GB2082866A

GB2082866A - Method for locating cable defects

Info

Publication number: GB2082866A
Application number: GB8115551A
Authority: GB
Original assignee: Howaldtswerke Deutsche Werft GmbH
Current assignee: ThyssenKrupp Marine Systems GmbH
Priority date: 1980-08-29
Filing date: 1981-05-20
Publication date: 1982-03-10
Also published as: FR2489527A1; DE3032519A1; FR2489527B1; BE888688A; DE3032519C2; GB2082866B

Abstract

A method of locating cable defects, especially defects in a cable which may not be burnt out, wherein an arc generator comprising a source (1), a periodically charging and discharging capacitor (3) and a spark gap (4) are connected via an inductance (6) and free running diode (5) to the defective cable (9), to which a pulse echo measurement device (8) is also connected via a high pass filter (7), and the inductance and current flow through an arc at the defect point determined by said inductance are selected in comparison with the pulse repetition time of the measurement device so that, alternately, one or more pulses are reflected from the arc and an equal or greater number of pulses are not reflected respectively during maintenance and breaking of the arc. <IMAGE>

Description

SPECIFICATION Method for locating cable defects This invention relates to a method for locating cable defects, in particular defects which may not be burnt out and in which, for the delivery of constant power to the cable defect, a capacitor is periodically charged and discharged to the defect point via a spark gap having an inductance and a free-running diode connected between said spark gap and the cable.

In order to be able to locate cable defects, in particular pre-location by pulse reflection, their contact resistance must be reduced to the magnitude of the cable surge impedance. For this purpose either the defect point is heated by the supply of electrical power in such a way that the hydrocarbons of the cable insulation are converted into conductive carbon (the defect is burnt out) or it is ensured that an electric arc is continuously sustained. The arc generating means used in this respect must be able to supply its-complete nominal output to differing defect resistances. This is carried out in a manner known per se by the switching over of current and voltage, or by constructing the arc generating means as a constant-power generator.

In the case of a simple embodiment of a constant-power generator of this type, a capacitor is charged via a voltage source with a currentlimiting internal resistance and is discharged via a spark gap to the defective cable. If, in this respect, the spark-over sequence and the spark-over voltage of the spark gap, which corresponds to the charging voltage of the capacitor, are maintained constant, the power supplied to the defect point in the defective cable is thus also constant.

However, numerous cable defects may not be burnt out. The defect point may in fact be subject to a spark-over if sufficient current is supplied from the arc generating means to an arc. However, after extinction of the arc, the defect is immediately high-ohmic again. In order to locate such defects a method has become known in which pulses from a pulse echo measurement device are reflected by stationary arcs, which behave in the same way as an ohmic resistance in the magnitude of the characteristic impedance of the cable. For this purpose, the pulse echo measurement device is coupled to the cable via a high-pass filter which enables the passage of at least the essential components of the pulse frequency spectrum.The arc current must be maintained as constant as possible and current variations, which include frequency portions which may be transmitted through the high-pass filter to the pulse echo measurement device, must be prevented, since they disturb or over-stimulate the amplifier in the pulse echo measurement device.

On the other hand, the arc current must be great enough to cause the arc resistance to fall within the magnitude of the characteristic impedance of the cable. This is a difficult requirement with respect to the conductivity of the arc generating means, which must supply a high voltage in order to ignite the arc and a high current for its maintenance. The current must be kept as constant as possible by suitable devices. This requirement may only be satisfied, in terms of economically viable means, by arc generating means with constant-power characteristics.

Nevertheless, in the case of the simple cable testers of the type described above, a capacitor is repetitively discharged to supply pulses to the cable, as a result of which high frequency portions arise with a corresponding feedback effect on the amplifier of the pulse echo measurement device.

With respect to the quenching behaviour of the spark gap, it is not possible to select the pulse repetition frequency to be high enough to produce, with viable expenditure on filtering means, a permanently flowing, sufficiently constant arc current.

In order to improve the power balance in testers for the location of cable defects with voltage pulses, a method has become known in which an inductance with a free-running diode is connected into the discharge circuit of the pulse generating capacitor. In this way the current is reduced and the discharge time is increased.

An object of the invention is to provide an improved method of cable testing, as compared for example with the methods known from DE-AS 20 27 942 and DE-AS 22 01 024, which enables simple detection of defects at the pulse echo measurement device.

In accordance with the invention, the inductance and the current flow time through an arc determined thereby are balanced in such a way in comparison with the pulse repetition period of a pulse echo measurement device that, for improved interpretation of the pulse reflection patterns, one or a plurality of pulses from the pulse echo measurement device is or are alternately reflected through the arc to the defect point and subsequently an equal or greater number of pulses are not reflected after breaking of the arc.

During the discharge period of the capacitor and inductance in the arc generator a current flows through the defect point, which current produces an arc which reflects pulses. When the inductance is discharged, the arc breaks. The pulses of the echo device are no longer reflected until the process is repeated with a renewed spark-over of the spark gap in the arc generator.

The display screen of the pulse echo measurement device therefore alternately shows the pulse pattern of the cable with and without defect points. This display clearly distinguishes the defect point from other points of reflection arising naturally from the construction of the cable assembly and considerably facilitates its location.

In this respect the invention may be used with defective cables of high capacitance since, by discharging the inductance, the current flow time through the arc continues for a sufficient period to enable use of the pulse echo measurement device.

It is also proposed that the period of the sparkover sequence of the spark gap is selected with respect to the current flow period through the arc into the cable defect in such a way that, as a result of the pulse ratio provided in this way, a sufficiently large current through the arc and a sufficiently small arc resistance arises for a comparatively low power.

An embodiment of the invention is shown in diagrammatic form in the accompanying drawings, in which: Figure 1 shows a cable testing circuit arrangement, and Figure 2 shows the display screen of a pulse echo measurement device.

The circuit arrangement shown in Figure 1 comprises a d.c. voltage source 1 which charges a capacitor 3 via an internal resistance 2 until it reaches the spark-over voltage of the spark gap 4.

When the spark gap 4 has sparked over, the capacitor 3 discharges via an inductance 6 to the defective cable 9, and the energy stored in the inductance 6 discharges via the diode 5. The pulses of a pulse echo measurement device 8 are connected through a high-pass filter provided with a capacitor 7 and the reflection pulses are discontinued when the inductance 6 is deenergised.

Figure 2 shows a display picture of the pulse echo measurement device 8, in which the defect point is shown by the point of separation 10 of the light traces 1 The defect point in the cable 9 is therefore clearly shown by this synchronous representation of the reference pattern and the reflected pattern.

Claims

1. A method for locating cable defects, especially defects which may not be burnt out, in which for the delivery of constant power to the cable defect a capacitor is periodically charged and discharged to the defect point via a spark gap with an inductance and a free-running diode connected between said spark gap and the cable, wherein the inductance and the current flow time through an arc determined thereby are selected in relation to the pulse repetition time of a pulse echo measurement device so that, for improved interpretation of the pulse reflection patterns at the measurement device, one or a plurality of pulses from the pulse echo measurement device is or are alternately reflected through the arc at the defect point and subsequently an equal or greater number of pulses is or are not reflected after breaking of the arc.

2. A method as claimed in claim 1, characterised in that the period of the spark-over sequence of the spark gap is selected with respect to the current flow period through the arc into the cable defect in such a way that, as a result of the pulse ratio provided in this way, a sufficiently large current through the arc and a sufficiently small arc resistance arises for a comparatively low power.

3. A method of locating cable defects substantiaily as hereinbefore described with reference to the accompanying drawings.