MCB Rating

[shaky]

I Had a job come up the other day and the paper work said measurde fault current execeeds that of MCB rating and im going to look at the job in a few days what would they mean by that

 

[beanzntoast]

hi shaky,i would take that as the PSCC or the PEFC (whichever's higher) is in excess of the prospective fault rating on the side of the mcb in a rectangular box (usually 6000).I may be way off so if anybody else has ideas i would also be interested too

 

[saveloy]

Yep Beanzntoast is right. The PFC/ PEFC must be lower than the rated breaking capacity of the MCB's.

 

[pevvers]

I agree with that, but if the breaking capacity of the main cutout upstream of the MCB is higher then it's not a problem. Usually this will be a BS1361 or BS88 fuse, or maybe even an MCCB depending on the installation, and they will normally cover this discrepancy, i.e. 16.5/33kA, 80kA or around 25kA.

Please correct me if I'm wrong of course!!

 

[beanzntoast]

I thought all the MCB's had to be high enough (either 6kv or 10kv) to cover the PFC reading, irrespective of what the main cutout is,

 

[pevvers]

I have read differently from many places, as the fact that if you're working on a big commercial installation, you'll maybe come across PFC's of around 0.02 ohms close to the incomer, which with a 3 phase installation would give a PFC of around 23kA. Unless you replace all MCB's with 1361 cartridge fuses or MCCB's then you'll never achieve breaking capacity above the PFC...

As I said though, please correct me if I am wrong as I am always happy to learn new things, which is why I frequent forums like this! I do believe that I have queried this before though and have come across this answer.

 

[zupos40]

The same topic is also being discussed in Inspection, Testing and Certification of Electrical Installations high PFC.

The BS7671:2008 Regulation some of you are talking about is 434.5.1

It would be great to get a definitive answer. if the BS88 or Bs1361 REC fuse limits the fault current, then why are we told to perform PSCC Pefc tests to find PSc at the supply and ensure our c.bs etc are capable of carrying this value of PFC.

Industrial/ commerical is different I done some design and all device were cordinated to such that they were capable of carrying the let through energy of the upstream device, asI have posted else where, I not sure Bs1361 or BS 88 meet this requirement with reference to domestic installation at let through energy.

 

[beanzntoast]

I have read differently from many places, as the fact that if you're working on a big commercial installation, you'll maybe come across PFC's of around 0.02 ohms close to the incomer, which with a 3 phase installation would give a PFC of around 23kA. Unless you replace all MCB's with 1361 cartridge fuses or MCCB's then you'll never achieve breaking capacity above the PFC...

As I said though, please correct me if I am wrong as I am always happy to learn new things, which is why I frequent forums like this! I do believe that I have queried this before though and have come across this answer.

This is true if reg 434.5.1 is achieved

Damn too slow

 

[pevvers]

I shall have to have a look then....

Cheers all!!

 

[electricAl]

AFAIA it`s agreed between IEE (IET) & the CU manufacturers that if the CU is to recent standards (60439-3 definately, plus an older standard too, i seem to remember) & an effective upstream device (1361 or 88) exceeds the max PFC, then compliance can be assumed upto 16KA, providing that installed breakers don`t exceed 50A Icn
Fairly sure one of the GNs makes mention of this (i`ll look later)

So for domestic atleast (assuming your MCB`s are not >50A ) you should be covered.

Oh, one caveat - not officially, but predictably, one or two of the manufacturers demand only their switchgear is used - otherwise they`ll turn away in event of trouble

But as Zupos said, some clear, unambiguous guidance should be made available to all.

 

[Darkwood]

The 6k or 10k ratings of an mcb is the effective current it can withstand before it fails to do its job so if the current exceeds this the mcb could fail and leave a connected fault path which for obvious reasons is very dangerous so to meet requirement if the pscc exceeds the rating of your c.u. fuses/mcbs then you can put a isolator upstream with a fuse of qualifying rupture value. then in fault situation if the mcb fails to cut the power the upstream fuse will take over due to its higher breaking capacity and safely cut power.

 

[zupos40]

I Had a job come up the other day and the paper work said measurde fault current execeeds that of MCB rating and im going to look at the job in a few days what would they mean by that

I have since been surfing the web and It seems that if the domestic installation CCU complies with BSEN 60439-3:1991 (and it seems most manufacturers do) and the ccu is protected by Bs1361 Type II upto 100A then the board and protective device are ok up to 16KA (do not matter whether the cb are 6kA, 10kA or 16kA).

So as long as the measured PFC do not exceed 16kA then it does not matter if measured PFC value exceed the kA rating of the protective device.

As other have said every day you learn something is a good day.

 

[Spudnik]

Not entirely sure i would be happy leaving an MCB in place if its had over its rating in fault current flashed through it.

Thats if its still in one piece

 

[electricAl]

Not entirely sure i would be happy leaving an MCB in place if its had over its rating in fault current flashed through it.

Thats if its still in one piece

I know where you`re coming from Jason - i`d be uncomfortable too, but the fact is that the powers that be decree our fears to be unsubstantiated (that`s a good un init )

Just looked in IEE Design Guide for reference to what i`d thought to be the case...

After pre-ambling about calculations to estimate the reduction in fault levels in downstream devices, it cuts to the chase when it states;

"For domestic premises, Consumer Units & fuseboards with conditional fault rating of 16Ka can be used - so further calculation is not necessary. So those to BSEN60439-3 annex ZA (specification for particular requirements of CU`s complete with fuses, Mcb`s & protective devices) are able to withstand the fault current for PFC levels upto 16Ka when the distributors fuse is a Type II fuse to BS 1361 rated at no more than 100A. The standard 100A fuse installed by DNO will meet this requirement.

Whilst the individual over-current devices in the CU or DB may not interrupt the highest fault currents, they will be able to carry the currents until the service cut-out operates.

Clearly, it`s better, as is usually the case, the fault rating of the protectice device in the CU/DB is of sufficient rating to clear the fault. Particularily faults downstream of the CU/DB.

Fault levels are rarely as high as 16Ka & rapidly decrease within the installation, so in practice the protective devices in the CU/DB will clear faults"


This is undoubtably what is referenced to by GN3 aswell... p54

"Where a service cut-out containing a cartridge fuse to BS 1361 type II supplies a CU which complies with BS 5486-13 or BSEN 60439-3, then the SC capacity of the overcurrent devices within CU may be taken to be 16Ka."

Except for major cities, the max FC for 100A supplies is unlikely to exceed this...

"The SC capacity of overcurrent devices incorporated within CU`s may be taken to be 16Ka where:

i) The In of breakers do not exceed 50A
ii) CU complies with above Brit Standards
iii)CU is supplied thru a BS1361 type II upto 100A"

 

[Spudnik]

Well i cant argue with that then!

Still not happy about it though.

16ka (i know, very unlikely) through a 6ka MCB and they are happy with it. Bizarre.

Just make sure your EFLI is well below the max or you will be hung (< sarcasm to the regs).

Anyhoo, im off to walk the dog now.