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Over-sheath Continuity Testing for Graphite-coated Cables

The oversheathing continuity test is designed to check the final outer layer of graphite coated Medium Voltage cables to ensure that there are no defects such as holes, fractures, or minute breaks in the length or breadth of the over-sheath. Such defects are usually too small to be seen by visual examination. 

Graphite coating on MV cables is used as an electrode to connect the over-sheath to the ground. It is particularly used by Network Rail for their trackside power NR/PS/ELP/00008 19/33kV and 24/45kV Cable.

The test methodology for oversheath continuity testing sees a cable prepared by marking a 30cm section on one end, then using an emery cloth or sandpaper to remove approximately 5-10cm of the graphite coating. The remaining 20-25cm of oversheath is removed to expose the copper wire screen. Where possible, a section of swellable tape is left in situ as an additional layer to prevent arcing. 

The copper wire screen is grouped so that the MV insulation resistance test equipment (VLF cable test) can be applied to the ends, and a copper spring clip is applied around the over-sheath approximately 30cm away from the ground down section.  On the opposite end of the cable 20 cm is measured and marked, and approximately 5-10 cm of the graphite coating is removed with the remaining 10-15cm of the over-sheath removed and the copper wire strands spread apart.

The test is conducted by applying a specified DC voltage for a specified duration to the final metal layer under the over-sheath, and to the outer surface of the over-sheath. The insulation resistance between the metallic screen and the graphite over-sheath is measured, with the required insulation resistance will be specified in the relevant cable specification.


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