What do you consider a sample to mean during an EICR

hmm!  I think maybe some folk are looking at the wrong end of the stick I was trying to point with. Generally, and especially domestically, we do not know anything  about who has done what so the independence or correlation between looking in one place and knowing the quality of workmanship in another where we are not looking could be almost anywhere between zero and 100%, either way..

At which case the whole sample 20% of the circuits if you have more than 100 to look at has no more mathematical basis than a game of pin a tail on the donkey. By all means do it, but be clear it is not backed up by sampling theorems, the central limit theorem or anything else out of the A level applied maths and statistics course - like assuming that only low impedance faults occur, it is simply  a fudge to make the cost benefit balance look better, but one that is better than nothing and gives reasonable coverage in practice. In general I suggest that poor work is clustered, not randomly distributed, and tends to form a trail behind poor workers.

When did you last roll the dice when wiring a fitting to decide if this was the one where you were not going to bother to tighten the earth screw, or to wire it reverse polarity ? I'd be amazed if anyone did... But only by that sort of method is bad work truly randomly distributed. And only then do the stats work properly.

Mike

mapj1 said:

At which case the whole sample 20% of the circuits if you have more than 100 to look at has no more mathematical basis than a game of pin a tail on the donkey.

I never said "circuits", I said "population" - and provided an example of how coverage could be met. In fact, I'm agreeing with the statement that 20 % of circuits has no real meaning.

The approach has to be a coverage that provides confidence overall, using a combination of inspection and testing.

I think we are misunderstanding what sampling (in large installations) really means. My take is that say 20% should be examined (tested and inspected fully), and depending on the findings, and if they are problem free we can reasonably assume that the other 80% are generally satisfactory. However, and this is the important point, there can never be any guarantee that this is actually true. The question really is "does this lack of confidence matter?", and this is entirely down to the client. In some high risk situations it may matter seriously, in the average relitively low risk situation (using the accident statistics) it may well be decided that this an acceptable level of risk.

The problem we have in present times is that there is probably NO acceptable level of risk, and this is a cultural change over the last 50 years or so. This attitude has a serious downside, that risk and levels of risk are no longer understood due to lack of ability to assess properly. Risk is always present, it is part of life, but many now think that everything is risk free or must be made so, in my view to the detriment of life itself.

I notice that even very low risk jobs on domestic houses are being said to require full scaffolding, whereas a few years back a ladder was fine. The effect is that costs have gone through the roof, there are not enough scaffolders, and scaffolding is a relitively high risk occupation so all that has happened is that accidents are transferred to scaffolders! The industry now has quite a few cowboys, because of the above causes. Mad!

Sampling is guessing and assuming. Engineering works properly on absolutes not roulette. Just inspecting or testing part of an installation only provides part of an answer.

Four of my car tyres are at the correct pressure. The spare wheel is difficult to access, but as the majority of tyres are o.k. I will assume that the spare is as well.................until on a dark wet stormy night I find that it is flat. That is sampling.

Z.

The first thing I need to mention is you do not sample circuits, you inspect and test all the circuits. You don't sample PA Test a number of appliances and then pronounce all the appliances to be satisfactory on that basis.

If you have a large building you may I&T say 20% of the boards each year over 5 years to spread the cost and disruption but all the boards get I&T in that cycle. All DBs and all circuits get inspected and tested.

You need to separate out the 2 processes, inspection and testing. BS 7671 Chapter 65 sets out the objective for a Periodic Inspection and Test. It majors on inspection, not testing.

You can sample inspect items on each circuit say 20% of accessories on a circuit by removing them and having a look behind and checking the tightness of terminations.

You can sample test say by testing all readily accessible sockets and 20% of light fittings on a circuit.

As I have said in my post above if you want my standard specification for a MINIMUM standard for inspecting and testing a dwelling I will send it to you free with no strings attached. 

Flippin Ek! Parallel evolution. Darwin and Watson and Crick.

Z.

652.2 comes to mind.

So, the "electrical installation" of 651.1 does not necessarily mean the WHOLE building. (See definitions).

Z.

John Peckham said:

If you have a large building you may I&T say 20% of the boards each year over 5 years to spread the cost and disruption but all the boards get I&T in that cycle. All DBs and all circuits get inspected and tested.

With all due respect, that is not sampling. It is testing all of 20% of an installation. It is valid only so long as a different 20% is tested each year.

So in let's say a 100 room hotel, you could test the accessories in rooms 1, 6, 11, etc. in year 1; 2, 7, 12 in year 2 and so on. That would tell you that rooms 1, 6, 11, etc. we're sound in year 1, but very little about rooms 2, 3, 4, 5, 7, etc. However, if you get out your random number tables and test 20% of rooms, you can make deductions about all of them.

Examined from the other end of the telescope, you ought to have some data about previous tests. If you want to be 95% sure that the whole installation is sound, the maths will tell you how many rooms you have to sample.

This is not an easy subject, which is why research institutions employ statisticians.

Zoom

What has been inspected and tested would have been agreed with the client before starting work on site. What has been inspected and tested should be set out in the "Extent" box in some detail to show what has been inspected and tested and what has not. Also  included in the "Limitations" will be the agreed exceptions to what has been inspected and tested.

Chris

If testing a 100 bed hotel you would agree with the client  before work starts what the extent and limitations are to the work. If I&Ting the whole hotel in one hit you may want to use the same method of testing the rooms as set out in my spec. for dwellings. They use a small consumer unit for each room or pairs of rooms with a 3 phase DB at the ends of the corridors either 1 DB or 2 DBs for a long corridor. The DBs being supplied from  a panel board. If not doing the whole hotel in one year maybe over  5 years with 20% a year agreed with the client. If there is a swimming pool present then the swimming pool every year. Industrial kitchens maybe every 3 years.

651.1 mentions previous documentation and the absence of documentation. IET GN3 provides extensive guidance on periodic inspection and testing.A standard textbook for inspectors in my view.

I worked in a large production factory once. It had several teams of fully qualified 24/7 maintenance electricians whose primary job was to keep the production machines running. If a fault developed on an individual machine, or a row of machines or a row of overhead lights, a team member would immediately be dispatched to investigate and hopefully repair the problem.

I believe that the whole installation was constantly inspected and tested most of the day and night on an ongoing basis. Faults just were not allowed to exist for long. Shut downs were not possible.

Definition. Sampling is a process used in statistical analysis in which a predetermined number of observations are taken from a larger population. The methodology used to sample from a larger population depends on the type of analysis being performed, but it may include simple random sampling or systematic sampling.

Edit. Add, I only have Guidance Note 3 for the 17th edition. SHOCK HORROR. But I can't see that it differs much from the newest version.

Table 3.3 suggests minimum sample sizes for inspection:

Main switch gear external inspection.

Main switchgear internal sections and cable terminations.

Main switchgear internal inspection of circuit breakers and control sections.

Final circuit distribution boards.

Earthing and protective bonding conductors.

All 100 per cent to be sampled.

Then.......

Final circuit accessories between 10 to 100 per cent.

Z.