In this pursuit, I would like to begin with the understanding of the different word "TRIP or RELEASE", which is in fact a combined "ELECTROMECHANICAL MODULE or MECHANISM that comprises of a "mechanical latch" and a "Electrical Relay / Release" which can OPEN THE SWITCHING DEVICE e.g. a circuit breaker, by electrical "energisation" or "de-energisation", according to their characteristics.
Now coming back to our main subject, i.e.; "SHUNT RELEASE" & "UNDER_VOLTAGE RELEASE", How they DIFFER IN THEIR FEATURES & CHARACTERISTICS and, what are the VARIATION IN APPLICATIONS & USAGE.
Both have got their individual characteristics and are used according to the specific circuit requirements, that I describe here below:
This Electrical Device operates by momentary energisation and opens the "Switching Device / Circuit Breaker" by activating the Mechanical Latch. Since, this require energisation hence you NEED A CONTROL SUPPLY either of "it's own inherent" or an "Independent" one, and the other important thing, that this "Release is wired through its NO (Normally open Contact" that ensures supply cessation immediately after OPENING / TRIPPING the "Switching Device. SHUNT Release is generally used for "REMOTE OPENING OF THE SWITCHING DEVICE" but, you can also use it for LOCAL OPENING / TRIPPING of the SWITCHING DEVICE through local Push Button OR through any number of parallel Control Signals. Further this is a MOMENTARY DUTY type Release whose Power supply needs to be interrupted after opening / Tripping the "Switching Device / Circuit Breaker".
NOTE: SHUNT RELEASE does not provide the feature of TRIPPING THE "Switching Device / Circuit Breaker" upon Power supply failure OR Voltage dips
UNDER VOLTAGE RELEASE:
This Electrical Device always NEED ENERGISATION to initially "Closing the SWITCHING DEVICE / Circuit Breaker ", OR to KEEP IT IN CLOSED condition. The SWITCHING DEVICE immediately TRIPS / SWITCHES "OFF" as soon as the Control supply is interrupted through "Local Push Button" or through interlocking Auxiliary Contacts that are wired in Series of its circuit path. Further, this is CONSTANT DUTY Type Release.
1. UNDER VOLTAGE RELEASE offers the feature of TRIPPING THE "Switching Device / Circuit Breaker " upon Power supply failure OR Voltage dips below its "Drop-out Voltage Design", which is generally 80 % of its rated Voltage. This is the reason "Under-voltage Release" is also used as an "UNDER-VOLTAGE RELAY" and most of the "Under-voltage sensitive Circuits", like Motor Feeders are incorporated with this "Release".
2. To avoid nuisance / undesired Tripping of the "Switching Device / Circuit Breaker " on the Voltage Dips OR needing to keep an operator to Re-closing same, a Different Variation of "Under-voltage Release" is available that is fitted with a Capacitor element across its terminals, which prevents the "De-energisation" of "Under-voltage Release" and "Tripping / Opening" of the "Switching Device", and the energy stored in the Capacitive unit is used while the Circuit is deprived of OR drops below the operating Control Voltage range.