[0] Earth fault loop impedance [IT] Highest [TN-S] Low [TN-C] Low [TN-C-S] Low [TT] 
High
H i g h
Y e s
L o w
N o
S a f e
L e a s t
H i g h l o o p i m p e d a n c e ( s t e p v o l t a g e s )
S a f e a n d r e l i a b l e