Graphite is generally used as a conductive coating to enable a sheath test in the field or on the drum, where the cable may not be in contact with an earth medium (eg in conduits, or direct laid in non conductive trenches, or maybe sitting in insulated cable racks in tunnels). It gives it a continuous potential for the sheath test so the whole of the sheath insulation can be tested for damage during installation.
Some graphite will be scraped off during installation, however the user must scrape down into the HDPE or other sheath material to completely remove the conductive graphite. This allows the cable owner to ensure the cable was pulled and laid properly before energising.
Plus it gives the jointers and cable pullers something extra to complain about, when they come home covered in black graphite.
If it is a higher voltage cable, the semiconducting screen is used for voltage stress relief to reduce the effect of cable insulation imperfections. The semiconducting varnish or extruded layer is in contact with a copper tape for earth fault current carrying capacity.
Firstly 144x - sorry, reading back through it I realise that my thoughts were not in line with my typing. I was just trying to make the point that even if you scrape some of the graphite off the cable whilst laying, you will still have quite a bit of graphite embedded in the sheath, making a sheath test still valid and a good indication of damage whilst laying. If you terminate the cable with a gland isolated bushing (for protective purposes), you will physically have to scrape a millimetre or two (depth) of the sheath away (all around the cable) to get rid of the graphite (hence the conducting path), otherwise you will be shorting out the isolation.
The semicon layer that I think paged and vector9 are describing is for stress control and corona minimisation which can be caused by the non circular nature of copper stranded conductors. This is described as the conductor screen, and is usually extruded over the conductors (in extruded cable technology) to provide a circular equipotential surface. The same applies to the insulation screen, where a semicon layer is applied over the bulk insulation (under the earth screen), again to provide a stress free surface. The graphite goes on the outside of the cable, outside the metallic earth screen, hence it is outside the insulation electric field.
a cable manufacturing company claims that he would use a semi conducting layer over pvc sheath instead of the graphite coating for after laying tests.is that possible?
Do you have to extrude or you get some spray or paste to be applied on the tube. I am thinking of this idea for making corona ring for outdoor application.
I can, perhaps, bend the tube and apply coat of this semiconducting material.
Can some one tell me if such material exists.
Suggestion: The cable structure is set by the manufacturer. It is not advisable to add anything without the manufacturer consent (especially, if there is any warranty on the cable).
