Load Distribution

What is UAE short for (is it possibly United Arab Emirates, the location of this installation or design)?

I am guessing MDB is Main distribution Board and SMDB is Sub-Main Distribution Board (an internet search produced a very dodgy acronym for SMDB).

At any over load or fault condition on the feeding zone of SMDB which breaker will trip first ( SMDB incomer breaker OR MDB outgoing breaker ).

They will probably both trip (unless perhaps the trip curves are different).

If it is SMDB incomer breaker How it happening ?

The bigger question is why design an electrical system like this with no protection discrimination. This is not the first time a question like this has come up, I still have had no good explanation as to why this is done in some cases.

Is it possible to set both breakers to trip at different OC/SC levels?.

Put simply it depends on the circuit breaker functionality. For simple circuit breakers with fixed current curves then all you can do is select circuit breakers with different trip curves in an attempt to get the best protection discrimination for your application. For more complex circuit breakers (or protection relays, etc) the curves may be selectable.

Yes in some cases it is not possible, but generally if you are powering multiple sub-distribution boards you don't want a single fault to cause a power loss to multiple sub-distribution boards (and multiple circuits). The case may be different if you are say only using the circuit breaker to protect the cable.

It is best you follow your local electrical standards and practices as this is all detailed in there, with possible differences (electrical standards can vary from country to country).

Re: The bigger question is why design an electrical system like this with no protection discrimination. This is not the first time a question like this has come up, I still have had no good explanation as to why this is done in some cases.

Well, with respect to molded case circuit breakers and short circuit faults, it is hard to make a selective system. They are selective for overload conditions.

The simple way to make a selective system without a lot of fancy relaying is with fuses, but then you have the problem of replacing blown fuses.

So in many cases (my experience in coal mining), the tradeoff was convenience in resetting a circuit after a fault with MCBs, vs. selectivity with the need to replace fuses after a fault. Usually, the MCBs won out.

As you have pointed out there is no single right answer and practices vary from country to country, industry to industry.

The right way is the protection descrimination should be zoning system. Not just guessing which one will trip first? Of course to where the fault occur such as if the fault occured between MDB and SMDB, the CB at MDB will trip not unless the CB is deffective and will not kick off, then if the fault occured at at SMDB to the load/s downstream then the CB at SMDB will trip. Since your electrical systems are not properly designed with protection descrimination, in case of failure to trip both your MDB and SMDB CB's during the system fault,, then its time to call the fire department.

Improper discrimination

Already stated here for rules and regs...but in addition, why would you want to lose the load on all other SMDBs connected to the MDB by rating them the same (when the MDB trips)?

Not a good idea.