Criteria for use of XLPE in lieu of PVC conductor

[DrDrreeeaaa]

Hi all,

What criteria do you practically apply for the use of XLPE over PVC for submains etc? I expect the critera to be interms of load amps??

Cheers

mb

 

[DrDrreeeaaa]

EDIT: The subject is supposed to be 'in lieu of PVC insulation' not 'PVC conductor'

 

[7anoter4]

hi 123MB
First of all PVC insulation maximum admissible temperature is 65-70 degrees C.
Secondly never the less PVC is self extinguishing material the fumes produced in case of fire are very toxic and corrosive.
Third, in a case of short circuit [for short time] PVC does not withstand more than 130 degrees C [xple does for 250 degrees C].
Some other properties are less as relative permittivity [3-7 for PVC] so the capacity between conductor will be more than for XLPE where permittivity is only 2.1-2.2. As moisture absorption of PVC will be 1-2% the permittivity will increase more in wet location[as water has 80 the permittivity].Also the dielectric losses of PVC are more than XLPE [tangent delta, a very important property for high voltage cable -due to heat increasing].
A property where PVC is better is the mechanical strength. So for cable jacket may be use PVC [if chlorine is not prohibited here].
Best Regards

 

[jghrist]

What criteria do you practically apply for the use of XLPE over PVC for submains etc? I expect the critera to be interms of load amps??

What do you mean by submains? What application? Utility substation or power plant? Industrial? Commercial? Residential? What voltage? PVC would not be used for MV or HV cable.

 

[DrDrreeeaaa]

jghrist, fair enough, I will provide more information in future posts. It would be useful I think, for the forum community, to answer in terms of the installation characteristics as the 'criteria' I was talking about.

The application is a 3-phase 430V 150A submain at the LV power distribution level in an industrial application.

The cable is installed on a cable ladder.

Cheers

mb

 

[7anoter4]

Hi 123MB
I am sorry I didn't understand well your question.
What I did is not fair with respect the PVC as I only criticized it.
The PVC has a very important property and I am sorry as I forgot it.
PVC is the cheapest polymer used for electrical cables. It contains 30-40% resin and 50-60% filler [usually chalk].It is thermoplastic material that means from the extruder direct to cable. XLPE is a thermosetting material that means after extrusion a complicate process [similar to rubber vulcanization] cures the material.
So, let us try to design a more appropriate criteria in order to state where the XLPE is really required [as jghrist already inquired].
For a cable buried in earth or concrete and does not pass through a stuffy cable spreading room the PVC would be accepted [except MV and HV cables –nevertheless the old PVC cables were insulated PVC up to 10 KV].
If the cable is laid in a large cable spreading room PVC should be avoided.
This is the case of a Power Station or Utility Large Substation.
For an industrial facility in which the equipment is not so expensive and fire extinction and smoke detection is provided, the cable crossing the wall and floor openings are fire stopped, the PVC would be also permitted.
For large commercial facility I should recommend halogen- free materials [no PVC at all, even the jacket has to be halogen-free material] .The same for residential multi- dwelling and mainly multi-level [where chimney effect may accelerate the fire].Fire resistant cables will be more recommended here.
For your application if the cables represent about 10% from over all investment, you may think how to reduce the cable expenses. I think instead of cable ladder a solid bottom cable tray should be provided [if PVC insulated cable would run through]. Also the cable drop should be protected by GRS [Galvanized Rigid Steel conduits].
For current carrying capacity [in metric units]on tray, a German Standard is available: DIN VDE 0298 Part 2.
Suppose your cable is only one cable on this ladder.
For a cable lying on cable racks the maximum permitted current will be the same as for a cable laid in free air [no derating for 30 degrees C air temperature].
Let say the air temperature will be 40 degrees C then a derating factor of 0.91 shall be applied for XLPE and 0.87 for PVC.
For PVC [NYY Cable-copper conductor PVC insulated PVC jacketed] in 30 degrees C environmental air is required 150/.87 =172 A and for XLPE insulated 150/.91=165 A .From the Standard 3*70 PVC current-carrying capacity is 212 A and for 3*50 XLPE 195 A. If the distance between cables may be maintained more than 2 cable diameter [but not less than 30 mm] any number of cables may be laid on the same rack with no derating.
For solid bottom the derating factor depends upon number of cables in the same tray:
For 1 cable will be 0.95 and for 3 cables 0.80[no matter the distance between one to another] that means PVC 3*70 and XLPE 3*50 is still good for 150 A at 40 degrees C.
Regards

 

[BritEng]

Here in the UK we would select our cables based on BS7671 - 17th Edition. One of the things to look out for is regulation 512.1.2 which states

Switchgear, protective devices, accessories and other types of equipment shall not be connected to conductors intended to operate at a temerature exceeding 70oC at the equipment in normal service, unless the equipment manufacturer has confirmed that the equipment is suitable for such conditions.

Therefore if you are sizing cables that are to be connected to equipment designed to operate at a temperature not exceeding 70oC then PVC values should be used and not XLPE in your calculations, refer to note adjacent to table 4E4A in BS7671.