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Join Date: May 2014
Posts: 5

Semiconductive Cable Screen End to End Resistance, Calculation Assistance

05/22/2014 4:35 AM

Hi all, I am trying to calculate the end to end resistance of a semiconductive screen material utilised in a subsea cable. I am struggling (i think) with the values given and appropriate calculation. My personal results look tangible but would like either some informal validation of my final figure or could some-one point me to a process to perform the calculation.

Key details

Semiconductive material - (DOW DHDA-7708 BK) Volume resistivity 25Ω/cm

Screen cross sectional area - 15.7347 mm² (14.66 mm OD x 0.7 thick)

Length - 5800 m

My calculation gives a resistance (end to end) of circa 9218 MΩ. Is this ballpark correct (allowing for rounding errors). If not could someone point me in the direction of the correct method of calcualtion as if this is wrong I'm not sure where I've dropped the ball, as this is the first time I've had to do this sort of calc from the available details.

Location - UK

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Guru

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#1

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 5:10 AM

Seems to me that you could treat the problem as a wire having the same cross-section and resistivity as the sheath and go from there.

unless:

1. It is in contact with seawater,

2. It is carrying an AC signal, in which case skin-effect will drive up the impedance as a function of frequency and geometry.

Is this how you modeled it? As a wire?

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#2
In reply to #1

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 5:22 AM

I think i've treated as a wire using the data on good on wikipedia (http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity) . And converting the 25Ω/cm to the appropriate value for the x-sectional area.

AC is involved but at present a simple DC equiv value will suffice and then look into the skin effect from there.

My process is what is troubling me and the final value result (from my calc) while tangible just 'looks wrong'.

Been far too many years since I had to do anything close to this type of calc hence my trepidation with my results. Unfortunately despite searching fairly well I can't find a 'simple' description of how to perform this calc.

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#3
In reply to #2

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 6:02 AM

OK just reworked this as I found a units error. Would I be correct with the following based on the above data.

Final value = 9.218 MΩ (end to end resistance, not allowing any skin effects)

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#5
In reply to #3

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 6:38 AM

Just a note: resistivity is in units of Ω cm, not Ω/cm.

Look at the definition of resistivity:

Now do a dimensional analysis (use cm for linear measurements to jive with given units):

ρ = Ω * cm2 / cm = ohm * cm * cm / cm = Ω cm

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#7
In reply to #3

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 6:48 AM

Where'd you go? If you're taking a test and if you left before my last edit, you're going to get this answer wrong.

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#4
In reply to #2

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 6:15 AM

Treating the screen as an annular cylinder, where

t = thickness

ro = OD/2 [outer radius]

ri = ID/2 = ro - t [inner radius]

A = πro2 - πri2 = π(ro2 - ri2) [Area of annulus]

------

t = 0.7 mm

ro = 14.66/2 mm = 7.33 mm

ri = 7.33 - 0.7 = 6.63 mm

A = π * (7.332 - 6.632) = 30.7 mm2

Converting to cm2,

30.7 mm2 / 100 mm2/cm2 = 0.307 cm2

----

Converting cable length, L, to cm,

L = 5800 m * 100 cm/m = 580000 cm

----

R = ρ * L / A

where

A = 0.307 cm2

ρ = 2.5 Ω-cm

L = 580,000 cm

and so

R = 2.5 Ω-cm * 580,000 cm / 0.307 cm2

R = 4,723,127 Ω or about 4.7 MΩ

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#6
In reply to #2

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 6:42 AM

Correction, ρ is not 2.5 Ω cm, but 25 Ω cm, so scale result by x10:

47 MΩ

Sorry 'bout that. My mistake.

Does this look reasonable to you?

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#8
In reply to #6

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 8:22 AM

Looks perfectly reasonable, thanks for clarifiying my math and the principle.

You did highlight another error I had made with respect to the area of the cross section. As I started with diameter I used Half D to give me the radius, but when allowing for the thickness I had halved this also, incorrectly.

Hence my 9.2MΩ aas opposed to your 47 MΩ which now I have amended my figures I concur with.

Thanks all for the input on this.

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#9
In reply to #8

Re: Semiconductive Cable Screen End to End resistance, Calculation assistance.

05/22/2014 2:10 PM

You're welcome! If you've any other questions, feel free to post them.

e2

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#10

Re: Semiconductive Cable Screen End to End Resistance, Calculation Assistance

05/23/2014 4:52 AM

Why do you want end to end values for a starter/ There is a simple calculation to measure and obtain the value of any semi con in XLPE. (Semi con screen over the XLPE insulation and for the inner core).. Simply refer to the IEC standards 60502, test method, and you will have the value for each core over the length of the cable.

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#11
In reply to #10

Re: Semiconductive Cable Screen End to End Resistance, Calculation Assistance

05/23/2014 7:39 AM

Just curious, what does that standard show for 5800 meters? I don't have access to the standard and wouldn't in my normal line of work, and I can't make heads nor tails of the results I'm getting from a Google search.

Thanks!

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#12
In reply to #10

Re: Semiconductive Cable Screen End to End Resistance, Calculation Assistance

05/23/2014 7:41 AM

IQ, this is a custom made cable and not sure it complies to the standard stated. It is for special use subsea and has gas / water blocking qualities but I will look at the standard and quiz the manufacturer accordingly.

End to end value is critical to the operation of our Line insulation monitor device. Also of concern to us is EM blocking which this will not provide as the cable also carries data.

We are just trying to understand the implications of the construction and relative values accordingly. Unfortuantely I have several 100's of km already manufactured and we need to input the effects into our system models.

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