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- 39 - SURFACE DISCHARGE INVF,STIGATIONS ON MINDUS SIGN CABLES L. F. Roehinann Anaconda Wire & Cable Company Hastings-on-Hudson, N. Y. Luminous sign caWes are high-voltage cables without being generally recognized as such. They have fit conductor of stranded copper, an insulation of natural or synthetic rubber (thermosetting), or of an extruded thermoplastic compound, and are finished i n a va r i e t y of finishes: w i t h saturated or lacquered braids ( t e x t i l e , glass^, or with a vulcanized or extruded synthetic jacket. For reasons of economy luminous sign cables have no metal shield. Routine development work i n t e n s i f i e d by occasional f a i l u r e reports from the f i e l d , led to a comprehensive investigation. F i r s t , the crest and root-mean- square voltages to which sign cables are subjected, were analyzed, being part of the c i r c u i t : sign transformer - cables - sign. I t was found that during sign operation, o s c i l l a t o r y di scbarges occur every h a l f wave of 60 cycles which are associated w i t h very high crest voltages, A sign transformer with a nominal r a t i n g of 15 Kv. (7,5 Kv. to ground) produces crest voltages as high as 11 and 12 Kv. to ground, at rated load. Therefore, the cables have to be designed f o r such voltages. The r e s u l t i n g stresses are essentially r a d i a l where the cables are i n - s t a l l e d i n grounded metal cond'-dts but essentially longitudinal at the end of the conduit run where the cable cormects to the high voltage electrode. For the com- monly used 15 Kv. i n s t a l l a t i o n s , the National E l e c t r i c a l Code specifies a clearance length of four inches. I f such clearance length becomes contaminated, as f o r instance by s a l t deposits i n outdoor i n s t a l l a t i o n s near the sea coast, end f a i l u r e s may occur. Simulated laboratory tests were made with cycles of alternate snrays of sea T"ater and fresh water, with "wind-drying" and "calm-drying" periods i n between. In these tests, a l l presently used comn;ercial cables f a i l e d between the f i r s t and f i f t h snray cycle, i n patterns which resembled very closely those obtained i n the f i e l d . Thereupon, e f f o r t s were directed toward development of an improved luminous sign cable having adequate surface discharge strength ( t e n t a t i v e l y set at ten spray cycles). Polyethylene i s such a material. Breakdown as well as surface discharge strength are exceptionally high'. Unfortunately, i t supports combustion, and i s therefore unable to meet the luminous sign cable specification f o r flame resistance. Work proceeded along the following l i n e s : 1. t o develop a jacketing compound which woul.d be flame resistant and would have adequate strength to surface discharges.
