Method and device for testing a telecommunication cable

Description

The present invention relates to a method for testing a conductor element, applicable to locating a continuity defect of the conductor element. The present invention relates, in particular, to testing the electrical continuity of the metal screen of a telecommunication cable.

Current telecommunication networks, that have a tree structure, are produced by means of cables

1

with a large section of the type represented in

FIG. 1A

, comprising several hundred or thousand electric wires

2

insulated from each other by a suitable wrapping and arranged two by two to form telephone pairs. The assembly is protected from electric disturbance by a metal sheath, or screen

3

, that is itself covered by a protective sheath

4

made of an electrically insulating material such as polyethylene, PVC, . . .

Such telecommunication cables, arranged in the ground or in the air, are subjected to various attacks the most frequent of which are caused by lightning, rodents, road works, the rubbing of tree branches . . . These various attacks can lead to a tear

5

of the protective sheath and to the penetration of water into the cable. From an electrical point of view, such deterioration results in an insulation defect of the screen

3

relative to the ground, represented in a diagram in

FIG. 1B

by a resistance Rd, and leads to the occurrence of a voltage Vc, or “screen potential”, generated, in particular, by the combination of the metal of the screen

3

with water and various metal oxides. When a tightness defect is not repaired in time, the deterioration of the cable extends to the wrapping of the electric wires and develops over a substantial cable length due to the spread of water.

Therefore, testing the proper tightness of cables is a major concern for telecommunication operators and more and more needs to be automated with a view to reducing maintenance costs and working time on site.

Another test that must be performed on networks of telecommunication cables is that of the electrical continuity of screens, that are generally connected to the ground at their ends to allow the flow of electric charges brought about by electromagnetic disturbance or rises in the electric potential in the ground. The testing of the electrical continuity of screens between two connection points to the ground is also currently a major concern for telecommunication operators, as the latter must guarantee optimal performances of their networks due to the increase in the rates of digital data transfer imposed by the development of the Internet network and other computer applications.

In recent years, the applicant has designed, developed and perfected an automatic surveillance system for telecommunication networks constituted by a set of measuring devices marketed under the reference “IMD” (Remote Measuring Interface). Such devices, described in the patent EP 408 480 and in the international application PCT/FR99/02288, are arranged at the connection points to the ground of the metal screens and linked by telephone pairs to a piece of local maintenance equipment, that is itself linked to a regional maintenance centre. They allow various tests and measuring operations to be conducted daily, in particular:

the disconnection of the screens relative to the ground and the measuring of the insulation resistance of the screens and of the screen potential,

the detection and location of an insulation defect of a screen by injecting a low frequency voltage, in compliance with a method described in the above-mentioned international application, and

the detection of a continuity defect of a screen, by a method called “ground loop”.

However, as telecommunication cables are of considerable lengths ranging from approximately one hundred meters to a few kilometres, locating an insulation defect in a segment of cable by means of IMD devices must be completed by a step of precisely locating the defect in the field in order to repair it.

In the above-mentioned international application, the applicant proposed an additional locating method allowing the anomaly affecting a suspect segment of cable to be located on the repair site. As a reminder, this additional locating method consists in injecting into the screen of the suspect portion of cable two currents of different frequencies, measuring the currents at various points of the screen, then calculating the real part of at least one of the two currents to be free from the influence of the leak capacitance of the screen. A sharp drop in the real part of the current at a determined point of the cable allows the insulation defect to be located.

A similar problem arises as far as electrical continuity defects are concerned, as the ground loop method allows a continuity defect to be detected in a segment of network without precisely locating the point of discontinuity. For a better understanding,

FIG. 2

schematically represents a network of telecommunication cables comprising a cable “A” comprising N wires the end of which is connected by means of a splice box

10

to two cables “B” and “C” comprising N/

2

wires each. The cable B is itself connected by means of a splice box

11

to two cables “D” and “E” comprising N/

4

wires each. The screen of cable A is connected to the screens of cables B and C and the screen of cable B is connected to the screens of cables D and E. The screen of cable A is connected to the ground at its point of origin through a device IMD

1

and the screens of cables C, D, E are connected to the ground at their end points through devices IMD

2

, IMD

3

, IMD

4

.

