Granny charger

[M44LCY]

That's not strictly true - it's obviously OK to use a 5A rated extension cable to supply a device rated at 1kW or less. Fuses are rated to protect the cable (and no other reason) in a fault condition. The rating for a ring main circuit is 32A (but the socket is obviously only rated at 13A. A '10A rated' cable should not have a 13A fuse in it but, as the standard fuses are 1A, 3A, 5A and 13A, the fuse will not fulfil its purpose of protecting the cable. In my opinion 10A rated (1mm2) extension cables should not be allowed to be sold.
There is also an earlier post which talks about not having 2 RCDs (RCBOs) stating that they will cause a 'conflict'. This is untrue. If you have 2 RCBOs rated at the same value it will be pot luck which one (or both) will break first in the event of a fault. It can be argued that it would be good practice to have a lower rated RCBO for devices outside as the user is likely to be in a more vulnerable position (could be standing on wet grass for instance). A separate RCBO would also prevent 'nuisance' tripping of the whole house because of an issue outside. The point is having 2 RCBOs doesn't make it dangerous.

With respect, I didn't say that having 2 RCBOs is dangerous. "If you have 2 RCBOs rated at the same value it will be pot luck which one (or both) will break first in the event of a fault." You're using "pot luck" to describe the scenario that I described as a potential conflict. Neither of these is strictly correct but both convey the sentiment that having 2 RCDs or RCBOs is not good practice.

"A separate RCBO would also prevent 'nuisance' tripping of the whole house because of an issue outside." Surely the whole point in having RCBOs in the consumer unit is that when a fault condition occurs, only the affected circuit will shut down rather than a bank of circuits or the whole house. I'm not sure why you would want more than one protecting a single device?

Anyway, this is way beyond what was needed to answer the original question so I'll leave it at that.

 

[pmknowles]

Consumer units normally have only 1 RCD/RCBO (unless it's a 'split load' type in which case it would still only have 2 - so only half the house would go off). A lot of modern devices (like LED lights and EV chargers) have DC residual current under fault conditions which would not cause a standard Type AC RCD to trip. IET recommend Type A RCDs for EV charger installations. They're more expensive and most houses won't have them.

There are 10A fuses in plug tops used in a lot of cheap extension leads and IEC cables.

The problem is that people don't understand the fusing. 10A fuses are difficult to source and will often be replaced by 13A. I've seen 13A fuses in plug tops with Maximum Rating 3A on them. I've also seen cable reels with the cable melted into one solid mass. People will buy a slightly cheaper 10A rated lead without understanding the implications.
I have a portable EV charger with 3 connectors on - 13A BS1363 and 16A and 32A Commando types. I know that someone else with that charger will try to charge their EV at 13A ( or even 16A or 32A) from a 13A socket on the end of a 10m long 10A rated extension lead with a replacement 13A fuse. A BS1362 13A fuse will withstand 20A for 30 minutes (the IET curve at 32A is around 7 minutes) without fusing so that 1mm² cable could run at 16A indefinitely without blowing a fuse. Even with a 10A fuse it would take a around 30 minutes to blow but would I trust that cable again?