Screw connector blocks
The main weakness of these is that they can sometimes loosen in time, creating a fire risk. This makes them forbidden for inaccessible locations, such as under tiling etc. Also many screw connectors are found that haven't been tightened up enough, creating reliability and fire risks. (Connector failure due to overtightening are far less common.)
There's a simple method to reduce this risk to some extent. The stripped conductor of both cables extends under both screws, so both cables are screwed down at 2 points. This can be done with the cables entering at opposite sides or both on the same side. Now a bad connection only occurs if both screws loosen. The bare cable ends aren't bent over. Make sure the bare ends don't stick out on the far side. Its generally only possible to get 2 cables into a connector used this way.
Wrapping screw blocks with insulting tape is not adequate. It provides no fire protection, and tends to fall off. The wiring ]regulations require a self extinguishing container for all fixed wiring junctions. Taped blocks are ok inside such containers, such as a junction box, pattress box or ceiling rose, but position them so they won't short to anything if the tape falls off.
Flex ends need twisting and folding over before use, and should not go in the same hole as cable, or the flex connection can be weak and electrically inadequate. Lone cables going into a connector terminal are bent double to ensure good grip.
Very popular in fixed wiring, these are screw connections (usually) in a self extinguishing enclosure. Rotating the lid exposes a varying number of entry holes.
The connection screws often screw into a split hollow metal insert, which is in turn restrained by the plastic housing around it. If screwed too tight the hollow metal part can split apart, creating an unsafe connection. Some junction boxes use a better terminal design.
The wiring regulations require junction boxes to be fixed in place, and wires entering them to either be clipped, or secured in the box with a cordgrip. The lack of cord grip on most junction boxes makes them unsuitable for wires that aren't clipped in place. Plenty of fixed wiring exists that fails to comply on this point.
A backbox and blanking plate plus a screw connector strip can be used like a junction box with lots of ways. Like junction boxes they normally lack a cord grip, though backboxes with cordgrips are available. Useful as a central heating wiring centre, and for complex flexible house wiring schemes.
After a long history of use inside lighting fittings, these are becoming more popular for higher current cable junctions now. These have been widely used for years in America, and don't have a great record for safety or reliability at the current levels used by sockets. They're quicker to use than choc blocks, are permitted in inaccessible locations, and a tool is only needed when releasing the wires.
The cable is clamped on these by pushing a lever down. Levers connectors are handy for temporary connections. Stranded wire works in lever connectors. If more than one wire is used in each entrance hole, they need to be identically sized, and check both are properly secured.
A variant on these just uses a pushbutton to release the clamping force on the cable. They're not often seen in house wiring.
Soldered joints are permitted in accessible and inaccessible locations. Practically you need to provide some sort of strain relief for the joint, as solder is very weak. The cable ends should always be twisted together before soldering, otherwise even very small movements during assembly can cause the bond to fracture. Twist at least a couple of full turns, otherwise some movement can still occur, and the joint break.
Soldering isn't a popular option for fixed wiring because of these issues, and because soldering takes a little equipment and skill. There's also no readily available junction box designed for soldering. Its a good enough option if you've got the skill to do the job properly.
Soldered wires should not be put into screw connections, the 2 aren't a safe combination. The first problem is that solder is soft and tends to creep under screw pressure, leaving a weakly clamped joint after a while. The other is that the soldering of stranded wire makes strands more likely to break. Soldering screwed wires increases the odds of fault & fire.
Crimped flex ends can be safely used in mains plugs and other screw connectors. Ideally the flex should be twisted before the bootlace ferrule is applied, to avoid concentrating stresses on a small number of strands.
A small number of old mains plugs use screw connections where the flex goes under the screw head and around the screw shank. There is a tendency for these screws to push strands out when tightened, and too often most of the strands end up not connecting. Make the stripped flex end long enough to go nearly all the way round the screw shank, and twist the strands fairly tightly to minimise strand pushout. A small washer under the screw head also minimises the problem.
Small diameter flexes and some cordgrip designs result in the plug cordgrip failing to grip the flex. This can be remedied by putting the flex 360 degrees round the cordgrip before tightening, like so:
__ wire follows a circular path /__\ wrapping round the cordgrip ______\__/_____ going under it twice
If using 13A plugs & sockets outdoors, its wise to use unbreakable ones, shatterable indoor sockets are usually soon broken.
Round pin plugs
Available in 2A, 5A and 15A sizes, these historic plugs are still used on lighting circuits to enable a wallswitch to control plug-in lamps.
These oval connectors are a good way to extend flex. They incorporate screw connections & cordgrips in a robust insulating case.
In-line plug & socket
They come in 2 and 3 pin versions. 2 pin connectors must not be used on 3 core appliances, on 2 core supply leads, or with appliances that require a polarised supply. There are also different pin configurations that have seen popular use over the years, so not all connectors are compatible with others. Many such connectors are rated at less than 13A, and should not be overloaded. A 5A connector is good for a 1kW motor.
A standard 13A plug and socket is more versatile, enabling a (3 core only) lead to be used on any appliance. But its bulkier.
Resin filled capsule
If a mains cable needs to be joined underground, the joint must be totally waterproof long term. The only realistic way to achieve this is to place the connection in a plastic housing, and fill it with resin. Suitable resin filled connector kits are available. Underground electricity distribution uses the same connectors, albeit in a larger size.
These are little plastic or ceramic hat-like items with a tapered hollow coarse screw thread on the inside. The cables are pushed in and the wirenut rotated to screw it onto the cables. As it screws up, the cables are pressed together.
These have been illegal to install on fixed mains wiring since the 1950s for reasons of reliability and safety. They're seldom seen here now (but the US still uses them). A crimped version of these is used inside some appliances, and is much safer than the original screw-on type.
Twist & tape
Much used in the past, twisting wires together and taping them is a recipe for fire. The UK has around 69,000 domestic fires a year, killing over 400 people. One problem stems from the fact that such joints aren't gas tight, so the copper oxidises in time, creating resistance in the joint. This causes heating, causing more oxidation and resistance, causing more heating and fire.
A variant on this used in the 1930s was the tobacco tin junction box. The wires inside were simply twisted together. Filling the tin with bitumen solved the wire oxidation issue and provided a cordgrip. However twisted connections have much higher resistance than the rest of the wire, so the junction boxes were prone to getting hot enough to melt the flammable bitumen and expose the joint to oxidation. These are rarely seen today, and if spotted indicate a seriously substandard installation.