Definite Time Overcurrent Relay 4. Inverse-Time Protection. The inverse time … 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: $t = T \left(0.18 + {5.95 \over {M^2 - 1}} \right) \hskip 30pt \hbox{Inverse curve}$ The type CDG 24 relay is a CDG 14 with an instantaneous unit. 1. 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 They are available with standard inverse, very inverse and extremely inverse characteristics. ADVERTISEMENTS: Depending upon the time of operation, overcurrent relays may be categorized as: 1. 3. Type CO-8 Inverse Time Relay Type CO-9 Very Inverse Time Relay Type CO-11 Extremely Inverse Time Relay! Inverse Definite Minimum Time (IDMT) Overcurrent Relay. The block diagram of the inverse-time overcurrent relay is shown in the figure. In American standard there are five different types of time overcurrent relay. I commonly use inverse-time, definite-time, and instantaneous elements, all on the same relay. Very Inverse Relay 6. Inverse-Time Overcurrent Relay 3. In this type of relays, operating time is inversely changed with the current. 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 Inverse Definite Minimum Time (IDMT) Relays 5. You can use combinations of curve types to achieve the design requirements. 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. So, high current will operate overcurrent relay faster than lower ones. Extremely Inverse Relay. 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. This means that the operating time decreases with increasing current magnitude. Instantaneous Overcurrent Relay 2. 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