GB2195512A - Telephone line tester - Google Patents

Abstract

An instrument for testing a subscriber's telephone line from the subscriber's end of the line has two terminals A' and B' respectively connectible to the corresponding conductors of the subscriber's line and across which is permanently connected a high voltage test circuit 1 for testing whether an undesirable high voltage is present on the subscriber's line. The instrument also includes a polarity check circuit 2 for checking whether or not a pair of lines have become reversed; a line voltage check circuit 3 for checking whether or not the D.C. voltage on the pair of telephone lines is within prescribed limits; and a loop impedance check circuit 4 for checking the loop impedance of the pair of lines. Each of the test circuits 2,3 and 4 can be selectively connected in parallel with the circuit 1 by operation of an associated push-switch 5. <IMAGE>

Description

SPECIFICATION Telephone line tester The present invention relates to an instrument for testing subscribers telephone lines. Telephone line testing equipment is well known and in common use but is generally intended for testing the subscribers lines from the telephone exchange end and is moreover of a fairly complex nature suitable for use by engineers and technicians.

It is an object of the present invention to provide a telephone line tester which is intended for use at a subscribers end of a telephone line and which can readily be used by an unskilled person to give a meaningful indication of the state of that subscribers line.

According to the present invention, an instrument for testing a subscribers telephone line from the subscribers end of the line comprises a circuit for detecting if an undesirable high voltage is present on the subscribers line, means for selectively connecting at least one other test circuit for testing another parameter in parallel with said high voltage test circuit, means for indicating the condition of the subscribers line in response to each test and means for connecting said instrument to a subscribers line.

Other test circuits which may be selectively connected to the subscribers line may comprise one or more of: a) a polarity check circuit for checking whether or not a pair of lines have become reversed.

b) a line voltage check circuit for checking whether or not the voltage on a pair of telephone lines is within prescribed acceptable limits.

c) a loop impedance check circuit for checking the loop impedance of a pair of lines.

According to a preferred form of the invention the telephone line tester includes all of the above test circuits a), b) and c), each of which may selectively be brought into operation.

The indicating means which indicate the condition of a telephone line may be visual, such as a lamp, light emitting diodes or a meter; or audible such as a tone. The con necting means preferably comprise a tele phone plug similar to that on a subscribers telephone so that the instrument can be plugged into the telephone jack, now commonly provided to terminate a subscribers -line, in place of the subscribers telephone.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block circuit diagram of one embodiment of telephone line tester according to the invention, Figure 2 shows one form of high voltage test circuit, Figure 3 shows one form of polarity check circuit, Figure 4 shows one form of line voltage test circuit, Figure 5 shows one form of loop impedance check circuit, and Figure 6 is an illustration showing how the telephone line tester may appear as an instrument ready for use.

It will be understood that the telephone line tester of the present invention is intended to be employed to test a subscribers telephone line (which consists of a pair of actual conductors or lines A and B) from the subscribers end. In practice, one line A is a grounded line and the other line B is at a fairly low D.C.

voltage, for example -50 volts D.C., when the line is in a stand-by state.

Figure 1 shows the basic circuit of the telephone line tester which has two terminals A' and B' respectively connectible to the corresponding conductors A and B of the subscribers line and across which is connected a high voltage test circuit 1 for testing whether an undesirable high voltage is present on a subscribers line. This circuit 1 is permanently connected between terminals A' and B'. The tester also includes a polarity check circuit 2 for checking whether or not a pair of lines A and B have become reversed; a line voltage circuit circuit 3 for checking whether or not the D.C. voltage on the pair of telephone lines is within prescribed limits; and a loop impedance check circuit 4 for checking the loop impedance of the pair of lines.Each of the test circuits 2, 3 and 4 can be selectively connected in parallel with the circuit 1 by operation of an associated push-switch 5 which only remains closed whilst its push button or the like is being pressed.

The whole instrument may optionally be protected by a safety circuit 6, shown in broken lines, and comprising series and/or shunt connected components, such as series fuses and a shunt gas discharge tube, which operate to protect a user against undesirable high voltages or lightning.

Figure 2 shows one embodiment of the high voltage test circuit 1 which tests for the presence of high A.C. voltages on the line pair A,B. This dangerous condition can occur through a high voltage cable, such as amains cable, coming into direct metallic contact with the telephone line for example due to a fault in a cable duct or an overhead cable. The circuit comprises a neon lamp N1 in series with a capacitor C1 and resistor R1. The component values are chosen such that the neon is not struck if only normal 50V exists on the telephone lines, but will strike at a voltage slightly higher than 50V A.C. and will pulse on and off if ringing current exists on the lines.

Figure 3 shows one embodiment of the polarity check circuit 2 for checking whether or not the two lines A and B of a pair have become reversed. Such reversal can occur due to incorrect wiring at any junction box or at the telephone line jack. Although some telephones will work on a reversed pair, others will not and it is hence desirable that the correct polarity of the lines is always maintained.

The circuit comprises a green light emitting diode LED 1 having a yellow light emitting diode LED 2 connected across it with reverse polarity between the -50V line terminal B' and a series resistor network R2, R3, R4, which is in turn connected to the grounded line terminal A'. A pair of zener diodes D1,D2 are connected back-to-back across the two light emitting diodes. The component valves are selected so as to work from the low line voltages but so as not to be damaged by the presence of high voltages, e.g. mains voltages.

If the line polarity is correct LED 1 is illuminated green, but if the polarity is reversed LED 2 is illuminated yellow.

