Undervoltage Relays

one coil can detect under voltage of connected phase two only ,for monitoring all three phase you need 2 coil,

UV relays are supposed to protect equipment, most often motors, from the dangerous effects of continuous under voltage conditions. But sometimes, they are used only to detect a complete power outage. So if that is the purpose, sensing only 2 of 3 phases may be sufficient. Not advisable, but sufficient if one of your other criteria involves being too cheap to pay a little extra for a 3 phase monitor.

But are you referring to UV coils in a circuit breaker? If that's the case, they are used not as UV monitors but as control devices that respond to a REMOVAL of control power rather than a APPLICATION of control power (as a Shunt Trip does).

Economy boss! In a three phase system one phase undervoltage is a very rare phenomenon. So, if you can monitor the voltage of any two lines (one phase) that would be sufficient.

However, if single line to neutral undervoltage is anticipated - may be due to improper load distribution in a three-phase 4-wire system - you can always go for three single phase undervoltage relays.

The UV coils are connected phase to phase, and sense the potential difference between the phase conductors. Consider a single UV coil connected between phases A & B. A low voltage in phase A will lower the voltage across the coil, causing it to de-energize. Likewise, a low voltage in phase B will also lower the voltage across the coil, also dropping out the coil. That single coil monitors both phases.

Connecting 2 coils will provide complete phase undervoltage protection, as long as each phase is connected to at least one coil. Two coils provide 4 points of connection, so one phase will be common to both coils.