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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 ?




  • 0
    Sparkingchip:

    What sort of question is that to ask of a guy with a lightning charger lead dangling over the arm of the settee immediately next to him?


    Being an Ipad lightning lead the terminals are exposed to touch and every so often I feel a tingle when I put by bare elbow on it, guess what it is plugged into?



     




    So the worst that can happen is?


  • 0
    Sparkingchip:
    Sparkingchip:

    What sort of question is that to ask of a guy with a lightning charger lead dangling over the arm of the settee immediately next to him?


    Being an Ipad lightning lead the terminals are exposed to touch and every so often I feel a tingle when I put by bare elbow on it, guess what it is plugged into?



     




    So the worst that can happen is?




    Depends what's causing the tingles. In terms of failure of protection against fault current, well, if there's a fault, but inadequate protection, the worst is fire and/or small explosion I guess.


  • 0
    Apparently a lot of people experience tingles with Apple chargers, and I've noticed it with others. Some of the time this can be due to static, or possibly "leakage" across isolation barriers.


    Some people complain about it on TVs with metal frames - particularly with antenna fed by a UHF distribution amp - this is probably combined "leakage" from all the connected devices (e.g. 4 no. Class II devices produce enough "leakage" to cause a perceptive shock of about 1 mA). This can be solved by earthing the antenna system at one point.
  • 0
    Sparkingchip:
    Sparkingchip:

    What sort of question is that to ask of a guy with a lightning charger lead dangling over the arm of the settee immediately next to him?


    Being an Ipad lightning lead the terminals are exposed to touch and every so often I feel a tingle when I put by bare elbow on it, guess what it is plugged into?



     




    So the worst that can happen is?




    If it's a genuine Apple charger, then the worst that can happen is a small tingle.  It's part of the way they are designed, with a class Y capacitor between the AC and DC sides.  That can leak a tiny current across.


    If it's a cheap charger of unknown origin, then the best place for it is the electronics recycling bin.


  • 0
    AJJewsbury:
    I'm still slightly lost as to what the actual question is. A single USB charger shouldn't draw more than about 15W (0.065A) under any circumstances. But you seem to be asking what will protect against some massive power surge, under some unspecified conditions.

    It'll draw an awful lot more than 0.065A if it suffers from an internal short circuit. No power surge required.

       - Andy.


    I think it's inherent in the design of a wall-wart that you can never get a dead short between the live and neutral pins.


    The worst case that I can think of is where a diode in the bridge rectifier fails shorted.  In that case, you might get another diode in the rectifier and the EMI filter, suddenly seeing the mains voltage across them.  One or both would go "pop" rather violently.


    So long as that "pop" is contained by the case, it's fine.  They aren't designed to be repairable, so if that happened, you'd throw it away and buy a new one.


  • 0
    Simon Barker:
    Sparkingchip:
    Sparkingchip:

    What sort of question is that to ask of a guy with a lightning charger lead dangling over the arm of the settee immediately next to him?


    Being an Ipad lightning lead the terminals are exposed to touch and every so often I feel a tingle when I put by bare elbow on it, guess what it is plugged into?



     




    So the worst that can happen is?




    If it's a genuine Apple charger, then the worst that can happen is a small tingle.  It's part of the way they are designed, with a class Y capacitor between the AC and DC sides.  That can leak a tiny current across.


    If it's a cheap charger of unknown origin, then the best place for it is the electronics recycling bin.




    Interesting ... does that align with the Ethernet standard ... particularly if the device is connected to a wired connection.


    I really worry about devices that are connected to Class II power supplies on the mains side, and then connected via conductive cabling to other devices. Far happier with my laptop with a PELV (functionally earthed) secondary.


  • 0
    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.
  • 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.
  • 0
    I can tell you why that capacitor is there, but I share your distrust of lightweight designs, and much prefer it when it is moved to between the windings.

    (some  'good' manufactures may use two caps in series to get double fault to danger, which is sort of cheating..)

    You cannot do  a widely split bobbin at tens or hundreds of KHz in the way you would at 50Hz, as the leakage of magnetic  flux is not insignificant (the ur of ferrite is some hundreds to thousands not  the tens of thousands of the traditional nickel Iron transformer steels,or the hundred thousand or so of the copper/molybdenum steels. )

    In effect the magnetic field induced by the primary breaks out of the core a bit, and some sneaks back without enclosing the secondary  - so in the circuit equivalent  you have a transformer with a rather indeterminate transformation that is not quite the turns ratio any more , and an inductance you cannot get rid of in series with the primary - so when you put a load on the secondary that inductance causes the volts to droop badly.


    So to get back to nice transformer behaviour, the primary and secondary windings have to overlay, or on a large transformer interleave either as alternating layers, or as alternating  disk like piles of winding.

    So the primary to secondary  capacitance is much higher than for the split bobbin counterpart. - so the 400V p-p square wave on the switching transistors gets to the output - very badly, as not only is the frequency that much higher, so for a given capacitance the displacement current is higher, but the higher  capacitance also increases the leakage.

    However  we can return the switching frequency back to the source by forming a voltage divider between the winding capacitance and  that capacitor from output to input  leaving us with a predominantly mains input  frequency signal on the output instead of one at tens or hundreds of KHz. (tingles may be 50Hz or 100Hz depending if the return is made to the DC bus, or pre-rectifier  mains)


    The far nicer solution used in stealth SMPS that must not disclose their location by radiating like mad and jamming sensitive receivers is to accept a larger transformer, and to interpose foils between primary and secondary windings, and then to connect the foils, and the cores to one side of the DC bus, but to leave the secondary floating. Such designs can be made to have very low mains leakage and to exceed the highest surge requirements

    . They do however cost a bit more, and may not fit in the size folk expect a phone charger to be, and whenever I have got involved in such things the idea you value non-radiating over size and weight  seems an alien language to so-called power supply experts..


    Smallest, Cheapest and high performance -  you can only have any two at most, and some times only one.


    regards Mike



  • 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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