£65 inc VAT, better than the £76.50 that WHSmiths want for it. Plus the 18th Edition does not come into law until the beginning of January 2019.
My new Megger MFT 1730
£65 inc VAT, better than the £76.50 that WHSmiths want for it. Plus the 18th Edition does not come into law until the beginning of January 2019.
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BS7671 never comes into law in England.
Ah. Right, that might make sense for the OSG and worst case scenario.
I don't think it good enough for BS7671 with no explanation as to original temperatures.
As we're just discussing - it doesn't.
John has just discovered that it probably relates to measurements taken at 10°C - so that would be slightly different at higher temperatures.
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Indeed.
Well, as I said, the 0.8 is a 'deemed to satisfy' figure, and such figures generally are at least as bad as 'worse case' (they perhaps assume that electricians don't/won't work in environments colder than 10°C!) - and, as you say, indicate that one can calculate the max Zs 'properly'/precisely if one wants to. That's the usual way for 'deemed to satisfy' provisions throughout the Building Regs (and other legislation) - just there for those who are too lazy (or unable) to work things out properly for themselves (and thereby almost always end up with less onerous requirements) - Table 54.7 in BS7671 is a good example.
However, I certainly agree that they should say what 'measurement temperature' the 0.8 figure relates to.
Kind Regards, John
Update: I have just rewired the 5 FCU's in the kitchen (as originally mentioned they are the first items on the circuit before the main sockets), and I found out to my horror the electrician who originally installed them had crammed 3 x 4mm² wires in the back of a single FCU for the line and neutral conductors what all popped out when I removed it! (Rather than looping the 4mm² T&E sequentially between each FCU, he T'd one of them.)
Since correcting that with two connector blocks and just tightening the wires in general what were very lose, the Zs on the first socket after the FCU's has improved from 1.23Ω to 0.98Ω. Still not great, and the next direct socket what is probably only 6 meters away in terms of cable length is giving a Zs of around 1.08Ω, however that is expected from the m ohms/m for 4mm² T&E. I only tightened the first socket after the FCU's, so I might try rewiring it tomorrow.
Since correcting that with two connector blocks and just tightening the wires in general what were very lose, the Zs on the first socket after the FCU's has improved from 1.23Ω to 0.98Ω. Still not great, and the next direct socket what is probably only 6 meters away in terms of cable length is giving a Zs of around 1.08Ω, however that is expected from the m ohms/m for 4mm² T&E. I only tightened the first socket after the FCU's, so I might try rewiring it tomorrow.
I've just been looking and, for some reason, that explicit statement (and a fair bit more) has disappeared in the 18th ed DPC ....
... for a start, (some of) what is currently Appendix 14 has been shifted into an early part of Appendix 3 in the DPC - but all that now appears is the explanation of how to do the temperature correction calculation, still with the 0.8 factor, and still without any explanation of how that figure came about or what measurement temperature it relates to. All of what were the following notes, starting with the comment you mention above, have seemingly disappeared. All that is left is one sentence saying that 'the above' is only one possible way of temperature correction and that "other methods are not precluded".
Kind Regards, John
Mmm. Strange.
They may as well do what you suggested and print the max Zs figures in Chapter 41 for 10°C (perhaps not unreasonable for Britain but not quite equivalent to the 0.8 value) and just state that adjustment for other temperatures may/should be calculated.
"other methods are not precluded"
There aren't really "other methods" are there? Just other temperatures for the measurements.
I can't think of anything else at the moment.
They may as well do what you suggested and print the max Zs figures in Chapter 41 for 10°C (perhaps not unreasonable for Britain but not quite equivalent to the 0.8 value) and just state that adjustment for other temperatures may/should be calculated.
"other methods are not precluded"
There aren't really "other methods" are there? Just other temperatures for the measurements.
I can't think of anything else at the moment.
Indeed. As I said before, that would make more sense than majoring on tabulations of 'max Zs' when measured at a temperature at which Zs is virtually never going to be measured. I suppose that, in Britain, something like 15°C or 20°C would be more realistic as the most common indoor temps. Indeed, that might not be unreasonable for most 'developed' countries, since higher indoor temps would probably be "AC'd down" to not much above 20°C.
Well, it would seem that the "more precise method" described in the current Appendix 14 (but seemingly not to be described in 18th) is 'more precise' (and almost certain to give higher 'max permissible measured Zs' figures) for two potential reasons. Firstly, they only apply the temperature correction to the the loop impedance within the installation - i.e. they do not correct the Ze component of total Zs. That probably makes sense, in as much as the assumption is presumably that the temperature of conductors in the DNOs cables is probably fairly constant.
Secondly, I think that the last bit of step (iv) is probably saying that one does not necessarily have to calculate on the basis of the maximum permitted conductor temp (e.g. 70°C) but, rather, can use the temperature of the conductors expected if the circuit's full 'design current' is flowing (which might include allowing one to use the 'after-diversity current equivalent' of a cooker circuit). I'm not quite sure how one would determine/estimate that temperature but, if one could, it again makes sense.
Certainly the first and (if I am interpreting correctly) sometimes the second of those considerations would result in an appreciable increase in the 'max permissible measured Zs', as compared with calculations which temperature-correct Ze and assume a potential conductor temp of 70°C.
Kind Regards, John
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I wasn't looking at Eddie's posts when I mentioned the spec. It has a load of functionality which a diyer will never use (or likely understand). I would say that the make/model of MFT is nowhere near as important as the requirement to know how to use it.
That may well be true, but it has nothing to do with Eddie's participation in the forum, so you maybe muddied the waters a little by mentioning it in reply to my first response to your criticism.
I'm sure no-one would disagree with that statement of the obvious. The same could probably be said of any 'tool' used by anyone (DIYer or otherwise) in any field.
Kind Regards, John
How much experience do you have of testing? It is so much more imortant that you understand what you are doing (and what the results mean) than the make/model of MFT. I'd suggest you read up on I&T (the OSG and GN3) would be a good starter. The NICEIC have a guide and Chris Kitcher has a decent book out, which has links to Youtube videos demonstrating the practicals. If you are on your own and intend to carry out electrical work, I'd suggest you look at the C&G courses available along. The I&T course has a high failure rate because I&T of an installation you don't know is, as you are quickly finding out, a complicated process. I did notice that you laid out the ontents of the box, minus the user manual. It is the most important item in the box and I'd recommend you read it.
The Part P Doctor book on testing is good.
Says the one who can not spell "important"
. But on a serious note, while I do understand what the measurements should be for the particular test I am carrying out and how the readings may be affected under certain circumstances, such as unwanted parallel paths during loop impedance tests for example; It is fair to say that I don't know everything, as it took me quite a while to learn and find out that the MK RCD's in my CU were affecting the accuracy of my No-Trip loop impedance measurements.I went for the model that I did because I wanted the data download and recharging facility's, not to mention the Megger MFT 17XX series are nicely laid out from an operators point of view. The manual in photo two I printed myself as it did not come with a physical copy of one, only a digital version on the CD.
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