Copper Tape Overlap for Short Circuit Calculation

[etb90]

Hi all,

Please advise whether it is okay to include copper tape overlap in earth fault calculation and is that true that earth fault and metallic screen resistance is reciprocal to each other.

Any other standard to refer rather than ICEA P-45-482.

Thank you

 

[7anoter4]

The line-to-Earth [earthing] fault calculation it is destined to check the withstand short-circuit current through main conductor and screen [sheath, armor, conduit or else]
The main conductor short-circuit current depends on screen[shield, sheath] impedance and the mutual impedance between main conductor and screen and the Earth. But even if the short-circuit it is not a ground fault the screen will get a short-circuit current by induction from the main current. This happened like in the case of continuous load current and is calculated using formula from IEC 60287-1-1 2.3.1 Two single-core cables, and three single-core cables (in trefoil formation), sheaths bonded at both ends of an electrical section. If the main losses pmain=Imain.sh.circ^2xRcond the total losses will be pmain x(1+ λ1) and screen losses only pmain x λ1.
Rs resistance depends on screen material as tapes [with overlap or gap],wire, braid, tube and other form and different metals as copper, aluminum, steel, lead and other.
For tapes with overlap [new tapes and good contact] it may be consider as tube then[ as per French Standard HN-33-S-52-for instance]:
Rs=[R1]=2.ρ1.L/(π.(d-T1).T1)
ρ1=resistivity of tape material [copper =1/58 ohm.mm^2/m at 20oC]
The maximum temperature for copper tape it could be 250 oC [for less than 5 sec]
L=cable length[m]
d=insulated core diameter [mm]
T1=tape thickness[mm]
The overlapping [%] it has a very small influence and it could be neglected.

 

[7anoter4]

The induced current in a screen it is as in attached sketch:

 

[davidbeach]

ebt, what are you trying to get at? What’s the real underlying problem to be solved? Mostly you seem to be worrying about rounding errors. What are you seeing that makes this important rather than being another one of life’s many uncertainties? Maybe it is important and you haven’t explained it in a manner anybody else gets, maybe it’s trivia that you’ve latched on to. Hard to know from what we’ve seen so far. But obviously you care, why?

 

[7anoter4]

I have not IEC 60949 with me, however, I have the impression that the equation for calculating the area should be the following[/ instead of x]:
(OD under + thickness x no of layer) x no of layer x pi x thickness / (1+OS/100)

 

[etb90]

dear 7anoter4, thank you for your answer.where can i refer this area of calculation from?

 

[7anoter4]

In my opinion, no one of area formulae is perfect for any overlapping and any lapping angle.
The formula from ICEA is suitable for resistance calculation using corrected resistivity [for instance for copper tape it is corrected for a tape length of 2.9 times the winding pitch and also a 4/pi() factor is employed].
The formula from the French standard is not referring to overlapping or winding angle and the IEC 60949 takes into consideration a number of tapes.
This area it is an equivalent area considering in a length of one pitch two resistance in series: one in the overlapped zone where the length is overlap%*W and the area is 2*π*(dm+thk)*thk and the second where the length is W-overlap and area is π*(dm+thk)*thk.
The tape resistance general calculation formula it is Rs=ρ*kT*length/crossection A.In IEC World length in m and A in mm^2. kT=1+α x (Tc-To) where Tc it is the maximum conductor temperature and To is the temperature of ρ measuring [or for copper- for instance -kT=(234.5+Tc)/(234.5+20) when ρ measured temperature was 20oC.]
By the way, this formula is not very accurate for elevated temperature like 250oC.
In my opinion, an error of +/-10% it is allowable.