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High PFC

Pete Murphy
Hello,
I’m looking for help interpreting readings at a single-phase domestic CU, TNC-S. V= 241.6
Ze = 0.06Ω, main switch open, main earth disconnected.
I’ve taken the PFC readings twice now.
  1. PEFC = 2.2kA, PSC =4.0kA

  • PEFC = 2.7kA, PSC =6.0kA

These readings were from the supply side of the closed main switch with main and supp. earth connected. I took one set of PSC readings direct from the (6kA) RCBO’s of which the highest was 5.9kA.
Seems a large difference for a combined earth and neutral.
Main fuse is BS1361 type 2 so no problems there but before I spend £300 plus for 10kA breakers am I missing something here?
Thanks for any comments,

Pete

  • 0
    If Ze = 0.06 Ω, then at 241.6 V, PEFC = 4.03 kA so I don't know where the 2.2 kA value comes from. With a TNC-S system, PEFC should of course equal PSSC because the route is the same except between the service head and the DB.


    Were you using the same method for both readings? (2-lead v 3-lead)
  • 0
    Hello Chris,

    Yes, 2 lead tests for all but as you say the arithmetic doesn't tally. I'm back in the (empty) apartment on Wednesday so time to do it again. It's a city centre not far from the transformer so the low Ze/high PFC makes sense but not the rest of it.

    Just thought, the meter's been recently calibrated so I'll check it against my known readings at home.

    Thanks for your time,

    Pete
  • 0
    Hi,

    My own inconsistencies with much higher then expected PFC readings have been due to my bad (inaccurate) zeroing of the lead resistance.


    Regards
  • 0
    Bear in mind the difference between the 2.2k and 6.0k readings corresponds to a difference in resistance of only 70 mOhm, so depending on how well your probes are making contact with terminals etc, you could easily see such variations.


    Also, a domestic CU to 61439-3 annex ZB with 6kA breakers should be able to break 16kA when backed by a <=100A 1361 fuse.
  • 0
    Thanks yes, there's a fair chance of that. I'd done it earlier that day using a wander lead it showed up!
  • 0
    Thanks wally,

    Aye, something I was aware of but not properly
  • 0
    wallywombat:


    Also, a domestic CU to 61439-3 annex ZB with 6kA breakers should be able to break 16kA when backed by a <=100A 1361 fuse.


    Same was in place for BS EN 60439-3 CUs before that. Provided it's a CU intended for the UK it should be OK.



    Earth fault loop impedance measurements can change between taking readings because of changes in local loading too - so if something is switched off next door for example the reading can change ... but in TN-C-S systems, I'd expect the PEFC and PFC readings to be very cloase, because the N and E are (should be) connected solidly together at the service head. The minor difference in the readings should only be the difference in cross-sectional area between the main earthing connection and the N tail.


    Having said all that, tests for loop impedance and pfc are not always very accurate anyway.


  • 0
    Thanks to all who commented. I was back on the site today; better connections, and properly nulled leads; sensible readings PEFC 2.2kA,

    PSC 2.1kA. My Fluke 1604 has a 'zeroed' icon which I always disregard and zero for resistance readings anyway but I think not last week when I took the PFC readings. The meter had just been calibrated which sowed some doubt but it was an end of day error in truth.

    Best regards,

    Pete
  • 0
    wallywombat:

    . . . a domestic CU to 61439-3 annex ZB with 6kA breakers should be able to break 16kA when backed by a <=100A 1361 fuse.


    Yes, but only once. They do not need to be re-usable after breaking more than their rated current. 


    Regards,


    Alan. 


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