How do I join two cables outside so that they're safe and kept dry?
Electricity is a funny thing and different applications apply for various situations.
THis is UK/EU/US/Asia/Africa.. its all the same.

If you are running a low voltage cable 0-48Volts DC/AC

then you can use a pvc outdoor joiner (the screw caps have a rubber seal on them to prevent moisture from entering)


If you are running high voltage (110AC/240AC) 

single phase then using a resin joiner is recommended!



You might be tempted to do something like this (this example is of low voltage coax,tv,internet, etc) and is not recommended for outside situations at all! over time water will get in.



That was undergorund application,

you can also use these above ground,

Notice the rubber seals- that is very important!