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Because s o p h i s t i c a t e d computer modeling i s n o t e a s i l y accessible, a one-dimensional thermal analogue i s o f t e n used f o r r o u t i n e c a l c u l a t i o n s o f v i r t u a l j u n c t i o n temperature o c c u r i n g i n t h y r i s t o r s when subjected t o f a u l t c u r r e n t s . Such models simul a t e t h e heat p a t h from w i t h i n t h e s i l i c o n wafer through t h e component p a r t s of t h e mechanical package o u t t o t h e e x t e r n a l case. The usual c o n f i r m i n g l a b o r a t o r y t e s t f c r t h e model takes advantage o f t h e JEDEC & I E C t e s t f o r thermal r e s i s t a n c e s whereby a c a l i b r a t e d l o w l e v e l s i g n a l c u r r e n t f l o w s between t h e l o a d c u r r e n t cycles. The v o l t a g e drop o f t h i s s i g n a l c u r r e n t i s c a l i b r a t e d versus temperature t h e r e b y a c ti n g as a thermometer. The l i m i t a t i o n s o f t h i s procedure [l] was found w h i l e e s t a b l i s h i n g and improving f a u l t suppression r a t i n g s o f h i g h v o l t a g e ( 5 kV c l a s s ) t h y r i s t o r s . The thermal model was upgraded t o i n c l u d e a number o f heat sources w i t h i n t h e s i l i c o n and some degree o f two dimensional heat spreading w h i l e r e t a i n i n g t h e r e l a t i v e s i m p l i c i t y o f a one-dimensional model. T h i s upgraded model d i s t i n g u i s h e s t h e thermal impedance o f t h e cathode j u n c t i o n f r o m t h e thermal impedance o f t h e anode j u n c t i o n . The s u p p o r t i n g bench t e s t was s e t up t o use t h e c a l i b r a t e d o f f -s t a t e VBO c h a r a c t e r i s t i c r a t h e r than t h e c a l i b r a t e d o n -s t a t e drop o f t h e s i g n a l c u r r e n t . I t was found t o be a more d i s c r i m i n a t i n g thermometer, responding t o any e x i s t i n g hot spot temperature caused by surge c u r r e n t l e v e l s . Using c e r t i f i e d t h y r i s t o r s , improved f a u l t supp r e s s i o n r a t i n g s were e s t a b l i s h e d and subsequently proven under r e a l power system c o n d i t i o n s a t t h e GE-Skeats High Power Laboratory.