Figure 4 shows one embodiment of the line voltage check circuit 3 for checking whether or not the line voltage on a pair of lines A,B is within prescribed acceptable limits. The level of voltage on a pair will normally give a good indication of a potential cable fault. If the nominal service voltage is say -50V, then voltages outside the range -48V to -57V can indicate a fault ranging from a cable short circuit or high leakage circuit to a high voltage power cross. As shown, the line voltage is fedviaterminals A', B' to a rectifier bridge RB1 whose output is fed via a resistor network R5,R6, to an input of a voltage level detector VELD 1. The voltage level detector is shown as powered from a battery B1, for example a 9 volt battery, but the circuit could also be line powered.Four light emitting diodes, namely two red illuminated LED 3, LED 4, one green illuminated LED 5 and one yellow illuminated LED 6 are connected via respective resistors R7 to RIO between the battery positive line and outputs of the voltage level detector VELD 1. This may comprise an integrated circuit comparator chip such as a type LM 239, a Schmitt trigger arrangement or a combination ofNAND and NOR circuits.The level indications given by the circuit are: Battery voltage low-LED 3 lights red Line voltage high-LED 4 lights red Line voltage correct-LED 5 lights green Line voltage low-LED 6 lights yellow The component values are chosen and the voltage level detector is set to allow for the natural voltage drop caused by the loop resistance of the telephone pair. The outputs from the level detector can be designed to give either a slow transition from one working level to the next or a snap change so that only one LED is illuminated.

Figure 5 shows one embodiment of the loop impedance check circuit 4 for checking the loop impedance of the pair of telephone lines A,B. If the loop impedance looking into the telephone line network is outside of a range of say 20 to 2500 ohms, it is likely that a cable fault exists. As shown, the input terminals A',B' are connected through a changeover switch S1 to a signal detector SD1 and a signal generator SG1. Three outputs of the signal detector are connected respectively to a red illuminated LED 7, a green illuminated LED 8, and a yellow illuminated LED 9. These light emitting diodes are respectively connected through resistors R11, R12 and R13 to the positive line of a supply provided by battery B2.By means of the changeover switch S1, an AC or DC signal from the signal generator SG1 can be fed onto one side of the pair of lines A,B and the detector SD1 which is connected to the other line of the pair detects the presence of the loop signals to give an indication loop impedance as follows: Open circuit or high impedance--1ED 7 lights red Normal impedance-LED 8 lights green Short circuit or low impedance-LED 9 lights yellow The test voltages and frequencies of signal used are chosen such that they will not be misread by the telephone network as commands to dial or other instructions.

Figure 6 illustrates one form which the telephone line tester, as described above, may take in practice. The various circuits are contained in a casing 10 of metal or impact resistant plastics material and connection to the circuits is made via an input cable 11 terminating in a plug 12 which fits into a usual type of telephone jack connected to the lines A and B of a subscribers telephone line.

When the line tester is to be used, all the telephones at the subscribers premises are disconnected and the instrument is then plugged into the first or primary telephone line jack and the various conditions on the line tested. The condition of the line is indicated by the light which is lit for each test. If a fault is shown to exist, then it is a fault on the incoming telephone line provided by the telephone service. Alternatively, if the tests show that the incoming line is satisfactory, the line tester can then be plugged into any subsequent line jacks in order to check the telephone wiring on the premises.

A telephone subscriber can thus readily check the state of both the incoming line and the line wiring on his premises in order to determine who is responsible for correcting the fault.

It will be understood that various modifications may be made without departing from the scope of this invention. Thus the instrument need not incorporate all of the test facilities described and the various circuits may differ from those specifically described.

Moreover the lights or other indicating means may also be interpreted to indicate other conditions on a subscribers line, such as A.C. on the line, ringing on the line, an open circuit line or no voltage on the line.

It will also be understood that although the circuits have been specifically described in relation to a telephone line of which one line A is grounded and the other is at -50 volts D.C.; the actual voltages employed and the polarity of the lines A and B may vary in different countries or territories according to local practices.

Claims (9)

1. An instrument for testing a subscribers telephone line from the subscribers end of the line comprises a circuit for detecting if an undesirable high voltage is present on the subscribers line, means for selectively connecting at least one other test circuit for testing another parameter in parallel with said high voltage test circuit, means for indicating the condition of the subscribers line in response to each test and means for connecting said instrument to a subscribers line.

2. The instrument as claimed in claim 1 wherein said at least one other test circuit comprises a polarity check circuit for checking whether or not a pair of lines have become reversed.

3. The instrument as claimed in claim 1 or 2, wherein said at least one other test circuit comprises a line voltage check circuit for checking whether or not the voltage on a pair of telephone lines is within prescribed acceptable limits.

4. The instrument as claimed in claim 1,2 or 3, wherein said at least one other test circuit comprises a loop impedance check circuit for checking the loop impedance of a pair of lines.

5. The instrument as claimed in the combination of claims 2,3 and 4, wherein each of said other test circuits may selectively be brought into operation.

6. The instrument as claimed in any preceding claim,wherein said indicating means which indicate the condition of a telephone line are visual, such as a lamp, light emitting diodes or a meter.

7. The instrument as claimed in any one of claims 1 to 5, wherein said indicating means are audible, such as a tone.

8. The instrument as claimed in any preceding claim, wherein said connecting means comprise a telephone plug similar to that on a subscribers telephone so that the instrument can be plugged into a telephone jack, provided to terminate a subscribers line, in place of the subscribers telephone.

9. An instrument for testing a subscribers telephone from the subscribers end of the line, said instrument being substantially as herein described with reference to the accompanying drawings.