Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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20~~7~4
Tel.eco~municati~ras lime test system
The present invention relates to a device capable of
investigating a fault in a telecom~uniaations system, par.
ticularly slang a telecommunications line between an exchange
and a subscriber.
It is clearly desirable that a fault be quickly located
in order that it be quickly put right. In addition to
determining location, by which we mean determining the
general area of a fault, it is also desirable to deteranine
the type of fault since this knowledge will help one to
locate the fault more precisely. Thus it is desirable to be
able to determine whether a fault is a short circuit, for
e~ta~ple between tip and ring (to use USA terminology)
telephone conductors, an open circuit or an earthed conduc-.
tor.
The increasing use by subscribers of their own equip~
ment, which is simply plugged into the telephone co~pany~s
system by the subscriber, has made it desirable that one can
determine on which side of the plug a fault lies. The
reason is that a fault on the exchange side of the plug and
socket is the responsibility of the telephone company, and a
fault the other side may be the responsibility of the
subscriber. A fault on the subscriber's side may be due to
faulty subscriber equipment or to faulty connection, and in
either case location of the fault can save time and the
telephone company's expense by avoiding unnecessary line
testing.
Thus, devices have been proposed for incorporation at
the boundary between the telephone company's and the
subscriber°s responsibility. In particular such devices may
be part of the socket into which a subscriber plugs his
telephone or other equipment.
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Japanese patent specifiation A 58-.b3~60 for e~:ample
describes a fault detector far communications equipment
which ena tiles precise determination of the nature of a fault
and the place of its occurence to be made from the telephone
exchange, and so makes rapid repair possible.
This prior art fault detector is said to comprise a
control part and a deteetor part, the control part comprising
diodes inserted in series between the pair of conductors of
a telephone~lane and the subscriber, and a varistor con-
nected across the two canductors. The varistor must have a
threshold value higher than the threshold values of the
diodese 'The~detector part comprises a variable voltage
power unit, a current detector which can detect the eurrent
flowing in the conductors as a function of the variable
voltage applied, and a voltage detector.
~Te haae found however that the circuitry of the control
part of the prior art fault detector has some significant
disadvantages. Firstly, the voltage required to be applied
to the telephone line has to be higher than is preferred for
saf~ty reasons (more than 100 volts is required), a sharp
current-voltage curve cannot be relied on, and the detec-
tor s performance is likely to fluctuate with temperature.
An alternative device has now been designed that can
overcome these problems.
Thus, the present invention provides a device capable
for use in investigating a fault in a telecommunications
system comprising a first and a second part interconnected
by a line, which device comprises:
(a) a first circuit that can be connected in series
with the line and comprising a first capacitor and a first
component, such that an alternating current can pass along
the line via the capacitor and a direct current can pass
Vf~ 91/11g'Y2 P~'1~119110~1~9
20~~'~~4
along the line via the first component in at least one
direction with a threshold valtage; and
(b) a second circuit that can be connected in parallel
across the line and comprising a second component having a
threshold voltage of greater than ~t8 volts and/or being
able to pass direct current only in one direction,
The invention also pravides a device capable for use in
investigating a fault in a telecommunications system
comprising~a first and a second part interconnected by a
line, which devise comprises:
(a) a first circuit that can be connected in series
with the line and comprising a first component, such that an
alternating current can pass along the line and a direct
current can pass along the line via the first component in
at least one direction with a threshold voltageg and
(b) a second circuit that can be connected in parallel
across the line and comprising a second compcanent~comprising
at least one zener diode and having a threshold voltage of
greater than 48 volts and/or being able to pass direct
current only in one directions
The invention further provides a device capable for use
in investigating a fault in a telecommunications system
c~mprising a first and a second part interconnected by a
line, which device comprises:
(a) a first circuit that can be connected in series with
the line and comprising a first component comprising a zener
diode, such that an alternating current can pass along the
line via the capacitor and a direct current can pass along
the line via the first component in at least one direction
with a threshold voltage; and
(b) a second circuit that can be connected in parallel
across the line and comprising a second component having a
threshold voltage of greater than ~d8 volts and/or being able
to pass direct current only in one direction.
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A telecommunications line generally comprises a pair of
conductors from the first part (such as an exchange) to the
second part (such as subscriber equipment?, the single pair
carrying information, such as speech, in both directions.
In this case the second component referred to above is con-
nected across those tcao conductors. A line may however
comprise four conductors, in two pairs, each pair carrying
information in a single direction. In that case a device of
the invention may be provided for each pair.
