difference between ACB and MCCB

Size and fault tolerance.

MCCs typically stop at around 1200A, some go as high as 1600A and maybe 2000A, but after that you must go to ACBs ACBs typically do not g lower than 1000A. Even if the trip ratings are lower, the basic frame (and therefore cost) is much larger.

In addition, ACBs (called Insulated Case Power Breakers in North America) are designed to be the LAST device to trip in a circuit, because it is usually feeding a very large part of a facility. So where you would coordinate the trip settings of smaller MCCBs to take faults off-line at the lowest level possible, you want the upstream systems to hold in long enough for them to do that. Therefore as your coordination trip times climb backwards up the system, by the time you get to the largest breakers the short circuit trip times may be measured in minutes rather than seconds. The breaker must then be capable of handling the extreme physical magnetic forces involved in handling that much power and trying to interrupt it should it need to. An MCCB would disintegrate long before an ACB would.

If only things were so simple.

Dipping into my Schneider Electric catalogue (who are the largest manufacturer in the world) MCCBs go all the way to 3200 amps @ 85 kA fault current. They take exactly the same control (trip) units, the Micrologic range, as ACBs.

ACBs go from 800amps to 6300amps in the Schneider range, with fault currents 42 to 100 kA.

So there is now a lot of overlap. The big difference is that ACBs are withdrawable, meaning the operating mechanism can be removed completely, for maintenance, safety and upgrade purposes.

The garding goes like this form low order to high order- MCCB. MCB and then ACB. There can be some overlapping depending on the rating and the manufaccturer. In addition MCCB and MCB are fixed but ACB can be drawn out for maintenance and long lasting.

Also ACB on and off is less compared to MCB and MCCB

shivaram

ACB and MCCB both are used for safe making and breaking of electrical system. In principle both serve the same purpose. Earlier concept was to use OCB/ACB / when large load currents were to be handled because they were very robust and could withstand high making and breaking capacity and allow many frequent safe making and breaking .Due to various fire hazards and high coast of maintenance,the use of OCB in LT distribution is getting eliminated .

With technological development a new version of circuit breakers were introduced with moulded case which were initially having only very low load break capacity.They were much cheaper and use and throw type and could be used for only limited number of operations.

With further development in technology the at present day MCCB are designed with larger make and break capacity and fault withstand capacity. As stated by Baby Guinness now MCCB with capacity of 3200 A and breaking capacity of 200KA with all control benefits and dependable quality are available from reputed manufacturers. They need less space in the panel compared to ACB.

ACB has robust enclosure, draw out facility, sliding shutters and capacity to withstand many break and break operation with out damage and proven technology. ACB is still preferred for on load switching operation of 800 Amps and higher systems.