When measuring a ground loop, for example in the segment ABE, the device IMD

1

maintains the point of origin of the screen A connected to the ground while the devices IMD

2

, IMD

3

, IMD

4

disconnect the end points of the screens C, D, E. The device IMD

4

measures the resistance of the loop constituted by the resistance of the screens ABE, the ground resistance of the device IMD

1

and its own ground resistance. If the loop resistance is very high, that may mean that the segment ABE has a continuity defect. However, the location of the continuity defect is unknown. The defect may for example be situated in one of the connection boxes

10

,

11

, and must be located to carry out the repair.

Now, to the applicant's knowledge, no really satisfactory method has yet been proposed to locate the continuity defect of a metal screen in the field, current methods being essentially based on a visual inspection.

The present invention aims to overcome this inconvenience.

More particularly, one general object of the present invention is to provide a method for testing a conductor element applicable to locating an electrical continuity defect, that is precise, reliable and simple to implement.

Another object of the present invention is to provide a device for testing a conductor element applicable to locating an electrical continuity defect, that is small in size and easy to use.

To achieve this object, the present invention is based on the observation that the additional locating method described in the above-mentioned international application is also applicable to locating continuity defects, by calculating the imaginary part of the bifrequency current instead of calculating the real part. A purely capacitive current value is obtained which allows a continuity defect to be located with great precision.

More particularly, the present invention provides a method for testing a conductor element applicable to locating a continuity defect of the conductor element, the conductor element having, relative to a reference conductor, an insulation resistance and a leak capacitance spread along the conductor element, the method comprising a step of injecting into the conductor element at least two currents of different frequencies by means of a current or voltage generator, one terminal of which is connected to the reference conductor, at least one step of measuring the amplitudes of currents at one measuring point chosen along the conductor element, and a step of calculating the imaginary part of at least one of the currents and/or of calculating the leak capacitance of the conductor element downstream from the measuring point, using the measured amplitudes of currents.

According to one embodiment, the method comprises a plurality of steps of measuring the amplitudes of currents at various measuring points, and a plurality of steps of calculating, after each measurement, the imaginary part of at least one of the currents and/or the downstream leak capacitance, an electrical continuity defect of the conductor element being located when the imaginary part calculated and/or the downstream leak capacitance proves to be zero before the end of the conductor element is reached.

The present invention also provides an application of the method above to locating an electrical insulation defect of the conductor element relative to the reference conductor, the method comprising a step of calculating the real part of at least one of the currents and/or the insulation resistance downstream from a measuring point.

According to one embodiment, the method comprises a plurality of steps of measuring the amplitudes of currents at various measuring points, and a plurality of steps of calculating, after each measurement, the real part of at least one of the currents and/or the downstream insulation resistance, an electrical insulation defect being located when the real part calculated shows a clear attenuation and/or when the downstream insulation resistance rapidly rises without this being justified by the topography of the conductor element.

According to one embodiment, the method comprises a step of calculating a formula of type: K

x

√[K

y

|i

1

|

2

−K

z

|i

2

|

2

], in which |i

1

| and |i

2

| are the amplitudes of currents and K

x

, K

y

, K

z

are constants.

According to one embodiment, the conductor element comprises several telecommunication cable screens (A-E) connected to each other, and the reference conductor is the ground.

The present invention also relates to a device for testing a conductor element, comprising a connection terminal to a contactless current sensor, an analog/digital conversion circuit of the signal delivered by the current sensor, and a calculation means, in which the calculation means is arranged to analyse the signal delivered by the current sensor and to extract the amplitudes of at least two currents of different frequencies present in the signal delivered, and to calculate the imaginary part of at least one of the currents and/or an electric capacitance by means of a formula of type: K

1

√[K

2

|i

1

|

2

−K

3

|i

2

|

2

], in which |i

1

| and |i

2

| are the amplitudes of currents and K

1

, K

2

, K

3

are constants.

According to one embodiment, the calculation means is also arranged to calculate the real part of at least one of the currents and/or an electric resistance, by means of a formula of type: K

4

√[K

5

|i

2

|

2

−K

6

|i

1

|

2

], in which K

4

, K

5

and K

6

are constants.

According to one embodiment, the device comprises selection means to choose between the calculation of the imaginary part of at least one of the currents and/or the calculation of an electric capacitance, on the one hand, and the calculation of the real part of at least one of the currents and/or an electric resistance, on the other hand.

According to one embodiment, the device is intended to be connected to a hook-on ammeter, and comprises calibration terminals intended to be interconnected during a calibration phase, and means for injecting onto one of the calibration terminals at least two calibration currents of different frequencies through a standard resistance.

