Just in case it adds anything, I've been looking back over the old regs. As far as I can tell the requirement for insulation testing started with the 3rd Ed in 1897 - then the limit was 10MΩ divided by the maximum demand of the installation in amps.
Andy, thank you for the interesting info.
They must have been doing well to measure IR in 1897 - the first Megger came a few years later.
I suspect that 1 MΩ was picked because it wasn't practically possible to test much higher and that value gave an adequate margin for error. After 100-odd years, why change?
Just in case it adds anything, I've been looking back over the old regs. As far as I can tell the requirement for insulation testing started with the 3rd Ed in 1897 - then the limit was 10MΩ divided by the maximum demand of the installation in amps.
Andy, thank you for the interesting info.
They must have been doing well to measure IR in 1897 - the first Megger came a few years later.
I suspect that 1 MΩ was picked because it wasn't practically possible to test much higher and that value gave an adequate margin for error. After 100-odd years, why change?
