Household electrical problems

Hi,
I decided to replaced a FCU with a new one, which i thought was going to be an easy job. However, it was not to be! I have two cable coming into the metal box. Both contain the usual earth, neutral and live. My issue is that both of the live wires are constantly carrying 240v, having tested them with my volt meter. As a result i am unsure which is the supply cable and which is the load cable to reconnect to my new FCU?
As a side question does it need a FCU or could i just connect the two cables together?
If anyone can give me some advice to overcome this issue i would be grateful