19.1ka psc reading on a 2 bed small house!

[ch1cki]

Any ideas??

 

[Guest 004]

When Ben decides to listen to the correct answer, and indee...oh feck it Im out.....

 

[egginyourface]

Ive seen fault currents like that blow main fuses then rip through rcd

 

[mart8235]

I've had a job like this before. very small industrial unit with tranny 5 meters away. PFC was over 20ka, the mcbs in the DB were only rated to 6 ka. main fuse was 100a 1361. in gn 3 it gives info on this situation. so basically in short we changed the board so we could install 16ka mcbs as gn 3 advises and all was good. you need to have the right type of main fuse for this to work. I'm sure someone is going to tell me this is overkill but I'm standing by it.

 

[IQ Electrical]

I've had a job like this before. very small industrial unit with tranny 5 meters away. PFC was over 20ka, the mcbs in the DB were only rated to 6 ka. main fuse was 100a 1361. in gn 3 it gives info on this situation. so basically in short we changed the board so we could install 16ka mcbs as gn 3 advises and all was good. you need to have the right type of main fuse for this to work. I'm sure someone is going to tell me this is overkill but I'm standing by it.

The conditional rating is 16ka as I posted way back but if your PFC was 20KA, why did you install devices rated at 16KA?

Could be underkill!

 

[mart8235]

The conditional rating is 16ka as I posted
way back but if your PFC was 20KA, why did you install devices rated at 16KA?

Could be underkill!

that's what I thought when we first looked into it. but after a lot of reading and checking with various peole that was the way to go. unfortunately I can't rember the full reasons why 16ka breakers were ok (was about 2 years ago) but I think you guys have touched on that already? gn3 has a lot of the answers but not all.
last thing I wanted to do is add more confusion to this thread.

 

[NoSparks]

I havn't read all this thread so please forgive me if I am repeating somthing.

The subject has been raised before and we were told that there was no concern because there is a reg allowing "cascading" that is any device can be used as a cutout if the let through energy of a cut out higher up the line is less than the device can withstand. Or in simple words any MCB can be used provided the service cutout affords it adequate protection.

I spent several months checking this out. You will find just about NO information on the let through energy of a BS1361 (if anybody has then I would be interested) or the energy that a BS60898 will withstand. I contacted the manufacturers of several well known MCB and they all confirmed that a 100A BS1361 would protect their standard 6kA MCBs. Finally I realised that the let through energy of a BS1361 is simply the integral of its time-current function (given in BS7671) and similarly for BS60898, and to my supprise the former does (theoretically) protect the later. I have no comment on higher rated BS1361 cutouts found in larger installations.

I was interested in one message in this thread where damage was done to the MCBs when the service cutout blew. Are there any more details please.

 

[IQ Electrical]

'Several months' are you serious?

I'll post UK Annex ZA up tomorrow, it's conditions, when satisfied, are the ONLY TIME A SERVICE FUSE CAN BE USED AS BACKUP SHORT CIRCUIT PROTECTION.

The service fuse must be a BS 1361 II (now called a BS88-3) with a maximum rating of 100A and even then, the conditional rating is only 16KA.

Maybe we need a 'sticky' explaining this as it just doesn't seem to be making sense to some.

 

[NoSparks]

Yes I am serious, several months because basically I didn't believe it. I will be very interested in your post tomorrow.

 

[Guest 004]

^ we wait with interest