In American standard there are five different types of time overcurrent relay. Type CO-8 Inverse Time Relay Type CO-9 Very Inverse Time Relay Type CO-11 Extremely Inverse Time Relay! 1. This means that the operating time decreases with increasing current magnitude. The type CDG 24 relay is a CDG 14 with an instantaneous unit. The time-current characteristic curve is different for inverse time, definite time, and instantaneous relays. Instantaneous Overcurrent Relay 2. Time Overcurrent Relays Fall 2018 U I ECE525 Lecture 11 Extremely Inverse Curve and 50E fuse Time Overcurrent Relays Fall 2018 U I ECE525 Example Lecture 11 Vs Z1 Local Load Local Load Local Load Faulted Line Bus #1 Bus #2 Bus #3 Z3 Z4 Z2 Source R2 R3 R4 • Want the relay on the faulted line, R4, to be the only relay to trip The trip time formulae programmed within a Schweitzer Engineering Laboratories model SEL-551 overcurrent relay for inverse, very inverse, and extremely inverse time functions are given here: $t = T \left(0.18 + {5.95 \over {M^2 - 1}} \right) \hskip 30pt \hbox{Inverse curve}$ Extremely Inverse Relay. Inverse-Time Overcurrent Relay 3. Inverse Definite Minimum Time (IDMT) Overcurrent Relay. Very Inverse Relay 6. ADVERTISEMENTS: Depending upon the time of operation, overcurrent relays may be categorized as: 1. So, high current will operate overcurrent relay faster than lower ones. Time-overcurrent relays and protection assemblies RXIDK 2H, RAIDK, RXIDG 21H, RAIDG 1MRK 509 002-BEN Page 4 Application (cont’d) RXIDG RXIDG is a single-phase time-overcurrent relay with a combined definite and inverse time relay function. Their time-current characteristic curves are: Definite minimum, CO-6 Moderately inverse, CO-7 Inverse, CO-8 Very inverse, CO-9 Extremely inverse, CO-11 These time-current characteristics are compared in Figure below. The block diagram of the inverse-time overcurrent relay is shown in the figure. Inverse-Time Protection. I commonly use inverse-time, definite-time, and instantaneous elements, all on the same relay. 3. These relays can replace the following GE electromechanical & static relay types that require a time overcurrent range of 0.5-15.9 Amps and an instantaneous overcurrent range of 1-159 Amps: IAC51 IAC53 IAC55 IAC57 IAC66 IAC77 IAC95 IFC51 IFC53 IFC57 IFC66 IFC77 IFC95 SFC151 SFC153SFC177 They are available with standard inverse, very inverse and extremely inverse characteristics. Definite Time Overcurrent Relay 4. Extremely Inverse Time Overcurrent and Earth Fault Relay A high set overcurrent unit (type CAG) can be fitted in the same case to provide instantaneous protection under maximum short circuit conditions (see Application Sheet R-5087). These relays operate when the current exceeds the pick-up value and with an operating time that varies inversely the magnitude of the current. Inverse Definite Minimum Time (IDMT) Relays 5. You can use combinations of curve types to achieve the design requirements. 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