We prefer that the first circuit campr.ises a capacitor
and a first component in series with each of the conductors
of a pair. In this case, we will refer to a first capacitor
and a first component in series with one conductor and a
second capacitor and third component in series with the
second,conductor. This terminology is used because the pre-
cise characteristics (such us capacitance and threshold
voltage) of the two capacitors and of the first and third
components may differ, althau~h they will Function as
defined above.
We prefer that the first, second and third components
each comprises a zener diode. In any case, each of said com-.
ponents may comprise two or more physically separate
electrical or electronic components in electrical connection.
The second component preferably has a threshold voltage
of greater than 48 volts, more preferably greater than 55
volts, especially from 6Q-75 volts. Where the threshold
voltage is less than 4~ volts, the usual operating voltage of
a line, some means should be provided to ensure that
the subscriber apparatus is not shorted out at the normal
operating voltage. This may be achieved by employing means,
such as a diode, for preventing current passage through the
second component in a direction opposite to that at which it
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exhibits its threshold voltage. Thus, the system is nor-
mally operated using a source of direct current such that
the second circuit is non-conductive. The polarity is
reversed for at least part of the test,. under which con-.
ditions the threshold.voltage may be exhibited. The
threshold voltage of the second component is preferably
greater than that of ~he first circuit (generally the sum of
that of the first and third components) unless said means
for preventing the opposite passage of current is-employed.
The rbason is that otherwise the second circuit will
not come into play. ~,~here the threshold. voltage of the
second component is greater than ~8 volts (the normal as
opposed to the test voltage) this problem need not arise and
the device will be operable with either polarity of normal
operating voltages between the conductors of the line. This
is desirable because in many countries the polarity that is
established on mating a telephone call is random.
The general principle of operation of the device can
now be seen. What is required is a unique voltage-current
curve corresponding to each of the fault conditions in the
line to be tested. ~ voltage is applied, either between the
two conductors of the line or between one of them and earth,
depending on the test; and the current flowing in the line is
measured. This is done for selected different voltages or
continuously over 3 range of voltages. The current voltage
curves will differ ,,~aepending on where a fault lies, because
the different faults will cause current to follow different
paths. For example the current may flow through the first
circuit, through ~he seCOnd circuit, through part of one or
both circuits, or not at all. The voltage is preferably
applied, and the current and voltage measured at the
telephone exchange. Thus the device of the invention may be
used for remote investigation.
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The first, second and third components will give rise
to characteristic sudden increases in current at their
threshold voltages, V1,UZ and V3 respectively and at those
voltages prefers bly become substantially conducting. For
example the total circuit may then have a resistance of less
than 2000, prefers bly less than 1100 ohms. A threshold
voltage is therefore a predetermined voltage or voltage
range at,or over which a component or d circuit exhibits a
sharp change in resistivity. If the current-voltage curve
shows a sudden increase in current at about Vl, then one may
conclude ttaat the current is flowing through the_ first com-
ponent. ~ Similarly, if an increase oocur~s at V2 + V3, then
current is flowing through the second and third components,
but not through the first (at least not in the direction in
~rhich the threshold voltage is exhibited). We prefer that
the threshold voltage of the second circuit is greater than,
particularly by at lease 15 volts, especially by at least 30
volts, that of the first circuit.
As an example, normal operation of the telecom-
munications system may involve current flooring through the
first circuit. The capacitors are preferably provided in
order that an alternating signal may pass substantially
unrestricted to allow a ringing tone (generally an AC signal
superimposed on a ~?C bias of usually ~t8 volts) and that a DC
audio signal or other signal may to be passed between the
first part and the second part. If the threshold voltage of
the first and third (where provided) components together is
much less than the operating voltage of the system, the
direct current for normal operation may be applied at either
polarity. .If the total of the threshold voltages is higher,
then normal operation will require connection such that
current flows in the opposite direction to that at which the
threshold voltages are exhibited. In that ease the polarity
has to be reversed for at least part of the test.
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It may be possible that the first and third components
exhibit their threshold voltages at opposite polarities, ie
they are connected in series back-to-back. In this case the
normal operating voltage will need to~be higher than only
one of the threshold voltages.
We prefer that the device be able to distinguish between
two or more of the following conditions:
1. A short circuit within the first part of" the system
(for example the telephone company°s property)
2. An open circuit within the first part
A short within the second part tfor example the
Subscriber°s property)
4. An open circuit within the second part
5. An earthed conductor within the second part
An earthed ring conductor within the first
part.
