434.5.1 Except where the following paragraph applies, the rated short-circuit breaking capacity of each device shall be not less than the maximum prospective fault current at the point at which the device is installed.
A lower breaking capacity is permitted if another protective device or devices having the necessary rated short - circuit breaking capacity is installed on the supply side. In this situation, the characteristics of the devices shall be co-ordinated so that the energy let-through of these devices does not exceed that which can be withstood, without damage, by the device(s) on the load side.
It is this let-through bit which is of interest reading Hager â€œ4 Curcuit_Protective_Devices.pdf it seems the incoming fuse means the MCBâ€™s do not need to stand the full 6.6kA page 75 details how this is calculated which is not easy reading. As to if it applies in your case will depend on actual values and because you donâ€™t have control of the fuse fitted by the supply authority it may mean you have to fit your own incoming fuse.
It was only after you raised the problem that I found the Hager publication and I was unaware on how to calculate this myself. If you did fit an extra main fuse if you used 2 meters of cable to connect it up the resistance of the 25mm cable would cause it to pass anyway.
Although you have given a lot of detail between the two posts I canâ€™t really work it out for you that needs doing by someone on site.
I of course have never seen your set-up. But from what you have said I think the tester is being a bit pedantic and I would not fail the installation with the figures you give.
There are two main books we use as reference when inspecting and testing. The main one is "Requirements for Electrical Installations" and the other "Book 3" the latter explains what is meant by what the first says. I have asked another electrician for help you may have seen the post.
One of the problems is consumer units tend not to have the British Standard number printed where it can be read. And web sites tend to give limited information. I noted I have a Siemans MCB which is rated at 10kA.
I have looked at the Wylex (part of electrium group) and there is no mention of the British Standard to which it complies.
I am sorry I could not be of more help.
Read "Sparx help please".
He seems to come to same conclusion as me in that your testers have only half read the books.
434.5.1 as stated in previous post has two sections. And it seems they stopped reading after first section and missed second section.
The Book 3 we talk about my son has my up to date copy. But it's just after Table 2.4 and before 2.7.16 Functional testing sorry don't know page number.
As Sparx says it is unusual to get reading this high and I have only come across it once with a massive supply to a tower crane which we wanted to tap into to charge the banksmans radio.
We are not on site so we can't say they got it wrong but I think there is a good possibility they have. All I can suggest is you print out our replies to you and ask them for their comments.
Being nice about it of course as you may have missed something when relating to problem to us. And being honest it is an easy mistake to make and does show they were testing and not just sticking on a label as many testers do.
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