Cable insulation
Cable insulation
(OP)
Dear all experts,
Is the thickness of the insulation for xlpe cable important to cable design? In other words, whether thick insulation is better than thin insulation?
Thank you!!
Is the thickness of the insulation for xlpe cable important to cable design? In other words, whether thick insulation is better than thin insulation?
Thank you!!






RE: Cable insulation
RE: Cable insulation
RE: Cable insulation
I agree with Donale....judging any insulation system is not as simple as just seeing which one uses more material.
RE: Cable insulation
RE: Cable insulation
I am not a cable guy, so please explain why a thinner insulation increases capacitive losses.
RE: Cable insulation
Sorry about the late reponse on this. I haven't been checking the forums as often as I used to...
The relation between capacitance and insulation does seem counterintuitive at first, but I think I can explain it in language that is more or less English.
Capacitance changes with respect to the electric field applied across the insulating dielectric. As the distance between the plates is increased, the region of space where the electric field is stored increases. As the distance increases, the electric field remains constant. This increases the potential difference between the plates, given by V = E*d where d is the distance between the plates. To keep the potential from increasing, charge must be removed from the plates. Hence, the capacitance decreases. Conversely, as you decrease the distance between the plates of the capacitor, the same electric field is distributed across a smaller distance, increasing the charge held on the plates.
You can also look at the following equation used to calculate capacitance for single conductor shielded cables, and note the relationship between capacitance and the thickness of the insulation...
C =(0.00736*K) / (LOG10(D/d))
(C is in units of microfahrads/1000ft in this case)
Where:
K = dielectric constant of the insulation
D = diameter over the insulation in inches
d = diameter of the conductor in inches (or over the conductor shield if present)
This relationship is used by some instrument manufacturing companies to measure insulation thickness if the dielectric constant of the insulation is known.
Dielectric losses for cables can be calculated by multiplying the capacitance by the power factor and the square of the operating voltage (and a conversion constant to get the losses in the units you want).
It's been a while since I had to talk in terms of electric fields, so my terminology may be a little rusty. Hope this helped.
Kraigb
RE: Cable insulation
Thanks,
Michael Sidiropoulos
RE: Cable insulation
For every voltage level IEC specifies a corresponding nominal insulation thickness. eg:- for 11 kv cables the specified nominal thickness is 3.4 mm. IEC further specifies that the minimum thickness at any point can not be less than 90% of the nominal value by 0.1mm. That means if the insulation thickness at any point if found less than (0.9*3.4 -0.1) mm ie, less than 2.96 mm for an 11 kv rated cable the cable is rejected.
Further the average thickness of insulation (rounded to 0.1 mm) at any cross section of the core shall not be less than the nominal thickness specified above.
If the thickness is on the higher side electricaly there is no problem. Effects of capacitance, dielectric losses etc. are negligible especially at medium voltage level. However higher insulation thickness has the following disadvantages-
1) It requires excess insulation material
2) Higher diameter of the cores makes the diameter of the laid-up cores high, which inturn increases the materials required for fillers,wrapping tapes,inner sheath, armour,outer sheath etc.
3) An excess outer diameter of the cable can make it not suitable for standard sized cable glands etc.
Advantages
1) Increase in outer diameter of the cable slightly improves the current rating of the cable.
Thanks
Janesh
RE: Cable insulation
You are correct that the losses go up as the voltage goes up, but what you consider to be negligable depends more on the economic philosophies of the company owning the lines.
Depending on the ROI (return on investment) horizon a company chooses, the yearly cost of losses ($/watt for every meter of cable installed) can become a factor in the decision. If the company is more interested in the life-cycle cost of the cable, it is quite possible that an increase of $0.03/foot for every year the cable is in service will make a difference. (This, of course, depends also on how long the cable is that you plan to install.)
RE: Cable insulation
Apologies if this has already been covered, but ANSI-type standards establish a nominal dielectric thickness based on voltage and insulation level, and tolerable variations. AEIC CS8 for medium-voltage cables is one reference.
RE: Cable insulation