why Pfc is 16kA in Installation Cert?

[bluesky]

In the Installation Certificate, the Pfc of supply is 16kA, but on the top of testing results, the Pfc normally is 1 or 2kA, normally less than 6kA. Why the 2 numbers are so different?

cheers

 

[pevvers]

I'm not quite sure what you mean to be honest, I've never seen an installation certificate with this rating stated on it. This is the breaking capacity of a BS1361 I type fuse (+0.5kA), and this is normally the value that the DNO with give you if you ask for the PFC for an installation.

Would you care to expand your question a bit?

Thanks.

 

[Des 56]

What I am guessing he means is the box asking for PFC in the characteristics for the type of supply fuse in the cut out
ie Bs 1361 16.5 KA for the type 1 or 33 for type II.

Then stating the actual Pfc recorded for the supply characteristics (1 or 2 KA)

Then comparing those to the mcb/fuses maximum short circuit capacity of the final circuit ( 6 KA )


!6.5 KA may be the breaking capacity of the suppliers fuse
1 or 2 KA is a typical measured Prospective fault current of the installation
6 KA is the breaking capacity of the mcb of the final circuit

 

[pevvers]

Nope, still don't get it... It must be getting late!! No worries, just wondered where the 16kA was stated on a installation cert, I've never seen it.

Only the PFC of the supply, and the short circuit capacity of the primary overcurrent protective device are measured in kA on the cert so I am stumped what the question is!

 

[2391bob]

it`s not the pfc that`s 16.5kA ,it`s pscc that`s 16.5kA and that`s only if it`s a BS 1361type I or a BS 88 gG fuse if it`s a BS1361 typeII then it`s 33kA.Now when you`re talking about 1ka or 2ka pfc on the breakers(pscc) that`s commonly BS 3871 type 2 breakers 5A 1kA,>5A 2kA.hope that make sense!

 

[pevvers]

it`s not the pfc that`s 16.5kA ,it`s pscc that`s 16.5kA and that`s only if it`s a BS 1361type I or a BS 88 gG fuse if it`s a BS1361 typeII then it`s 33kA.Now when you`re talking about 1ka or 2ka pfc on the breakers(pscc) that`s commonly BS 3871 type 2 breakers 5A 1kA,>5A 2kA.hope that make sense!

Huh? PFC is prospective fault current, which is the highest of the readings you would take between either line and neutral or line and earth, in domestic this would just be one reading for each at the origin.

What you're talking about is completely different which is the rated short circuit capacity or breaking capacity of the protective device, which can be anywhere between 1kA up to 120kA depending on the protective device.

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Also, a BS88 fuse normally has a breaking capacity of 80kA, not 16.5kA as you stated. A rated breaking capacity of only 16.5kA would be pretty useless when used in large installation that can often have very small impedances (therefore large prospective current) at the source due to the fact the transformers are sometimes on the other side of the wall.

 

[bluesky]

sorry, i meant Periodic Inspection Report, there are some characteristics of the supply. like voltges, frequency, Ze,also there is Pfc there, for Ze, it is normally 0.35, for Pfc, it is normally 16.5kA. I dont understand since we have to measure it again, why use 16.5Ka there, also it is not acceptable for CBa which are 6kA?

 

[pevvers]

sorry, i meant Periodic Inspection Report, there are some characteristics of the supply. like voltges, frequency, Ze,also there is Pfc there, for Ze, it is normally 0.35, for Pfc, it is normally 16.5kA. I dont understand since we have to measure it again, why use 16.5Ka there, also it is not acceptable for CBa which are 6kA?

I see... I've never seen a PIR sheet that already has the values on it. These are the standard values that the DNO will give you Ze for a TN-S - 0.80, TN-C-S - 0.35, PFC - 16.5kA. But your report should always have the values that you have taken unless you've gone the acceptable route of finding out what the characteristics are by enquiry, but I've never done this myself as why not measure it if you can measure it?

 

[Forde]

sorry, i meant Periodic Inspection Report, there are some characteristics of the supply. like voltges, frequency, Ze,also there is Pfc there, for Ze, it is normally 0.35, for Pfc, it is normally 16.5kA. I dont understand since we have to measure it again, why use 16.5Ka there, also it is not acceptable for CBa which are 6kA?

Read pevvers post above. People are confusing the short circuit breaking capacity of MCB's with the PFC at the cu and the breaking capacity of the main fuse at the primary supply.

 

[bluesky]

Read pevvers post above. People are confusing the short circuit breaking capacity of MCB's with the PFC at the cu and the breaking capacity of the main fuse at the primary supply.

like i said, i am talking about pfc, not breaking capacity...

 

[pevvers]

like i said, i am talking about pfc, not breaking capacity...

I understand that, but you say on an installation certificate the PFC is 16kA. Where and on what installation certificate have you seen this? PFC and breaking capacity are completely interlinked, as you check that the PFC is lower than the breaking capacity of the device, so in the event of a catastrophic fault the device will not weld itself together and will break the current. Just tell me where you've seen the 16kA and I may be able to help!!

 

[sparrow]

Hi I've come across this this thread a little late, but if it helps.. P19 in Scadden " For supplies up to 100A the supply authorities quote a value of PSCC of 16KA, at the point where the service cable is joined to the distributor cable. This value will decrease significantly over only a short length of service cable."

 

[bluesky]

Hi I've come across this this thread a little late, but if it helps.. P19 in Scadden " For supplies up to 100A the supply authorities quote a value of PSCC of 16KA, at the point where the service cable is joined to the distributor cable. This value will decrease significantly over only a short length of service cable."

thanks sparrow, that is the answer i wanted, but i did not ask clearly and could be very confusing...

so i guess that is why some people put in 16kA as the supply characteristics in the certificate...and this should be allowed...

 

[confused-fuse]

take alook at the pdf file lots of info

 

[johnboy6083]

thanks confused fuse, thats a very helpful crib card to have! would save a lot of time!