?. An earthed tip conductor within the first part,
(The terms °~ring°° and~~tip'~ are standard terms used to
distinguish between the two conductors of a line. In a non-
symmetrical system, for example as used is the USA, the tip
is generally at 0 volts and the ring at -~.$ volts. In
Europe and elsewhere, the system is symmetrical and the
polarity applied is random.)
The device should also be substantially transparent,
preferably exhibiting a voltage drop of less than 15 pre_
ferably less than 10 volts, during normal direct current
operation of the line. It should also be substantially
transparent to alternating current in order that a ringing
tone may be transmitted.
The invention is further illustrated with reference to
the accompanying drawings, in which:
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Figure 1 shows a telecommunications system;
Figure 2 shows a prior art device;
Figures 3 and a show devices of the invention;
Figure 5-7 show various faults in a telecommunications
system;
Figures 8 and g show current-voltage relationships
obtained from devices of the invention; and
Figures 10 and 11 show a preferred device and related
current-voltage relationship. __
A telecommunications system is represented schemati-
cally in Figure~t. An exchange 1 is connected to a
subscriber ~ by a line 3 comprising tip and ring conductors
and 5 respectively. The dotted line shows a possible
division between the telephone company's responsibility and a
subscriber°s responsibility. A device of the invention may
be planed in the line 3 at the position of the dotted line in
order that one can deter~aine whether a fault is the respon-
sibility of the telephone company or the subscriber.
Preferably the devioe of the invention is incorporated in a
connection block which also comprises a housing and a
telephone socket. The block may be mounted on a wall of a
building, The invention may be used at other positions in a
telecommunications system and "1°' may be regarded as a first
part of a system (as referred to above ) and ~~~°° as a second
part.
A prior art device for locating a fault is shown in
Figure 2. This device comprises an electrical circuit
of two pairs of diodes b, the pairs being in series with the
tip ~J and ring 5 conductors respectively. A varistor 7 is
connected across the tip and ring conductors. The diodes of
each pair are connected in opposite fashion. We have found
that this circuit is somewhat susceptible to temperature
variation and the current_voltage relationships for the
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various different faults to be investigated are not suf-
ficiently distinct. We have also found that ~he circuit
causes some attenuation to a signal during normal use of the
line, and requires unacceptably high test voltages.'
The present invention adopts a different approach, as
is ill ustrated in Figures 3-g.
In,Figure 3 capacitors 9 and zener diodes 10 in
parallel are together connected in series with respective
conductors of the line. For some investigations a
capacitorlzener diode pair may be required in only one of
the lines. A zener diode 10 and ordinary diode 11 (ie a diode
that is substantially nonconductive in one direction over
the voltage range at which it is to be used, preferably up
to 80 volts) are connected in opposite fashion between the
tip conductor ~i and ring conductor 5. In a modification of
this design, component 11 is also a zener diode.
The first and second circuits can now be seen. The
first circuit comprises the wen diode Z1 (the first
component referred to above) the telephone 8 (or rather a
connections therefor since the telephone is not part of the
circuit per se) the zener diode Z3, and the two capacitors
9. The second circuit comprises the zener diode Z2 (the
second component referred to above) and the ordinary diode
11.
For nornual operation of the telephone a voltage of
generally 48 volts is applied between the tip and ring con
doctors. (Tip and ring is terminology used in the USA where
the line is non-symetric, but it will be used here also to
refer to symetric lines although designation of each line
will be arbitiary.) The telephone company will first have
sent a ringing tone (where the telephone 8 is receiving a
Ball) by means of an alternating current, and this can flow
~~ 91/1172 PC.'I°/G1391/0 :9
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in the first circuit through the capacitors 9 with little or
no attenuation. The capacitors preferably are substantially
transparent to audio frequencies, and preferably have a
capacitance of at least 5 microfarads~,especially at least 10
microfarads. The second circuit will not be in use because
the threshold value of Z2 is 70 volts, ie greater than the
48 volts operating voltage, and because of the diode 11,
In the embodiment illustrated, the zener diodes Z1 and
Z3 have threshold voltages of 20 and 30 volts respectively,
Since the voltage applied for normal use (48U) is less than
the total voltage drop for reverse operation of these diodes
(ie the total threshold voltage of the circuit) the circuit
will only work at signal frequencies at which the capacitors
are substantially transparent. Thus, the capacitors may be
chosen.to allow passage of normal audio frequencies.
