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#1
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Jamal Wafi
09-21-2013 12:38 PM

Why we connect Earth lead with metal tape shield in the cable?

at cable terminations for 3 Core plastic cable 240 mm2, Why we connect Earth lead with metal tape shield in the cable?
09-21-2013 03:10 PM
Top #2
Omer A.Osman
09-21-2013 03:10 PM
Your query not clear, plz revisit if possible, but let me clarify the following:-
1. I guess that the cable you are talking about is an armour cable with 4 core, is't it?
2. The method of Earthing arrangement depends on Earthing system adopted. ie for TN-S system the cable armour act as the main earth conductor and therefore the cable glands at the termination have to be fixed to the armour and to be connected to the earth bar (for the installation), while for TT, IT the arrangement is different.
3. The earth loop impedance at a particular fault point for TN-S system, as above, can be calculated as follows:
Z= Loop impedance of the Armour up to the Source + Risistance of the phase conductor + Risistance of the circuit protective conductor(CPC).
Note:
1. CPC is the earting conductor between the branch circuit & Earthing bar (feeder glands).
2. CPC is different from Equipotential bonding conductor.
09-21-2013 05:21 PM
Top #3
Lester Philip Lualhati
09-21-2013 05:21 PM
hmmm... you question is somehow vague... i suppose you are referring to shielded XLPE cables... yes, earthing in a cable is connected with the metal shield tape of the cable in protecting the equipment/ panel from leakage current (poor XLPE quality or wrong terminal lugs/ shrinkable tube assy)...
09-21-2013 07:46 PM
Top #4
Allan Stevens
09-21-2013 07:46 PM
I am presuming your question is in regards to shielded medium voltage cables having an XLPE insulation. In that case, the copper tape shield is connected to earth (Grounded in ANSI/IEEE following countries) in order to provide a means of dissipating induced charge on the outer layer of the insulation and to create a uniform distribution of the electrical field in the cable. This also prevents the outer jacket of the cable from getting elevated above a low earth potential and thus becoming a shock hazard to personnel.

Note that in the case of MV cables, unless the cable length is very long -- or it is a utility installation -- only the sending/source end of the shield is grounded. There are good reference calculations available from the various cable manufacturer websites which will help in determining how long a length can be used before the shield voltage has reached 50 volts or so, at which point one needs to look at inserting a break in the shield and reconnecting to ground; sometimes this is done at a splice in the mid-point between the ends and the ends are then grounded with the shield left floating at the splice.
09-21-2013 10:35 PM
Top #5
Norman Chambers
09-21-2013 10:35 PM
Trying to break down the question but it is difficult.
At what point of termination are you talking about. Transformer.. What point?
09-22-2013 12:36 AM
Top #6
Chandra Sekhar Nuli
09-22-2013 12:36 AM
What is a plastic cable ?
09-22-2013 03:12 AM
Top #7
Norman Chambers
09-22-2013 03:12 AM
Exactly, what is a plastic cable?
09-22-2013 05:29 AM
Top #8
Richard Poulter
09-22-2013 05:29 AM
Assume he means xlpe insulated, often referred to as plastic or polymeric
09-22-2013 07:41 AM
Top #9
Meynardo Custodio
09-22-2013 07:41 AM
Mr Allan Steven's explanation on the purpose of the metallic shieding around the 3 core cable is correct. I would like to add some more explanations to make the clarification more broader. The metallic shield, aside from confining the electrostatic field generated by voltageto within the sheath of the cable shield also collects and drain the induced current to the ground (earth). There are other secondary functions that the metal shield accomplish. It also serves as a mechanical protection. In medium voltage cables, by US standards the cable shield is required for each conductor if the voltage is 2.4 KV and above. The cable shield is grounded (earthed) at every point where a medium voltage splice is installed. It is also required to be grounded at each termination (grounding the streaa relief cones). Because the cable shield is the confining cover over the insulation of the cable (it is between the insulation and the cable jacket) , the cable shield also becomes the first ground path in a cable arcing fault / line to ground fault when the cable insulation fails. That is also he prime reason why it should be grounded at multiple points (in every cable splice and manholes).
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