KA FAULT LEVEL Ask questions and find answers to many subjects relating to electrics and electrical work

[danny001]

HI,
i have had a periodic inspection done and 2 things i dont know what they mean can anyone help
first its says circuit protection not appropriate for actual kv fault level

and also protective device over rated for circuit / cable
i think the second has a 32 mcb and should have a 20 mcb

but the KV FAULT LEVEL i dont know what it is and its in a shop iam looking to rent
any help would be great thanks

[ericmark]

On your subject you say KA but in text you say KV the latter I have no answer but the former refers to the prospective short circuit current. On each one of the MCB within an oblong is a number for example 6000 or 4500 this refers to the max current which can be switched by the MCB the prospective short circuit current is measure twice with a special meter once Line to Neutral and once Line to earth and the highest reading is recorded. This must be lower than the figure inside to oblong. It is not very often there is a problem I remember on a tower crane used on the building of T5 with a 150mm feed cable we were unable to use MCB's and had to use fuses. In a house in some areas this may mean you can't use RBCO's and very close to the transformer you may still require fuses which will be more like 10000 amps rated. The old Wylex board was originally designed for fuses and a conversion was available to MCB's and these were only rated at 3000 or 3KV and I would think near to the transformer there may be a problem. What was the PSC reading he has recorded?

[danny001]

hi,
thanks for the reply i understand a little more, it says Circuit protection not appropriate for actual kA fault level and he has put this in the OBSERVATIONS AND RECOMMENDATIONS FOR ACTIONS TO BE TAKEN box but it is just under a general observation with no ref to a mcb

but below he has pointed out 3 mcb which are 32a and says thay are Protective device over rated for circuit / cable and these have a code which on a second page refs to a over door heater

if i got a electrician in to change the mcb to a 20a or the corect type would this sort out the kA fault level

and what does psc mean as i cant see it on the paper work
thanks again for your help

[ericmark]

If it follows the BS7671:2001/8 - IET 16/17th Edition you should have a:-
SCHEDULE OF TEST RESULTS
At the top about 3/4 way along you should get:-
Type of Supply: (3 options crossed out two)
Ze at origin ....... ohms
PFC: ...... kA This is what you are looking for if under 6000 ohms or 6Kohms then no real problem if over 6000 and under 10000 ohms the MCB's will need careful selection if over 1000 ohms then a lot bigger breaker will be required called a Moulded Breaker or use fuses.
Although strictly speaking he is correct most people would consider if the whole board was protected by a BS88 fuse the PFC Prospective Fault Current on individual breakers is not that important there are calculations to work it all out but I have not used that method since I left University and I would have to wade through my notes.
Prospective short circuit current and Prospective Fault Current are the same thing.

[danny001]

Hi,
i have found what you mean the number in the box is 6.60 ka which i believe is 6600

the reason iam concerned - he has put it down as a cat 1 which says on the paper work as requires urgent attention

how can it this be changed ? is it down to the mcbs being to high on the heater

i want to get the problem fixed and then tested again, i just want to know what will or can be done
any help would be great thanks

[ericmark]

Did you find PFC: ...... kA on his paperwork? If so what is the value?

[danny001]

HI,
Iam again a bit confused - In a section that is called
Supply characteristics and earthing arrangements it says
Prospective short circuit current 1.90ka amd the internal loop impecence ze is 0.24


And then in another section called board test is says ipf 6.60 ka + a zs of 0.24

Which ka should I be looking at
thanks

[ericmark]

Your not the only one who is confused if “Prospective short circuit current 1.90kaâ€

[danny001]

hi,
say the prospective fault current is to high
when i get someone in, and he blinds me with science,

can you tell me who he will get it down to below 6000 or 6ka
will he change as mcb or will i need a new board
thanks

[ericmark]

According to our training the only place the PFC is likely to be too high is Central London. To handle the extra current with trips would require moulded breakers and the expense would be excessive so the only reasonable option would be a fuse box and revert to cartridge fuses. Which is basically a consumer unit change.
From the reading you have given I don't think the PFC is too high I believe the instruments he used were faulty.
There is only one way to prove this and that's to get some independent reading.
The supply authority are duty bound to provide you with these reading so it should not cost.
Also you are only interested in reading at the consumer unit so would not cost much anyway will only take a few minuets its not a full inspection and test that you need.
Remote I could be wrong of course you may have the main transformer right behind the shop but I would think unlikely and I strongly advise a retest and I would expect that will be the end of your problems I don't think the PFC is really too high.

Let me know how you get on please.

All best Eric

[danny001]

hi,
thanks for all the help i will get another reading done and go from there
and we will see, thanks again