Electromechanical RelaysBy Steven McFadyen on April 2nd, 2012 Electromechanical relays have been the traditional backbone of electrical protection systems. While over recent years these have been replaced by microprocessor based numerical devices, there are still many older electromechanical relays in service. Contents [hide] 1. Evolution 2. Tripping Curves 1. IEC 60255 Characteristics 2. North American Characteristics 3. 4. 5. CDG11/16 Curves GEC / English Electric / Alstom /Areva 1. Stabilising Resistors Evolution Mechanical relays developed in the 1800s were the first form of electrical protection. While still being reliable and widely used these were superseded by static relays in the early 1980s. Static relays have no moving parts (hence the name) and operated on the basis of analogue circuitry. More recently static relays have been superseded by first digital relays and now numerical microprocessor based devices. Tripping Curves IEC 60255 Characteristics The IEC 60255 standard defines four standard current time characteristics – standard inverse (SI), very inverse (VI), extremely inverse (EI) and long-time inverse. Each characteristic can be calculated from: where: t I Is = tripping time in (S) = fault (actual) secondary CT current (A) = relay pick-up current setting) TMS = time multiplier setting Characteristic Standard Inverse Very Inverse Extremely Inverse α 0.02 1.0 2.0 K 0.14 13.5 80 95 0. The relay pick-up current value is set at 0.0 2. IEEE Very Inverse.14 .0 0.61 5.8s/10] Long Time Standard Earth Fault[13.02.02 2.01694 0.491 0.2 0.0515 19. [3s/10] .3s/10] North American Characteristics Current time characteristics in North America as classified as IEEE Moderately Inverse.0 2. Tripping times for the various relays are: Standard Inverse (SI) Very Inverse (VI) Extremely Inverse (EI) [3s/10] or [1. IEEE Extremely Inverse. K = 0.18 K 0.3s/10] [1. US C08 Inverse and US CO2 Short Time Inverse. What will be the tripping time? . the tripping time is: . time multiplier setting is 7 and the fault current is 8000 A.02394 Setting Example (IEC 60255) An 1000 Amp breaker protected by relay with Standard Inverse characteristic.0 120 Relay characteristics are sometimes classified according to the tripping time at 10 times the setting current (i.8 = 800 A .5s/10] [0.e.1217 28.from the table α = 0.114 0.a standard inverse curve which will trip in 3 seconds at 10 times the current setting).8.Long-time Inverse 1.pick-up current setting = 1000 A x 0.02 β 0.using the IEC 60255 equations. These are given by: where: t = tripping time in (S) I = fault (actual) secondary CT current (A) Is = relay pick-up current setting TD = time dial setting (multiplier) Characteristic IEEE Moderately Inverse IEEE Very Inverse IEEE Extremely Inverse US CO8 Inverse US CO2 Short Time Inverse α 0. 2= long time delay. 2= 'metricated') suffix ('5' is for 50Hz only relays.directional) number of units (ie CDG3x is a 3 element / unit CDG relay) characteristic (for CDG. D . stabilising resistor|resistors are added to increase the minimum relay operating voltage. E .static) Application (G.) identification (identifies rating. '6' for 60Hz) For a full list refer to publication MS/5100/2 from English Electric The company history has changed over the last few years. 5.earth.We also have an online IDMT Tripping Time calculator. Recently the company has now been acquired by Areva.differential.55s). Rule of thumb for sizing of resistor: try to drop: at . F . using number CDG31 etc. 2=D c=G etc. D . contact arrangement etc. T . with three extremely inverse elements and is a 50 Hz unit. 10 terminal etc.CDG 34EG0022A5 is a current operated. U . 3=very inverse (1.) case mounting (F=flush etc. 7. saturation of one or more of the transformers during transient events may result in large spill currents. 8.general or generator. That is the time for the 1. Stabilising Resistors If current transformers are connected in a residual circuit. 6. you can just multiply the time with 1. 2. 4. CDG11/16 Curves If you have the 3sec relay's trip curve. 9. 1= std inverse (3s). English Electric became GEC and subsequently GEC-Alstom. this may cause the the relay to falsely operate. To achieve stability under these conditions. M .induction disc. induction disc general relay.3 and divide the answer with 3.balanced armature. M .: 1. 6=Long Time Standard Earth Fault) case size (15 different cases.3sec relay. 1=C. Particularly with high impedance relays. example .flag.6s). operating quantity (C . GEC / English Electric / Alstom /Areva Labelling the model from left to right.current. 4=extremely inverse (0. 3.voltage) basic movement (D .motor.definite time. A=size 1 draw out. V. the tripping time is: .02.com/notes/entryid/159/electromechanicalrelays#sthash. K = 0. The relay pick-up current value is set at 0.See more at: http://myelectrical.14 .from the table α = 0.8 = 800 A . .using the IEC 60255 equations. time multiplier setting is 7 and the fault current is 8000 A. What will be the tripping time? .dpuf Setting Example (IEC 60255) An 1000 Amp breaker protected by relay with Standard Inverse characteristic.8.aNJuGnyy..pick-up current setting = 1000 A x 0. that is: Where: R Vk Ipk = stabilising resistor value = knee voltage of CT = relay pickup current Alternatively the stabilising resistor can be calculated by using: where: VA = relay burden Ir = relay setting current Note: The power rating of the stabilising resistor should be chosen taking into consideration the expected magnitude and duration of the current through the resistor.