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Fabric Tape Conductor Shield

May 4, 2012

Fabric Tape Conductor Shield

Q:

I am wondering if you can provide some thoughts or comments on a cloth fabric semi-con we have here on some older #2 Cu cables, 15kV. Does the Cablecure 732/733 [Ultrinium] fluid harm this fabric? Does the fluid react with any semi-conducting materials like carbon? Does Novinium think the injection process disturbs or harms the fabric?There is no reaction between carbon black and alkoxysilanes.A:

The fabric tapes used on pre-1980 vintage cables were carbon-black dispersed on cotton fibers. Neither the cotton nor the carbon black in these semiconducting tapes react with the silanes used with Cablecure 732/733 fluid or Cablecure iXL fluid. In fact, cables with fabric tapes have been treated with alkoxysilane rejuvenation fluid for over two decades. Novinium has not experienced a single failure of a cable with a taped conductor shield.

The use of taped conductor shields in medium voltage distribution applications all but halted by the mid-1970’s in North America. Bartnikas and Srivastava relate in Power and Communication Cables, page 83 …“Semiconducting carbon black tapes were…used as shields in the early linear polyethylene (PE) insulated cables. Due to poor adhesion between the tapes and the PE as well as occasional breaks or gaps between butting edges of carbon black tapes themselves, partial discharges often occurred within the voids formed at these faults. Polyethylene cables using carbon black shielding tapes were also found to be highly susceptible to tree growth.”The fact that you have some of these cables in service today is testament to the absence of partial discharge and hence these particular cables do not suffer from the poor adhesion, occasional breaks or gaps which Bartnikas and Srivastava warned about in their book. However, there are certainly water trees and I’ll bet they’re doozies. Fortunately, ameliorating the pernicious effects of water trees is precisely what Cablecure 732/733 and Cablecure iXL fluids are designed to do.

There are two operational considerations when injecting taped conductor shield cables. First, there is much more room in the strand interstices, so the fluid will flow faster and the cable will hold more fluid—these are both good things.  Second, when fluid flows through the strands it will entrain some un-adhered carbon black, so the outlet fluid will be black. Not to worry, there is much more where that came from and there is no need to try to flush it all out either.