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Ever thought about ... ?

gkenyon
I was asked a series of interesting questions this week about fault protection and overload protection for a particular application. Some of these really make you think, and the physics doesn't always lead you where you think you'd go.


Dropping out of all this, was me pointing out something interesting which I wonder whether it's ever crossed the minds of contributors to this Forum ... so here goes.


Ever thought about what, in typical UK installations, protects the electronics in a plug-in [to a standard BS 1363-2 socket-outlet] phone charger / wall-wart type power converter against:

(a) Fault current (consider both cases of L-N and L-PE); and

(b) Overload current ?




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  • 0
    AJJewsbury:
    HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.

    Interesting. Do you have numbers for BS 3036 fuses? (I (or rather Google) have failed to locate any so far).


    Given that the permitted Zs values for a rewireable are if anything a little higher than for either a BS88-3 (e.g. according to table 41.2) I would have though BS 3036 ones would have if anything slightly quicker disconnection times for the same current than an BS 88-3 and therefore (broadly) a lower I²t.


       - Andy.


    Well to see what I'm saying, have a look at Tables in 3A1 and 3A2, for disconnection times < 0.4 s ... I've put the results, along with the calculated I2t, in the table below:



    Fuse

    0.1 s disconnection time

    0.2 s disconnection

    Current

    I2t

    Current

    I2t

    BS 3036

    450 A

    20,250 A2s

    300 A

    18,000 A2s

    BS 88-3

    320 A

    10,240 A2s

    280 A

    15,680 A2s

    This is because the curve for the BS 3036 fuses below 1 second compared with the BS 88-3 - so by the time you get to 0.1 s disconnection time, the let-through of a BS 88-3 is half that of the BS 3036.


    To see this visually, you could always plot the lines of "maximum disconnection time for current" for a few conductor sizes (say 0.22 through to 25 sq mm) on the graphs ... basically, a 30 A BS 3036 won't protect 1.0 sq mm under any conditions, but a 32 A BS 88-3 will protect 1.0 sq mm for disconnection times of about 0.1 s or less. A 32 A Type B does better again ... provided, of course, the prospective fault current is not too high (because then we'd have to rely on manufacturer's data which would drive up the CSA again.


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  • 0
    AJJewsbury:
    HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.

    Interesting. Do you have numbers for BS 3036 fuses? (I (or rather Google) have failed to locate any so far).


    Given that the permitted Zs values for a rewireable are if anything a little higher than for either a BS88-3 (e.g. according to table 41.2) I would have though BS 3036 ones would have if anything slightly quicker disconnection times for the same current than an BS 88-3 and therefore (broadly) a lower I²t.


       - Andy.


    Well to see what I'm saying, have a look at Tables in 3A1 and 3A2, for disconnection times < 0.4 s ... I've put the results, along with the calculated I2t, in the table below:



    Fuse

    0.1 s disconnection time

    0.2 s disconnection

    Current

    I2t

    Current

    I2t

    BS 3036

    450 A

    20,250 A2s

    300 A

    18,000 A2s

    BS 88-3

    320 A

    10,240 A2s

    280 A

    15,680 A2s

    This is because the curve for the BS 3036 fuses below 1 second compared with the BS 88-3 - so by the time you get to 0.1 s disconnection time, the let-through of a BS 88-3 is half that of the BS 3036.


    To see this visually, you could always plot the lines of "maximum disconnection time for current" for a few conductor sizes (say 0.22 through to 25 sq mm) on the graphs ... basically, a 30 A BS 3036 won't protect 1.0 sq mm under any conditions, but a 32 A BS 88-3 will protect 1.0 sq mm for disconnection times of about 0.1 s or less. A 32 A Type B does better again ... provided, of course, the prospective fault current is not too high (because then we'd have to rely on manufacturer's data which would drive up the CSA again.


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