who can then take appropriate action (e.g. do nothing, dispatch an engineer etc.) depending on whether or not there is a ULR fault on the line. 
   With reference to  FIG. 10 , the process of automatically performing a sequence of passive and loop line tests (an automatic line test) will now be described. In order to detect ULR faults, a user  900  inputs (step  1001 ), via user interface  125 , the customer line  107  that is to be tested and the nature of the line test that is to be carried out; in this case, an automatic line test. User interface  125  forms a message containing the user inputted information and forwards the message (step  1003 ) to Test Controller  123  which then sends a message (step  1005 ) to LTS Testhead  121  requesting access to the specified customer line  107  in order to carry out the specified automatic line test. LTS Testhead  121  then accesses (step  1006 ) the specified customer line  107  via switch  105  and sends a message (step  1007 ) to Test Controller  123  informing it that the specified customer line  107  has been accessed. Test Controller  123  then sends a message (step  1009 ) to LTS Testhead  121  requesting that a passive test be carried out on the specified customer line  107 . LTS Testhead  121  then sends a signal (step  1011 ) to the NTD  117  that terminates the specified customer line  107 , which signal causes NTD  117  to remain in its passive mode of operation. NTD  117  sends a signal (step  1013 ) to LTS Testhead  121  in order to acknowledge that it is ready for the passive test at which time LTS Testhead  121  can perform the passive line test (step  1015 ) and send the results (step  1017 ) of the test to Test Controller  123 . Test Controller  123  then interprets the test results (step  1019 ) (in this case, according to the rules described above in relation to the automatic line test) and in the case where the result of the passive test indicates that the line is fault free, Test Controller  123  sends a message (step  909 ) to LTS Testhead  121  requesting that a loop line test be carried out on the specified customer line  107 . The process then continues through steps  911  to  921  as described above in order to determine whether or not a ULR fault exists on the customer line. It is to be understood that some fault is assumed present, since it is this that will have caused the customer to complain. 
   It will be apparent from the foregoing description that many modifications or variations may be made to the above described embodiments without departing from the invention. 
   For example, in a further alternative embodiment of the loop line test, as shown in  FIG. 8 , a resistive load  801  (or alternatively a short circuit) is applied to customer line  107  inside exchange  101  and the customer line  107  is terminated in an open circuit. The application of the resistive load  801  and/or the termination of the customer line  107  can be carried out by an engineer and/or LTS Testhead  121 . The termination of a customer line  107  can be carried out by either manually disconnecting CPE  119  or by operating NTE  117  to isolate CPE  119 . Portable test equipment  803  is then applied across the customer line  107  by an engineer. Suitable portable test equipment is the CopperMax®/OSP metallic portable test system available from Spirent™ Communications, Hamilton International Technology Park, High Blantyre, Glasgow, G72 OFF, UK. Portable test equipment  803  is operable to carry out passive line tests in addition to the same resistance measurements as described above in relation to the other embodiments of the loop line test. It is also operable to interpret the measurements in order to display to the engineer whether or not a ULR fault is present on the customer line  107  under test. The engineer can apply portable test equipment  803  to the customer line  107  at any point along customer line  107  between customer premises  111  and exchange  101 . 
   Although the above embodiments have been described in relation to a transmission line in a telecommunications network comprising a pair of wires twisted together, the invention is equally applicable to other types of transmission line including, for example, a transmission line supplying power to a sensitive, low power device where the current flowing in the transmission line is sufficiently small (e.g. less than 30 mA). For example, a current of between 4 mA and 20 mA used for industrial sensor current loop signalling. 
   An alternative embodiment is shown in  FIG. 12  where a transmission line, in two sections connected by a joint (modelled as non-Ohmic resistor  301 ), connects to a transformer  1201 . In this case, it is not necessary to apply a resistive load or short circuit in order to carry out the loop test. 
   Although in the above described embodiments a current was applied to the transmission line by LTS Testhead  121 , in other embodiments a current may already be flowing in the transmission line and hence applying the current is not necessary.