GB2234598A

GB2234598A - Testing multiconductor cables

Info

Publication number: GB2234598A
Application number: GB8916968A
Authority: GB
Inventors: Peter Mayo; John Tilson
Original assignee: PM COMMUNICATIONS Ltd
Current assignee: PM COMMUNICATIONS Ltd
Priority date: 1989-07-25
Filing date: 1989-07-25
Publication date: 1991-02-06
Also published as: GB8916968D0

Abstract

Cable test apparatus for testing a multiconductor cable having a termination device (Fig 3) connected to the conductors at one end of the cable (not shown) and a test device (Fig 4) connected to the conductors at the other end. The termination device has inputs (9) each connected to a common conductor (12) via a predetermined resistance (10). In the test device a voltage source Vcc is sequentially applied (D1-D8) to each of respective outputs (1-8) causing current to flow through the conductor connected to that output, through the appropriate resistor in the termination device and back to the test device through the other conductors. The amplitude of the current is monitored (20-24) in the sequence to detect an open circuit, a short circuit or a crossed wire. The latter is detected by the current level undergoing a decrease during the sequence. <IMAGE>

Description

CABLE TEST APPARATUS The present invention relates to a cable test apparatus.

There are many circumstances in which it is desirable to test the continuity and integrity of installed cabling systems. For example multi-pair cables are used to interconnect networked computer terminals, the cables generally being concealed in ducting. It is necessary to check that such cabling systems have been correctly wired up before a system is commissioned. The traditional approach to this is to apply signals to one end of a cable and monitor the signals appearing at the other end of the cable. The objective is to detect open circuits, short circuits and crossed wires. This traditional approach requires two people working in synchronism even if they are sometimes in different rooms and direct visual or spoken communication between them is not possible.

Thus not only is the test work difficult to conduct effectively but also the need to have two skilled technicians to test one installation represents a major cost to the system installer.

It is an object of the present invention to provide an improved cable test apparatus which obviates or mitigates the problems outlined above.

According to the present invention, there is provided a cable test apparatus for testing a multiconductor cable, comprising a termination device for connection to one end of the cable to be tested, and a test device for connection to the other end of the cable, wherein the termination device comprises inputs for connection to respective conductors of the cable, each input being connected by a respective impedance to a common conductor within the termination device, and wherein the test device comprises outputs for connection to respective conductors of the cable, means for sequentially connecting the outputs to a voltage source, means for monitoring the amplitude of current flowing from the source through the respective output to the termination unit and back to the test device through one or more of the other outputs, and means for indicating a fault in the cable connected to the output if the monitored current amplitude is different from a predetermined current amplitude.

Preferably the cable provides a multiple path return to the test device such that useful results can be obtained even if one or more conductor in the cable fails to an open circuit condition.

Preferably means are provided for indicating when the monitored current exceeds an upper threshold (indicating a short circuit) or falls below a lower threshold (indicating an open circuit).

Preferably the impedances in the termination device are all different and are arranged such that if the cable is appropriately connected they are connected to the voltage source through the test device in an order corresponding to the magnitudes of their impedances. Thus if the impedances are simple resistors the current level returned to the test device assuming all is in order gradually rises or gradually falls through each test cycle. Means are preferably provided to integrate the monitored current signal and to indicate a fault if the monitored current does not change in a progressive manner as expected.

Preferably the test device also comprises means for detecting earth faults.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a front view of a test device forming one part of an embodiment of the present invention; Fig. 2 is a side view of the arrangement of Fig.

1; Fig. 3 is a circuit diagram of a termination device which forms a component of an embodiment of the present invention; Fig. 4 is a diagram of the circuit of a test device of the type illustrated in Figs. 1 and 2; and Fig. 5 is a circuit diagram of a switching component incorporated in the circuit of Fig. 4.

Referring to Fig. 1 and 2, the illustrated device comprises a casing 1 receiving a circuit to be described in detail below, an earth clip 2 to enable the device to be connected to a convenient earth point such as for example a central heating radiator, a connector 3 to which a cable 4 to be tested is in use connected, and a button 5 which when depressed initiates a test cycle. A switch 6 is also provided to enable the user to select a test mode appropriate for a two pair, a three pair or a four pair cable. Eight green LEDs 7 are arranged in pairs PR1, PR2, PR3 and PR4 and four red LEDs 8 are arranged to indicate crossed wires, short circuits, open circuits and earth faults respectively.

Referring now to Fig. 3, this shows the circuit of a termination device which is connected to the far end of the cable from the device illustrated in Figs.

1 and 2. Thus the termination device has eight inputs 9 each of which is connected to a respective conductor of a four pair cable and each of which is connected by a respective resistor 10 and diode 11 to a common conductor 12. Typical values for the resistances are indicated in Fig. 3 and it will be seen that they increase in resistive value from 110 ohms to 580 ohms.

Each of the inputs 9 is allocated a number corresponding to a number on the output connector 3 of the test device. If the test device and termination device are correctly interconnected by a cable the resistance connected to each output of the test device will always have a predetermined value.

