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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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    When the BS3036 fuse holders were being designed all the modern subtleties of PFC/PSSC were not high on the list. Unless marked otherwise assume it will break no more than 1kA totally safely if inserted live onto a fault, and rather  more without damage with the cover fitted, and quite a bit more without actually damaging anything but itself internally.


    For a higher official rating it needs to be the kind that is either shrouded or makes it hard to push a re-wired fuse in onto a fault, in some designs, that is done by interlocking the switch to make sure it can not turn on until the cover is in place.

    The problem that limits the breaking capacity is that in some more open versions, it can be that as the wire is vapourised by the fault, hot metal is ejected from the holes/gaps in the fuseholder over the  finger tips and hand pushing it in, not that it fails so violently that it demolishes the building.

    Assuming the BS88 is a sensible rating, then to a degree it will energy limit if there is a gross fault close to the BS3036 fuse, which the same thinking that allows 6kA MCBs in such a case and in practice in this case the danger is not great, so long as when the fuse is changed it is not refitted with the load switched on.


    As an aside, 19kA is quite high for this sort of thing (usually its the other way about, electric showers dim the lights etc) unless the transformer is in the basement below, and before panicking, I presume you have verified that your meter leads are in good order, correctly nulled and the meter is in cal and not low on battery.

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  • 0
    When the BS3036 fuse holders were being designed all the modern subtleties of PFC/PSSC were not high on the list. Unless marked otherwise assume it will break no more than 1kA totally safely if inserted live onto a fault, and rather  more without damage with the cover fitted, and quite a bit more without actually damaging anything but itself internally.


    For a higher official rating it needs to be the kind that is either shrouded or makes it hard to push a re-wired fuse in onto a fault, in some designs, that is done by interlocking the switch to make sure it can not turn on until the cover is in place.

    The problem that limits the breaking capacity is that in some more open versions, it can be that as the wire is vapourised by the fault, hot metal is ejected from the holes/gaps in the fuseholder over the  finger tips and hand pushing it in, not that it fails so violently that it demolishes the building.

    Assuming the BS88 is a sensible rating, then to a degree it will energy limit if there is a gross fault close to the BS3036 fuse, which the same thinking that allows 6kA MCBs in such a case and in practice in this case the danger is not great, so long as when the fuse is changed it is not refitted with the load switched on.


    As an aside, 19kA is quite high for this sort of thing (usually its the other way about, electric showers dim the lights etc) unless the transformer is in the basement below, and before panicking, I presume you have verified that your meter leads are in good order, correctly nulled and the meter is in cal and not low on battery.

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