I suggest you are saying (and  I agree) a single figure is always a compromise, and rather depends what issues we are trying to detect.

A small flat wired in PVC  where we can be sure no wiring goes outdoors should pass a higher limit.

Large rambling installations in 'pyro' wire such as are often found in churches and other old public buildings, may well read low on the megohms test, and yet have no real fault - it is just a feature of mineral insulated cables. Indeed if enthusiastic one can test the circuit in sections and find nothing.

Arguably looming problems with failing equipment or the wrong kind of load, that may  soon give tripping issues with RCDs are better found with a clamp meter with the system energised.


What is of interest is the sort of fault that if it were all in one place would be hot enough to cause a fire - and that is probably a dissipation of a watt or two - so even the 100k figure (2.3mA on 230v,  about half a watt dissipation) is no more heat than a small torch bulb of the kind not used in modern torches, and worse could be tolerated.


Your approach may not be the official technique, I suspect it is just fine in reality, and a lot better than a row of 'lim' or ''n/a"

The old textbooks used to quote the example of uninsulated live parts only being some fraction of a hair's breadth apart - so the test voltage was chosen to be more demanding than normal service to reveal such faults before the 'bang test' did - the worry being that lower test voltages may not arc across small gaps and so give false pass results.


Given that 230V nominal can be anything up to 253V and being a.c. those are r.m.s. values, the peak voltage in normal service is going to be something like √2 higher - say about 360V - a 250V d.c. test alone perhaps isn't that reassuring.


Personally I do a 250V test first - just in case there's something still connected that shouldn't be (which usually isn't an issue with L+N - PE tests) - and if that shows reasonably clear, then try the proper 500V test.


  - Andy.

It rather depends what failure mode you are expecting, actually neither 230V nor 500V will 'jump' very far without assistance - much like a welding rod, contact has to be made either mechanically or by a signifiant over-voltage, and then an arc can be drawn out to a volume of hot gas limited only by the available power. Usually when this finally goes out the conductors have burnt back well beyond any re-striking distance. Equally for finding nails in the wire, trapped wires in backboxes or similar, a battery and bell set would probably do.

I agree a 500V test will find a few things a 250V test will not, and a 1000V test will find a few more.

In some ways the standard 500V test looks a bit feeble, given the overvoltage  categories defined in IEC 61010-1

1500V for Cat I, (operation with protective low voltage), battery-operated devices, car electrics)

2500V for Cat II  (household appliances, portable electrical appliances) and

4000V for Cat III (loads with direct fixed connection, distribution, fixed installation appliances in the distribution system)

Perhaps we should be testing at 2500V on new work and on retests when we are sure that all loads really are removed.

I agree with Mike that 500V is a bit of a compromise, but is certainly better than using 250V dc. Andy has already pointed out that 250V dc is less than the RMS value of the mains. Certainly at LV, you are more likely to get a reading from the carbon produced by a fault than an arc between conductors. Certain appliances are more likely to be damaged than others - satellite boxes are a favourite, as the power supplies are (or certainly were) rated only just outside the normal mains tolerance. In fact, one manufacturer supplied a “power supply repair kit” which contained the components only just in voltage tolerance, and therefore those that usually failed. 


Regards,


Alan.

Chris


You have fallen in to the usual trap of not understanding the major difference between Initial Verification and Periodic Inspection and Testing.


Initial Verification is the I&T of a new installation before being put in to service and periodic inspection and testing is for an installation already in service.


The tests for Initial Verification are done in sequence and if any test fails then the process stops and any defects are rectified before re-commencing testing and repeating any test that could have been influenced by the defect, see Regulation 643.1. IR testing being one of the dead tests that must be satisfactory before energising the installation for the first time for the live tests. Regulation 643.3.2 allows for 250V IR testing where equipment could be damaged or SPDs cannot be removed.


Periodic Inspection takes place with the installation having been in service and there are no prescribed tests in BS 7671 and any tests need not be done in any set sequence. Periodic Inspection and Testing (Chapter 65) majors on Inspection and testing is an ancillary activity as Regulation 651 .2 says "supplemented by appropriate tests and measurements from Chapter 64". 



I see >200M for large installation Live/Live on EICRs and ask for an explanation as how that test was carried out. knowing full well that is a blatant fabrication!

Chris, that is a fantastic answer! Exactly what I was looking for.


Thank you

I went south of the river last Thursday to do what should have been a straightforward job, until I saw the cable that supplied the central heating system that I was planning to extend.


You do have to appreciate just how bad the insulation is on some cables that are in use without tripping a RCD or MCB in a relatively new dual RCD consumer unit.

I actually disconnected the existing circuit completely and tapped into another one which meant taking the landing carpet and floor up, then relaying them, which was not part of the plan.


That photo was taken whilst I was deciding what to write on the certificate as comments on the existing installation!


 Andy B.

No rubber cable is visible within the consumer unit at that house, so from looking in the CU you could assume the whole installation is wired in PVC insulated cable. So a bit of cable like the one in the photo is one of the things you are looking for when doing an EICR, I did not mess about doing an insulation test on that cable, but I know from experience that they can pass an insulation test even in that state.


Inspection trumps testing.


Andy Betteridge