Differentiate:-Relay, Contactor and Circuit Breaker

                  Relay and contactor have the same function, to close a circuit with a logic signal (applied to the coil). Circuit Breaker has another main function, to protect a circuit for over currents. In most cases, the breakers are operated manually. The relays and contactors are not operated manually. Relays are used in circuits to close a single circuit, contactors to close a three phases circuit (motors).

                  Relays and contactors are control elements, i.e. you control the current (On/Off) in the attached device with a control signal. Normally, relays are for low power applications (voltage x current) while contactors are for higher voltages and currents. You might see components called "Contactor Relays" which in fact are contactors for control applications. 

                  Circuit Breakers are protection elements. They protect the downstream connected devices from overload current, short circuit current, leakage current.. There are also several types of Circuit Breakers: Miniature CB (which you have at your appartment in the power distribution board. There are also Molded Case CB's and Air Circuit Breakers.. and so on to Medium and High voltage breakers.

Most circuit breakers are thermal devices that when excess current run through them they heat up and trip (open up).They can also be used as a switch to open a circuit.A contactor is a electric relay that is essentially a solenoid and a pair of electrical contacts. The solenoid is activated and it closes the contacts 

Relay is the device acttached to any circuit which makes it work automatically.it has less operating voltage but in contactor it works on the principle of cantact making that either single phase voltage or 110v ac supply , the plunger makes the other three phase contact and thus operates on some high operating voltage

Contactor is a switching device. Unless & until you give command it wont operate. Contactor is not a sensing device. on the other hand Relay is the device which sense the abnormality & give command accordingly. Relay differentiate normal & abnormal condition which contactor can not.In short if only contactor is connected in circuit.. in case of abnormal like OV or OC it will burn.. but if it is connected in coordination with relay.. relay will sense the fault & give signal or command to contactor to trip  

Relays and Contactors are electromagnetic switches. The only difference is that relays are usually used for low voltage applications while contactors are used for higher voltage application. Contactors usually have Overload protection.

Some examples :-

Relays are use as electromagnetic switches to control certain parts of a circuit (low voltage application). A better example would be such as if you want your circuit to sound a horn when a certain abnormality is happening at a certain period of time, then depending how you set up your circuit, a specific relay will either open or close and you horn will start making the sound. -Contactors can be used for example, to start motors or a number of motors that require a higher voltage in order for the motor or motors start working. In the example that I just gave, contactors work as starters. 

The first question should be the LOAD conditions to determine which component to use - relay, contactor or breaker - and the function of the product. 

-Relays are somewhat limited to the load characteristics. 
-Contactors in simple terms are just BIG relays able to handle larger loads. 
-Breakers are a safety device, when load conditions exceed contact capability and the breaker "opens". 

                                                                    RELAY

                  

                      4 Channel Relay Card 

                                Contactors

 
                                              Siemens Contactors

                           
                          Circuit Breakers