KA FAULT LEVEL STILL TO HIGH


Postby danny001 » Sat May 24, 2008 4:59 pm

HI,
back again i have had another test done.
in the general section the tester has writen

CIRCUIT PROTECTION NOT APPROPRIATE FOR THE ACTUAL KA FAULT LEVEL

AND IN A BOX ON THE PAPER WORK THERE IS

IPF 6.60 KA

iam told this is to high, is there a simple way of getting it down

any help thanks
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm

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Postby ericmark » Sun May 25, 2008 12:24 am

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 “4 Curcuit_Protective_Devices.pdf it seems the incoming fuse means the MCB’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 don’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 can’t really work it out for you that needs doing by someone on site.

All best Eric
ericmark


Postby danny001 » Sun May 25, 2008 7:51 am

HI,
i think i understand what i need to get done

a possibility may be to add another main fuse/box from the incoming supply to the original fuse box with all the mcbs with a extra 2 metres of 25mm cable
thanks again,
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


Postby ericmark » Sun May 25, 2008 10:36 am

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.

All best Eric
ericmark


Postby ericmark » Sun May 25, 2008 8:49 pm

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.

All best Eric Palmer
ericmark


Postby danny001 » Mon May 26, 2008 10:38 am

hi,
i have got in another company to do a reading and they are going to explain the report to me after,
they seem a lot more willing to help then the first company

thanks for your help again
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


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