These and other objects, advantages and features of the present invention shall be explained in greater detail in the following description of the theoretical foundations of the method of the present invention, of an example of the application of the method of the present invention to locating a continuity defect in a network of telecommunication cables, and of an example of an embodiment of a measuring device according to the present invention, in relation with the following figures, in which:

FIGS. 1A

,

1

B described above represent a telecommunication cable,

FIG. 2

described above represents a network of telecommunication cables,

FIG. 3A

schematically represents a conductive screen and shows the method according to the present invention,

FIG. 3B

is the equivalent wiring diagram of the screen in

FIG. 3A

,

FIG. 3C

is the equivalent diagram of the screen in

FIG. 3A

seen from a measuring point,

FIGS. 4A

,

4

B are vector representations of currents involved in the method according to the present invention,

FIG. 5

represents, in a simplified manner, the network of cables in FIG.

2

and shows one application of the method according to the present invention,

FIGS. 6

to

8

represent in the form of curves the results of measurements of current carried out on a segment of the network in

FIG. 5

,

FIG. 9

is an external view of a measuring device according to the present invention,

FIG. 10

is the wiring diagram in block form of the measuring device in

FIG. 9

, and

FIG. 11

shows a calibration step of the measuring device.

FIG. 3A

schematically represents the screen

20

of a telecommunication cable the electrical continuity of which must be tested. The points of origin and the end points of the screen are first disconnected from the ground (GND) by means of two switches

21

,

22

, for example the switches present in IMD devices. The equivalent diagram of the screen

20

is represented in FIG.

3

B. The screen

20

comprises a plurality of basic cells R

1

C

1

, R

2

C

2

, R

3

C

3

. . . R

i

C

i

. . . R

m

C

m

spread between its point of origin and its end point. The basic resistances R

i

together form the insulation resistance of the screen

20

relative to the ground and the basic capacitances C

i

form the leak capacitance of the screen

20

relative to the ground.

According to the present invention, at least two low frequency currents i

1

, i

2

of respective frequencies f

1

, f

2

and of pulsations ω

1

, ω

2

are injected at the point of origin of the screen

20

, for example currents in the order of a few milliamperes oscillating respectively at 1 Hz and 2 Hz. The currents i

1

, i

2

are delivered by a voltage or current generator, preferably a voltage generator

23

delivering alternating voltages u

1

, u

2

of the type

u

1

=

U

1

Cos (2

πf

1

t

)

u

2

=

U

2

Cos (2

πf

2

t

)

having known amplitudes U

1

, U

2

in the order of a few volts. The ground terminal of the generator

23

is connected to the ground so as to form a conduction loop passing through the screen

20

and the ground. The amplitudes of currents i

1

, i

2

are measured at various points P

1

, P

2

, P

3

. . . P

i

. . . P

n

along the screen

20

, by means of a hook-on ammeter

30

connected to a measuring device

31

.

These first steps of the method according to the present invention are similar to those of the method described in the international application PCT/FR99/02288, the purpose of which is to locate an insulation defect. It will be shown below that the detection of the amplitudes of currents i

1

and i

2

allows the imaginary part of each current to be calculated, and that the calculation of the imaginary part of currents i

1

, i

2

at several measuring points P

1

, P

2

, . . . P

i

, . . . P

n

allows a continuity defect to be located.

Seen from any measuring point P

i

, the screen

20

can be represented as shown in

FIG. 3C

, in the form of a resistance R

A

and a capacitance C

A

in parallel. The resistance R

A

is the insulation resistance of the part of the screen

20

located downstream from the measuring point P

i

, depending on the flow direction of the currents i

1

, i

2

, and is constituted by all the basic insulation resistances R

i

downstream from this point. The capacitance C

A

is the leak capacitance of the part of the screen

20

located downstream from the measuring point P

i

, and is constituted by all the basic leak capacitances C

i

downstream from this point. As the linear resistance of the screen

20

is negligible, the voltages present at the terminals of the equivalent circuit R

A

C

A

are the voltages u

1

, u

2

of amplitudes U

1

, U

2

applied to the point of origin of the screen.

The vector representation of the currents i

1

, i

2

in

FIGS. 4A

,

4

B shows that each current comprises a real part i

r1

, i

r2

in phase with the voltage u

1

, respectively u

2

, and an imaginary part i

c1

, i

c2

out of phase by 90° relative to the real part, which complies with the following relations:

|

i

1

|

2

=i

r1

2

+i

c1

2

(1)

|

i

2

|

2

=i

r2

2

+i

c2

2

(2)

|i

1

| and |i

2

| being the modules, or measurable amplitudes, of currents i

1

, i

2

.

