FREQUENCY monitoring relays are used in circuits where equipment can be damaged if the voltage exceeds or falls below the normal level. . The relay won't allow the emergency power to be connected until the correct number of cycles is present thus avoiding possible damage to equipment.

45UFR - Detects Under Frequency Condition
45OFR - Detects Over Frequency Condition
B1UFZ - Detects Under Frequency Condition
B1OFZ - Detects Over Frequency Condition