Armour Earthing & Metallic Screen Earthing

[AKassem]

Dear all,

Please provide help in these questions ?

What is the reason of earthing the armour?

What is the use of metallic screen in HV & MV cables,and why it is earthed & why not used in lv cables?

What is the use of conductor screen (placed directly over the conductor) & (insulation screen placed directly over the insulator)

Why in MV & HV cables,we use Raychem at the terminations,what is it and what is its function?

 

[meeko]

I know that you need to ground the armour because you get induced voltage in the armour, and when a cable has a concentric neutral, it is also grounded for induced voltages again.

Im not sure about your other questions, but i beleive it is to seal the copper conductor, with ZERO air gaps, and completely sealed, so the cable will not get tracking and No partial discharges from the high voltage which leads to cable failure. This insulation, semi-con, is to keep the cable to be in service for over a lifetime with no harmonics and no tracking and no partial discharges.

A cable is like a capacitor, should have a phase shift of 90degrees of voltage and current, the more the cable is broken down, the less the phase shift, as resistive leakage current is added.- (can test this with the tan delta)

 

[7anoter4]

The conductor it is not a smooth cylindrical surface so the electric field close to the conductor in some location could be higher and would
damage a part of insulation. Then the conductor shield, of extruded semi-conductive material, creates a smooth surface and
restore the field shape.
The insulation shield behaves as a capacitor armature confining the electrical field in the dielectric [the insulation] also, on a smooth surface,
creating so an even radial field. This way reduces the electrical stress in the insulation and avoid the field spread outside.
The low voltage electrical field is not so strong and the shield is not necessary in order to reduce the stress in insulation.
But,since the shield avoid field spreading around in vicinity, low voltage cable some time is shielded too. See:

http://www.google.co.il/url?sa=t&rc...ce=web&cd=1&cad=rja&ved=0CDUQFjAA&url=http://

http://www.anixter.com/AXECOM/AXEDo...hIHACA&usg=AFQjCNGtAQXm4CaOKCHcCeqRzWSDLviIDA

The main conductor a.c. current flowing in the other adjacent conductors[ or single-core cables] produces a magnetic field which in turn induces
an E.M.F.-induced voltage-in the shield or armor of the first considered cable.
IEEE-525 indicate a maximum 65 V for this built-up voltage but only 25V at terminations.
The termination protect the cable against moisture ingress and in the same time increase the length of the creeping path
avoiding surface discharge flash arcing and protecting the cable end.

 

[7anoter4]

Considering E electric field intensity [as vector] constant across the insulation thickness
V12=E*d12 where V12=potential difference between conductor [1] and the grounded shield[2]
and d12 the insulation thickness.
E=V12/d12 [approx.] If V23=0 then E=0.
If the shield is grounded no electric field will be between the shield and any other grounded object[included armor] since the potential difference will be null.