Cable Sheath Fault Location Using Bridge Methods

Cable sheath faults are defects in the outer protective sheath of cables, such as PVC jackets. These type of faults do not necessarily influence the electrical performance of a shielded cable initially, but do have a negative side effect on the medium to long-term operation of the cable. Damages to the outer sheath can allow soil and moisture to penetrate into the cable, which facilitates corrosion of the cable sheath, or of the neutral wires (neutral corrosion), and may lead to the development of water trees that could lead to breakdown of the cable in the future. As a result it is important to test the integrity of a cable sheath and repair any faults that may be present to ensure the long-term performance of a power cable. 

When cable faults occur between two defined cores, such as cable conductor and cable screen, methods using time domain reflectometry (TDR) can be used to pre-locate the fault positions (See Syscompact Blog Post). However, in some cases and in certain cable structures, faults can happen between core to the cable sheath or surrounding soil. This is especially the case in unshielded cables such as high voltage DC cables used for railway power supply, low voltage cables, and signal or pilot cables. Since the soil or surrounding medium cannot be accessed like a grounded metal sheath or screen, TDR can no longer be used. A TDR pulse can only travel along a path as long as two parallel conductive paths are given. Therefore, pre-location for cable sheath faults is conducted using measuring bridge techniques. HV TECHNOLOGIES, Inc. offers this solution with the Shirla Sheath Test and Fault Location Device, which uses the measuring bridge principle according to Murray and Glaser. 

The Bridge Principle – Wheatstone Bridge

The Murray and Glaser measuring bridge principles used in the Shirla Sheath Test and Fault Location Device are based upon the simple Wheatstone Bridge measuring principle. This bridge consists of two voltage dividers, which when balanced, have a ratio of R₁/R₂ = R₃/R_Xand the meter reads zero. In order to determine the fault resistance (R_X) of the faulted cable, R₂ must be adjusted until the measuring device reads zero. In this case, R_X = (R₂/R₁) * R₃

Murray Loop Bridge

The measuring bridge circuit according to Murray is applied when only one additional healthy conductor (auxiliary line) is present. Ideally this additional conductor has the same diameter and conductor material so as to not have different resistance values affect the measurement. The resistance of the loop bridge at the cable ends is also included in the measurement, and therefore, this must be of very low resistance. In this case R_X =  (R₂/R₁) * (R₃ + R_{Loop Bridge} + R_3b), where R₃ is the resistance of the auxiliary line, R_{Loop Bridge} is the resistance of the loop bridge, and R_3b is the resistance of the cable to the fault from the loop bridge. The distance to the fault can then be calculated.

Using the Shirla, the Murray measuring bridge is connected to the cable in the following way. It is important to remember that when trying to force a current flow from the sheath to earth through the fault only, both ends of the cable sheath need to be disconnected from earth.

Glaser Bridge

The bridge measurement according to Glaser requires operation via two auxiliary lines. The advantage is that different parameters of the conductors, such as material, cross-section and length, will not affect the accuracy of the measurement. The major difference to the Murray method is that the forward path defined via the two auxiliary lines is compensated and the remaining effective external circuit is the sheath only. The influence of the R₃ auxiliary line is eliminated and the bridge only sees the ratio of the voltage divider in the cable. This compensation is very helpful, as the sheath is always of different diameter compared to the core. In this case R_X =  (R₂/R₁ + R_H) * R_3b

Using the Shirla, the Glaser measuring bridge is connected to the cable in the following way. It is important to remember that when trying to force a current flow from the sheath to earth through the fault only, both ends of the cable sheath need to be disconnected from earth.

Advantages of Bridge Measuring Techniques using Shirla

The main advantage of using bridge measuring techniques when determining sheath faults is the simple fact that the resistance of the actual fault has no influence on the measurement and only the resistances of the cable conductors (core, sheath, auxiliary lines, etc.) are taken into account. Therefore, an unstable resistance behavior in the earth is negligible. This is contrary to the voltage drop method, in which the resistance of the fault has a full effect on the measurement as it is part of the entire measurement loop. If the earth loop value alternates during a measurement, the resulting calculated resistance ratio will alter as well. Furthermore, if the sheath fault is located within 5-10% of either cable end, the voltage drop method becomes extremely inaccurate.

Features of the Shirla Sheath Test and Fault Location Device:

• Automatic zero balancing and evaluation of the fault distance
• Ideal for locating sheath faults and areas of neutral corrosion
• Ability to enter various conductor materials, cross-sections and lengths, increasing the accuracy of the measurements
• Supplied with high quality, very low resistive test leads and linking bridges to ensure accurate measurements 
• Lightweight and portable device
• Operation via mains power or with internal rechargeable battery 
• Cable sheath testing up to 10 kV
• Simple operation and intuitive user interface
• Data export via USB interface