The real parts i

r1

, i

r2

of currents i

1

, i

2

are linked by the following relations:

i

r1

=U

1

/R

A

(3)

i

r2

=U

2

/R

A

(4)

R

A

=U

1

/i

r1

=U

2

/ir

2

(5)

i

r1

=i

r2

(

U

1

/U

2

) (6)

The imaginary parts i

c1

, i

c2

of currents i

1

, i

2

are linked by the following relations:

i

c1

=C

A

ω

1

U

1

(7)

i

c2

=C

A

ω

2

U

2

(8)

C

A

=i

c1

/ω

1

U

1

=i

c2

/ω

2

U

2

(9)

i

c1

=i

c2

(

U

1

ω

1

/U

2

ω

2

) (10)

By injecting into the relation (1) the expressions of currents i

r1

and i

c1

given by the relations (6) and (10), the result is:

|

i

1

|

2

=i

r2

2

(

U

1

/U

2

)

2

+i

c2

2

(

U

1

/U

2

)

2

(ω

1

/ω

2

)

2

(11)

By multiplying the relation (2) by the term (U

1

/U

2

)

2

:

|

i

2

|

2

(

U

1

/U

2

)

2

=i

r2

2

(

U

1

/U

2

)

2

+i

c2

2

(

U

1

/U

2

)

2

(12)

By subtracting the relation (12) from the relation (11), the result is, after simplification, the expression of current i

c2

:

i

c2

=(ω

2

/U

1

)√[[(

U

2

|i

1

|)

2

−(

U

1

|i

2

|)

2

]/(ω

1

2

−ω

2

2

)] (13)

By combining the relation (10) and the relation (13), the result is the expression of current i

c1

:

i

c1

=(ω

1

/U

2

)√[[(

U

2

|i

1

|)

2

−(

U

1

|i

2

|)

2

]/(ω

1

2

−ω

2

2

)] (14)

By combining the relations (7) and (14) as follows:

i

c1

=C

A

ω

1

U

1

=(ω

1

/U

2

)√[[(

U

2

|i

1

|)

2

−(

U

1

|i

2

|)

2

]/(ω

1

2

−ω

2

2

)] (15)

the result is the expression of the downstream leak capacitance C

A

:

C

A

=(1

/U

1

U

2

)√[[(

U

2

|i

1

|)

2

−(

U

1

|i

2

|)

2

]/(ω

1

2

−ω

2

2

)] (16)

Finally, as the parameters U

1

, U

2

, ω

1

and ω

2

are constants, a general formula of the type:

K

1

√[K

2

i

1

|

2

−K

3

|i

2

|

2

] (17)

in which |i

1

| and |i

2

| are the currents measured by means of the hook-on ammeter and K

1

, K

2

, K

3

are constants, covers the relations (13) (14) (16) and allows the current i

c1

or the current i

c2

or even the downstream leak capacitance C

A

to be calculated, according to the value given to the constants K

1

, K

2

, K

3

.

It will now be shown using a practical example shown in

FIG. 5

that the relations that have just been described are directly applicable to locating a continuity defect.

FIG. 5

schematically represents the screens A, B, C, D, E of a network of telecommunication cables similar to the one in FIG.

2

. The screens are connected and form conductive segments AC, ABD and ABE. The point of origin of the screen A and the end points of the screens C, D, E are disconnected from the ground to implement the method of the present invention. The generator

23

is connected to the point of origin of the screen A. The currents i

1

, i

2

circulate in the screen A and are divided in the screens B and C, then in the screens D and E.

In

FIG. 6

, the curve

60

represents the profile of the imaginary currents i

c1

, i

c2

along the segment ABD when the latter does not have a continuity defect, and the curve

61

represents the profile of the currents i

c1

, i

c2

in the presence of a continuity defect

24

situated here on the screen B. The curves

60

,

61

also represent the profile of the downstream capacitance C

A

along the segment ABD, the relations (13) (14) and (16) being proportional. These curves are traced by means of the hook-on ammeter

30

and the measuring device

31

, an example of an embodiment of which will be described below.

