Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 16 replies
  • Subscribers 32 subscribers
  • Views 2122 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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!
Parents
  • 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.


Reply
  • 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.


Children
No Data

Community Rules & Guidelines