As stated the transformer used is often two bobbin to ensure good isolation and not that good of a quality, because it is poor quality the losses are high, but by putting a switch activated by the plug being pushed in, and switching off the transformer when not in use the use of a poor quality transformer does not really matter, leaving items plugged in, even if only the lead and no shaver on the end of the lead can cause the units to over heat.
You can get rechargeable shavers and these have a shaver plug so you can still use them when flat in the bathroom, you have the option of using direct. However charging them in the bathroom is not recommended, in the main as all too easy for them to end up in the sink, as to why a shaver plug was ever fitted to a tooth brush I don't know, likely not really a shaver plug BS 4573 plug
is very similar to the CEE 7/16 Alternative II "Europlug" (Type C)
but not quite the same. The wikipedia page linked to Shows the plugs. Although rechargeable toothbrushes are in the main a very low power, the non rechargeable tooth cleaner is often higher power the UK shaver socket can deliver,
so in the main when sold in the UK they are rechargeable type. There is really no difference between plugging in a toothbrush or a shaver, if used in the bathroom plugged in and below the 20 VA maximum the socket can provide there is not a problem, but once you have finished shaving or cleaning teeth it should be unplugged, it should not be re-charged in the bathroom simply as there is a chance it could fall into water without the user realising.
I would not expect a shock problem should a shaver or tooth cleaner be dropped into the sink, the current would flow local within the device, however what current would run within the unit and as to if when retrieved if still plugged in it would give you a shock I don't know, I would assume a shaver socket would auto disconnect with excessive load, but never tried it, as to the heat both the item in the water and the socket that is also something I don't know. But if the socket delivers 20 VA for an extended time it would likely overheat.
I have never seen a mains powered electric toothbrush, I am sure there is one, but I have not seen one, I have seen many mains powered shavers. The big question is should you recharge any battery operated device in the bathroom? It may be permitted, and you may get shaver sockets with the tooth brush icon. However the question is if it is safe? In my bathroom the answer is no, there is no area the items could be charged without being a chance of being knocked into sink or bath. Clearly some bathrooms will be different, my wife's bedroom is now technically a bathroom as it has a shower in the corner. Having a shaver socket in the bedroom would not be a problem, however there are normal sockets 3 meters from the shower and much easier to use these.
I think we rely on regulations too much, what we need to do is return to common sense, and the is in the main no need to recharge tooth brushes or shavers in the bathroom, the bathroom socket is to allow you to use a shaver or tooth brush directly off a 230/120 volt supply, not to recharge them from that supply.
I've used the shaver socket for years without any trouble. When you have a fault though and the toothbrush stops charging make sure you check if someone has plugged it into the 110v first before ripping apart the wiring to find a fault. Not that I'm talking from experience or anything.
I don't think I've seen 120/110v anywhere but North America and a few places very close to, or strongly influenced by, the US. Almost everywhere else uses voltages in the 220/230/240 range. I believe Japan still has 100v
Some hotels catering to international travellers provide a variety of outlets.
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