whilst removing A LIGHT SWITCH with a tester screwdriver i was amazed to see the indicater light come on..i did a test on the earth screw and found 40 volts going thru it..i didnt have time to check current and was wandering why this didnt trip the fuse board.the fault is only present on this light fitting and not on others.can this be a fault in the general construction of the lamp or do i have a problem with a phase to earth some where.