The RCD is tripping after few minutes of the light is on.

[biswajit]

The above won't eliminate anything if the fault is between neutral and earth.

I need help. This is first time I have done electrical wiring after my 42 year old Electrical Engineering understanding. So please help how I can resolve the issue. I have now done the test as you advised and found one particular circuit in a MCB is having fault. Like if I put some load in there the RCD trips. I need help to understand how to find the cause. Please be considerate as I am new to this DYI thing. I have also noticed the other two MCB's are causing the fault as I previously mentioned. Like RCD trips after few minutes of light switched on. And now I found that those two circuits MAY NOT be faulty at all. I am sauying this because I have also tested with just RCD is on and all three MCB's are off. And still my RCD trips after few minutes. That suggests my incoming power supply wire may have faults. How can I find what is causing this RCD trip while all MCB's are OFF.

 

[biswajit]

RCDs can trip on N to E faults and the OP being DIY doesn't magically prevent this from happening.

Please help me to solve my problem. like I may not understand your abbreviation so please be patient and kind with me.

 

[mainline]

Please help me to solve my problem. like I may not understand your abbreviation so please be patient and kind with me.

You need to do an IR (insulation resistance) test of each circuit.

 

[UNG]

I need help. I am sauying this because I have also tested with just RCD is on and all three MCB's are off. And still my RCD trips after few minutes. That suggests my incoming power supply wire may have faults. How can I find what is causing this RCD trip while all MCB's are OFF.

You obviously have a neutral earth fault which without the correct test kit will be difficult to find
As a DIY your best option would to completely discoonect all the circuits and reconnect them one at a time and switch on the RCD after connecting each one this should find the problem circuit

 

[westward10]

Have you notified Building Control of these works?