Son has moved to 50s house, there was only one surface single sockets in each of the three bedrooms, there is a new consumer unit fitted, he has now installed three double sockets in each b/room on the same ring main, today switched on the 32a rcd and he tested each socket with a "test plug" confirming that all wiring was correctly installed, he then plugged in a radio to test, as soon as he switched on, the main rcd tripped, he has checked all the sockets for "Shorts" but seem ok, when he tested for live on the pos. there was also live on the neutral, the system only trips when you actually switch on the appliance, help please