Further investigation IR below 2 Megohms or 20 Megohms??

Any single value is a bit of a fudge as of course it depends on how long and rambling the circuit is and what the wiring method is - pyro (MICC) cable is always a bit leaky on insulation test, as is paper insulation, but it is not faulty, but clean PVC should be giga-ohms per metre.
I'd suggest the use of a bit of experimental  knowledge rather than blind adherence to a single figure. As far as I know it has never been 20Meg, but I do know that 1M, 2 M and 0.5M have been used by various authorities at different times. It may be that someone will chip in  with a situation that uses  20M as well ;-)
Mike.

RobW said:

he current online IET OSG says that further investigation is required below 20Mohms rather than the previous versions advising further investigation below 2 Megohms?

Yes, it now aligns with Guidance Note 3 - see page 69 of the 2022 version.

HOWEVER

OSG talks about 'latent defect', i.e.  something like a snagged cable - relating to new install.

And GN 3 clearly relates this figure to NEW installations.

Both, of course, without connected equipment which can also affect the reading.

mapj1 said:

Any single value is a bit of a fudge as of course it depends on how long and rambling the circuit is and what the wiring method is

Agreed ... and connected appliances may also affect the reading.

Ah thanks, perhaps I should update my notes... ;-)  Mind you I bet I can damage a cable in a way that still passes at 20Meg as well as at 2. If you can, nothing beats a visual check before boxing or plastering things in.

Mike.

mapj1 said:

Mind you I bet I can damage a cable in a way that still passes at 20Meg as well as at 2.

Definitely ... even perhaps using a techs screw to penetrate just the line conductor in a flat T&E cable, making the steel frame of a steel-framed plasterboard wall live in the process, which then doesn't come to light until someone kneels in a puddle of water that's seeped under the wall, killing a plumber who was fixing the leak.

A non compliance with 522.6.203 and 522.6.204 then.

Z.

Yes, but still happened in real life in the UK, possibly preceding current 'rules', and not identified by testing (apparently)

I was asked to investigate a low value of IR in a large, rambling GP complex. An EICR conducted by others had an unsatisfactory designation because the value recorded for a number of circuits was 0.16Mohms for which the contractor awarded a FI code. The surgery manager needed the report returned to satisfactory for statutory purposes. 
Three-phase installation with several 16-way TPN boards all with 3pole isolators with circuits neatly installed using single core cables in metal trunking/conduit. Hundreds of items of equipment connected. My view was that it was madness to conduct IR tests in the first place and expect a meaningful outcome. 
My rate for Saturday isn’t eye watering but it ain’t cheap either. I agreed same with manager and insisted that an electrician was on hand to assist. Real pain to disconnect the equipment and had to remove circuit neutrals but eventually tracked it down to these fellas. Several on individual circuits pulled the IR down. At 250v the individual reading was around 0.5Mohms. 
So if the guys doing the EICRs continue to do bonker things, I reckon there is a few quid to be made sorting the nonsense that is generated. I wonder how much these poorly conducted EICRs are costing clients across the U.K.! 

In my view, a NEW installation should be expected to reach 20 Megohms, unless there is some exceptional reason to expect and accept a lower figure.

For an EXISTING installation, a much lower figure could be reasonably accepted.

Sounds similar to the Emma Shaw case. (Tried to post this under GKs post 2 up but system not having it).

Relatively similar, but different cases. There have also been incidents of cables run underneath roofing supports, where roofing fixings (perhaps for metal roofing materials) have penetrated If I remember correctly, dry/stud wall and roofing incidents have led to H&S prosecutions ... if I get a moment I'll have a look as they ought to be searchable on public record on the web.

All perhaps providing evidence for the fact that the "concealed < 50 mm from surface" rules are necessary.