The curves

60

,

61

are parallel and show a regular decline with a clear reduction of the currents i

c1

, i

c2

and of the capacitance C

A

at each division of the network into two different branches BC and DE. The significant difference between the two curves is that the curve

60

reaches the value zero at the end of the screen D, while the curve

61

, of a lower level, reaches the zero at the point of discontinuity

24

. Therefore, in practice, it is easy to locate the continuity defect

24

by observing the evolution of the currents i

c1

, i

c2

and/or of the leak capacitance C

A

. When the currents i

c1

, i

c2

or the capacitance C

A

become zero before the end of the conductive segment tested is reached, that indicates the presence of a continuity defect.

Compared to a simple detection of current, the method of the present invention provides the advantage of great measuring sensitivity and excellent precision, and only requires currents i

1

, i

2

and voltages u

1

, u

2

of low value, oscillating at low frequency and not disturbed by electromagnetic disturbances present in the region of 50 Hz. The calculation of the imaginary part of currents i

1

, i

2

allows, furthermore, to be free from losses of sensitivity due to an insulation defect, when the insulation defect is not very high and is for example in the order of a few kilo ohms. Moreover, the method of the present invention allows the leak capacitance C

A

to be calculated and meets a long-felt need to analyse the electric characteristics of telecommunication cables. It is, however, clear that the calculation of the capacitance C

A

is not essential to locating a continuity defect, as the method can be implemented by calculating the imaginary part of one of the currents i

1

, i

2

.

Furthermore, in one variation of an embodiment of the method, the voltages U

1

, U

2

are equal and the relations (

13

) and (

14

) are simplified:

i

c2

=ω

2

√[(|

i

1

|

2

−|i

2

|

2

)/ (ω

1

2

−ω

2

2

)] (13)→(18)

i

c1

=ω

1

√[(|

i

1

|)

2

−|i

2

|

2

)/ (ω

1

2

−ω

2

)] (14)→(19)

In this case, it is not necessary to know the applied voltage U to calculate the imaginary parts of the currents i

1

, i

2

and to locate a continuity defect.

The teaching of the international application PCT/FR99/02288 will now be completed and it will be shown that the method that is described in it allows the real part of each current and the insulation resistance R

A

to be determined, in the general case in which the voltages U

1

and U

2

have any values that are not necessarily identical. By multiplying the relation (2) given above by the term (ω

1

U

1

/ω

2

U

2

)

2

, the following relation is obtained:

(ω

1

U

1

/ω

2

U

2

)

2

|i

2

|

2

=(ω

1

U

1

/ω

2

U

2

)

2

i

r2

2

+(ω

1

U

1

/ω

2

U

2

)

2

i

c2

2

(20)

By subtracting the relation (21) from the relation (11), the result is:

i

r2

=(1/

U

1

)√[[(ω

1

U

1

|i

2

|)

2

−(ω

2

U

2

|i

1

|)

2

]/(ω

1

2

−ω

2

2

)] (21)

By combining the relations (6) and (21):

i

r1

=(1/

U

2

)√[[(ω

1

U

1

|i

2

|)

2

−(ω

2

U

2

|)

2

]/(ω

1

2

−ω

2

2

)] (22)

By combining the relation (5) with one of the relations (21) or (22):

R

A

=U

1

U

2

/√[[(ω

1

U

1

|i

2

|)

2

−(ω

2

U

2

|i

1

|)

2

]/(ω

1

2

−ω

2

2

)] (23)

As the parameters U

1

, U

2

, ω

1

and ω

2

are constants, a general formula of the type:

K

4

√[K

5

|i

2

|

2

−K

6

|i

1

|

2

] (24)

in which K

4

, K

5

, K

6

are constants and |i

1

|, |i

2

| are the currents measured, covers the relations (21) (22) (23). Depending on the value given to the constants K

4

to K

6

, this formula allows the current i

r1

, the current i

r2

or even the resistance R

A

to be calculated (by reversing the result).

As above, the amplitudes U

1

, U

2

of voltages u

1

, u

2

applied to the point of origin may be chosen to be identical. In this case, the currents i

r1

, i

r2

are identical, the relations (21) and (22) are simplified and become independent of the value of the voltage U applied to the point of origin, which can allow the currents i

r1

, i

r2

to be calculated without knowing the voltage U.

