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Discuss Ze / PFC conundrum at factory in the Industrial Electrician Talk area at ElectrciansForums.co.uk.

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  1. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    Hi all,
    I recently went to install a new circuit & when trying to obtain a ze reading at the origin (TN-C-S) I got 0.01 ohms with a pfc of >19.9kA... This is when testing from the main distribution board which is in a hut next to the building where the work took place 15/20 meters away. Anyone came across this type of scenario before!?
    Cheers
     
  2. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Hi - perhaps a measurement issue. You could remeasure with a known R in series and see if it adds as it should?
     
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  3. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Modern testers are not good at measuring low impedance values of Ze caused by being close to the substation of which a factory may have its own. As Wilko said introduce a known resistance to the test and deduct this from your reading but if you are close to the source substation it will be low.
     
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  4. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    Cheers Wilko, Westward... when I test from DB1 in the factory itself, the readings go back to normal which tells me its not my tester. It is close to the substation actually, didnt realise this would give me a high pfc & low ze! So a possible option to find a more accurate reading would be to calculate the difference between the R1+R2 & Zs @ the first DB in the factory? If so, how would I go about calculating a lower pfc taking into consideration its a three phase supply?
     
  5. Vortigern
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    Vortigern Electrician's Arms

    Location:
    England
    Business Name:
    F.H. Electrical
    Yes I have come up with low Ze in a domestic. Sub station 100 yards away. If you look at your manual you may see it specifies that the reading you are taking is within six figures. Meaning your reading may be .07. I or the customer had a new cut out by the DNO and when they read it it was .11 (Ze) I spoke with the tech at Kewtech and they pointed the figures thing. So my reading was .03 and of course there were concerns regards the Ics/Icw of the MCBs. But if I/you think about it if I add the six figures I am not that far away from the DNO figure. It would seem we are at such a fine level of measurement it is very hard to really get an accurate figure in those circumstances.
     
  6. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    I wouldn't worry about the ka ratings of the mcbs too much as back up protection can be with manufacturers guidance be achieved by the fuse in the cut out which will clear the fault
     
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  7. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    You could use the R1+R2 of the board submain but I would personally use a known resistance of say 0.10 and introduce this to the test then deduct it. It is bound to be low if you are in close proximity to the transformer.
     
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  8. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    Ok great, so would you record the ka as you get it at the main consumer unit?
     
  9. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Calculate it from your Ze. My old Robin tester rarely gets thrown by low impedance Ze, don't make em like they used to.
     
  10. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    Ok great, when you say calculate with a ze of 0.01, how do you go about this to get a relative kA reading?
     
  11. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Use ohms law, divide it into the measured nominal voltage.
     
  12. Risteard
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    Risteard Electrician's Arms

    Location:
    Derry, Ireland
    Business Name:
    Walsh Electrical Services
    The conditional rating is 16kA so beyond that you cannot assume that they will withstand the fault current. I would ascertain the figures by enquiry - measurement simply won't be accurate.
     
  13. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    A type 2 bs 1361 for example has a typical ka rating of 33ka so I think it will.
    Like I've said manufacturers should be consulted if back up protection is used
     
  14. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    Yeah I did but got over 40kA still and was wondering if i'd missed a trick is all!
     
  15. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    How did you get 40ka when if using nominal 230 volts
    230volt/ 0.01=23000amps 23ka?
     
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  16. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    I divided 400v instead of 230v! well spotted!
     
  17. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    If it's 3 phase then it could be more like 40ka as you would do a pssc between live and neutral and double it or if your meter allows test between phases.
     
  18. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    True, thanks for the help all. I'll be going back next week to finish off. One more thing (i'll try not to make it sound too confusing), i'm carrying out an installation cert (see the consumer unit layout below)
    Consumer unit @ origin (4way tp) > (feeding) DB 1, & DB 2 (via 2x D63a mcbs)
    -DB1 feeds new circuit 1 out of 2.
    DB 2 > DB 3 & 4 (via C63a mcb) *this 4x16mm swa "piggy backs" in & out of DB3's main switch before entering DB4 (perfectly acceptable I presume but not something i've seen before?)
    -DB3 feeds new circuit 2 of 2.
    My main question is, with the continuation sheets I currently total 12 pages, this is because the only consumer unit I havent worked on is DB4, so I was hoping not to have to record any readings at this consumer unit & to make a note in comments instead saying "DB4 not tested" but in all honesty, because the supply from DB2 ends @ DB4 (piggy backed into main switch @ DB3) this is where I should record another ZS on a seperate continuation just to segregate the 2 boards, would you agree? If DB3 fed DB4 via a circuit breaker, could I then write on the circuit details of DB3;
    Circuit details for new circuit installed & then
    "all other circuits not tested"?
    Sorry to ramble on!
     
  19. Wilko
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    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    If I've understood correctly... it's a new supply to DB4 but no changes in DB4. If so, perhaps an update to the DB4 details showing the new Zdb ?
     
  20. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    No way I would "piggy back" one board from another.
     
  21. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    Yeah i'll go with that so you can see all consumer units are on certificates for the sake of the install, wont hurt for future reference either. I suppose in even bigger installations where there are multiple DB's not associated with one you are installing a new circuit in, it's acceptable not to include these in your installation cert providing the one worked on has certs right back to the origin?
     
  22. Dunc88
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    Dunc88 EF Member

    Location:
    West Midlands
    I've asked a couple of people about this as it didnt look right to me & i'd never seen it before & they seemed to think it was perfectly acceptable. Seems a very lazy/odd way to do it as DB3 & 4 arent too far away from each other & DB3 has plenty of space in it! Frustrating coming across things like this that you havent had anything to do with but thats how it goes sometimes (/most the time!!)
     
  23. Risteard
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    Risteard Electrician's Arms

    Location:
    Derry, Ireland
    Business Name:
    Walsh Electrical Services
    The 33kA is for the fuse - not what the distribution board assembly is conditionally rated to owing to Annex Zb (previously Annex Za).
     
  24. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    We are not talking about domestic installations
    Were it's highly unlikely to see such fault currents
     
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