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Commonly used terms in the Electrical industry.
reclosing relay
an auxiliary relay that initiates circuit breaker closing in a set sequence following fault clearing. Reclosing relays are typically employed on overhead lines where a high proportion of the faults are temporary.
relay
a device that opens or closes a contact when energized. Relays are most commonly used in power systems, where their function is to detect defective lines or apparatus or other abnormal or dangerous occurrences and to initiate appropriate control action. When the voltage or current in a relay exceeds the specified "pickup" value, the relay contact changes its position and causes an action in the circuit breaker. A decision is made based on the information from the measuring instruments and relayed to the trip coil of the breaker, hence the name "relay." Other relays are used as switches to turn on or off equipment.
residual overcurrent relay
an overcurrent relay that is connected to sense residual current. Residual current is the sum of the three phase currents flowing in a current transformer secondary circuit, and is proportional to the zero sequence current flowing in the primary circuit at that point.
seal-in relay
an auxiliary relay that remains energized through one of its own contacts, which bypasses the initiating circuit until deenergized by some other device.
synchro-check relay
a device used to monitor the frequency and phase angle of the voltages across an open circuit breaker.
Synchro-check relays are commonly used to prevent breaker closing or reclosing on excessive voltage or frequency difference.
synchronizing relay
a relay that monitors the voltage across an open circuit breaker to determine the frequency and phase relationship of the voltage sources on either side of the breaker. Synchronizing relays are used on generator breakers to bring the generator to the system frequency and to match the phase angle between the generator and system prior to closing the breaker.
time overcurrent (TOC) relay
an over-current relay that has intentional, selectable, time delay. The time delay is chosen so that the relay will operate more slowly than downstream relays or fuses, and more quickly than upstream relays or transformer fuses. Relay and fuse curves are generally displayed on time-current curves.
time-delay relay
relay that responds with an intentional time delay.
1. in control circuits, time-delay relays are used to cause a time delay in the state of the relay when power is applied or removed to the relay actuator;
2. in power system protective relays, the response time usually depends on the magnitude of the measured value. If the measured value is a large multiple of the pickup value, then the relay operates or trips after a short time delay. For smaller multiples of pickup, the relay trips after a longer time delay.
TOC relay
See
time overcurrent relay
transformer differential relay
a differential relay specifically designed to protect transformers. In particular, transformer differential relays must deal with current transformer turns ratio error and transformer inrush and excitation current.
underfrequency relay
a protection device that curtails loads in an area that is deficient in generation. Lower generation compared to load demands give rise to lower frequency and a frequency threshold can be used by the relay to initiate load shedding in order to balance generation and demand.
undervoltage relay
a protective relay that operates on low voltage or loss of voltage.
voltage regulating relay
a voltage regulating relay senses RMS voltage level and issues commands to devices such as load tap changers, which then adjust the tap position to bring the voltage back to the desired level.