One example of locating an insulation defect, here a defect

25

present on the screen C in

FIG. 5

, is shown by the curves

62

,

63

in

FIGS. 7 and 8

. The curve

62

represents the profile of currents i

r1

, i

r2

and the curve

63

represents the profile of the downstream insulation resistance R

A

along the segment AC. Starting at the point of origin of the screen A, the curve

62

has a profile that is roughly flat up to the defect point

25

, without any noticeable reduction in the currents i

r1

, i

r2

at the point of division of the network into two different branches BC. In fact, the majority of the current passes in the screen C due to the insulation defect

25

, which is much lower than the insulation resistance of the screens. However, a sharp drop of the currents i

r1

, i

r2

can be observed when the defect

25

is reached. This reduction is not linked to the topography of the network and is significant of the presence of the defect

25

, through which the currents i

r1

, i

r2

leak into the ground. The curve

63

in

FIG. 8

leads to the same conclusion, the downstream insulation resistance R

A

sharply increasing when the insulation defect

25

is reached.

Now an example of an embodiment of a measuring device

31

according to the present invention will be described, intended here to calculate the leak capacitance C

A

and the insulation resistance R

A

of a conductive screen.

FIG. 9

is an external view of the device

31

. The device

31

comprises a connection plug

32

of the hook-on ammeter, two calibration terminals

33

,

34

, a display

35

, an on/off key

36

, a display

35

light-up key

37

, a calibration key

38

, a filter key

39

, a key

40

-

1

to trigger a calculation of insulation resistance R

A

and a key

40

-

2

to trigger a calculation of leak capacitance C

A

.

FIG. 10

represents in block form the electrical design of the device

31

. This comprises a sampling chain

41

, a microprocessor MP fitted with a memory

45

, a calibration circuit

46

, a storage battery

50

, a battery

50

load circuit

51

and a power supply test circuit

52

delivering various voltages V

1

, V

2

, V

3

required by the operation of the various parts. The display

35

and the keys

36

to

40

are also represented and are connected to the microprocessor.

The sampling chain

41

comprises a current/voltage amplifier

42

the input of which is connected to the plug

32

and receives the current Ip delivered by the hook-on ammeter (not shown). The amplifier

42

provides a substantial gain for the detection of low currents in the order of a tenth of a microampere in the presence of insulation defects of several hundred KΩ. The output of the amplifier

42

delivers a voltage Vp and is connected to the input of an analog-digital converter

44

through a low-pass filter

43

, a Butterworth filter for example having a 15 Hz cut-off frequency and a high cut-off slope (−55 dB to 50 Hz) to remove the disturbances at 50 Hz. The converter

44

is of the sigma-delta type with a serial output and comprises an internal digital filter of the 6

th

order. The sampling frequency is chosen at 512 Hz with a resolution of 16 bits. The output of the converter

44

delivers a sampled signal Vpn sent to an input of the microprocessor.

The calibration circuit

46

comprises a digital/analog converter

47

the digital input of which is connected to the microprocessor and the output of which is connected to the calibration terminal

33

through a voltage amplifier

48

and a standard resistance

49

of value R

cal

.

The memory

45

comprises a volatile RAM-type memory, to store temporary data, and a non-volatile EPROM-type memory. Principally, the EPROM memory comprises the operating system OS of the microprocessor, a Fast Fourier Transform (FFT) program PGR

1

, an amplitude calculation program PGR

2

, a voltage/current conversion program PGR

3

, a leak capacitance C

A

or insulation resistance R

A

calculation program PGR

4

, a calibration program PGR

5

, and a main program PGR

6

ensuring the coordination of all the programs and the execution of the instructions received by the keys

36

to

40

and the testing of the various parts of the device

31

.

By means of these various programs, the microprocessor extracts from the sampled signal Vpn two signals Vpn

1

, Vpn

2

the respective frequencies of which correspond to the measuring frequencies f

1

, f

2

, here equal to 1 Hz and 2 Hz, and calculates the amplitudes Vpnmax

1

, Vpnmax

2

of these signals Vpn

1

, Vpn

2

. The microprocessor then converts the amplitudes Vpnmax

1

, Vpnmax

2

into amplitudes of currents |i

1

|, |i

2

| by means of conversion coefficients KVI

1

and KVI

2

:

|i

1

|=KVI

1

Vpnmax

1

(25)

|i

2

|=KVI

2

Vpnmax

2

(26)

The microprocessor then calculates the capacitance C

A

or the resistance R

A

by means of relations (16) or (23).

