An essentially is to be applied as noted in (a)--(c).
(a) between each different type of primary circuit (for example,the input and output circuits of a control both of which are intended to be conductively connected to the branch circuit in the end application).
(b) Between terminals of opposite polarity on a capacitor that is connected across the line
(c) Across any other insulation or spacing involving a primary circuit that is required to reduce the risk of fire,electric shock,or injury to persons(for example,at the input and output terminals of an isolation device)