Calculating combined insulation resistance tests - C&G 2391

I recorded this a while ago so I apologise for the audio.

One of the common calculations and formula used in the City and Guilds 2391 course involves combining multiple fractions to determine an overall value of insulation resistance.

Whilst many think that this is one of those daft formulas you need to know just to pas in the course, I wanted to explain and illustrate exactly how the wording in the Wiring Regulations suggests that we should actually be carrying out this calculation on regular inspections. ... https://www.youtube.com/watch?v=MLTM-OJE0Lo