According to one aspect of the present invention, the conversion coefficients KVI

1

, KVI

2

are determined by the calibration program PGR

5

, which is activated by one press on the key

38

“CAL” (calibration). The program PGR

5

contains a table of data comprising the sampled values of a bifrequency signal of frequencies f

1

, f

2

. These data are sent to the input of the converter

47

, used as a frequency synthesizer, the output of which delivers corresponding analog voltages. At the output of the amplifier

48

, there are two calibration voltages U

cal1

, U

cal2

oscillating at the measuring frequencies f

1

, f

2

. During the calibration phase, the terminals

33

and

34

are linked by a conductor wire

53

as shown in

FIG. 11

, and the hook-on ammeter

30

is arranged around the wire

53

. The wire

53

is passed through by two calibration currents I

cal1

, I

cal2

equal to:

I

cal1

=U

cal1

/R

cal

(27)

I

cal2

=U

cal2

/R

cal

(28)

the values of which are known by the program PGR

5

. These currents are detected by the ammeter

30

and the microprocessor calculates corresponding values Vpnmax

1

, Vpnmax

2

using the sampled signals Vpn

1

, Vpn

2

. The values Vpnmax

1

, Vpnmax

2

allow the microprocessor to calibrate the conversion coefficients by means of the following relations:

KVI

1

=(

U

cal1

/R

cal

)/Vpnmax

1

=

I

cal1

/Vpnmax

1

(29)

KVI

2

=(

U

cal2

/R

cal

)/Vpnmax

2

=

I

cal2

/Vpnmax

2

(30)

Tables 1, 2 and 3 below summarize the operation of the measuring device

31

. It will be understood that variations and different embodiments of this device may be made. For example, it may be limited to the production of a measuring device calculating only the imaginary and real parts of a bifrequency current without calculating the capacitance C

A

and the resistance R

A

. Therefore, the keys

40

-

1

and

40

-

2

could be used to trigger the calculation of the real part and the calculation of the imaginary part of one of the currents i

1

, i

2

, which would be sufficient to locate insulation or continuity defects. The calculation of the capacitance C

A

and of the resistance R

A

is an additional advantage allowing the electrical characteristics of a telecommunication cable to be analysed.

Generally speaking, the method of the present invention is applicable to any conductor element other than a metal screen of a telecommunication cable, insofar as the conductor element has, relative to the ground or relative to any other conductor used as reference conductor, a measurable leak capacitance and insulation resistance.

Also, although the method described above is based, for the sake of simplicity, on the injection of two currents, it will be understood by those skilled in the art that the principles and methods of calculation set out above are transposable to a detection done by means of three, or even four currents or more having different frequencies.

TABLE 1

Measurements of insulation resistance R

A

Press on the key 40-1

E1

Sampling the signal Vp for 4.25 seconds at 512 Hz

(i.e. 512 samples a second).

E2

Removal of the samples of the sampled signal Vpn

corresponding to the first 25 hundredths of a second

(start-up time of the low-pass filter 43)

E3

Fourier transform of the signal of remaining samples

Vpn with an analysis fineness of 0.25 Hz in a

frequency band ranging from 0 Hz to 256 Hz and

extraction of the signals Vpn1 and Vpn2 at 1 Hz and

2 Hz

E4

Calculation of the amplitude Vpnmax1 of the signal

Vpn1 at 1 Hz and of the amplitude Vpnmax2 of the

signal Vpn2 at 2 Hz.

E5

Conversion into current values of the values Vpnmax1,

Vpnmax2, by means of conversion coefficients KVI1 and

KVI2:

|i

1

| = KVI1 Vpnmax1

|i

2

| = KVI2 Vpnmax2

E6

If filter mode selected (key 39): steps E1 to E5

repeated 4 times; calculation of the average of the

last four results:

|i

1

| = Σ

(4)

|i

1

|/4

|i

2

| = Σ

(4)

|i

2

|/4

E7

Calculation of R

A

according to relation (23)

E8

Display of R

A

TABLE 2

Measurement of leak capacitance C

A

Press on the key 40-2

E1

Step identical to the one in table 1

E2

Step identical to the one in table 1

E3

Step identical to the one in table 1

E4

Step identical to the one in table 1

E5

Step identical to the one in table 1

E6

Step identical to the one in table 1

E9

Calculation of C

A

according to relation (16)

E10

Display of C

A

TABLEAU 3

Calibration

Press on the key 38

F1

Send the calibration voltage U

cal

to one of the

calibration terminals of the device

E1

Step identical to the one in measuring mode

E2

Step identical to the one in measuring mode

E3

Step identical to the one in measuring mode

E4

Step identical to the one in measuring mode

F2

Determination of conversion coefficients KVI1 KVI2:

