Sheath testing, Sheath fault location and Location of earth faults

Since many years the main reason of faults in cables with plastic insulated outer sheath is the damage of this sheath. This permits the penetration of water into
the cable and as a consequence it enhances the growth of ,,Water Trees" and other corrosion based damages in power cables. Water trees are one of the
primary reasons of cable faults. In communication cables the ingress of water significantly reduces the transmission quality which is in today's high performance data transmission requirements almost equal to a loss of the line. In cables, which are not protected against longitudinal water migration there is the additional danger of joint faults due to water spreading though the conductors into the joints. The following article describes the different test procedures for plastic insulated and the prelocation and pinpointing of sheath faults.

Table of Contents
1. Introduction ... 3
2. Why Sheath testing ................................................................................................ 3
2.1 Sheath test devices .......................................................................................... 4
2.2 Norms and regulations for sheath testing ......................................................... 5
2.3 User definitions ................................................................................................. 5
2.4 Conducting the test ........................................................................................... 6
2.5 Safety ... 7
3. Sheath fault prelocation .......................................................................................... 8
3.1 Prelocation with the comparison or voltage drop method ................................. 9
3.1.1 Bipolar Measurement of the voltage drop ................................................ 11
3.1.2 Advantages and disadvantages of the voltage drop method ................... 11
3.2 Prelocation with a measuring bridge ............................................................... 12
3.2.1 Prelocation ............................................................................................... 12
3.2.2 Prelocation with MVG 5 ........................................................................... 12
3.2.3 Two wire measurement according to Murray ........................................... 13
3.2.4 Three wire measurement according to Graaf ........................................... 13
3.2.5 Advantages and disadvantages of bridge methods ................................. 14
4. Sheath fault pinpointing ........................................................................................ 15
4.1 The different methods ..................................................................................... 15
4.2 Voltage gradients ............................................................................................ 15
4.3 The DC Method .............................................................................................. 16
4.3.1 Pulsed DC and current limits .................................................................... 18
4.3.2 Power ... 18
4.3.3 Specialties of the DC - Method ................................................................ 19
4.3.4 Influences ................................................................................................ 19
4.3.5 Advantages and disadvantages of the DC - Method ................................ 19
4.4 Surge Method ................................................................................................. 20
4.5 Localisation of earth contacting faults in LV plastic insulated MV systems ..... 20
4.6 Audio frequency methods ............................................................................... 21
4.7 Equipment combination FL 50 with Step voltage Probe DEB 3-10 ................. 22
4.8 Audio Frequency Method with 4,8 Hz and SFL2 A-Frame ............................. 22
4.9 Localisation of earth contacting faults in LV and illumination systems ............ 24
5. Wording ... 25

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