Overcurrent fault tripped two MCBs

[JosephandAmber]

So customer reporting she plugged in a faulty LED light and it tripped both upstairs and downstairs lighting MCBs indicating that the circuits are connected (the board has no RCDs - yet). Before I go and do a CU change to RCBOs I need to identify the source of this fault. I'm interested to hear how others might approach this scenario in terms of the fault-finding method? Is it just a case of testing continuity between MCB 1 and the fixtures of circuit 2 until you find a leg that shows a connection is present?

 

[Dave OCD]

It's also rare but possible that the lighting was originally wired as a ring then at the first CU upgrade some bright spark decided to put the 2 cables into seperate MCBs without any testing.