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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
    I am not sure that the physics does not apply Graham.

    There are several variations on the protection, so here goes.


    Wall Warts with 5W transformer supplies: The TX is would on the mains side with the wire as thin as can be wound at a reasonable price, say 46SWG or smaller. This will fail at a few hundred mA as a fuse. The output voltage regulator if fitted will have electronic current limiting, otherwise the transformer winding fails open circuit (Experience!).

    Ditto but electronic and USB sockets: Output overcurrent protected by electronic current limiting (Part of the USB spec). Electronic failure will either stop any current flow or the low power parts used will fail open circuit. In all cases fire is extremely unlikely due to the low power levels. Electronic parts may not look like fuses but are very effective all the same. Some may have small fuses in the circuits, but I have not seen many.  From another post, this is about foreseeable faults, and shorts between pins or to the usual plastic class 2 Earth pin are impossible due to the construction. Is someone suggesting these devices are not safe?
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  • 0
    I am not sure that the physics does not apply Graham.

    There are several variations on the protection, so here goes.


    Wall Warts with 5W transformer supplies: The TX is would on the mains side with the wire as thin as can be wound at a reasonable price, say 46SWG or smaller. This will fail at a few hundred mA as a fuse. The output voltage regulator if fitted will have electronic current limiting, otherwise the transformer winding fails open circuit (Experience!).

    Ditto but electronic and USB sockets: Output overcurrent protected by electronic current limiting (Part of the USB spec). Electronic failure will either stop any current flow or the low power parts used will fail open circuit. In all cases fire is extremely unlikely due to the low power levels. Electronic parts may not look like fuses but are very effective all the same. Some may have small fuses in the circuits, but I have not seen many.  From another post, this is about foreseeable faults, and shorts between pins or to the usual plastic class 2 Earth pin are impossible due to the construction. Is someone suggesting these devices are not safe?
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