pfc

[i=p/u]

how do you get pfc measurement, is it u/ze? it says 0.9KA and im geting 0.4KA, its 230v and Ze is 0.58??? hmmmm help

 

[Geoffsd]

NO.....they are not.

Yes

 

[Sparky Ninja]

But it can't be -can it?? Ze & PEFC are measured in completely different ways.

Did you not read my earlier post??

They can be the same. I measure them at the calculation values every day of the week.

It's on a test rig, where the main bonds are connected to a 200mm piece of copper that provides no different potential what-so-ever.


They don't go anywhere near the floor though

 

[topquark]

They can be the same. I measure them at the calculation values every day of the week.

It's on a test rig, where the main bonds are connected to a 200mm piece of copper that provides no different potential what-so-ever.


They don't go anywhere near the floor though

Is that 200mm squared mate

 

[WayneL]

So as not to confuse the 'learners' - who were mentioned before.

If you had a warehouse supplied by a single phase TN-C-S system and your measured Ze was 0.35 ohms.

By the calculation the PEFC would be 230/0.35 = 657 Amps.

But there are a load of 'bonded' metal supports for this warehouse, which give a Zs at the Distribution Board of 0.2 ohms...........giving a measured PEFC of 1150 Amps (1.15 KA).......because PEFC is measured with the 'bonding 'connected.

A bit of a difference, wouldn't you agree?

 

[Sparky Ninja]

Is that 200mm squared mate

Err... I meant 20cm in length



ish.

 

[topquark]

So as not to confuse the 'learners' - who were mentioned before.

If you had a warehouse supplied by a single phase TN-C-S system and your measured Ze was 0.35 ohms.

By the calculation the PEFC would be 230/0.35 = 657 Amps.

But there are a load of 'bonded' metal supports for this warehouse, which give a Zs at the Distribution Board of 0.2 ohms...........giving a measured PEFC of 1150 Amps (1.15 KA).......because PEFC is measured with the 'bonding 'connected.

A bit of a difference, wouldn't you agree?

Yep, often a big difference between what's taught as "early theory" versus "later theory" and real world values.

 

[Sparky Ninja]

A bit of a difference, wouldn't you agree?

Most definitely.

That's exactly why we call it 'maximum prospective' and not 'calculated or apparent'.

I agree. Whilst I understand why the ohms law relationship is illustrated so much (pfc=Uo/Ze) for design. It is very rarely clarified on the importance of measuring the PFC because of the effect from parallel paths etc. I would expect this is probably due to EWR regulation 14 again

Bloomin thing.

 

[Geoffsd]

So as not to confuse the 'learners' - who were mentioned before.

???

If you had a warehouse supplied by a single phase TN-C-S system and your measured Ze was 0.35 ohms. By the calculation the PEFC would be 230/0.35 = 657 Amps.

Yes but you said it couldn't be calculated.

But there are a load of 'bonded' metal supports for this warehouse, which give a Zs at the Distribution Board of 0.2 ohms...........giving a measured PEFC of 1150 Amps (1.15 KA).......because PEFC is measured with the 'bonding 'connected.

Correct but Ze is measured at the DB where the PEFC is 657. As you have done what I said - Uo/Ze.
At fixtures PEFC is Uo/Zs.

A bit of a difference, wouldn't you agree?

Yes but they are all PEFCs.

 

[Guest123]

A designer would not have the benefit of a measured PFC to work with when designing a new installation,he may only have the declared Ze from the supplier.

Calculation is then your only option.

 

[WayneL]

This thread has made me think - damn!

My meter doesn't do it, but you know the meters that advertise that they give you simulltaneous Ze and PEFC readings on the screen............they must be using Ze in the calculation then???

 

[Geoffsd]

.because PEFC is measured with the 'bonding 'connected'

Not at the DB. Is that where you're confused?

 

[topquark]

I would expect this is probably due to EWR regulation 14 again
Bloomin thing.

Had to look it up

Work on or near live conductors

14.  No person shall be engaged in any work activity on or so near any live conductor (other than one suitably covered with insulating material so as to prevent danger) that danger may arise unless–

(a)it is unreasonable in all the circumstances for it to be dead; and

(b)it is reasonable in all the circumstances for him to be at work on or near it while it is live; and

(c)suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent injury.

 

[Sparky Ninja]

This thread has made me think - damn!

My meter doesn't do it, but you know the meters that advertise that they give you simulltaneous Ze and PEFC readings on the screen............they must be using Ze in the calculation then???

They do. When you set a tester to pfc, it actually just does the math from the Ze/Zs test and displays the figure for you.
I've caught many guys writing both Ze and PEFC down at the same time because they both appear on the screen, this is a big defect IMO.

 

[Sparky Ninja]

Not at the DB. Is that where you're confused?

Whats 'Not at the D.B.'?

 

[WayneL]

.........but Ze is measured at the DB where the PEFC is 657. As you have done what I said - Uo/Ze.
At fixtures PEFC is Uo/Zs.Yes but they are all PEFCs.

No.

PEFC is measured at the incomming 'Line' conductor on the main switch and the MET - with the 'Earthing Conductor, 'bonding' and CPCs etc connected - this will always give the highest value for PEFC - which is the worst case scenario.

Ze is measured between the incomming 'Line' conductor and the 'Earthing Conductor' (disconnected from MET.) - different value altogether if used to calculate PEFC- unless there are NO extraneous conductive parts or parallel paths, in which case they would then be roughly the same.