Any wire connector can fail. Screws can loosen with vibration, stress, corrosion, temperature fluctuations, and so on. Push-in (or stab-in) connections can fail for the same reasons. Crimp and pin connections, friction lock, and even soldered joints can fail.
The real question is: what's the most effective wire connection given the operating conditions, parts and installation cost, lifetime expectancy, use case, and so on.
For example, where I work (we manufacture large electric signs), we use all manner of connection methods, and they vary based on these factors. One such factor is serviceability. If a connection is expected to be rugged and reliable but disconnected for safety reasons, we may use screw terminal blocks (which only accept straight wire) or friction lock (which requires use of a screwdriver to release, similar to the push-in connectors on outlets and switches).
Outlets shouldn't require frequent service nor replacement, nor should experience extreme vibration. They also won't (usually) have wide temperature swings, which leaves the stress from normal usage (connecting and disconnecting plugs) which can wiggle things loose over time.
A well-made outlet can have quite good push-in connectors; by "good" I mean they hold solidly for years and aren't likely to fail with normal use. However it can be difficult to tell during installation whether the inserted wire has made good contact or not. Also if one happens to be faulty, it may accept the wire but provide little feedback that the locking mechanism is less than optimal. Pulling on the wire after installation is one way to tell, but if you're using the push-in connectors, are you spending the extra time to do the pull test? Probably not. Screw terminals are easy to visually inspect and determine if solid contact is being made and that the wire is firmly held by the screw or plate.
So while push-in or friction lock connectors can be quick and useful in certain applications (with appropriate confirmation of operation), I recommend wrapping wire around the screw terminals for the most durable connection that's easily verifiable. Usage and vibration can still loosen the screw, and in such case the wire hook will be less likely to separate from the outlet than a straight wire. In all cases where the screw comes loose, however, arc fault can occur, causing problems.
It looks like you need to connect:
- Red to the 'L' terminal (brown)
- Black to the 'N' terminal (blue)
- Green to the earth terminal (green / yellow)
Your wiring is the in old colour scheme and the socket is marked with the new (harmonised) colour scheme.
(By the way, from the photo, the existing wiring looks like it could be the (very) old rubber insulated type of cable, which you may well need to seriously think about replacing).
Best Answer
These are a type of splice that in your case was a safe way to terminate the wire in the bottom photo. In my area they are referred by there brand name Wago. I only use these types of splices or connectors in fluorescent fixtures but they are or can be used as they are a listed connector. Some contractors use them because they are quick to install and take up very little room in the box. I have had to replace many outlets and switches that have this same kind of connection called back stabs so I usually cut them off. If your wire is on the short side they can be crushed to release & save the wire.