Breakdown in polyethylene

It is well known that a type of predischarge condition, in the form of a tree-like pattern,1–5 may be observed in solid synthetic insulation, especially if the insulation is transluscent. It has long been established that the simplest way to form such a tree-like pattern is to subject the insulation to a very nonuniform electric field, applying either ac or pulse voltage. Hence, the so-called needle test, based on essentially point to plane electrode configuration, has been adopted by many laboratories as a criterion for evaluation of quality of polyethylene or crosslinked polyethylene insulation. The needle is usually molded into a specially prepared sample, which is later subjected to a voltage stress. This procedure may yield certain information about the tested polymer, but does not necessarily provide any relevant information about a cable product made of this polymer. This is primarily true because the dielectric strength of a molded sample can be very different from that of the cable product, whose quality depends directly upon extrusion conditions and control.