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V.High PFC at switchfuse

Horace Horace
Hello guys

I was looking at a job today at an old house divided into flats where a BS 88 dist board supplies my switch fuse with 60A (which is adjacent to it). The switch fuse is a dated MEM with a porcelain 60A BS3036 but looks in fair condition. Im getting 0.01 ohms ze at the switch fuse and due to this the PFC is off the scale.

Am i right in contemplating that as its a single phase supply under 100a, that the switchfuse's 60A rewireable 3036 breaking capacity can be overlooked as the upstream BS88 will take the brunt of any potential fault and let through no more than 3-4KA which the rewireable can then handle. While were on the topic of breaking capacity's, anyone know the BC of a ceramic MEM 60A 3036 ? I do need to record a suitable PFC value at the switch fuse, my multi tester is is reading 19.9KA (which likely calculates as off the scale). The PFC of the CU in the flat is an easier 3.5KA !


Thanks all



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    Chris Pearson:
    Horace Horace:

    The Ze of 0.01 is not helped by the fact that earth from the switchfuse is via the metal trunking, there is no earth cable to speak of. So I cannot disconnect any parallel paths easily. 


    You don't want to disconnect the parallel paths - you need to know the highest possible PFC.


    3.5 kA at the flat equates to a Zs of 0.07 Ω and an R1 + R2 of 0.06 Ω. If CSA of the conductors is 16 mm², that equates to a length of about 25 m. If the real length is significantly different, you need to question the accuracy of the measurements.




    The submain is t&e, probably imperial size between 10 and 16mm, replacing this would be a ballache. The bonding to the water goes back to the landlords cupboard on the stairs. Length is not 25m, somewhere up to 20m max.


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  • 0
    Chris Pearson:
    Horace Horace:

    The Ze of 0.01 is not helped by the fact that earth from the switchfuse is via the metal trunking, there is no earth cable to speak of. So I cannot disconnect any parallel paths easily. 


    You don't want to disconnect the parallel paths - you need to know the highest possible PFC.


    3.5 kA at the flat equates to a Zs of 0.07 Ω and an R1 + R2 of 0.06 Ω. If CSA of the conductors is 16 mm², that equates to a length of about 25 m. If the real length is significantly different, you need to question the accuracy of the measurements.




    The submain is t&e, probably imperial size between 10 and 16mm, replacing this would be a ballache. The bonding to the water goes back to the landlords cupboard on the stairs. Length is not 25m, somewhere up to 20m max.


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