what is the graphite coating & Why it is used for power cables ?

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

Cable sheath bonding

Single-core underground power cables with two-points bonding induce currents in their metallic sheaths. The sheath induced currents are undesirable and generate power losses and reduce the cable ampacity.  The values of the sheath losses in some cases could be greater than conductor losses, depending on various factors. Such these factors are type of cable layouts, cable parameters, cable spacing, sheath resistance, phase rotation, conductor current and cable armoring

SHEATH BONDING ARRANGEMENTS

Cable trenches installations

Trench Construction:

• The cable trench may be precast (Preferred) or poured in place (Non Preferrred).
• Poured-in-place trench walls to be formed, do not cast against existing soil.
• Exposed edges to be finished with edging tool.
• No customer conductor, including bare ground/bonding conductors, is to be installed in the trench. 

Photos for some cable trenches installations