GB2345975A - A non-intrusive method of transmission earth detection and analysis - Google Patents

Abstract

This invention is a new method to determine the earthing configuration applied to the screen of a co-axial transmission cable 4, without disconnecting the cable and without interruption any carried signal. That is, it is a non-intrusive method of transmission earth detection and analysis. The method involves injecting a known test signal which does not degrade telecommunications information being conveyed by the cable. The cable 4 may be a jumper cable interconnecting an equipment line driver 1 with a remote equipment line receiver 7. The test signal dc current is injected into the cable screen at a frame terminal 3. Depending on how, or if the earthing connection has been made i.e. at A (correct), E (incorrect) or neither, the current divides differently to flow between the local and remote equipment through the screen and centre conductor. A magnetic loop pickup around a cable 2 senses the currents and the earthing configuration can then be deduced.

Description

A NON-INTRUSIVE METHOD OF TRANSMISSION EARTH DETECTION AND ANALYSIS Application This invention relates most directly to the telecommunications industry.

In telecommunications centres, multiplexed signals are commonly conveyed from many equipments to a flexibility frame by co-axial"permanent bearer"cables. On the flexibility frame a permanent cable from any specific equipment can be connected to a permanent cable from any other equipment by running an interconnecting"jumper cable", also co-axial.

A signal will therefore originate upon one piece of equipment, be conveyed by a permanent bearer cable to the flexibility frame where, via a jumper cable, it is connected to another permanent bearer cable which conveys it to its destination piece of equipment. In practice a transmission link actually comprises a pair of cables to and from the flexibility frame, one for the transmitted signal and one for the received signal."Transmit"and"receive"are with respect to a particular equipment, because the transmit bearer from a"local"equipment is the receive of the"distant"equipment, and vice-versa.

International convention [reference I] recommends that the local equipment should "connect to earth"the co-axial screen of the bearer conveying its transmit signal.

Similarly the distant equipment should"connect to earth"the screen of the bearer cable conveying its transmit signal. By this arrangement both the transmit and receive coaxial bearer cables for a particular piece of equipment are provided with an"earth screen".

But in practice it is possible that the earth may be omitted entirely, it may be applied at the wrong end of the bearer cable, it may be applied at both ends, and it may degrade over time. All of these conditions may result in interference to the telecommunications signal conveyed.

A feature of the flexibility frame is removable links. A link connects the inner conductor of a permanent cable to the inner conductor of a jumper cable, and the coaxial screen of that permanent cable to the co-axial screen of that jumper cable.

Typically, actual earthing configuration may be determined by removing frame links and measuring screen resistance to frame ground, measuring towards the local and distant equipments separately. But removing the frame link breaks the circuit, interrupting the signal being carried and thereby affecting telecommunications service.

This invention is a new method to determine the earthing configuration applied to the screen of a co-axial transmission cable, without disconnecting the cable and without interruption to the carried signal. That is, it is a non-intrusive method of transmission earth detection and analysis.

The Method The method involves injecting a known test signal which does not degrade the telecommunications information being conveyed by the cable. The test signal is applied to the cable's co-axial screen and then the way the test signal divides is identified and interpreted in order to determine which of the possible earthing configurations exists. The five possible configurations are; -"circulating current"present (see detailed description, below) -no earth at either end -earth applied at the local equipment -earth applied at the distant equipment -earth applied at both ends but no"circulating current"present The essential features of the method are; -generation of a known test signal which does not degrade the telecommunications information being conveyed by the cable -a means of injecting the test signal onto the co-axial screen of the cable under test -a connection to a reference earth -a sensor to observe the magnetic flux generated by the test signal -discrimination between flux generated by the test signal and other output from the flux sensor -interpretation of the detected flux as one of the five possible states given above Interpretation is an analysis based upon Ampere's Law for equal and opposite current flows in the conductors of a co-axial cable (i. e. the magnetic flux outside of the cable is zero) and upon Kirchoff's laws for electrical circuits. (See the detailed description below).

