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Topic Title: Prospective Short Circuit Current at Intake
Topic Summary: Too High for 60898 mcbs
Created On: 29 October 2013 12:16 PM
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 29 October 2013 12:16 PM
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keithredpath

Posts: 415
Joined: 30 March 2002

On measuring the PSCC at the intake of a new installation I get over 20.000 Amps with my Megger 1534. All the local Mcbs are rated at 10.000 Amps. Is this a problem?

-------------------------
keithredpath
 29 October 2013 12:48 PM
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Parsley

Posts: 960
Joined: 04 November 2004

The tester is probably inaccurate, you would have to be very near the source to record that high reading.

What fuses are in the cutout and main switch/panel etc they may offer back up protection see 434.5.1

Regards
 29 October 2013 12:53 PM
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Legh

Posts: 3472
Joined: 17 December 2004

What value of psc do you get when measured at the
(a) input at the MCBs ?
(b) the first load on the largest cct?
(c) What sort of protection have you got upstream of the MCBs?

That might help establish whether or not there is enough resistances to reduce the psc and whether your upstream protective device is able to offer protection.

Legh

-------------------------
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 29 October 2013 09:29 PM
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leckie

Posts: 1669
Joined: 21 November 2008

No it's not a problem. The construction of the board and type testing, allows for up to 20ka to the board AND the mcb's at the main intake position. Can't remember the BS ref numbers.
 29 October 2013 10:17 PM
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Zs

Posts: 2777
Joined: 20 July 2006

'The construction of the board and type testing' said leckie. Leckie, even if by pm, can you tell me more about this please? that's not a challenge by the way, it's a curiosity.

Keith, as parsley said, get back to us about the incoming fuse details.
Loads of sheds and garden kennels etc. do this, but not to this extent.

Normally I'd find at least one fuse which is going to pick this up in the chain, or I'd be inserting something grand like a good old fashioned BS88 into the equation, if indeed it was worthy ( not the kind of thing I know by heart, nor do I intend to) but this is a big number. We need some more information. I do not doubt your meter.

Send us some more and I'll run it through Amtech for you....errr, except that I'm off to play blues for a few days. See you on the other side of Peter Green and Gary Moore.

Zs
 30 October 2013 12:23 AM
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alancapon

Posts: 5745
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Originally posted by: Zs
'The construction of the board and type testing' said leckie. Leckie, even if by pm, can you tell me more about this please? that's not a challenge by the way, it's a curiosity. . .

Consumer Units are manufactured and tested to BS EN 60439-3.

The type approval for UK consumer units often relies on Annex ZA of BS EN 60439-3. This specifies that "a type tested assembly made of the manufacturers own parts may meet 16kA breaking capacity as long as it is supplied from either a BS1361 type 2 or the equivalent BS88 fuse rated at 100A or less". A good example are the Wylex rewireable fuses, which are rated at either 2kA or 4kA depending on their type. Annexe ZA gives them a conditional rating of 16kA. The requirement is for an mcb / rcbo / fuse to break the fault current - it does not have to continue to be serviceable!

The next issue is that of mixing devices of different manufacturers in a consumer unit. This effectively voids Annexe ZA and the type testing. Rather than typing it all out or making a statement of my own, I will refer you to the BEAMA Technical Bulletin - Safe selection of devices used in assemblies dated 10th June 2013, which strongly advises against mixing devices.

Regards,

Alan.
 30 October 2013 11:26 AM
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keithredpath

Posts: 415
Joined: 30 March 2002

The only back up protection are the service head cut out fuses which are sealed. (No rating available)

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keithredpath
 30 October 2013 01:58 PM
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AJJewsbury

Posts: 11251
Joined: 13 August 2003

(No rating available)

Don't you have an idea of type & maximum possible rating? (e.g. with domestic the carriers are usually rated 100A max - which gives a worst case for the fuse that could be inside).
- Andy.
 30 October 2013 03:53 PM
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christait

Posts: 24
Joined: 08 February 2013

If the cutout fuse is to BS1361 or BS88 then you should be ok. BS 1361 will withstand approx. 33kA and BS88 will withstand approx. 80kA.(If it is a sealed DNO cutout as you say then it will almost certainly be to BS1361) High quality installation work to prevent potential short circuit near to the DB will also reduce the risk of this protection being operated.
 30 October 2013 05:20 PM
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HarryJMacdonald

Posts: 254
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Have you taken several readings?

I once had great difficulty getting a consistent reading of PSCC at source, which I put down to a noisy supply. These things take a brief high current and measure the voltage dip. If there are comparable spikes on the system and a small dip then you can get very odd readings, like negative PSCC!

I also understand that there are different kA ratings of the same MCB depending on the standard. Some demand that the MCB is unaffected by the short circuit, others that it works once, but perhaps never again!

Edited: 30 October 2013 at 10:27 PM by HarryJMacdonald
 30 October 2013 08:59 PM
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alancapon

Posts: 5745
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The BSEN that I referred to, requires that the device interrupts the current without an explosive failure outside the CU enclosure, but does not require that in the case of an mcb or rcbo it is suitable for re-use.

Regards,

Alan.
 31 October 2013 09:55 AM
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HarryJMacdonald

Posts: 254
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Thanks Alan, I think most MCBs are only suitable for 6kA if you want them to survive undamaged. How you are supposed to know that one has gone over this limit and should be replaced even if it doesn't appear damaged I don't know!
 31 October 2013 06:18 PM
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OldSparky

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Joined: 28 June 2011

surely your fault current was taken at the tails? why would this effect the MCBs..

have you done fault current on the out going mcbs as well?
 31 October 2013 11:49 PM
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alancapon

Posts: 5745
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Originally posted by: HarryJMacdonald
. . . How you are supposed to know that one has gone over this limit and should be replaced even if it doesn't appear damaged I don't know!

Usually it will appear damaged - it could be welded closed with the cutout fuse operated, no longer electrically continuous between the terminals, or the operating lever no longer operating the mechanism. Often there will be scorch marks round the operating lever too.

Regards,

Alan.
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