Preferably the capacitors have a capacitance of at least 10
micrafarads. 4Jhere polarity is predeterztined, preferred
values for Z1 and Z3 are 5-40 especially 1S-25 volts for Z1,
and 15-~5 especially 25-35 volts for Z2. The total
threshold voltage of the first circuit is preferably less
than 70 volts, especially 40-60 volts.
A test is made by reversing the polarity so that the
tip conductor 4 is negative with respect to the ring conduc-
tor 5. The voltage is then increased from 0, up to a
value somewhat above the greater of the threshold voltages
of the first and second circuits, in this case 70 volts.
The current flowing is measured as a function of voltage.
The current-voltage relationship may be determined
continuously over, say, the range 0-75 volts, or it may be
sampled at values for example slightly below and above each
of the threshold voltages, ie 18,22,28,32,48,52,68 and 72
volts. Ldhat is desired is a different current-voltage rela-
tionship for each fault. The path the current takes, and
'~'~ ~l/1i~72 PCf/~~9~/~129
1 1 -
therefore the zener diode or diodes whose threshold value or
values must be reached, will depend on the location of the
fault. Examples are given for various faults in Figures 8 and
9.
The device shown in Figure 4 is a modification of that
of Figure 3, designed for normal telephone operation under
either polarity. Again, component 11 may be a zener diode
thus making the device completely symetric. __
Since the telephone 8. must work under either polarity,
zener diodes 21 arid Z3 must not provide an excessive voltage
drop for either direction of current flow in this ease we
prefer that each has a threshold voltage of 15 volts or
less, preferably less than 10 volts, more preferably less
than ?,volts, and that their combined threshold voltage is
less than 2p volts. It is also desirable that their
threshold voltages be different (to be explained below) and
we prefer values of 1.5~4, especially 2.5.3.5 volts, and 4-7
especially 4.5 to 5.5 volts.
Again, since current may flow in either direction during
normal telephone operation, the threshold voltage of Z2 must
be greater than the normal operating voltage, usually 48
volts. If this were not so, the telephone would be short
circuited by the second circuit during normal operation under
one of the two polarities.
In Figure 4 a telephone or other equipment 8 is repre-
sented by its resistance, and the resistances of the lines
are shown as 12. A reverse polarity switch 13 connects the
line to a source of variable voltage power 14 for testing,
During testing the applied voltage is measured at X and the
current flowing is measured at Y, for example by measuring
the voltage drop across a known resistor. 'The outputs from
voltmeter X and ammeter Y may be plotted as the X and Y axes
of a graph, as shown in Figures $ and 9.
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20~3~64
Figure 4 therefore shows a control unit C, a line
(and exchange etc) L a device of the invention D and a
subscribe rs telephone S. The device D will determine
whether a fault falls in region G or region S and whether
that fault is a short. circuit, an open circuit or an earth.
Figure 5 shows an open circuit 1~ at the subscriber.
On application of a voltage for testing, any current must
flow thr~ugh the second circuit, and this will be_indicated
by a current-voltage above showing, if line 5 is made posi_
tine, the threshold voltage of the second component, ie
zener diode Z2. If line 4 is made positive, no current will
flow due to diode 11. An open circuit on the exchange side,
ie region L of Figure ~d, would prevent current flowing
completely,
Figure 6 shows a short 16 on the exchange side, The
current-voltage curve will. now show no threshold voltage
since the current path involves neither the first nor second
circuit. The slope of the curve will be substantially
constant, and will give the resistance of the line (together
with that of the short itself). The same result will be
obtained for either polarity.
In Figure 7 the fault is an earth connection 17 at the
subscriber. This is revealed by applying the test voltage
between one of the conductors 4 and 5, and earth. In
general it will be necessary only to apply the voltage bet-
ween one of the conductors and earth, but the test may be
continued using the other conductor. The current will now
flow through part of the first curcuit, ie through the zener
diode Z1, but not through the zoner diode Z3~ The current-
voltage curve will therefore exhibit the threshold voltage
of Z1 when conductor 5 is positive and no threshold voltage
when conductor ~ is negative with respect to earth. If the
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voltage were applied between the other conductor and earth,
the theshold voltage of Z3 would be exhibited. It will
not always be necessary to extend the test over both polari-
ties since this will depend on the amount of information
required, and on the ahreshold values of the circuits. An
earth fault on the exchange side will mean no current will
flow through the first or second circuits, and the current-
voltage,curve will not show a threshold voltage. This fault
is however distinguishable from the fault of Figure 6
because different tests are required to produce the similar
current-voltage curves. Also, the curves will be slightly
different since the resistances of the two resulting current
paths are likely to be different.
