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Topic Title: pfc figures
Topic Summary: neutral being distributed
Created On: 08 November 2012 05:23 PM
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 08 November 2012 05:23 PM
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tillie

Posts: 801
Joined: 03 May 2006

Hi , I have been involved in a complete fit out of an office block for the last three months .

The building had been gutted and all that was reinstalled was a lighting system and an underfloor electrak system.

I have been installing new circuits and adding to existing ccts etc.

I have been testing and certifying each floor as it has been finished.

I test Zdb and pfc at each distribution board on each floor ( six floors ).

My Zdb readings are fine ( 0.02 to 0.05 ) as we go further up the rising busbar.

My query is with the Pfc.

It is obviously a three phase installation so I measure line to neutral and also directly across the phases ( Megger 1552 ).

Now my line to neutral comes out at around 6ka to 7ka , so if I double this figure it comes to around 12ka/14ka.

My test between line conductors comes out at roughly 10.2ka which is the figure I put on my certificate.

Now Yesterday we had a meeting with the Landlord, agents and contractors who did the original job earlier this year.

The O+M manuals were produced and so was the original electrical installation certificate as requested.

On the certificate I noticed that the Pfc was recorded at 6.5ka, roughly the same as my line to neutral value.

When I queried this I was told by the contractor that he does not record the line to line value because it is a 4 wire system and the neutral is distributed.

I know this has been a very long winded post to get to my point but I am now curious.

Is he right ? If the neutral is distributed do we use the line to neutral value pfc instead of the line to line pfc ?

Advice appreciated.

Regards
 08 November 2012 05:32 PM
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John Peckham

Posts: 7616
Joined: 23 April 2005

The contractor who did the original job is talking BLX. If you were kind hearted you could let him have a look at your GN3.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 08 November 2012 05:36 PM
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OMS

Posts: 19900
Joined: 23 March 2004

Ask him if he's ever seen a p_p_p fault - or perhaps he doesn't have that fault on his systems -

Complete nonsense of course - the prospective short circuit current or prespective fault current is just that - it'll be the largest of the L-E, L-N, L-L or L-L-L values

regards

OMS

-------------------------
Failure is always an option
 08 November 2012 05:43 PM
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tillie

Posts: 801
Joined: 03 May 2006

Thanks for replies.

I thought as much but this chap looked too intelligent for me.

I said it was the first I had heard of it and that I will always put down the highest figure but there was something niggling away at me , probably something I had read regarding neutrals being distributed and using 230v in calculations instead of 400v.

This chap had a suit and a briefcase , I had ripped Dickies on and smelly boots.

Regards
 08 November 2012 05:47 PM
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Ricicle

Posts: 851
Joined: 23 October 2006

Originally posted by: tillie

This chap had a suit and a briefcase ,


So do double glazing salesman but they still know FA about electrical engineering

-------------------------
Empty barrels make the most noise.
 08 November 2012 06:23 PM
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OMS

Posts: 19900
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This chap had a suit and a briefcase , I had ripped Dickies on and smelly boots


I've got a suit and a briefcase and I've also got ripped Dickies and smelly boots - your man with the suit is a bit adrift from the mark.

In very specific cases looking at zero sequence impedance (a generator under subtransient conditions with a fault close to the winding as an example) the P-N fault may exceed the value of a 3 phase fault. But you certainly aren't looking at that in the system you describe.

Regards

OMS

-------------------------
Failure is always an option
 09 November 2012 07:04 AM
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ebee

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I remember us all being told on a 2391 course a few years back back that a three phase pfc could only be the same as as on the single phase by virtue of the fact one phase one winding two or more phases more windings therefore the impedance rising in the same proportion as the voltage therefore current stayed constant throughout the equations.

It was only after coming on here for the very first time that it confirmed that it was an increase on three phase (easy rule of thumb being twice).

This chap though was NICEIC at the time later becoming a NAPIT field officer so his edicts were relied upon by some. (Martyn Duerden knows who I mean)

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 12 November 2012 10:09 PM
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MAXMIRA

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I have experienced such ignorance on many occasions even with so called experienced engineers. The resistance to doubling the figures seems to get stronger when the value exceeds 10kA and often more expensive distribution systems are required.

Anyway, tillie you are 100% correct and your man in a suit is wrong!
 13 November 2012 01:21 PM
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krishanc

Posts: 142
Joined: 24 September 2003

Originally posted by: Ricicle

Originally posted by: tillie



This chap had a suit and a briefcase ,





So do double glazing salesman but they still know FA about electrical engineering



hahahahah

 13 November 2012 07:56 PM
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rocknroll

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Oh dear, the poor old guy in the suit got some stick again , anyway I think you will find in a LV earthed installation the zero sequence is generally lower than the positive sequence so the L-E fault would be slightly greater than the L-L-L fault, in a MV unearthed, indirectly or impedance earthed installation the zero sequence is around 2 to 4 times greater than the positive sequence so the L-L-L fault would be greater.

So the man in the suit is actually 'technically' correct although if you did your sums it generally comes out at around 1.02999............ times the L-L-L fault at the service cutout which is miniscule in comparison so can be disregarded.

Thats why he wears a suit and your still slumming it around in your worn out dickies and boots. LOL

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 13 November 2012 at 08:06 PM by rocknroll
 13 November 2012 09:02 PM
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weirdbeard

Posts: 1652
Joined: 26 September 2011

Originally posted by: rocknroll

Oh dear, the poor old guy in the suit got some stick again , anyway I think you will find in a LV earthed installation the zero sequence is generally lower than the positive sequence so the L-E fault would be slightly greater than the L-L-L fault, in a MV unearthed, indirectly or impedance earthed installation the zero sequence is around 2 to 4 times greater than the positive sequence so the L-L-L fault would be greater.



So the man in the suit is actually 'technically' correct although if you did your sums it generally comes out at around 1.02999............ times the L-L-L fault at the service cutout which is miniscule in comparison so can be disregarded.



Thats why he wears a suit and your still slumming it around in your worn out dickies and boots. LOL


Hi Rocknroll, no disrespect intended, but you can buy suits that might have been previously inhabited by the greatest electrical minds for a few quid in charity shops.

Would you mind explaining the sums which give the 1.02999 figure?

.Thanks
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