This is very informative, Thanks to all for taking the time to explain. I guess I am in the lap of the gods once the max Zs requirements are this low, without a much more detailed and intrusive inspection.  I may well need to get the design info, to try and prove there isn't an issue that needs addressing, I worry that our insurer may not agree if it is classified as a C2 and we do not rectify. Would I be correct to say Items like this, unless the Zs is significantly higher than would be expected, should be classed as FI ?


Thanks.

It is not really the measurment accuracy that is the problem, it is the measurment method Mike. I could make an MFT with a 24 bit A to D converter, which might have an inherent accuracy of about 22 bits. This is by no means the limit, digital meters with 10 or more decimal digits are made and used (around 36 binary bits) but they are frightningly expensive. However Andy is right in that the method of measuring the EFLI is fundamentally inaccurate, and that is why I carefully described the proper method. Adding a tiny load to an unstable supply and trying to measure millivolt or microvolt differences is a fools paradise, but the MFTs are believed to be accurate. I rather doubt (from experience) that an MFT ELFI of less than 0.1 Ohms is anything like 5% accurate, the metering method might be, the practical result is not. Calibration is not carried out with a mains derived supply, it is a very carefully controlled oscillator and an exact resistance. In other words variations in the mains supply are eliminated. That may give 5% plus a few counts, I am sure the mains will not. This is not a critism of MFTs, but there are significant difficulties with the results when we get onto more than 100A supplies, with much higher PSCCs, much lower Ze etc. At the final circuit DB you are on much firmer ground. There are a lot of other types of A to D converter inside chips, offering much faster conversion, greater accuracy etc than the old discrete types, just try a dual slope type with an audio signal! A to D of at least 6GHz is easily available today, and the number of bits probably unlimited given enough money and effort (and a completely noise free signal to measure, which of course doesn't exist). I am sure MFT design will improve with time, and the modern bells and whistles have not really made the measurments much better, just quicker and easier to use. It is still necessary to understand exactly what is really being measured, and why it might be useful. I would really like my MFT to take 50A for perhaps 10ms right at the waveform peak, but the resulting power loss can be a problem, and then compute the ELFI. I do have one meter which does something like this but it only allows about one measurement per minute to allow for cooling! It is usually better than the Megger in its estimates compared with calculation.

It is a problem that most  loop test instruments struggle to read low values of Zs really well, and the trend towards lower test currents in the interests of safety and not disturbing RCDs has made it worse (I have an old Edgecombe Peebles unit  from about 1970 that uses a 25 A test current, which I press into service if the newer one's reading is supect and you can sometimes hear the wiring twitch if it is delicate - the thinking back then was that any single strand hangers on deserved to fail during the test.)  Even so an instrumented test may well miss something that is electrically connected, but just hanging by one or two strands say.


A fair approach would be to set the fail limits a little slacker for an inspection than for the original installation - there is not really a credible degradation mechanism that will make the copper cores just a bit thinner in the middle of a cable run over time, so you are looking for things like earths coming detached.

I agree a 'FI' is the correct observation if the test is not a clear pass or fail within the limit of the instruments,  What form the further investigation takes is another matter.

In response to Handyosprey, you are really looking for deterioration in the kind of install you are describing. So it is inspection not the measurment which is most important. Loose, corroded or damaged connections are your key, not some magic number which you may not be able to measure. As I said above, it may have been designed incorrectly and the disconnection is not exactly 5 seconds, it probably doesn't matter. What does is the burnt loose connection which you will not measure at all! FI or a C2 is not indicated because you have not found anything wrong, you have tried to measure in place of inspect. You have measured close to the correct number, that is the best you can do. You don't really know how accurate the number is, and finding out with another half a days work and 10 grands worth of kit, is not the way to go. If you tried that on a major installation you would never finish, and your EICR full of FIs is worthless. The original designer is still responsible for the design (errors or not), it is a bit like an MOT with brakes which do not exactly reproduce a new vehicle, a 5% error is not important, worn out pads or disks is. The EICR is to find the worn out or damaged items, not a certificate to replace the installation with new (although many treat it this way).

Hi Dave,

I agree we could make a more accurate meter, (oddly there is a ten digit HP one flickering away on the bench beside me, but given the tests I am doing I only write down the first 3 or 4, as the others just fruit machine)  but as we are agreed, in this case it ends up being a futile test rather like  measuring walnuts with a micrometer - a different very precise 'right' answer every time.  Meanwhile, in the interests of a trade off of battery current and cost of instrument, the ones in the MFTs are dual slope or SAR designs (the SAR ones being capacitor balance types that are a part of the microcontroller chip itself, which is another design compromise ), and there is no great need of temperature compensation, or 4 wire probing, or all the things you would need to do to get the next place figures to mean something.