I may be totally misunderstanding, this has been a long thread.

But if I get this, there's a main breaker that trips while the four breakers it feeds (that turn out to be really two circuits, each with two breakers in series) do not trip.

That seemed strange if they were tripping on an overload. Really the individual breaker should trip first.

Now we know the main breaker is RCD, so it can trip on small unbalanced current. But the two subcircuits are protected by RCDs (obviously added in series later to meet the requirement) so why do they not trip? One or both of those RCDs must be defective.

Kind of nice a power panel in Thailand is neatly labeled in English.