FC Contactor and Breaker

Let's say contactors are faster (and more expensive) than breakers. The order of magnitude would be a contactor switching every second and a breaker switching every minute.

For Low Voltage applications you must have a breaker for protection and a contactor for switching. You turn on once a day the machine (breaker) and you start it and stop it 10 times more (contactor).

The control circuits that use a separate relay are for amplifying the control signal, due to high impedance or low power of the source signal.

I will assume that you are referring to a vacuum fused contactor.

A circuit breaker is intended for short circuit protection only, a contactor would still be required to provide motor starting facilities and overload protection which it would do via attached or separate overload relay.

Some circuit breakers can be also opened remotely by overload relays, but they generally do not have the ability to withstand continuous making and breaking (typically a few tens of thousands of operations and much fewer for S/Cct conditions) whereas a fused vacuum contactor can withstand hundreds of thousands of operations even under S/cct conditions.

Circuit breakers can be operated either manually or, as mentioned above, can be opened electrically. Contactors are only operated electrically.

Fused contactors are generally vacuum type where a bellows is used to close the main contacts. The bellows tries to close the contacts under atmospheric pressure, but is held in the open position by a mechanism consisting of springs and a rocker arm. A separate control relay is used to energise an internal coil to move the arm and thus allow the bellows to close the contactor.

When the relay is off, the coil is de-energised and the rocker mechanism opens the contacts. Power for the controls is often provided by a small transformer in the contactor.

A fused contactor has the advantage of very fast disconnection during high overload or short circuit conditions - a circuit breaker will normally not open in less than 3 cycles under short circuit conditions, a vacuum contactor will do so in 1/4 of a cycle.

The HRC fuses used limit the current to a preset "let through" value melting while the overload current is still rising

A contactor can also provide "No volt" protection which a manual circuit breaker generally can't do

Vacuum contactors are ideally suited for starting large motors or switching transformers, and large ones can be cheaper and smaller than equivalent capacity circuit breakers.

Under short circuit conditions, the fuses on the contactor melt before the contacts open thereby protecting the contacts from severe damage - simply replace the fuses and all is ready to go again. By contrast, a circuit breaker carries considerably more current for a longer period before it opens, and even with arc quench chambers, some burning takes place inside the mechanism.

Hope this helps.

..." Medium-voltage circuit breakers are favored when: Typical loads include transformers, capacitors, larger motors, generators or distribution feeders Ratings required exceed those of vacuum contactors (400 A or 720 A at up to 7.2 kV) Continuous load current is high (e.g., larger transformers, larger motors) Switching is not very frequent (e.g., weekly or monthly); high endurance (1,000s of operations) is satisfactory Process continuity is critical (e.g., no time for fuse replacement) Reduced-voltage (RV) starting is not needed (RV starting complicates switchgear bus arrangements).

Medium-voltage NEMA Class E2 controllers (fused contactors) are favored when: Typical loads include motors or smaller transformers Continuous load current is low or moderate (e.g., smaller motors or transformers) Switching is very frequent (e.g., daily or several times per day); very high endurance (100,000s of operations) is needed Process continuity is compatible with fuse replacement time Reduced-voltage starting is needed to reduce starting duty (and voltage fluctuation) on system."...

"Historically, circuit breakers have been used for mediumvoltage motors in certain industries, especially in utility generating stations. As these stations have aged, and station operation has changed from base-load to peaking service, many of these motor-starting circuit breakers have experienced total operations well in excess of the endurance required by the ANSI/IEEE standards. As a result, these applications have had higher maintenance costs than if medium-voltage fused contactors had been used originally. In contrast, users in the process industries have long favored the use of fused contactors for such applications, and have enjoyed long service with lower maintenance costs. When applied properly, both medium-voltage circuit breakers and medium-voltage fused contactors should provide decades of reliable service. Applied incorrectly, either can lead to major headaches."

https://w3.usa.siemens.com/us/internet-dms/btlv/PowerDistributionComm/PowerDistribution/docs_MV/TechTopics/ANSI_MV_TechTopics42_EN.pdf