PFC testing at 2nd DB

If you are testing a garage board that is fed from a MCB on a house consumer unit, when testing for the PFC do measure this from the main consumer unit in the house or would you test at the garage unit ? I assume that it would be at the garage board because that is what I am testing but just want to clarify. Thanks 

You test at the garage board ...

so you have the Ze at the house board

you have the R1 + R2 for the new circuit 

so add these together and that should give your ZDB at the garage board

your new garage board is no different to the end of any radial

are you installing this Then testing it?

Yes I get Ze and Zs at DB but in regards PFC , prospective fault current I assume that is tested at the garage board to ? 
 

I will be installing this in my own garage and test it. However as I am a trainee my boss will be checking it and signing it off as 3rd party. But I want to install and test as if I was doing it. 
 

Traineeboy said:

Yes I get Ze and Zs at DB but in regards PFC , prospective fault current I assume that is tested at the garage board to ? 
 

I will be installing this in my own garage and test it. However as I am a trainee my boss will be checking it and signing it off as 3rd party. But I want to install and test as if I was doing it. 
 

Click to expand...

You need to forget PFC on the sub board - the PFC is at the origin , but the test you need to do at the garage board is the zbd 

Yes its a bit confusing at first , especially when you are looking at the EIC for the origin and the schedule of results at the garage board.

Murdoch said:

You need to forget PFC on the sub board - the PFC is at the origin , but the test you need to do at the garage board is the zbd 

Yes its a bit confusing at first , especially when you are looking at the EIC for the origin and the schedule of results at the garage board.

Click to expand...

thanks. 
 

so just to Clarify when I fill in the schedule of results for the garage DB where it says PFC at the top - I put in the PFC I measured at the origin of the main house consumer unit ? 

You put the PFC in the schedule for the origin

Murdoch said:

You put the PFC in the schedule for the origin

Click to expand...

yes but on the schedule of test for the garage CU where is says PFC do I leave that blank N/A  ? Or do I just write the same PFC I got at the origin ? 

Traineeboy said:

thanks. 
 

so just to Clarify when I fill in the schedule of results for the garage DB where it says PFC at the top - I put in the PFC I measured at the origin of the main house consumer unit ? 

Click to expand...

On a domestic installation any difference in PFC readings will be negligible with reference to exceeding maximum limits!    

However...

If you are using the model forms as given in BS7671..

On page 463 it asks for Ipf in the box headed  "Nature of Supply Characteristics"   (i.e. prospective fault current at supply) 

And page 483 on the schedule of test results it asks for Ipf at DB...   (i.e. the prospective fault current taken at the DB)

This page also asks for the location and reference of the DB...

So if you want to belt & braces your EIC..

Add a continuation sheet with a second 'Schedule of results' page..

one for main house board and one for garage..

Then you've covered you rear on all bases to satisfy your boss..

It all depends on how petty someone wants to be checking your paperwork.. 

   :C

you can do a PFC at the garage, but it is a bit meaningless as it really is for the 'origin of supply',  ie the house incoming point.  The garage sub-main is just  a radial cct from the house, and you wouldn't PFC every cct off that board.  

Just to try and explain a bit more for you Traineeboy.....

Remember that the reason for checking what the PFC is...

is to verify that a worst-case short circuit fault should not generate a fault current greater than the breaking capacity of any protective devices connected to the supply...

And..

Back to your basic science theory, the nearer you are to the supply...

The higher the PFC will be... 

So the further into an installation you go the PFC will either be equal or less than the PFC at the origin..

As an exercise...  do the PFC at you main DB and your garage DB..

see what, (if any) difference you get?

Which is why as others have said..

You would normally just note down the PFC at origin...

Even though....  for some reason that I don't understand...

Back in the 17th Ed, Red Book, (valid from July 2008 -> ),

and earlier, the Schedule of test results model form just says "PFC.............kA"

Whereas from the Green book, (valid from Jan 2012 -> ),

onward it then said   "Ipf at DB (kA).............................."

Guinness

dunno if that helps.. or not???

:C

But keep your questions coming...

always good to see someone who genuinely wants to learn, asking questions whilst clarifying what they do or do not already understand..

:Salute