How can I stop main switch turning itself off when I turn on gas heater?

Not what you were hoping for and in other two sections explain which switch.

Difficult to know where to submit this question.
And you don't explain which switch? is it one on the heater? or is it the mains Elecs fuse box keeps tripping?.

1. Add by copy and paste to HEATING home heating systems.
2. Also to DIY. Switches and sockets.
WHY!
ON 1. A central heating engineer may know why.
ON 2. An electrician may have had to deal with the same type of problem.
This way you have three chances of a reply.

BOB
October 2010