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Range and noted no unusual power supply requirements. Once precise-power c i r c u i t s were e s t a b l i s h e d i t became imperative t h a t s t r a y electromagnetic f i e l d s not generate voltage spikes i n these c i r c u i t s . C a r e f u l and thorough t e s t i n g i n high i n t e n s i t y f i e l d s i n d i c a t e d that e l e c t r i c c i r c u i t s completely enclosed i n properly grounded metal conduits keep voltage s p i k e s w e l l below the G u i d e l i n e s v a l u e s . The e x i s t i n g type of l i g h t n i n g a r r e s t e r s t h a t h eld the spikes to approximately 250 percent of rated voltage were found to be the best a v a i l a b l e . The best type of i n t e r n a l - t r a n s m i s s i o n c i r c u i t s (power- bus c i r c u i t s ) f o r s a f e t y , f o r electromagnetic s h i e l d i n g , and f o r r e l i a b i l i t y were obtained by: 1. Using double busses f o r power d i s t r i b u t i o n . 2. Using separate c i r c u i t breakers f o r each c i r c u i t . 3. E n c l o s i n g a l l c i r c u i t s i n properly grounded m e t a l l i c conduits to prevent mechanical abrasion and prevent e x t e r n a l electromagnetic f i e l d s from inducing voltage s p i k e s . The e f f e c t of f a u l t s was determined and i t was found that a two-phase-to-ground f a u l t caused the g r e a t e s t v o l t a g e - peak surge. T h i s was approximately 300 percent of the r a t e d transmission voltage. ESTABLISHMENT OF SHIELDING REQUIREMENTS Large models of power p l a n t s t r u c t u r e s were exposed to -20-
strong electromagnetic f i e l d s to d e t e m i n e the most e f f e c t i v e method of s h i e l d i n g . The r e s u l t s were: 1. The hardened underground r e i n f o r c e d concrete s t r u c t u r e , with the r e i n f o r c i n g s t e e l , major m e t a l l i c components, and a l l conductors, conduits, p i p e s , e t c . grounded, provided s u f f i c i e n t p r o t e c t i o n f o r a l l but the most s e n s i t i v e instruments. To provide an environ- ment e n t i r e l y f r e e from electromagnetic d i s - turbance, the instrument room should be completely enclosed by s h i e l d i n g m a t e r i a l . 2. A grounding g r i d under the e n t i r e b u i l d i n g , l e a d i n g to one c e n t r a l ground, was found to be e s s e n t i a l to e f f e c t i v e l y ground a l l con- ductors , m e t a l l i c components, and equipment room s h i e l d i n g . 3. A d d i t i o n a l s h i e l d i n g , above that required f o r the present t h r e a t l e v e l , could be provided by welding the r e i n f o r c i n g bars i n t o a low r e s i s t a n c e c i r c u i t attached to the ground g r i d . 24 The o r i g i n a l c r i t e r i a were developed by General E l e c t r i c , 25 and the design i n s t r u c t i o n s were i s s u e d by Black & Veatch. -21-
