What is the difference between relay and circuit breaker?

Fundamental difference is that Relay is a sensing device where as Circuit Breaker is a fault interrupting equipment
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Relay is a protective device which senses the faulty conditions in a power system line and initiates the operation of circuit breaker to isolate the faulty part from the rest of the healthy power system.Examples of relays are Over Current relay,Directional relay,etc..

Circuit Breaker is a device which can

• Make or break circuit under normal conditions either manually or through remote control.
• Break circuit automatically under faulty conditions.
• Make or break circuit under faulty conditions either manually or through remote control.

A Circuit breaker is a protective device, used to interrupt current when a overcurrent fault occurs.
A relay is a switching device, it can be controlled remotely and used to make a small signal control a larger signal (amplification of sorts). It is used to control currents in normal operation.

Relay :- It is a sensing device which is connected to power circuit and when any parameter of circuit crosses safe limits, it sends signal to the CIRCUIT BREAKER to break the circuit.

Circuit Breaker :- A part of switchgear which is connected in circuit and it opens the circuit ( or you can say break the circuit) on receiving signals from RELAYs.

Therefore relays do only the sensing part and makes the circuit breaker open its contacts to break the circuit.