Contactor: it is to use on the main circuit system, control of a single device, such as an electric motor, is also a kind of switch, but it can be controlled, such as relay control, suitable for frequent movements, but average amount is small, no protection function, can't own tripping when accident;
Circuit breaker: is also used in main circuit on the system, can control a single device, can control multiple devices, but is not suitable for frequent action, can make a large capacity, have protection function, when the accident to their trip;
Relay: it is an auxiliary power circuit. It cannot pass large current. It is a typical "small" thing to control "big" things.
For example, a line with a small motor and a motor drive five minutes to five minutes; The circuit breaker to export in line installation, problems arise when the motor trip in time, the motor power series with the entrance of contactor, with its actual control motor drive and park, with 5 minutes delay circuit of relay contactor to control.
That is to say, relay, contactor should say it's the same type of device, but because of different current, has been applied in different lines, they are achieved through the coil and small current to control large current. The circuit breaker is usually the automatic switch, which is a switch device which has the effect of breaking the load current.
Relay with one switch handle on the following three wiring pile, relay and contactor principle is the same, just contact working current is larger voltage is higher, is generally used in high voltage; The relays are small and cannot drag large loads directly; 3 is relatively easy to distinguish the relay only up and down three connection among pile and a handle, relay and contactor, contactor generally larger than the relay, contactor is wiring pile up and down three other smaller, is as big as relay, and relay is likely to be transparent, contactor didn't; If it's a big old contactor, it might have a ceramic cap on it.
The contactor only ACTS as a switch but it can be controlled frequently.
Circuit breaker is arcing overload, short circuit under-voltage protection switch, such as commonly used in place of do not frequently switch can provide protection for some circuits, and contactor are generally current control, and circuit breaker is a manual control