Detailed Description example) Detailed description of method can be explained by example: Application to analysis of the earthing conditions of a co-axial bearer cable carrying a 2048 kbit/s digital telecommunications signal between two equipments in a network operator's switching centre.

Figure 1 shows a schematic of the cabling arrangement for one half of the communications link, the co-axial cable conveying the transmit signal from the line driver of a"local"equipment 1 to be received by the line receiver of a"distant" equipment 7. (The other half of the cable pair is identical, but in reverse direction i. e. from the line driver of the distant equipment to the line receiver of the local equipment). The permanent co-axial bearer cable 2 is connected via a flexibility frame link 3 to a co-axial jumper cable 4. This connects via another flexibility frame link 5 to another permanent bearer cable 6 terminating at the"distant"equipment 7.

Figure 2 shows the equivalent dc electrical circuit: R, is the resistance of the co-axial screen, and R2 the resistance of the inner conductor, of the permanent cable to the local equipment 1.

R4 is the resistance of the co-axial screen of the jumper cable plus that of the permanent cable to the distant equipment 7. R5 is the combined resistance of the respective inner conductors.

R3 is the resistance of the line driver's cable interface in the local equipment 1.

(Depending upon the coupling circuitry employed in the driver, R3 may approach infinite resistance to direct current).

R6 is the resistance of the line receiver's cable interface in the distant equipment 7.

(Depending upon the coupling circuitry employed in the receiver, R6 may approach infinie resistance to direct current).

Characteristics of the circuit are that the permanent and jumper cable resistances are all unknowns which will vary according to the length of the cables. Similarly the line driver and receiver resistances can vary according to the types of equipment and their design.

By the international convention [reference 1], the equipment 1 ought to connect the screen of the cabling to earth at point A of figures 1 and 2.

In reality both point A and point E may be earthed. In such circumstance, because switching centre earths are rarely potential free, a small potential difference may exist between point A and point E causing a"circulating current"to flow through the coaxial screen, and perhaps another to flow through the centre conductor. Because these currents flow in the same direction they will generate an additive combined magnetic flux around the co-axial cable.

The instantaneous equivalent dc circuit is either as in figure 3 or as in figure 4, depending upon whether point A is at a positive or negative potential with respect to point E.

This must be considered in the design and operation of the test instrument which will detect the consequent flux and give an output voltage Vs proportional to I3 + I4, if a flux sensor is clamped around the cable at location B, figure 1.

If there is a"circulating current", the earthing configuration is necessarily"earthed at both ends". If there is no"circulating current" (and therefore no consequent flux), the cable earthing arrangement has one of the four states; -no earth at either point A or at point E of figure 1 -earth at point A of figure 1 ("local earth") -earth at point E of figure 1 ("distant earth") -earth at point A and at point E with no"circulating current"between these points (i. e. point A and point E are at equipotential) These are the states differentiated between by use of a known test signal. In this example, with reference to distribution frame earth, a signal can be injected onto the co-axial screen by touching a probe to the conducting flexibility frame link at point C, figure 1.

The equivalent instantaneous dc circuits are shown in the figures Figure 5-no earth.

Figure 6-earth at point A of figure 1.

Figure 7-earth at point E of figure 1.

Figure 8-earth at point A and at point E with no"circulating current"between these points.

