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HT & LT Cable Construction

08/22/2013 2:29 AM

Hi everybody.

I would like to know in detail the construction dfference between HT & LT cables,like why HT cable are circular in shape & LT are segmental type.I tried to gooogle it but could not get clear answer.If anybody knows about it please let me know.

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#1

Re: HT & LT cable construction

08/22/2013 3:02 AM

Hint: it has something to do with voltage and current. Keep working on it.

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#2
In reply to #1

Re: HT & LT cable construction

08/22/2013 4:17 AM

Is it voltage & current

or Voltage only?

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#3
In reply to #2

Re: HT & LT cable construction

08/22/2013 4:56 AM

Hint: consider a cable carrying 100kW. What is the difference between the needs for a HV cable and an LV cable for the same duty?

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#4

Re: HT & LT cable construction

08/22/2013 8:57 AM

HT cable always carry high voltage where you need to have a very good thick insulation between the phase cable that is combine together to form a 3 core cable. Therefore, the XLPE insulation need to be thick and that is why the XLPE insulation is circular and thus the cable are also circular in shape.

For the LT or LV cable the voltage are at a low voltage where the PVC insulation material used is thinner. Thus, the cable are not in a circular shape. It is more towards triangle shape as the manufacture try to save the outer layer of the insulation by squeezing all the 3 phase cable in to smaller diameter.

For HT cables the thickness of the insulation for each of the phase cable is the main criteria to prevent the voltage flash over between the phase.

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#8
In reply to #4

Re: HT & LT cable construction

08/23/2013 12:10 AM

"It is more towards triangle shape as the manufacture try to save the outer layer of the insulation by squeezing all the 3 phase cable in to smaller diameter."

Is it not true that the circle has minimum overall diameter for a given conductor area & more economical?.By sqeezing,does the overall circumference decrease & hence cost?

Thanks a lot for the inputs.

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#19
In reply to #8

Re: HT & LT cable construction

08/26/2013 4:29 AM

It appears that you are not keen on going into the depth of any topic.

Read my comments carefully and note that the reason for using sector shaped on LT cables. It is same simple English and what you are trying to teach me back.

Also you have failed to realise why not sector shape in HV Cables - I can not help you but I shall never comment in furture to any of your posts.

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#5

Re: HT & LT Cable Construction

08/22/2013 9:49 AM
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#6

Re: HT & LT Cable Construction

08/22/2013 11:11 AM
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#7

Re: HT & LT Cable Construction

08/22/2013 10:13 PM

So no one has come across shaped conductors in MV (11KV) cables? Cables above 35mm² will be shaped.

I must have been dreaming for the last 40 years.

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#9

Re: HT & LT Cable Construction

08/23/2013 4:18 AM

OK. Here's the deal: it's because each shape uses the minimum amount of materials to do the job in each case.

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#10

Re: HT & LT Cable Construction

08/23/2013 5:04 AM

What is a "segmental" type LT cable? Most LT conductors I've seen are circular.

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#11
In reply to #10

Re: HT & LT Cable Construction

08/23/2013 6:26 AM
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#12

Re: HT & LT Cable Construction

08/23/2013 7:25 AM

Being in the business, it is quite simple. No1, not all cables are concentrically round, so the statement you make is not true. No2, it has nothing to do with voltage or amperage per say. Some HT cable are triangular in shape. (For a good reason).

Segmented shape is found in 4 core LT cable. It naturally forms a round shape. Hence filler tapes or strings are installed to round the cable off to go through round dies to extrude the inner sheath, lay up armour wires and extrude an outer sheath.

HT cable are 3 core, no neutral conductor, so sometimes fillers tapes are instaled to make the cable round. But in some cases the 3 core is extruded through round dies and left as a triangular shape, as triangular is easier to install in ducts as there is less friction build up of the cable against the duct walls. Larger HT cables are round due to extrusion process of the XLPE insulation on to a rounded conductor, which is layed up on a machine which spins in circular orbits. Hence round single core cables.

It is all to do with design parameters, machines, and costs. To make a 2 core cable round costs a lot of money in filler tapes and to expensive for you to buy. So they are flat. So look again at cables and you will note they are not always concentrically round.

Volts and amps are only a small part of the design, crucial I admit to insulation thickness, type of insulation used, but it is dictated by equipment to manufacture the cable, costs, what the client wants, and what can be manufacture on the equipment.

No rocket science here with volts and amps, sorry lads.

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#13

Re: HT & LT Cable Construction

08/23/2013 9:33 AM
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#14
In reply to #13

Re: HT & LT Cable Construction

08/23/2013 10:05 AM

Interesting, but the opposite of the OP's thesis....

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#17
In reply to #13

Re: HT & LT Cable Construction

08/24/2013 1:18 AM

interesting, I learned something new from this,

Thx, 7anoter4

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#15

Re: HT & LT Cable Construction

08/23/2013 10:12 AM

Low Voltage cable [ up to 600 V according to UL and 1/0.6 kV according to IEC] conductor shape could be : solid, circular stranded, compressed and compacted, and compacted shaped .

The purpose of compacting is in order to reduce the conductor diameter and in turn to reduce the insulation material-a slight effect will be on skin and proximity effect also. Shaped

conductor facilitate to reduce the overall dimension of the cable and so reducing the jacket material and duct diameter.

The Medium Voltage cable conductor has to be round in order to equalize the electric field all around and permitting accommodation of the shield.

In order to reduce the skin effect in a large conductor ,the conductor is build round from a number of shaped [stranded] conductors bare or even insulated.

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#16

Re: HT & LT Cable Construction

08/23/2013 9:46 PM

Aim of cable designer is to reduce the area of x-section of cable without taking any risk of damage to cable.

In LT cables, maximum rated voltage is 1100V. At this voltage if we select the sector shaped individual cores, the insulation is not going to breakdown at service voltage. At the same time we reduce the overall diameter of cable and reduce cost by avoiding filler etc. to keep circular shape.

In High voltage, reason for selecting round cross section is to reduce Electrostatic Stresses in insulation. To know more open your basic physics book of 9th standard and read what is electrostatic stess - why it is maximum at a sharp point and why it is uniform in case of spherical or circular surface (except at the edges - where live conductor comes out of insulation and the reason for using stress relieving tube or Stress cone).

Pl remeber that practical is nothing but application of what you learn in school and professional college and apply that in your life. I dislike common phrase used by Engineers that what we studied was theoritical and is of no value - we have to now do the job and learn practical - which is not correct - as you must have noticed that clue to your doubts lied in what we study in high school.

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#18
In reply to #16

Re: HT & LT Cable Construction

08/26/2013 3:23 AM

Thanks a lot to all.....especially the last answer gives a lots of message. :)

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