| 摘要 |
A trace wire is placed alongside an underground utility for long distance transmission of locating tones. The trace wire has a conductive core and an electrically insulating sheath on the core. The sheath has an outer layer of solid dielectric material surrounding the core, the outer layer having a predetermined weight per unit length and an inner layer comprising a solid foam of dielectric material filling the space between the core and the outer layer. the flexural rigidity ratio of the insulating sheath is given by: Frb/Fra>=4 where: Frb is the flexural rigidity of the outer layer Fra is the flexural rigidity of a minimum outer diameter sheath of said solid dielectric material. This combination of a core conductor with insulating materials in a multi-layered design provides significantly improved properties over current commercially available insulated conductors when used as trace wires. In the presently preferred embodiment of the invention, the trace wire construction includes a. 1.6 mm (14 AWG) hard drawn copper conductor to provide low resistance along with high break strength. The inner layer of the sheath is gas injected foamed polyethylene (PE) insulation applied to an overall diameter of 7 mm. The outer layer is solid, medium or high density PE applied over the first layer to an over all outer diameter of 8.5 mm. This dual-layer insulation exhibits an effective relative dielectric constant of about 1.6. The attenuation constant at 500 Hz is 0.227 dB per km maximum. The break strength is about 135 kg and the trace wire is light weight at about 36 grams per meter.
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