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)
a) For example, primary current (input L) and isolated secondary circuit;
b) For example, line-voltage current carrying parts(input L) and grounding G;
c) For example,at the input and output terminals of an isolation device;