Steve...
GS Ohm's approach certainly has merit! But, it only works if one is able to plot leakage-current, vs time! The following discussion may help:
An IR test involving the application of a DC voltage and measuring leakage-current over time can be used to assess IR quality. For example, had you tested two cables of equal length but different insulation thickness, the thicker one would require a longer test-time. Following is a brief explanation.
When testing insulation resistance, R=E/I, where E is the applied DC voltage, and I the observed leakage-current. Omitting temperature and humidity factors, then, what other time-related factors are there? The time to reach a certain current magnitude through leakage-paths over the insulation surface, and through the insulation's volume. Three current components are present; one transient and two steady-state:
1. Capacitance Charging Current: Contrary to popular belief, while it starts out high, it decays relatively quickly; in the order of tenths of seconds.
2. Absorption Current: It too starts high, but then decays over a period involving minutes.
3. Leakage Current: It is essentially a small but constant current both over and through.
Steve, I don't know if this will solve your specific problem... but it might answer a future "Why!"
