![]() ![]() When a fault occurs in a particular zone, the device dedicated for its protection will sense the current and isolate the fault from the remaining system. They only operate on faults that lie within their “zone of protection”. Each protective device is dedicated to a particular zone. Selective coordination is accomplished by adjusting and rearranging the time current curves of protective devices such that their settings or curves have minimum or no overlapping. In case, the breaker C.B-5, due to any reason, does not clear the fault, then C.B-2 clears it after some delay and if, due to any reason, C.B-2 is not able to clear fault, then C.B-1 issues a trip (which could be the worst case scenario). In this case, C.B-5 should be able to clear the fault in the least possible time and no other breaker (in this case C.B-2 and C.B-1) should trip during this time. The above figure shows a fault that occurs below circuit breaker 5 (C.B-5). In order to understand how protective devices are coordinated, let us take an example: “ Localization of an over-current condition to restrict outages to the circuit or equipment affected, accomplished by the choice of over-current protective devices and their ratings or settings.” So, you won't have the power outage if there is a fault somewhere downstream.Īccording to NEC article 100, Selective coordination is defined as: The fuse or a circuit breaker closest to the fault opens without opening the fuse or the circuit breaker that feeds it (from the upstream side). Complete selectivity means that the protective devices will minimize the effect of a short circuit or other undesirable event on the power system. Selective Coordination is defined as a method of adjusting the opening times of overcurrent protection devices so that the fuses or breakers nearest to the faults open first. The magnetic part of the breaker senses high overcurrent or short circuit and issues a trip signal. Instantaneous Trip: There is no intentional delay in tripping.As the current increases, heating goes on and overcurrent clearing time decreases. Bimetallic strip in the breaker heats up on high current causing the contacts to break up after a delay. Delay Trip: This trip is due to overcurrent sense by thermal part of the breaker.Thermal magnetic breakers have slightly different characteristic graphs than electronic (solid state) breakers as they have only two settings: Maximum Clearing Time: It is the time at which breaker issues a trip signal.Minimum Clearing Time: It is the time at which breaker senses a fault.This thickness in the graph has its own meaning which is described by two terms known as: Its setting can vary from 2 to 40 times of long-time ampere rating.Īs seen in the graph below, the breaker curve has a wide thickness. Instantaneous Pickup : Used when tripping is required without any delay.Short Time Delay: Delay given due to check if downstream devices cleared the fault so no trip issues or else after reaching delay breaker trips.The setting at which breaker tends to trip after some delay. Short Time Pickup: It is 1.5 to 10 times the long time ampere rating.This delay is given in the form of slope. Long Time Delay : This setting refers to delay due to inrush current of transformer and starting current of motor.Therefore, long-time ampere setting will be adjusted to 800 A. For example, a breaker is rated at 1000 A and maximum current that will flow through the breaker is 800 A. Long Time Ampere Rating : It is the continuous current rating at which breaker shows no tripping.Check out Power System Protection Fundamentals Course in which we briefly discussed "Types of protective relays & design requirements".īelow are some key points that are reflected in graph shown above. They are typically used in MV and HV systems. Relay curves are sharper and thinner than fuse and breakers because relays are only used to sense a fault and then issue a trip signal to the breakers. ![]() Logarithmic scale in TCC ensures that both extreme values of current and time are present. For example: in a system, a minimum fault of 100 A should be cleared within 10 s and for a system with a maximum fault of 5000 A it must be cleared within 50 ms. A TCC is plotted on a logarithmic scale so that all values of current and time are easily incorporated.In a TCC, current is mentioned on the x-axis while time on the y-axis.To ensure that all the downstream and upstream protective devices are coordinated, current versus time (I versus t) curve is used which is also known as TCC or Time Current Curve.įollowing are the characteristics of TCCs: It is desired that as fault current increases the Fault Clearing Time or FCT should be decreased. Fault intensity in power systems is proportional to the magnitude of current.
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