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Rolling partial I&T or sampling?

Chris Pearson
This subject came up in the recent thread about the church with a poor EICR. It was suggested that inspecting and testing ¼ of the installation every 4th year might be preferable to testing 100% every 4 or 5 years. I suggest that this is a misunderstanding of the principal of sampling.


The principle of sampling is that if inspection or testing of 20% of an installation reveals no defects, it may be inferred that the other 80% is in a similar condition. A statistician could calculate the degree of confidence of the assumption. The sample must be identified in advance and may be selected randomly, or systematically - say every 5th circuit, or every 5th socket. What is not permissible is to sample a sample, so it would be either 100% of sockets on every 5th circuit, or 20% of sockets on all circuits. If the sample has not been selected randomly, the same one should not be used repeatedly - so circuits 1, 6, 11, etc. on one occasion and 2, 7, 12, etc. on the next. Self evidently, the method of sample selection must be recorded on the EICR so that the next inspector may select a different sample.


If the sampled circuits (or sockets, or what have you) give cause for concern, the sample size may be increased: the purpose of this is to increase the level of confidence, not to find more defects.


If the sampled inspection and testing is satisfactory, the EICR is signed off and the normal inspection interval is recommended. It should not be reduced on account of the sampling. To test 20% every year is not sampling because in fact the whole installation is tested every 5 years and some of it, e.g. the origin and main panel, ends up being tested 5 times in 5 years.


Now I stand by to be shot down in flames!
  • Former Community Member
    0 Former Community Member
    Hi ChrisP thanks for your interest. What type of contract would you prefer, a couple of scheduled regular days per year checking out a planned section of an installation along with a general inspection for wear and tear and some additional bits such as pressing 100% of all RCDs buttons ,looking at the bonding etc, or having the chance to quote once every 5 years to fluff out an EICR with a raft of at best pointless C3 “observations”. 


  • Former Community Member
    0 Former Community Member
    https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=61562
  • 0
    Ryan Dempsey has created the The Compliance Workbook  


    If you have a major stock of housing or a large campus to manage it's no good going at it without a plan. 


    Andy Betteridge
  • 0
    Here’s a quote for you to consider:


    “Our software performs all of the difficult and time consuming compliance checks. A full EICR which used to take 30 minutes, now takes seconds.”


    Andt B.


  • 0
    I suggest that this is a misunderstanding of the principal of sampling.

    I would agree it's not sampling - but I think what was being suggested wasn't sampling, merely dividing one large job into several smaller ones.


       - Andy.
  • 0
    Yes this is not an issue of sampling but one of "Extent".


    Instead of carrying out a full inspection and test of a whole installation in year one it may be be better to carry out a full I&T in sections each year for larger installations. 


    So if the period of inspection is 5 years for a whole installation then this could be spread over 5 years doing a 1/5 each year. So for a church with an MCCB and 4 distribution boards the Words written in the " Extent" for each year might be:-


    Year 1. The incoming supply, earthing and bonding arrangements, the MCCB panel and the distribution circuits from the main panel only.


    Year. 2 Distribution board No. 1 and the final circuits supplied from that board only.


    Years 3-5 same as Year 2 for DBS 2,3 and 4.


    Note the use of the word "Only" to ensure what has been done and what has not been done.


    You need to agree the extent and Limitations with the client before work commences. You can adapt my wording from my standard specification for dwellings for this. It describes what and how the inspections and tests will be done at what sampling rate. It describes the differences between inspecting and testing.


    If you want a copy email me on

      info ( the sign for at)astutetechnicalservices.co.uk.
  • 0
    I`m thinking if you did do this 25% x 4 times or say 20% x 5 times then you`d have to proceed your one year interals immediately after the EIC or full EICR not say 5 years afterwards then you would complete a "full" 100% sample test  completed  by the 5 year anniversary.

    If you commenced on year 4 or 5 you would be extending the first term to 9 or 10 years correspondingly.
  • 0
    weirdbeard:

    Hi ChrisP thanks for your interest. What type of contract would you prefer, a couple of scheduled regular days per year checking out a planned section of an installation along with a general inspection for wear and tear and some additional bits such as pressing 100% of all RCDs buttons ,looking at the bonding etc, or having the chance to quote once every 5 years to fluff out an EICR with a raft of at best pointless C3 “observations”. 


    I can certainly see the advantage of a rolling partial I&T in terms of cash-flow for both client and inspector and it would be easier to keep up momentum.


    I wonder how many inspectors send out reminders, possibly with a quote?


    My apologies if my OP was a little unclear - my thoughts were not based upon just one thread, but it did seem to me that the question of sampling was getting a bit lost in the conversation.


  • 0
    Sparkingchip:

    Here’s a quote for you to consider:


    “Our software performs all of the difficult and time consuming compliance checks. A full EICR which used to take 30 minutes, now takes seconds.”


    It looks like a handy package, but it seems to be no more than an Excel spreadsheet.


  • 0
    John Peckham:

    So if the period of inspection is 5 years for a whole installation then this could be spread over 5 years doing a 1/5 each year. So for a church with an MCCB and 4 distribution boards the Words written in the " Extent" for each year might be:-


    John, thank you. Where, if anywhere, would you describe a sample on an EICR? And let's say that you have sampled every fourth circuit, what do you put in the schedules? Do you include just the circuits that you have inspected and tested? (A lot of N/As seems inappropriate to me.)


    ETA: with the example that you give, what would you put in Sections I and J in years 2 - 5 for things like Ze. Would it be what you measured in year 1, "N/A", or something else?


    I might add that it seems to me that at the domestic end of the scale, sampling needs to be treated with great care. 100% of circuits, but unscrew say every fourth or fifth socket and ceiling rose and similarly for Zs.


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