Hi sifus

My new house RCCB 63A 0.1A tripping randomly.
Did the usual diagnosis and trace the problem to one line supplying the front lights.
When the MCB (6A) for this in ON, cannot reset the RCCB, trips immediately.
When I OFF this MCB, then only can reset the RCCB.
After sometime however can reset back with that MCB ON.

The problem is transient, sekejap ada, sekejap OK.

The faulty line has a timer switch, which I thought was causing the problem, so call back the wireman.

He checked and found that the one line is carrying 33 light points (including 16 T5s and 10 mLED downlights) and 1 fan.
But I very seldom on everything. usually max 7-10 lights only.

He thinks it overload, and 'partition' some load over to the nearby 20A MCB. Don't know what it means.

But the RCCB still trips randomly. Although now can reset.

Any ideas how to approach? I suspect some wire are overheating, that why after a while cooling down the problem goes away, only to happen again later.