p.f.c

[c.boy]

Howdi,

Changed a d/b on fri and carried out a full test on a top floor flat,3 storeys high. Most of my readings were pretty bang on appart from the p.f.c which was 7.1 ka my ze was 0.10.Doing the formula it should have been around 2.3 ka as if im correct pfc= 230/ze . Ichecked the reading 3 times and still 7.1 ka was the reading. My gaffer has phoned me today to tell me that this is an unacceptable result.without trying to find the main earth in the field outside how would i be able to get a better reading????? the system was tncs with a bs1361 type 2 which has a rated short circuit capacity of 33ka.yeah 7.1 is a bit high but surely its well within the capacity of the bs 1361????

 

[Spudnik]

..... has a rated short circuit capacity of 33ka.yeah 7.1 is a bit high but surely its well within the capacity of the bs 1361????

Yes but if your MCB's are only 6K, then they could be incinerated if there was a fault.

 

[c.boy]

hi
thanks for the response.
yeah true!!! all breakers are type b rcbos which is 10k and the type c are 6k (could be wrong tho) how would a be able to reduce the reading on the p.f.c tho??

 

[electricAl]

Out of curiousity, what were the respective PEFC/PSCC readings? If it`s TN-C-S as you say, they should be the same -were they?

 

[platty1005]

Ze is measured with the earthing conductor disconnected from the bonding conductors to avoid parallel paths
PFC is measured with earthing conductor connected to bonding conductors etc so there will be a different loop impedance hence the difference between measured and calculated values.
If you read OSG page 88 It is not recommended that installation designs are based on the measured values of PFC as changes to the distribution network, subsequent to completion of the installation may increase fault levels.
OSG page 56 - for domestic single phase supplies up to 100A the PFC is unlikely to exceed 16.5 kA
con units incorporating protective devices complying as a whole assembly with BS 5486-13 or BSEN 60439-3 are suitable for fault currents up to 16kA when supplied through a type II fuse to BS 1361

DO NOT try to reduce the PFC reading it is high probably due to the sizes of pipework in the building and the steel frame contruction providing low resistance for fault currents.

I understand the OSG as explaining in domestic properties if the Fault current is high the Main fuse will blow before damage is done to MCB's
I agree it all sounds a bit confusing but your 6kA MCB's should be OK
If you are still not Happy then change MCB's to 10kA

 

[c.boy]

hi thanks for your reply.

yeah a mentioned that to my gaffer!! but he said it was totally unacceptable!!! and stormed off!!!no explenations as per usual!!!all fuses are rcbos type b which are 10k a think pal???new to the testing not done it since ma trade test,seem to find all the older sparks get the honour!!!!! a did say because its a flat top floor readings may be higher but he wants everything bang on if you are .02 out on a zs he ???? it!!! maybe a should just start doing the formulas LOL!!!am the only one out o 12 guys who ???s this and really getting a bit ****ed off as its my name on on the cert, at the end o the day!!! but a am right in saying that without finding the supply earth a would never be able to sort this yes????

 

[Darkwood]

hi thanks for your reply.

yeah a mentioned that to my gaffer!! but he said it was totally unacceptable!!! and stormed off!!!no explenations as per usual!!!all fuses are rcbos type b which are 10k a think pal???new to the testing not done it since ma trade test,seem to find all the older sparks get the honour!!!!! a did say because its a flat top floor readings may be higher but he wants everything bang on if you are .02 out on a zs he ???? it!!! maybe a should just start doing the formulas LOL!!!am the only one out o 12 guys who ???s this and really getting a bit ****ed off as its my name on on the cert, at the end o the day!!! but a am right in saying that without finding the supply earth a would never be able to sort this yes????

Direct your boss to this site so we can give him a few lessons about his job, is he an armchair boss you know the penpushing type or does he do his fair share on site the place where you learn things lol.

 

[c.boy]

armchair all the way!!!! does a couple o courses and knows everything!! never on the tools mind you the only way you get a gaffer job wi the council is that you cant work ont the tools!!! a wish a never left the petro chem industry!!!!!!!!!!!!!

 

[Monkey1984]

all fuses are rcbos type b which are 10k a think pal????

The Majority of the RCBOs I'm using are rated to 6kA so you might want to confirm that what brand RCBOs are you using and did you specifically ask for 10kA?