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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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    I'd not feel too bad about a B or C MCB of 32 A, but worse about the BS3036 fuses.

    I'm probably less worried.


    MCBs tend to have increasing energy let-though with higher fault currents - simply because it takes a finite amount of time to get the mechanical contacts to move apart. Fuses on the other hand continue to get significantly quicker with increasing currents so the energy let-through doesn't tend to increase greatly with increased fault currents.


    Ignoring niceties like current limiting, and just doing a crude calculation of I²t based on the times/currents tabulated in Fig 3A2 of appendix 3 of BS 7671 for a 30A rewireable, gives an energy let-though of 19,600A²s for 140A (1s); 17,640A²s for 210A (0.4s), 18,000A²s for 300A (0.2s) and 20,250A²s for 450A (0.1s) - obviously there's a bit of "experimental error" in those numbers - but they seem to be hovering around the 20,000A²s or so mark, and we'd need to extrapolate a bit for higher fault currents but all the same they don't look too bad to me when compared with the MCB figures above.


      - Andy.
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  • 0
    I'd not feel too bad about a B or C MCB of 32 A, but worse about the BS3036 fuses.

    I'm probably less worried.


    MCBs tend to have increasing energy let-though with higher fault currents - simply because it takes a finite amount of time to get the mechanical contacts to move apart. Fuses on the other hand continue to get significantly quicker with increasing currents so the energy let-through doesn't tend to increase greatly with increased fault currents.


    Ignoring niceties like current limiting, and just doing a crude calculation of I²t based on the times/currents tabulated in Fig 3A2 of appendix 3 of BS 7671 for a 30A rewireable, gives an energy let-though of 19,600A²s for 140A (1s); 17,640A²s for 210A (0.4s), 18,000A²s for 300A (0.2s) and 20,250A²s for 450A (0.1s) - obviously there's a bit of "experimental error" in those numbers - but they seem to be hovering around the 20,000A²s or so mark, and we'd need to extrapolate a bit for higher fault currents but all the same they don't look too bad to me when compared with the MCB figures above.


      - Andy.
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