Why does wire insulation have a working voltage and peak voltage?
High voltage insulators have an impulse rating that can be more than 10x the working voltage due to the rise time. This is normally for lightning protection with a specified rise time and tail time.  I'm not sure if that is what you meant by peak voltage.

Each commodity item may have different specs and ratios.  It takes longer to ionize air or surface contamination or charge up wire insulation dielectric with a series inductive wire.   

There can be spark tests at 60 Hz, 3kHz and Impulse test (peak) which increase in this order.

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