Difference Between RCBO and {RCD + MCB in Series Connection}

Space.

First cost.

Standards.

Those are the differences.

(1) Same space (4 Modules)

(2) RCBO very expensive

(3) Same Standards (Specifications) and same Ref. number and Make.

2) Your problem, not mine.

3) Installation standards, not equipment standards.

You are right! It is my problem to reduce cost for my company and country.

The Electricity Provider in our country allows both systems.

But where is the difference electrically?

It depends. One RCCD can feed several MCBs. In a retrofit where other circuits are not affected, it may be better to swap a MCB for a RCBO. Only the individual doing the design can work that out.

What does the building code of your country permit?

That should be your guiding principal, not cost.

the building and land use permit guide - Ministry of Local ...

Dealing with Construction Permits in Mauritius - Doing ...

Functionally, RCD + MCB = RCBO. Note that you probably only need a 1P MCB unless your electrical wiring rules require the MCB to be 2P.

As Crabtree says, it is largely a question of cost. You can use a 40A or 63A-rated 30mA RCD to protect several circuits, ie RCD + 3 MCBs, and this is usually quite a bit less costly than 3 x RCBOs. Here in NZ, until they tightened up on the rules, it was common to see entire houses wired with just 1 or 2 RCDs and maybe 10-15 MCBs. Now we can only protect 3 circuits per RCD.

Personally, I will be using 1 x RCBO per circuit when I replace my home switchboard. Having just one circuit die from an earth-leakage trip instead of half the house seems well worth the extra cost IMHO.

RCBO's will only match RCD + MCB's if the RCBO's are DP or SPSN, many of them are SP only.
A SP RCBO will sense a N→E fault but only switch the live. Not ideal especially if on a TT or PME (CNE) system.

I've got two SP RCBO's in the board in my flat, as soon as I get around to it they will be replaced by a RCD and two MCB's.