By application of Ampere's Law and kirchoff s laws it can be deduced that for a flux sensor clamped around the cable at location B figure 1; -if there is no earth (figure 5) probe output current Ip = 0 and sensor output voltage VS = 0 -if there is an earth at point A of figure 1 (figure 6) probe output current Ip = (Il + I2) and sensor output voltage VS + I2) where k = a constant dependent upon probe sensitivity -if there is an earth at point E of figure 1 (figure 7) probe outputcurrent Ip= (II +I2) and sensor output voltage Vs # k(I1 - I1) . : sensor output voltage Vs-zero flux at location B -if there is an earth at point A and at point E with no"circulating current"between these points (figure 8). probe output current Ip > I1 and sensor output voltage VS ~--kIl A sensor output voltage Vs < kIp Circuitry can be designed to measure Vs and Ip and therefore to discriminate between the different earth configurations. Some form of output display can then indicate the states to the instrument user.

measured Ip and Vs-and earth configurations I V earth configuration no test signal Vs # k(I3 + I )"circulating current"detected applied therefore"earth at both local and distant ends" (Fig 3 or Fig 4 Ip 0 0 current flows therefore"no I earth" (Fig 5) Ip = (I1 + I2) Vs # k(I1 + I2) all probe current is detected by the flux sensor therefore"earth at local equipment" (Fig 6) Ip = (I1 + I2) Vs = 0 probe current flows but no resultant flux is detected therefore"earth t distant equipment" (Fig 7) Ip = (I1 + I2) Vs # kI1 probe current flows but flux detected is equivalent to only a proportion of j therefore "earth at both local and distant equipments" j "Circulating current"] (Fig 8 Notes on instrument design In practice there is a possibility that a small flux may be wrongly detected as no flux, or vice-versa.

Similarly a small"circulating current"may be undetected and cause a misinterpretation between"no flux"generated by the test probe current and"small flux" generated by the test probe current.

These factors must be accommodated in the design of any instrument invented to exploit this new, non-intrusive method of analysis. Such an instrument may constitute a separate invention.

Accompanying Figures Fig 1 schematic of 2Mbit/s cabling between two equipments in a telecommunications network switching centre.

Fig 2 the equivalent dc electrical circuit.

Fig 3"circulating current"superimposed on this circuit (ground potential at "local"equipment > ground potential at"distant"equipment).

Fig 4"circulating current"superimposed on this circuit (ground potential at "local"equipment < ground potential at"distant"equipment).

Fig 5 the equivalent dc electrical circuit with a test signal applied if there is no earth at either"local"or"distant"equipments.

Fig 6 the equivalent dc electrical circuit with a test current applied if there is an earth at the"local"equipment.

Fig 7 the equivalent dc electrical circuit with a test current applied if there is an earth at the"distant"equipment.

Fig 8 the equivalent dc electrical circuit with a test current applied if there is an earth at both the"local"and the"distant"equipment.

Reference Ref 1 International Telecommunication Union ITU-T Recommendation G. 703 paragraph 6.4 End ofdescription

Claims (2)

1. CLAIMS 1. A method to determine the earthing configuration applied to the screen of a c-axial transmission cable without disconnecting the cable and without interruption to the carried signal, i. e. a non-intrusive method of transmission earth detection and analysis.
2. 2. A method as claimed in claim 1 where magnetic flux generated by the non-disturbing test signal is used to indicate the earthing configuration applying to the co-axial screen of the cable under test.

   2. A method as claimed in claim 1 which utilises a test signal which does not degrade the telecommunications information conveyed by the cable.

   3. A method as claimed in claim 1 or claim 2 where magnetic flux generated by the application of a test signal is used to indicate the earthing configuration applying to the co-axial screen of the cable under test.

   4. A method as claimed in claim 3 which utilises Ampere's Law for equal and opposite current flows in the conductors of a co-axial cable and Kirchoff's laws for electrical circuits to interpret magnetic flux generation as one of five possible earthing configurations.

   Amendments to the claims have been filed as follows 1. A method to identify whether a co-axial transmission cable's outer-screen is earthed by the transmitting equipment connected to it at one end or by the receiving equipment connected at its other end, or by neither or both of these equipments, without disconnection of the cable and without interference to a transmission the cable is carrying, by injecting a non-disturbing test signal into the outer screen of the co-axial cable and investigating the current flows resulting from it.