Q:Do you know of a blocking resin that will not affect the electrical and physical properties of foam/skin-insulated conductors?
Mario Cesar da Silva
Telebras Research and Development Center
Campinas, Brazil
A:Foam/skin-insulated, air-core cables are designed for underground installations where duct congestion is a major concern. The extrusion process creates two layers in high-density polyethylene (hdpe) foam/skin insulation for conductors in an outside-plant telephone cable, an inner layer of foamed polymer and a thin outer layer of solid polymer. The thin, solid outer layer, or skin, provides the mechanical properties and protects the foam. The inner foam layer decreases the electrical capacitance of the overall insulation, which allows closer spacing of the insulated conductors in the cable. Air in the voids of the hdpe cable has a dielectric constant of 1.00 compared to a dielectric constant of approximately 2.32 for hdpe.
Although these cables make more efficient use of duct space and reduce overall splice bundle size, they are problematic to block. Jim Schroeder, technical service representative at 3M Telecom Systems Div. (Austin, TX), says, "I know that there is not a compound available that will work on this cable. Any block that I have tried will leak air pressure between the insulation and the copper conductor."
Air leaks through the thin skin into the foam layer, travels along the foam layer through the block and then out again once past the block. According to Jim, the only way to block foam/skin cable is to splice in a piece of standard plastic insulated cable, install the block around the polyethylene-insulated conductor sheath and then splice back to the foam/skin cable. Others in the industry, however, maintain that the usual blocking compounds can be used with foam/skin cables (see "Mixed messages for end-user").
Mixed Messages for End-user.
While blocking compound manufacturers, such as 3M Telecom Systems Div. (Austin, TX), say that regular blocking compounds will not work with foam/skin conductors, some cable manufacturers claim you can use the same blocking compound that you have always used. According to Alan Dwyer, senior engineer at Cable Systems International (Phoenix, AZ), OGenerally, any blocking compounds or encapsulants that are used for solid hdpe-insulated conductors can be used for dual expanded polyethylene-insulated conductors, or depic.O He suggests that users contact a manufacturer of these compounds, such as 3M (800-745-7459), or a supplier that sells the materials, such as gte Supply (800-723-8285), for more information.
Until cable manufacturers and makers of blocking compounds resolve their differences on this topic and communicate a consistent message to the industry, the end-user is the loser.
Donna Ballast is a communications analyst at The University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, The University of Texas
