Circuit Breaker

ACBs are available in Fixed Version & in Draw-out Version. In fixed version, all the power and control connections are made to respective terminals mounted on the ACB itself. Thus, as and when a maintenance work has to be attended on the ACB, it is necessary to take a total shut down as both the incoming & outgoing power connections as well as the control connections have to be disconnected & removed from the ACB. Then the requisite maintenance activity on the ACB can be completed. And, again, a total shut down has to be taken to resume the power & the control connections on the ACB. Thus, the time taken for maintenance is more. Fixed ACBs are preferred in installations, where prolonged maintenance activity shall not cause a huge financial loss. Examples could be residential complexes, selected commercial complexes, temporary power supply installations, stand-by power supply installations, etc. However, a case to case study has to be undertaken before opting for a fixed ACB.

In draw-out type ACBs, the power and control connections are made to terminals fixed to a cradle. The ACB itself is mounted on a trolley, which can be inserted in or drawn out of the cradle by a suitable tool/handle. There can be male/female terminals for the power connections on the ACB which would mate with their respective female/male terminals in the cradle, when inserted into the service position. A safety shutter assembly would close down when the ACB is drawn out of the SERVICE position, thus preventing any inadvertent access to the live terminals in the cradle. When the ACB in inserted into the SERVICE position, this safety shutter assembly would automatically open and thus enable the ACB power terminals to mate with the cradle power terminals. Similarly, there are Secondary Isolating Contacts (SIC) on the ACB which will slide into the respective control contacts in the cradle, when the ACB is into the TEST/SERVICE position.

A lot of safety interlocks are provided in draw-out ACBs to prevent any mal-operation. Some of these interlocks are:

i) It is not possible to open the access shutter for inserting the trolley push-in/draw-out handle unless the mechanical stop button on the ACB is pressed. Thus, unintentional drawing out when the ACB in ON condition is prevented.

ii) It is not possible to draw-out the ACB when it is in ON condition. A mechanism would trip the ACB when such an attempt is made.

iii) Also, it is not possible to insert into SERVICE position, an ACB which is already in ON condition. Again, a mechanism would trip the ACB when such an attempt is made.

iv) It is not possible to close the ACB in any intermediate position. The ACB must be in either ISOLATED or TEST or SERVICE position, to enable closing of the ACB.

In draw-out ACBs as and when a maintenance activity needs to be taken up on an ACB, it is not necessary to take a shut down or disturb the power connections. Only the ACB is switched OFF and pulled out of the cradle. After completing the maintenance, the ACB can again be inserted into service without disturbing any power connections. Thus, the time taken for maintenance is relatively lesser than that in a fixed ACB. Hence, draw-outs ACBs are preferred in installations where prolonged maintenance would cause undesirable financial and/or production losses. Examples are industrial installations, hospitals, transportation services, etc.

Molded Case Circuit Breaker is referring to the type of construction, all components used in the switching are enclosed in a molded thermoplastic case. It is a lower cost construction method than Air Circuit Breakers or Vacuum Circuit Breakers but has limitations in terms of the ability to withstand fault energy. An Air Circuit Breaker is a "Power Circuit Breaker", generally larger than what you can get in Molded Case (there is overlap) but intended to be one of the FINAL lines of defense against catastrophic failure in a coordinated system.

So what that means is, if there is a fault, you want the fault cleared at the LOWEST possible level, i.e. closest to the fault. That way, it affects the least amount of equipment in your plant and helps facilitate continued operation or at least an orderly shut down. So lets take a look at an example:

1. You have a chiller rated 500A, protected by an MCCB rated 750A.
2. That 750A MCCB is fed from another 750A MCCB in a switchboard that is protected by a 1600A MCCB main.
3. That switchboard is fed from a 1600A ACB in your primary Low Voltage Distribution Switchgear.
4. The Primary Gear has a 5000A Main ACB coming off your main incoming transformer secondary.
   
5. The transformer is capable of delivering 200,000A into a fault.
6. A fault happens in the chiller motor. As the 100kA attempts to pump through the circuit at the speed of light, you want the fault cleared AT THE 750A MCCB, not the 1600A Main of the switchboard, or the 1600A Primary ACB or the 5000A Main ACB, because if it did, it would shut down the ENTIRE plant.
7. So let's say it takes 6 cycles for the 750A breaker to sense the current, unlatch the trip coil, open the contacts and extinguish the arc, stopping the current flow. The impedance of that total circuit may have limited the 200kA down to 60kA at the 750A MCCB. But the 1600A MCCB in the switchboard had to endure maybe 85kA for that 6 cycle time, and the 1600A and 5000A ACBs had to endure the full 200,000A for that same period of time because they are closer to the source (the transformer). So the trips in the ACBs had to be set for a longer period of time than the down stream components in order to give them time to react and act on the fault, which means those devices will be subjected to far greater mechanical stresses than all of the down stream devices. MCCBs just are not built to handle that stress, ACBs are.

Draw-out has to do with the switchgear construction, the breakers inside are no different. Your options are draw-out or fixed mount breakers. If you have fixed mount, and one nreaker gets damaged in a fault, you must shut down the entire line up of switchgear to remove it and fix or replace it. With draw-out, you can withdraw that ONE bad breaker, remove it and put in a replacement all while the entire line up is energized. It's about continuity and the cost of down time.