I am going to take a guess that this is not a plc question.
If it is, then it is related to a plc that I haven't seen, or don't remember anymore.
Now, on the other hand, if it general electricity...
MCB normally stands for a "Molded Case Breaker. Typically a large frame circuit breaker in a large molded (plastic) case. General 100-1200 amps, though I believe I have seen them up to 2000 or 3000 amps. I like to use some of GE's, as the case for 15-100 amps is the same size, 4.88 x 8.00 I think. Mount and look nice in panels
MCCB is often "Molded Case Circuit Breaker".
ELCB = "Earth Leakage Circuit Breaker", or interuptor. Similar (not neccessarily identical to a GFCI.
Is this the type of info you're looking for?
regards.....casey