Hi all. I am new to the site so please bear with me. I am not a registered electrician however my background is armed forces electrical technician and then medium voltage supply and terminations bu tnot a jointer so I am electrically savvy in most aspects. I am now retired. Got up the other morning and found the rcd protecting two circuits had tripped. All the other house circuits are protected by another rcd which was fine. One circuit was for all the house lighting and the other was for a couple of sockets in a cupboard and one of these sockets was used for the microwave. Reset the RCD which made but as soon as I reset the light mcb rcd tripped. Left the two mcb's tripped and reset the rcd which held for about 4 mins and then tripped. As the rcd tripped with mcb's off I assumed a dicky rcd or a neutral to earth fault. Rcd would not reset straight away with mcb,s tripped but did eventually after a period of time. Remade the sockets mcb and rcd held but as soon as I tried to make the lights mcb the rcd tripped immediately. So deduced a neutral - earth fault on the light circuit. No work has been carried out to suggest a possible area of fault however I have had some problems with one of the led lights in the bathroom. I decide therefore to try and chase down the problem by going to the neutral connections for the lights which were in the loft and disconnected the neutral leads for the lights. RCD still tripping so decided to remove the neutrals for the upstairs lights and the bathroom fan extractor. Still tripping so decided the go into the CU and disconnected the neutral for the light circuit from the neutral bar and fully expected the rcd to hold. No such luck so disconnected the earth from the earth bar and as expected the rcd held even with the other circuit mcb made. Connected the neutral and left the earth off and tried every appliance on the circuit and nothing tripped the rcd however one of the security lights didn't work so i thought the fault was internal to this so disconnected the supply to this, reconnected the earth on the cu and tried the rcd again but still tripping. I am now a bit stumped as to where to look and why the fault is apparently just on the earth. Any suggestions would be welcome.