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