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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
    Now, why would I want a capacitor between the mains and output of a class 2 supply, when this is an obvious dangerous failure point? The answer is simple, someone has had an EMC issue which they cannot fix without an Earth connection, so have used the supply instead! The mains tingles might well be expected, although not dangerous, unless the capacitor fails, which is not completely unknown. In my opinion, this is just an unsatisfactory basic design, as a class 2 device needs proper insulation, not a very thin capacitor dielectric, usually by a 2 segment transformer bobbin, which cannot fail by design because the dielectric strength is very high. In fact, I doubt that these are properly class 2, although they may meet the basic flash test when new. What happens if we do the test with 10kV? I expect failure, although this is outside the basic spec, but other class 2 items pass just fine. The design is making the most of the specification.
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  • 0
    Now, why would I want a capacitor between the mains and output of a class 2 supply, when this is an obvious dangerous failure point? The answer is simple, someone has had an EMC issue which they cannot fix without an Earth connection, so have used the supply instead! The mains tingles might well be expected, although not dangerous, unless the capacitor fails, which is not completely unknown. In my opinion, this is just an unsatisfactory basic design, as a class 2 device needs proper insulation, not a very thin capacitor dielectric, usually by a 2 segment transformer bobbin, which cannot fail by design because the dielectric strength is very high. In fact, I doubt that these are properly class 2, although they may meet the basic flash test when new. What happens if we do the test with 10kV? I expect failure, although this is outside the basic spec, but other class 2 items pass just fine. The design is making the most of the specification.
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