Although not shown in the diagram, the protective relay will actuate a contact that will tell the motor’s control circuitry to cut power to the motor in the event of a ground fault. For the sake of familiarity of protective relay, we have to understand some important terms. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Power Consumption of a Relay – It is the value of power consumed by the relay circuit at the rated current or voltage and expressed in VA for AC and watts for DC. NPN Relay Switch Circuit: A typical NPN relay switch circuit has the coil driven by a NPN transistor switch. A more complicated circuit, in which a 555 timer IC controls a flood lamp via a relay is another option. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof. Overload relays are used in a motor circuit to protect motors from damage caused by prolonged periods of overcurrent. Protective relay work as a sensing device, it senses the fault, then known its position and finally, it gives the tripping command to the circuit breaker. Some relays have a graded response to one or more quantity called characteristic quantity. At the same time, relays are able to control electrical and electronic circuits of many different c… A differential protection relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined amount. Your email address will not be published. What this means is the relay will prevent someone from attempting too many successive re-starts of a large electric motor. The differential relay shown protects against ground faults inside the motor. Some relays are used for the protection of the power system. Therefore, it is important that short circuit protection devices such as protection relays are properly configured. Relay Time – The interval between the existence of fault and closure of relay contacts is called the relay time. Insulated distribution box HO-55 £ 5.47. One of the more common forms is the differential current relay. Protective relays are special power-sensing devices whose job it is to automatically open or close circuit breakers in large electric power systems. Explain why a large electric motor would need to “rest” after several successive start-up events. The main features of a good protective relaying are its reliability, sensitivity, simplicity, speed, and economy. Save my name, email, and website in this browser for the next time I comment. The relay employed for protection of the apparatus and transmission lines are as follows. Some of them are primary relay meaning that they are the first line of defence. Motor protection circuit breaker MS32 1,6-2,5A £ 16.18. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value. Don't have an AAC account? Relay contact protection is needed as the switching of inductive or capacitive loads produces switching sparks due to transient voltage, which can influence the electrical service life of the relays. Overshoot Time – This is the time during which stored operating energy is dissipated after the characteristic quantity has been suddenly restored from a specified value to the value which it had at the initial position of the relay. If the motor is started and stopped several times over a short period of time, the relay will prevent the person from starting it again until a sufficient “rest” time has passed. When students consider the magnitude of the currents involved, and also the fact that most electric motors are fan-cooled and therefore lacking in cooling during the initial moments of a start-up, the reason for automatic lock-out after several successive start-up events becomes obvious. Hence the transient state stability limit of the system is greatly improved, permanent damage to the equipment is avoided, and the possibility of developing most simple fault such as single phase-to-ground into most severe fault such as double phase-to-ground fault is reduced. Protective Relays. The main operation of this device is to make or break contact with the help of a signal without any human involvement in order to switch it ON or OFF. It is mainly used to control a high powered circuit using a low power signal. The most important objective of power system protection is preventing human injury. Electromagnetic field generated by a small current is called an element relay that controls one or more key groups (switches on or off) to operate a switchoperating at high power or current . When the Base voltage of the transistor is zero, the transistor will be in cut-off region and acts as an open switch. This relays works on both the AC and DC supply. Pick Up – A relay is said to pick up when it moves from the off position to the on position or the operation of the relay is called the relay pick-up. winding faults). A very common example of a differential current relay - so common, in fact, that nearly every house is equipped with at least one - is the GFCI, or Ground Fault Current Interrupter. Some protective relays are designed to be used directly with large electric motors to provide sophisticated monitoring, shut-down, and start-up control. The relay failure is because of three reasons such as wrong setting, bad contacts and open circuit in the relay coil. The functional requirements of the relay: Reliability. Overvoltage Protection Circuit using Timers; In the normal operating conditions, (Between 160 and 250 V) the output of the timer 1 is held low so the transistor 1 is in cutoff state.As a result, the reset pin of the timer 2 is high which causes to output at pin 3 is high, so the transistor 2 conducts, and then the relay … A straightforward way of obtaining selective protection is to use time grading. Explain what sort of electrical fault this differential current relay protects against: Also, explain what this relay will do to protect the circuit if it detects this kind of fault. In a more general sense, a differential relay protects against conditions where two or more electrical quantities (usually current) are not in phasor balance. Motor protection circuit breaker MS32 6.3 - 10 A £ 18.07. When the coil of the relay is at rest (not energized), the common terminal and the normally closed terminal have continuity. I must say that I am indebted to C. Russell Mason’s wonderful text. Its a relay that you can connect directly directly with the output of your power supply if you are dealing with low current (2A) or you can connect it to a bigger relay … The backup relay has longer operating time, even though they sense the fault along with the primary relays. Whilst the common emitter relay circuit will be the most popular, it is sometimes useful to use a common collector or emitter follower configuration for the relay circuit. Such type of relays works only on ac quantities. Trip Circuit – It is the circuit that controls the circuit breaker for opening operation and comprises of trip coil, relay contacts, auxiliary switch battery supply, etc. Reset time – It is given by the time which passes away between the instant when the current or voltage (actuating quantity) becomes less than the reset value at the time when the relay contacts are closed. Protective relays: an expanded definition - An electric device that is designed to respond to input conditions in a prescribed manner and, after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits. The ratio of the drop-out voltage or reset value to the pick or operating value is called the drop-out or reset ratio. Drop Out or Reset Level – This is the value of the current or voltage, etc. Setting – It is an actual value of the energizing quantity at which the relay is made to operate under given conditions. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, overvoltage, reverse power flow, over-frequency, and under-frequency. In summary, small currents and voltages provide greater current and voltage control. 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