Encyclopedia of Grounding (CA09040E)

Cable

Over the years, many cable tests have been con ductedandagreatdeal isknownabout itselectrical and mechanical properties. Cable manufacturing processesarewell establishedandwhenconsistent provide a reliable interconnection. The ultimate ratings shown in Table 8-1 were originally calculated from an equation developed by Onderdonk [6] . They are based upon the time a known current with an asymmetry factor less than 20% can flow causing the cable to melt and sepa rate,much like a fuse, thereby interrupting theflow of current. The withstand rating is approximately 70% to 75% of the ultimate rating. It was included in theASTMF855 standard to emphasize the need to include a margin of safety when developing a personal protective ground system. The values shown in Table 8-2 are based upon reduced values taken from EPRI Project RP2446 Computer Program RTGC, "A Desktop Computer Program for Calculating Rating of Temporary Grounding Cables" using an X/R ratio of 30. A crimp ferrule should be used to interface the cable to the clamp. While it is possible to strip the cable insulation and insert it into the compression terminal of a clamp, this results in significant risk of equipment failure and should not be done. While copper strands are new and shiny, tests show that such an assembly functions at the rated cur rent. However, as time passes, individual strands exposed through the clamp compression fitting become corroded and start to break. Resistance between the exposed strands can increase sub stantially when this happens. Passing a high level of fault current through this increased resistance generates a substantial amount of heating. Test results have demonstrated the separation of cable and clamp due to this heating. In some cases, heat was so intense that the pressure terminal actually meltedandburnedaway fromtheclampbody. This results in a complete loss of worker protection. Ferrule size must match the conductor size. Fer rules are made both with and without a shroud. See Figure 8-7. The shroud slips over the insulation and is crimped. By covering the cable insulation, it Ferrules

provides protection against the entry of dirt and some contaminants. Ferrules often are used with a short length of clear heat shrink material placed over the cable jacket and the base of the ferrule. This also helps to prevent the entry of moisture and other contaminants and provide stress relief.

Unshrouded ferrules

Shrouded ferrules Two crimps

Cable

Section A

Sect. B Cable

Figure 8-7

Ferrules are available in aluminum, copper, and tin plated copper and are normally specified by the preference of the end-user. A properly crimped ferrule reduces the entry of contaminants and provides a strong, durable, and low resistance connection between the cable and clamp. There are sufficient variations of clamp, ferrule and con ductor sizes and styles to meet every need for personal protective grounding. Many applications and the accompanying theory are presented in later sections.

8-6

ENCYCLOPEDIA OF GROUNDING