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According t o the experience o f some power u t i l i t i e s , the a p p l i c a t i o n o f i n d u s t r y recomnended high voltage d.c. f i e l d t e s t s on 5-35 kV extruded d i e l e c t r i c cables, containing water trees, sometimes causes f u r t h e r d e t e r i o r a t i o n o f the i n s u l a t i o n . Tests conducted on laboratory aged 15 kV ethylene propylene rubber (EP) and 15 and 28 kV c r o s s l i n k e d polyethylene (XLPE) i n s u l a t e d cables i n d i c a t e t h a t d.c. proof t e s t s i n accordance w i t h A E I C s p e c i f i c a t i o n s and IEEE t e s t guides w i t h o u t flashover do n o t appear t o cause f u r t h e r d e t e r i o r a t i o n . Depending on the degree o f cable aging and the l e v e l o f t e s t voltage, when flashovers take place, damage may be i n f l i c t e d t o XLPE cables. No damage was observed on aged EP cable, a t the same t e s t levels. Because o f t h e aforementioned power u t i l i t y experience, some users have requested an a l t e r n a t e f i e l d proof t e s t . Tests conducted on new XLPE and EP cables i n d i c a t e t h a t damage t o the i n s u l a t i o n s t r u c t u r e can be detected using VLF (0.1 Hz) voltage a t approximately onet h i r d the d.c. voltage l e v e l . F i e l d t e s t s conducted on severely t r e e d e t e r i o r a t e d 15 kV polyethylene (PE) cable using AEIC recommended d.c.

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