KVI1 = (U

call

/R

cal

) /Vpnmaxl = I

call

/Vpnmax1

KVI2 = (U

cal2

/R

cal

) /Vpnmax2 = I

cal2

/Vpnmax1

F3

Repeat steps E1 to E4 and F2 three times

F4

Calculation of the average of the last four results:

KVI1 = Σ

(4)

KVI1/4

KVI2 = Σ

(4)

KVI2/4

F5

Store the conversion coefficients in the memory

Claims

1. A method for testing a conductor element applicable to locating a continuity defect of the conductor element, the conductor element having, relative to a reference conductor, an insulation resistance and a leak capacitance spread along the conductor element, the method comprising:a step of injecting into the conductor element at least two currents of different frequencies by means of a current or voltage generator, one terminal of which is connected to the reference conductor, a plurality of steps of measuring the amplitudes of currents at various measuring points chosen along the conductor element, and a plurality of steps of calculating, after each measurement, the imaginary part of at least one of the currents and/or the downstream leak capacitance of the conductor element, an electrical continuity defect of the conductor element being located when the imaginary part calculated and/or the downstream leak capacitance proves to be zero before the end of the conductor element is reached.
2. The method according to claim 1, applicable to locating an electrical insulation defect of the conductor element relative to the reference conductor, further comprising a step of calculating the real part of at least one of the currents and/or the insulation resistance downstream from a measuring point.
3. The method according to claim 2, further comprising a plurality of steps of measuring the amplitudes of currents at various measuring points, and a plurality of steps of calculating, after each measurement, the real part of at least one of the currents and/or the downstream insulation resistance, an electrical insulation defect being located when the real part calculated shows a clear attenuation and/or when the downstream insulation resistance rapidly rises without this being justified by the topography of the conductor element.
4. The method according to claim 1, comprising a step of calculating a formula of type:Kx√[Ky|i1|2−Kz|i2|2]in which |i1| and |i2| are the amplitudes of currents and Kx, Ky, Kz are constants.
5. The method according to claim 1, wherein the conductor element comprises several telecommunication cable screens connected to each other, and in that the reference conductor is the ground.
6. A device for testing a conductor element, comprising a connection terminal to a contactless current sensor, an analog/digital conversion circuit of the signal delivered by the current sensor, and calculation means arranged to analyze the signal delivered by the current sensor and to extract the amplitudes of at least two currents of different frequencies present in the signal delivered, and to calculate the imaginary part of at least one of the currents and/or an electric capacitance by means of a formula of type:K1√[K2|i1|2−K3|i2|2]in which |i1| and |i2| are the amplitudes of currents and K1, K2, K3 are constants.
7. The device according to claim 6, wherein the calculation means are also arranged to calculate the real part of at least one of the currents and/or an electric resistance, by means of a formula of type:K4√[K5|i1|2−K6|i1|2]in which K4, K5 and K6 are constants.
8. The device according to claim 6, further comprising selection means to choose between:the calculation of the imaginary part of at least one of the currents and/or the calculation of an electric capacitance, on the one hand, and the calculation of the real part of at least one of the currents and/or an electric resistance, on the other hand.
9. The device according to claim 6, intended to be connected to a hook-on ammeter, and comprising calibration terminals intended to be interconnected during a calibration phase, and means for injecting onto one of the calibration terminals at least two calibration currents of different frequencies through a standard resistance.

Priority Claims (1)

Number	Date	Country	Kind
00 00970	Jan 2000	FR

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No. PCT/FR01/00171, filed Jan. 19, 2001, and the disclosure of which is incorporated herein by reference.

US Referenced Citations (2)

Number	Name	Date	Kind
4103225	Stephens	Jul 1978	A
6538450	Bussinger	Mar 2003	B2

Foreign Referenced Citations (1)

Number	Date	Country
0 408 480	Jul 1990	EP

Non-Patent Literature Citations (1)

Entry
E.C. Bascom et al., “Computerized Underground Cable Fault Location Expertise”, Proceedings Of The Power Engineering Society Transmission And Distribution Conference, pp. 376-382, (1994).

Continuations (1)

	Number	Date	Country
Parent	PCT/FR01/00171	Jan 2001	US
Child	10/196070		US

Method and device for testing a telecommunication cable

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Term Extension

Abstract