A C16  breaker is normally assumed as the worst case for things being supplied by a Schucko socket. In reality the B32 is not a million miles off.

For vaporising PCB links, the 6kA or whatever you think that is the peak current is not as serious in terms of damage as how long it flows for - and the power supply designs with the fusible resistors in series do not have that problem anyway, as a dead short in the electronics "just" puts full mains across the few ohms - so for the time it takes to go 'phut' the current is in the high tens of amps up to perhaps a couple of hundred or so.


  examples     "The resistor will fuse safely if 120 or 240 V is applied. The time to fuse depends on the resistance value"

No, you are all considering this in the wrong way. The fault current available on a ring circuit is extremely unlikely to be several kA, not many domestic premises even reach 2kA at the intake. The point is that the electronics have very thin PCB tracks, and the fusing current is quite low. The semiconductors are small so also have very small fusing currents in wire bonds and silicon. The heat available from any of these failures is very small. I have a lot (probably a hundred wall warts) and have never had any flames, although some fail occasionally, usually at very inconvenient times. They rarely even make any smoke, although the cases are fairly well sealed and may not show it. Short circuit failure times will be similar to BS3036 wire fuses unless there is a fusible resistor, which has a higher resistance and therefore more power dissipation, so may be quicker. The cross-section of a track 3mm wide and 0.05mm thick is only 0.15mm2, (and that is a big track by electronics standards) which is similar to a 5A fuse wire. It would probably fail open circuit before tripping a B32. A wire bond is only 0.01mm or so in diameter with a correspondingly low fusing current. I suggest that the danger is not from fire at all, but this is easily proved by a few experiments, using deliberately damaged components. There are only a few grams of material in the electronics, and much of this is not inflammable at 400 degrees C, there is very little to burn to make a fire sufficient to burn the plastic case. There is also a very limited supply of oxygen (less than 1 gram) in these sealed up units which will also prevent much ignition.


Overall I think the transformer units are worse than the electronic ones, in that a reasonable overload can get the transformer very hot without failure, although many have a thermal switch in the windings. This could melt the case exposing mains voltages as well as providing more oxygen for fire.

Many modern electrical appliances presumably have conductors smaller than 3-amp fuse wire, so electricians installing 3-amp fused connection units doesn’t make much difference does it?


Does BS7671 The Wiring Regulations recognise any BS13363 cartridge fuses other than 13-amp and 3-amp as listed in Table 41.4?


Why aren’t electricians installing other sizes of fuses in fused connection units or manufacturers installation instructions telling them to?

I carry 13, 5 and 3 amp BS1362 fuses on my van, because oil fired boiler manufacturers say fit 5 amp fuses.

The ratings available from Bussmann are 1, 2, 3, 5, 7, 10 and 13 amps and IR = 6 kA, as the fuse wire is enclosed, which is probably more relevant than anything else.

https://www.electricalworld.com/Images/ModelDescriptions/Bussmann Consumer Fuse Specification Data Sheet Technical Information Electricalworld.pdf


A Polish electrician asked me “What is it with the English electricians and switched fused connection units” I laughed and said good question.


All these FCU are installed then the fuse doesn’t provide as close as protection as it could do, often being rated three times higher than it needs to be. But our European neighbours are quite happy protecting the same appliances with a 16-amp circuit breaker, often a C type just to compound things.


This is wandering off from Graham’s specific question about USB chargers, but maybe the answer is it doesn’t matter if they fail without blowing a fuse or tripping a MCB, so long as they fail safe.

That Bussmann data sheet does say what the BS1362 fuses are used for:


Applications: protection of the cables connected to domestic equipment against overloads or small overcurrents.

Indeed ... Sparkingchip and Davezawadi have worked out the real answer, in a round about way.


In general, pluggable appliances are to assume a circuit protective device is located in the electrical installation, that may offer protection to the appliance connecting cord only.


Fixed-wired product may rely on a specific type and/or rating of the protective device upstream in the installation ... but only where stated in the installation instructions (we've had this discussion in another thread).


The difference in the UK being that we use a B32 and most other countries use either B16 or B20 for their socket-outlet circuits.


My worry is really where the manufacturer of a wall-wart designed abroad assumes we have a 20 A protective device looking after the circuit the device is plugged into.


So, I looked into this a bit more closely. Provided we are using a B32 mcb, the disconnection time is met by the overcurrent protective device (not an RCD, because of L-N faults), and the fault current does not (as DaveZ has said is unlikely) exceed 3 kA at the socket-outlet, there is really no problem. This mirrors what we all know - for most direct shorts on appliances or appliance flexes, a B32 will operate before 13 A and 5 A fuses ... and sometimes 3 A fuses.

HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.


My conclusion is that things are probably OK for the conditions in most installations with B20 and B32 mcb's, but perhaps not OK in installations with rewireable fuses, where the use of an extension lead (to provide BS 1362 fusing at 13 A) is probably recommended.

I really should get into the habit of unplugging charger leads from the wall sockets when they are not being used.

Going completely off at a tangent, my latest purchase is a Klein Jobsite Speaker  Bluetooth Speaker with Magnetic Strap - AEPJS2 | Klein Tools - For Professionals since 1857 USB charging suddenly made appliances international as the specification is universal, so no one is worrying about voltages or plug types, I can just use the same products as people in America.


I get home and start charging my tester, work light, tool batteries, phone, Ipad, power bank, card reader, update the sat nav which has to be plugged in to do so and so on and so forth. There's a proliferation of USB and other chargers in my house and I doubt it's that unusual, apart from in the homes of age group most of my customers are in, the over 80's.

I too have my doubts about plug-in wall-warts, especially those that come with the more......'budget' devices. I've openned up a couple of cheap Chinese ones that came with items from a well known auction site... and, after seeing the component count and they way they had been built - never even plugged them in - they went in the bin & were replaced by units from RS or Farnell.


I like to use a plugin type three way adapter (I use the three way because they almost always have an outlet on the front and are always fused). I replace the 13A fuse with a 1A fuse & blank off the other two outlets. I especially insist on doing this for wall-warts which operate out-of-sight & out-of-mind.