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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

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  • 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.


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  • 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.


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