Cable Outer Sheath Fault Locator Portable High Voltage Murray Bridge and 0

The Cable Outer Sheath Fault Tester is built on the MURRAY bridge principle and is used to locate the breakdown points of wires and cables as well as defect points that have not yet broken down but show a low insulation-resistance value. The high-voltage source and the bridge are integrated in a single portable protective case, so the equipment combines high voltage with light weight, convenient operation and safe use.

The equipment uses a motorized voltage regulator and an R-type transformer to form a high-voltage constant-current source, with keys to raise and lower the voltage. A high-sensitivity amplifier and a galvanometer give the balance indication and, together with a proportional potentiometer, form a balanced bridge placed at high potential. The measuring cable is a specially designed two-core high-voltage rubber cable, and the four-terminal resistance measurement method avoids the error introduced by lead resistance, while the panel as a whole is at low potential. With one-key operation, once the cable length is entered the fault distance is calculated automatically.

With the full spread of urban grid renovation, many power supply bureaus, construction parties and design units are working with 110kV and above XLPE cables for the first time. Because of time pressure and inexperience, many cables fail the sheath withstand test (10kV/1min) after laying. Cable outer sheaths mostly use HDPE sheath material that has already passed a DC 25kV/5min withstand test in the factory, with a leakage current as low as tens of μA, so post-laying defects mostly come from external-force damage during laying, including the filling and cover-plate work. After the cable is put into operation, defects usually come from termite bites, water ingress in grounding boxes, deterioration of existing defect points and moisture penetration in the branched part of the grounding wire that lowers the insulation resistance — and when a cable is buried too deep or the surroundings are complex, positioning becomes very difficult and some defect points remain unsolved for years.

This tester is dedicated to the outer sheath (protective layer) of high-voltage cables: locating sheath breakdowns and low-resistance defects and carrying out sheath withstand tests. It suits site teams that face post-laying sheath problems, and it is especially suited to:

For projects meeting 110kV and above XLPE cables for the first time, where laid cables fail the sheath withstand test (10kV/1min) and the defective points must be found and dealt with.

For locating outer-sheath defect points caused by external-force damage during laying, filling and cover-plate work, using the bridge-balance method that has no blind zone.

For locating linear high-resistance breakdowns at cable intermediate joints, and for troubleshooting reduced insulation resistance caused by termite bites, water ingress in grounding boxes and moisture penetration in the branched part of the grounding wire.

The output voltage is 15kV and below, fully meeting the cable outer-sheath withstand-test requirements of the Chinese national standard GB50150-2006, including the sheath handover test and preventive test for cross-bonded systems. It quickly and accurately finds outer-sheath ground faults and excessive leakage current in 10kV–500kV single-core and three-core high-voltage and extra-high-voltage cables, and can carry out 5kV–15kV, 1-minute DC withstand tests on HDPE and PVC cable outer sheaths. It is essential dedicated equipment for the cable maintenance of 10kV–500kV single-core and three-core high-voltage and extra-high-voltage cable operation units, extra-high-voltage cable engineering companies and power transmission-and-transformation engineering companies.