Referring now to Fig. 4, terminals identified by numerals 1 to 8 indicate the outputs to which respective wires of the cable are connected. Each of these terminals is connected to an associated driver device D1-D8 and each of these driver devices D1 to D8 receives a respective output of a decoder 13 having outputs Y0 - V7 normally changed to voltage Vcc via resistor 14. A timer circuit 15 drives a counter the output of which is decoded by the circuit 13. Thus each of the driver circuits D1 to D8 receives an input in a predetermined sequence with no two driver units receiving an input simultaneously.

Fig. 5 illustrates the circuitry of the driver units. The signal from the decoder 13 appears at a terminal 16 and the respective conductor of the cable is connected to terminal 17. Each driver circuit comprises a first transistor 18 and a second transistor 19. All of the source terminals of the transistors 19 are interconnected to a common conductor 20. The arrangement is such that the transistors 19 are turned off when selected by the decoder 13 but are otherwise conductive and the transistors 18 are turned on when selected by the decoder 13 but are otherwise non-conductive. Thus only one of the outputs 1 to 8 carries the positive voltage applied to the emitter of the transistor 18.

The voltage appearing on the single selected output 17 causes the current to flow through the conductor attached to that output, through the resistor in the termination unit to which that output is connected, and back to the test device through the other seven conductors. A signal corresponding to the total magnitude of the current appears on output 20 and is passed through amplifier 21 to an arrangement of comparators 22, 23 and 24. Comparator 22 indicates a fault if the monitored current is greater than 43 mA. This fault is deemed to be a short circuit.

Comparator 23 indicates a fault if the monitored current is less than five mA. This fault is deemed to be an open circuit. Comparator 24 compares the voltage at the output of the amplifier 21 with the voltage across a capacitor 25 which in effect integrates the signal 20. The relationship between the outputs 1 to 8 and the resistors in the termination unit assuming that all connections are made appropriately is such that the current level will increase gradually during each eight step switching cycle. If on the other hand wires are crossed the monitored current will decrease from one step in the cycle to another and this can be detected by comparator 24 to indicate a crossed wire fault.

An earth leakage detection circuit has terminal 26 connected to the earth clip of Fig. 1 and provides an earth fault output in the event of an earth leakage current being detected.

The various faults are indicated on LEDs 27, 28, 29 and 30, these LEDs being illuminated in the event of crossed wire, short circuit, open circuit and earth fault respectively.

Assuming that no faults are detected a transistor 31 is turned on providing a conductive path to eight green LEDs 32. Each of the LEDs 32 is connected to a respective one of the conductors 17 at the output of the driver circuits D1 to D8. Thus current is drawn through each of the green LEDs 32 in turn assuming of course that there are no faults in the system. A repetition rate of 1.5 kMz for the test sequence gives a continuous display of the cable status.

Thus the termination device of Fig. 3 provides a unique impedance for each conductor within the cable and current is returned to the test device through all the other conductors of the cable. This guarantees a usable return path to the test device providing at least two conductors are intact even if those two intact conductors are not connected correctly. The conductors within the cable are scanned sequentially until all conductors have been tested and the results of the test are clearly displayed on the test device.

The circuit is reset after the eighth test and is then ready for a fresh cycle.

To avoid confusion to the operator in the event of multiple faults being located a priority system is provided by the logic illustrated in Fig. 4 such that diode 33 will supress any crossed wire indications until all open circuits have been cleared, and diode 34 will suppress crossed wire indications until all short circuits have been cleared.

Cables of four or six conductors can be tested by selecting the appropriate position using the switch 35. The operation of the switch simulates the correct connection of conductors to terminals 7 and 8 if a six cable test is desired and to terminals 5, 6, 7 and 8 if a four cable test is required.

Thus the described instrument is an automatic scanning device providing unique testing facilities for a variety of cable types and capacities. The termination device is entirely passive and its use eliminates the need for a two man test team. This is of fundamental significance from the economic point of view.

Claims

CLAIMS:

A cable test apparatus for testing a multiconductor cable, comprising a termination device for connection to one end of the cable to be tested, and a test device for connection to the other end of the cable, wherein the termination device comprises inputs for connection to respective conductors of the cable, each input being connected by a respective impedance to a common conductor within the termination device, and wherein the test device comprises outputs for connection to respective conductors of the cable, means for sequentially connecting the outputs to a voltage source, means for monitoring the amplitude of current flowing from the source through the respective output to the termination unit and back to the test device through one or more of the other outputs, and means for indicating a fault in the cable connected to the output if the monitored current amplitude is different from a predetermined current amplitude.
2. Cable test apparatus according to claim 1 wherein the cable provides a multiple path return to the test device such that useful results can be obtained even if one or more conductor in the cable fails to an open circuit cond-v-on
3. Cable test apparatus according to claims 1 or 2 wherein means are provided fo indicating when the monitored current exceeds an upper threshold (indicating a short circuit) or rallys below a lower threshold (indicating an open circuit).
4. Cable test apparatus according to claim 1 wherein the impedances in the termination device are all differert and are arranged such that if the cable is approprnâtely connected they are connected to the voltage source through the test device in an order corresponding to the magnitudes of their impedances.
5. Cable test apparatus according to claims 1 or 3 wherein means are provided to integrate the monitored current signal and to indicate a fault if the monitored current does not change in a progressive manner as expected.
6. Cable test apparatus according to any preceding claim wherein the test device also comprises means for detecting earth faults.
7. Cable test apparatus substantially as hereinbefore described with reference to the accompanying drawings.