being monitored. For instance, a differential element refers to the logic required to monitor two (or more) currents, find their difference, and trip if the difference is beyond certain parameters. The term element and function are quite interchangeable in many instances. For simplicity on one-line diagrams, the protection function is usually identified by an ANSI device number. In the era of electromechanical and solid state relays, any one relay could implement only one or two protective functions, so a complete protection system may have many relays on its panel. In a digital/numeric relay, many functions are implemented by the microprocessor programming. Any one numeric relay may implement one or all of these functions. A listing of device numbers is found at ANSI Device Numbers. A summary of some common device numbers seen in digital relays is: 11 – Multi-function Device 21 – Distance 24 – Volts/Hz 25 – Synchronizing 27 – Under Voltage 32 – Directional Power Element 46 – Negative Sequence Current 40 – Loss of Excitation 47 – Negative Sequence Voltage 50 – Instantaneous Overcurrent (N for neutral, G for ground current) 51 – Inverse Time Overcurrent (N for neutral, G from ground current) 59 – Over Voltage 62 – Timer 64 – Ground Fault (64F = Field Ground, 64G = Generator Ground) 67 – Directional Over Current (typically controls a 50/51 element) 79 – Reclosing Relay 81 – Under/Over Frequency 86 – Lockout Relay / Trip Circuit Supervision 87 – Current Differential (87L=transmission line diff; 87T=transformer diff; 87G=generator diff) See also Polyphase system Overhead power line Power outage Three-phase electric power References External links "Understanding Microprocessor-Based Technology Applied to Relaying" Working Group Report, IEEE Power System Relaying Committee http://www.scielo.org.za/pdf/jesa/v25n2/10.pdf A review on protective relays' developments and trends Video that explains overcurrent protection testing: https://www.youtube.com/watch?v=bQ6fZrrP0H4 Electric power infrastructure Electric power distribution Relays