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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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    Given the submain is T and E and that means a reduced earth core, 60 milli-ohms seems entirely credible for a distance of 20m.


    The BS88 fuse will not limit the current exactly in the same way that a resistor would. Rather it cuts the fault current off early - so the total damage done, which is related to the energy dissipated at contacts and other pinch  points. The parameter I2t is actually joules per ohm, and relates to the energy that is dissipated. Reducing 't' clearly reduces the peak energy, so volume of metal vaporised or melted, damage to insulation etc.

    Luckily we do not need to calculate this exactly, as the fuse makers have done this for us.  an explanation here for example.
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    Given the submain is T and E and that means a reduced earth core, 60 milli-ohms seems entirely credible for a distance of 20m.


    The BS88 fuse will not limit the current exactly in the same way that a resistor would. Rather it cuts the fault current off early - so the total damage done, which is related to the energy dissipated at contacts and other pinch  points. The parameter I2t is actually joules per ohm, and relates to the energy that is dissipated. Reducing 't' clearly reduces the peak energy, so volume of metal vaporised or melted, damage to insulation etc.

    Luckily we do not need to calculate this exactly, as the fuse makers have done this for us.  an explanation here for example.
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