kfh:

If they are so safe why do so many cheap clone power supplies burst into flames?


I realise that cheap components used in a design based on lowest possible price rather than safety or longevity will be primarily to blame but my question is a genuine one. 

Because they are cheaply made, with no attempt to follow recognised safety standards.


A well-designed power supply will provide its rated current all day without overheating.  It will ensure that the power supply shuts down if it's overloaded.


A cheap power supply will use under-sized components that run very hot, and will have no overload protection.  The gaps between the mains side and the ELV output may also be under-sized.

mapj1:


In good designs by reputable makes the PCB will often have a link designed to melt, or the inrush resistor is a fusible one. The glued case should be strong enough to contain all the flying parts during the highest energy operation of these mechanisms.

 

So, this is really pertinent ... a fault on the mains side, what sort of let-through energy should be considered ...


 ... in the UK use-case this is ???

 ... in most other countries' use-cases, this is ???

Sparkingchip:

Recharging Ecigs could overload the USB socket

gkenyon:

mapj1:


In good designs by reputable makes the PCB will often have a link designed to melt, or the inrush resistor is a fusible one. The glued case should be strong enough to contain all the flying parts during the highest energy operation of these mechanisms.

 

So, this is really pertinent ... a fault on the mains side, what sort of let-through energy should be considered ...


 ... in the UK use-case this is ???

 ... in most other countries' use-cases, this is ???

The same as used for the design of the new RCD sockets and FCU?

gkenyon:

mapj1:


In good designs by reputable makes the PCB will often have a link designed to melt, or the inrush resistor is a fusible one. The glued case should be strong enough to contain all the flying parts during the highest energy operation of these mechanisms.

 

So, this is really pertinent ... a fault on the mains side, what sort of let-through energy should be considered ...


 ... in the UK use-case this is ???

 ... in most other countries' use-cases, this is ???

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.

So, this is really pertinent ... a fault on the mains side, what sort of let-through energy should be considered ...

... in the UK use-case this is ???

... in most other countries' use-cases, this is ???

• UK - say a B32 - 45,000 A²s

• Euro - say a C16 - 42,000 A²s, or a C20 - 55,000 A²s

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.

gkenyon:
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 ?

A very good question!


Yes - I have thought of it (with some worry), and have wondered how many manufacturers give it a moment's thought and what standards there are for this case.  The last time was just last weekend, when I got a new wifi box whose adaptor gave a choice of a UK or Euro plug end to clip onto it.  I wondered how many manufacturers realise that in contrast to the Euro plugs, which are likely to have 10 A to 16 A protection probably by a good fuse or an MCB, the UK socket might even have 45 A protection (extreme cooker-socket) or a BS3036 fuse. The BS1363 standard grew up at a time when everything could be expected to have a fused plug.  I'm not aware of any other system in the world that has such high current.  

(I don't have access to BS3036 .. is a maximum let-through specified? It would amaze me if it's not a good deal higher in the 'hundreds to thousand-or-two' amps range, compared to modern MCBs or good [shaped element and sand] fuses).


Overload doesn't seem very important: even 3A plug fuses aren't a serious overload protection for small electronics.  Most 'wall wart' adapators I've seen don't even have a metal PE pin, so high L-PE currents wouldn't arise.  But L-N short-circuit faults such as condensation and tracking, a flying lead from a plug pin breaking at the solder and touching the others, etc, don't sound nice.  I'd not feel too bad about a B or C MCB of 32 A, but worse about the BS3036 fuses.  I accept it's unlikely that these events would happen, and likely they could blow themselves out without much damage beyond the plastic box. But I seriously wonder whether even reputable manufacturers supplying the UK realise the conditions.  If anyone would have a good idea of what requirements there are in standards covering wall warts, I think you would!  (I'm often impressed by your standards awareness): is there anything that covers all such devices and requires suitable tolerance of high protection settings? That would be comforting.

 

Sparkingchip:

 

The same as used for the design of the new RCD sockets and FCU?

This may appear to be a bit of a random comment, but I have it in mind that one of the selling points of these new fittings is that they are rated similar to consumer unit devices for big bangs.


I have tried an internet search to find the ratings, but have not come up with an answer, though I remember seeing it somewhere.


In my garage there’s a couple of double sockets immediately under the consumer unit connected to a B 32 MCB with around 300 mm of  4.0 mm twin and earth, the consumer unit is adjacent to the meter and intake. 


I’m not poking about out in the garage tonight measuring the PSC on the sockets, but it’s probably the same as the consumer unit.


So if I swap one of those sockets in the garage for one with a built in USB charger the potential current flowing in a big bang event is probably around 1.500 kA, which is probably going to make it disappear in a puff of smoke and may not do the adjacent consumer unit  with a plastic enclosure much good either.


It’s probably best to install a USB socket with a few metres of cable between it and the consumer unit. 

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