Which protective relay functions when a given voltage value falls below a predetermined value?

The protective relay that activates when a voltage dips below a specified threshold is known as the undervoltage relay, often identified by the designation 27. This type of relay is crucial for protecting equipment from damage that can result from insufficient voltage levels, which may lead to overheating or operational failures in motors, transformers, and other electrical apparatus.

When the voltage falls below the predetermined set point, the relay produces an output signal, which can activate alarms, initiate circuit disconnection, or trigger other protective measures. This ensures that the equipment remains safe and prevents potential failure due to under-voltage conditions.

Understanding the function of different relays is essential for those preparing for the NETA Level 1 Certification, as each relay type serves a specific purpose in the overall protection scheme of electrical systems. The other options represent different types of protective relays: for instance, 24 typically relates to a specific relay classification not focused solely on undervoltage, while 50 is a relay used for overcurrent protection, and 59 is an overvoltage relay. Each of these has distinct operational criteria and applications separate from that of the undervoltage relay.