What sort of values are you getting? And for what sort of circuit lengths?

I'd start by pointing out that 1M is the minimum standard expected for the initial verification of new installations - BS 7671 doesn't explicitly say that the same should be expected for periodic inspections. Do the readings suggest that safety of the installation is impaired?

   - Andy.

Thanks for the reply. The report just shows <1M so that's all the information I have. The main culprits are the lighting circuits and the building is 70m x 40m so the runs are typically 30m(ish). The circuits are all protected so imho, the safety of the installation isn't impaired. As you say, the BS7671 doesn't explicitly reference the difference between new and existing wiring, but how do I present that to our landlord?

Mineral cable is famous for absorbing moisture if not kept warm and dry and this lowers the IR. However it does not usually matter unless very severe. A few hundred K ohms over tens of metres is fine. The same resistance over a few feet less so.

Be wary of anyone too young to have installed it - modern plastic cables are very different with gigohm resistances.

It can often be dried by gas torch, starting a metre or so back and moving the hot spot to a free end. It is important the end can vent, as pressure from trapped steam can split the tube or fire the fittings off... The past is a different place in terms of what is considered reasonable.

Thanks for the reply. 

These pyros have been quietly doing their stuff for 60 years so they're allowed to be a bit moist and as you say, a few K ohms over the entire run is OK. I just need to convince my landlord...

The report just shows <1M so that's all the information I have.

Sounds like you need a better EICR.

Chances are that it's still several kΩ over tens of metres, probably caused by a small bit of damp sneaking past the seals over the years. If so the resulting leakage current would be in the milliamps range - a tiny waste of power, and no shock hazard provided everything's solidly earthed. You probably need some solid numbers to make a decent argument though - above a certain level (10mA say) there might be an argument that the circuits should have high integrity c.p.c.s to assure safety (which is likely to be there for free in a MICC installation, but the details might need double checking for any possible weak points). On the other hand very low readings might indicate a hidden fault (N-PE faults on circuits without RCDs can go unnoticed for years) or some other insulation issue (e.g. insulation breakdown in the light fittings themselves) which would need attention.

One day we'll get people who don't have a financial interest in the remdials to do EICRs.

    - Andy.

I'm not desparately familiar with MICC and appropriate values and wouldn't venture an opinion in your particular case, but if suspecting low values are due to moisture I might think about finding a test device that will allow you to measure Dielectric Absorption Ratio and Polarisation Index... these are fancy names for a prolonged insulation resistance test and watching how the resistance varies over a prolonged test (DAR = 60s, PI = 10min) and using the knowledge that if the issue is moisture in the cables rather than a resistive fault the IR should drop over time. Conversely if it is stable at an unacceptably low IR there's a real, resistive fault somewhere.

It sounds fancy but it can be a case of having a stopwatch, a calculator and a multifunction tester that reads out continuously rather than stopping the test as soon as it's happy with the answer.

(You can do cleverer things, particularly if you can plot the IR during the test and re-test every year, so you can compare plots over time, but that's the general idea)

PS - Sorry just re-read your OP in which you say you're not an electrician, but this might help someone

its slightly amusing that the younger types who have not had much to do with it consider it remarkable

https://www.electriciansforums.net/threads/super-magic-pyro.111368/

mind you if you want icky insulation results try cotton covered wire or dry  paper insulated (oiled paper is far better and is still quite common in UK street mains.)

(page 8 of this PDF of a paper from 1913 makes a fun read for the interested in the moisture and insulation problems faced by a generation now passed.)

Mike.

I would be interested to hear how much experience with micc cable the tester as under the belt. 

The idea the entire site needs rewiring sounds somewhat extreme as I have seen 'low' readings dealt with with a blowtorch and re-potting the end(s).

Is that reading for the full board then transposed against each circuit on the schedule of test results? Do you have any previous EICR against the installation?

I would think the landlord is responsible for the fixed wiring or am i missing something here ?

Gary

Sounds more like an industrial let being vacated that has to be returned to the landlord 'as supplied' - which normally means for tenants taking out any thing you have added, maybe repainting and in some cases even dilapidation work - that is putting back in  older dated and even sometimes broken equipment and fittings  that were removed when you moved in - to return it exactly to the original condition.

The laws of a badly worded contract can act in strange ways - usually common sense kicks in but not always.

Mike.