Figure 8 shows the current-voltage curves produced from
the device of Figure 3 under the following faults
Figure 8a short at exchange or line
Figure 8b short at subscriber
Figure 8c earth at exchange or tip conductor of line
Figure 8d earth at exchange or ring conductor of line
Figure 8e earth at subscriber
Figure 8f open circuit at exchange or line
Figure 8g open circuit at subscriber.
The tests were carried out as explained in relation to
Figures 5-7. In each case the X-coordinate of the graph is
the voltage measured as X in Figures 5_7 with positive values
refering to a positive ring conductor, and the Y~coordinate
is the current measured as at Y in Figures 5-7.
The graph of Figure 8b is similar to what would be
obtained if the test were carried out in the absence of the
fault. It will not, however, generally be necessary iden-
tify a lack of a fault by means of a test, The graph may
be less steep in the absence of a subscriber fault due to
the added resistance of the telephone.
fyt'~91/~1g72 PCf/G~9yl~4. ,9
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The threshold values of Z1 + Z3 can be seen in Figures
8b and 8e, of Z1 in Figure ~e and of Z2 in Figure 8g.
Figures 9a-9g correspond to Figures 8a-8g, but apply to
a modified c.ireuit of Figure 3 where the threshold voltages
of Z1,ZZ and Z3 are 3,4 and 5 volts respectively, and where
Z3 is reversed. This shows that Z2 need not be higher than
both Z1 and Z3.
A particularly preferred device is shown in Figure 10,
and the various current-voltage relationships it generates
are sh~~an in Figues 11A-ilE. 'this device may be entirely
symmetric. and is therefore ideal for European telephone cir-
cuits where the imposed polarity is random. The circuit of
Figure 10 includes four zener diodes 10, namely Z1,Z2,Z3 and
Z4. A short circuit during normal telephone operation and
during the ringing tone is avoided by the threshold voltages
for Z~ and Z4 being greater than the operating voltage, far
example greater than 46 volts which is the DC voltage on
which an AG ringing signal is often impressed. The
threshold voltages of Z2 and Z4 may conveniently be 100-130
volts. The z~ner diodes Z1 and Z3 may if desired have low
threshold voltages, for example in the range of a few volts,
for example less than 10 volts, preferably less than 5
volts, especially less than 3 volts. This avoids an unaccep-
table DC voltage drip across one of them (across which will
depend on the operating plarity) of the DC bias voltage
during normal operation. The zener diodes 21 and Z3 may
have the same threshold voltage, during but they may nave
different voltage in which case earthing at the subscriber
of respective lines will be distinguisliable.
The capacitors 9 preferably each have a capacitance of
Greater than 5, especially greater than 10 microfarads. The
pairs of diodes Z1, Z3 and/or Z2,Z4 may be reversed where
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15 _
the device is to be used on lines of fixed polarily (as is
the case in the USA) one only of the diodes Z1 and Z2 may be
reversed.
Because the circuit is symetrical, the current/voltage
graphs will be in principle symmetrical. E~ience in Figures
11A-.11E only the top right quadvants of the graphs have been
completed, the lower left hand quadrants will be similar,
although~the precise threshold voltages may be different.
Figure 11A shows the normal situation when the telephone
is not being used. The threshold voltage of Z2 or Z4 (which
may be different 5ut are preferably substantially equal)
will be apparent.
In Figure 11B an open circuit at the exchange or line
results in no current flow.
Earth leakages of two different resistance at the
exchange or line is shown in Figure 11C.
The threshold voltage of identical diodes Z1 or Z3 is
shown in Figure 11D where there is earth 1°akage on the
subsscribers side. Again two different leattage resistances
are shown. This system cannot distinguish between a leakage
from line 4 and from line 5.
In Figure lltr zener diodes Z1 and Z3 have different
threshold voltages allowing the position of a subscriber
earth leakage to be determined. The graph shows two leakage
resistanees at line a (left hand pair of curves) and at line
(right hand pair of curves).
For the avoidance of doubt it is here stated that the
invention provides a device for investigating a fault in a
telecommunications system where a useful current voltage
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relationship can be generated, particularly a threshold
voltage which is distinguishable from noise often to be
found in telecommunications lines. 'the device may camprise
any one or more of the various components interconnected in
any of the various ways described herein. nor example any one
or more of the zener diodes, capacitors, switches, power
souroes, and measuring devices may be selected.
