What is wrong with my power supply?

There are two main approaches to the problem of how to disconnect power when a live wire comes into contact with metalwork attatched to the earthing system. One way is to get the resistance through the fault path and back to the supply very low by having a metalic connection from the earth back to the supply transformer (a TN system). Then when a fault happens a very high current will flow rapidly blowing a fuse (or tripping a MCB).