Brian Gaff wrote

I would ask you this, Have you ever looked inside of a usb charger.

There are plenty of teardowns on youtube.

Most have no fuse

The best designed ones do.

and the switch mode supply is driven directly from the mains via a
semiconductor driving the primary of a transformer a very tiny
transformer,

Perfectly possible to do those so that whatever
fails you never get mains voltage on the USB.

at a high frequency, the other side is connected to a very simple
rectifier and voltage stabilising chip.
You are in effect relying on a cheaply made tiny transformer to isolate
you from the full mains voltage.

Perfectly possible to do those so that whatever
fails you never get mains voltage on the USB.

I'd no chance it knowing what I know about these things.

I would with a USB charger from operations
that know what they are doing like Apple.

I have had more than one fail one very spactacularly and trash the device
on it. Crap.

So don’t buy the crap, buy well designed USB chargers instead.

polygonum wrote

How dangerous would it be to use a phone or tablet in the bath whilst it
is connected to a mains-powered USB charger?

Obviously it could be dangerous to the device itself if you drop it into
the water!

Obviously a 5 volt low current supply isn't usually life threatening.

Obviously we have rules about sockets in bathrooms. But a long USB cable
to a socket in another room seems a likely way round that.

But if something is wrong somewhere, such as a mis-wired USB charger,
could there be any appreciable danger?

Is there in future likely to be a bathroom-safe USB socket next to the
shaver and toothbrush socket?

--
Rod