I have a Keston boiler that trips my RCD at the point of firing. I have checked everything that I can see but cannot stop it from tripping. It seems to me that the gas is ignited by a "spark plug" which has one lead to the top of it and an earth lead at the side. Now, if the spark jumps the gap from the HT lead to earth surely this would trip the RCD. BUT, it has only just started happening.
Any ideas folks? I am getting cold and last time it did it I was away and the freezer melted!

Neville Hobbs, January 2009

Thanks for your replies. I discovered the fault in the end. It turned out that two things were happening at the same time as the RCD was tripping ie the gas valve was opening and the ignition spark was sparking. Having fully checked the spark lead was not visually tracking I looked at the gas valve which is situated at the top of the boiler just below the cold air intake. During this cold period, water was condensing in the short plastic air intake pipe and dripping onto a plastic cover over the electrically operated gas valve. This cover is designed to keep the connections dry but SOMEONE had put the cover back around the wrong way leaving a screw hole in the cover directly over the terminal which was now wet and shorting to earth. I dried the terminal and replaced the cover the right way around and all is well.
I went abroad in early December to find that the RCD had tripped whilst away melting my freezer- we had a very cold spell then too so that explains why.
Cold damp outside air condenses slightly in the air intake pipe causing the problem on both occassions.
Thanks for your replies guys. I hope that this information will help someone else later.

Neville Hobbs, January 2009

disconnect pump and try firing boiler up again, if it doesnt trip theres your fault. If not do same with fan.

tom, January 2009