When were shutters introduced on socket outlets in your country?
In Sweden they were introduced in the 1960:s. In the beginning the designs were akward. One design required you to slide the plug (applying equal pressure to the pins) from the top of the socket down to the holes. The shutters probably jammed the day after they were installed. (We had these in the house where I grew up. I was taught never to remove a plug from a socket, since it was almost impossible to get them back in...)
Another solution featured a turnable cover. You inserted the plug a few millimeters, turned it 45° and then inserted the plug completly.
Today sockets utilize a very simple and easy to use solution: In order to open the gates you have to push two pins simultaneously into the socket.
Hi C-H BS 1363 13A sockets have always had safety shutters, it was part of the standard, I think 13A sockets first appeared around 1948. The older BS 546 sockets are usually found with shutters too, I don't know if this was required by BS, as I've seen some really old installations (1930's)with unshuttered 5 & 15A sockets. I think the Brits may have been first with safety shutters! Also British sockets almost always incorperate a switch, so that you do not have to unplug appliances on load. BS = British Standard.
I cannot be entirely sure, but I think that it was around 1990, that I saw the first safety-shuttered outlets. Maybe Old Appy has a better idea?. But one thing is for sure, if they actually had safety-shutters in plug-in multi power boards, it would have saved the life of an infant over here,a couple of years ago, the child stuck an Auto key into the Phase aperture, this type of power board (like most of them these days) was imported from China.
I don't remember exactly when, but we also had the turntable model and now all sockets HAVE to have shutters. These new models exist already from around 1980's. Maybe this was introduced in the major code reshuffle in 1981.
Here in the USA, there is one manufacturer that I know of that has "kiddie-resistant" sockets with spring loaded shutters behind the two plug holes. Apparently it works on the principle of equal pressure from the plug pins. I don't know, never bought one.
If you don't want to go that route, however, there are wallcovers with spring loaded turnable covers that spring shut when you pull the plug out. These go in place of the standard cover.
I've seen a shuttered schuko receptacle advertised in an early 1960ies magazine, but never actually saw one. There are child-proof covers available for sale. They're stuck into the receptacle and usually have an adhesive strip (remember, Schuko receptacles are recessed, so you just put a plastic disc into the opening). There are 3 systems I know: Slide down, turn 90 degrees (both as C-H described, just the slide ones were a little bit better). They have a rubber band as a spring which naturally ages by time. We've had many of them around when my brothers were young. The worst (and probably oldest) type were just plain covers which were inserted and required a special key to remove them before a plug could be inserted. I've seen many of these covers but not a single key. They always get lost, so I got lotsa training removing these covers with screwdrivers.
As David has pointed out, British 13A sockets have had shutters since they were introduced in the late 1940s.
I'm not sure when shutters were first fitted to BS546 (round pin) sockets either. Could it have been earlier? It certainly isn't a bad idea with the 15A types, as the pins and socket apertures on these devices are really large, and could easily accept a metallic object the diameter of, say, a pencil.
For many years (decades) the shutter mechanism was operated by a pin which extends into the top (earth) contact so that as a plug is inserted the shutter is pushed downward to open the phase & neutral apertures (a few types operated in a slightly different way, but still operated from the earth pin). I'm sure David will confirm that in the U.K. we soon master the knack of opening the shutter with meter probes!
Some more modern types have a rotating shutter pivoted between the phase & neutral and need equal pressure on those two pins before it will open. Still possible to get in with probes, but a little trickier.
I believe that some manufacturers (MK?) now have 3-way shutters which won't open without equal pressure at all three points. I've not encountered one of these yet -- Looks like probing will get more difficult though!
The worst (and probably oldest) type were just plain covers which were inserted and required a special key to remove them before a plug could be inserted. I've seen many of these covers but not a single key. They always get lost, so I got lotsa training removing these covers with screwdrivers.
Here you can buy something like this as add-ons for old sockets. Some time ago I bought one and blocked one of the sockets in the lunch-room kitchen and then threw away the key. Thing is that there are three sockets, three 2 KW kettles and lot's of people but only one 10A fuse. I got tired of replacing the fuse... (20A is fine on a 10A fuse, since a fuse is rather forgiving.)