Difference Between Relay and Release

I'm not completely sure of your question, but I think you are asking what is the difference between a "relay" and a "release" when describing a protective function in a piece of distribution equipment?

If so, a relay would be a stand alone device that monitors for the condition and sends a signal to be acted upon by another device, such as a contactor or circuit breaker. A "release" of the same name is usually referring to the fact that the circuit breaker has the protective function built-in to it.

So for example, a circuit breaker may have an "Under Voltage Release" (UVR) built into it. That means if the voltage to the coil of that UVR is eve below the hold-in threshold, it lets go and activates the trip mechanism of the breaker. But an Under Voltage Relay (which may also be abbreviated UVR) would be a separate relay monitoring for a specific threshold of line voltage and taking action based on programmed parameters. Those actions however may not necessarily involve tripping the breaker, it may just send an alarm to a SCADA system.

The same could be said for an Over Current Relay, although the concept of an Over Current Release would be inherent in any circuit breaker, so I have never actually seen that listed separately.

JRaef is quite right. Here is the definition from IEC 60947-1:

Release is something which release your CB to operate or like a mechanical interlock.

Relay is a protective device and it comes into the picture only after the CB closed.

Relay:

Mounted external to the breaker

Needs shunt trip coil on the breaker

Needs trip supply

Needs interconnecting wiring

Relay operationg time is more tahn a release

Release:

Integral part of the breaker

Direct acting (does not need a shunt trip coil on the breaker)

No need of trip supply

No need of inter connecting wiring

Operating time less than that of relay

For UV does UV release auto close upon resumption of voltage?