Electric protective devices
protective device An external support applied to vulnerable joints or other body parts to guard against injury. Protective devices include helmets, braces, tape or wrapping, and padding. A respiratory protective device (RPD), also known as a respirator, is a piece of safety equipment used for personal protection. Respirators are designed to prevent the inhalation of contaminated air and belong in two main categories: Air-purifying respirators: These respirators are designed to filter or clean contaminated air from the workplace before it is inhaled by the RPD user. They are.
Equipment applied to electric how to cure stomach flu naturally systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth. From time to time, disturbances in the normal operation of a power system occur.
These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of decice service, to limit injury to people, and to limit damage to equipment when problem situations develop.
Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system. These are compact analog or digital networks, connected to various points of an electrical system, to detect abnormal conditions occurring within their assigned areas.
They initiate disconnection of the trouble area by circuit breakers. These relays range wwhat the simple overload unit on house circuit breakers to complex systems used to protect extrahigh-voltage power transmission lines.
They operate on voltage, current, current direction, power factor, power, impedance, temperature. In all cases there must be a measurable difference between the normal or tolerable operation and the intolerable or unwanted condition.
System faults for which the relays respond are generally short circuits between the phase conductors, or between the phases and grounds. Some relays operate on unbalances between the phases, such as an open or reversed phase. A fault in one part of the system affects all other parts. Therefore relays and fuses throughout the power system must be coordinated to ensure the best quality of service to the loads and to avoid operation in the nonfaulted areas unless the trouble is not adequately cleared in a specified time.
See Fuse electricityRelay. For the purpose of applying protection, the electric power system is divided into five major protection zones: generators; transformers; buses; protecyive and distribution lines; and motors see illustration.
Each block represents a set of protective relays and associated equipment selected to initiate correction or isolation of that area for all anticipated intolerable conditions or trouble. The detection is done by protective relays with a circuit breaker used to physically disconnect the equipment. For other areas of protection See GroundingUninterruptible power system. Fault detection is accomplished by a number of techniques, including the detection of changes in electric current or voltage levels, power direction, ratio of voltage to current, temperature, and comparison of the electrical quantities flowing into a protected area with the quantities flowing out, also known as differential protection.
This is the most fundamental and widely used protection technique. The system compares currents to detect faults in a protection zone. Current transformers on either side of the protection zone reduce the primary currents to small secondary values, which are the inputs to the relay.
For load through the equipment or for faults outside of the protection zone, the secondary currents from the two transformers are ia the same, and they are directed so that the current through the relay sums to devive zero.
However, for internal trouble, the secondary currents add up to flow through the devcie. This must be provided on all systems to prevent abnormally high currents from overheating and causing mechanical how to potty train a maltese on equipment.
Overcurrent in a power system usually indicates that current is being diverted from its normal path by a short circuit. In low-voltage, distribution-type circuits, such as those found in homes, adequate overcurrent protection can be provided by fuses that melt when current exceeds a predetermined value. Small thermal-type circuit breakers also provide overcurrent protection for this class of circuit. As the size of circuits and systems increases, the problems associated with interruption of large fault currents dictate the use of power circuit deevice.
Normally these breakers are not equipped what is a protective device elements to sense fault conditions, and therefore overcurrent relays are applied to measure the current continuously. When the current has reached a predetermined value, the relay contacts close. This actuates the trip circuit of a particular breaker, causing it to open and thus isolate the fault.
See Circuit breaker. Distance-type relays operate on the combination of reduced voltage and increased current occasioned by faults. They are widely applied for the protection of higher voltage lines. A major advantage is that the operating zone is determined by the line impedance and is almost completely independent of current magnitudes. Lightning in the area near the power lines can cause very what is office project professional 2007 overvoltages in the system and possible breakdown of the insulation.
Protection for these surges pprotective of lightning arresters connected between prorective lines and ground. Normally the insulation through these arresters prevents current flow, but they momentarily pass current during the high-voltage transient to limit overvoltage. Overvoltage protection is seldom applied elsewhere except at the generators, where it is part of the voltage regulator what goods and services are gst free control system.
In the distribution system, overvoltage relays are used degice control taps of tap-changing transformers or to switch shunt capacitors on and off the circuits. See Lightning and surge protection. This must be provided on circuits supplying power to motor loads. Low-voltage conditions cause motors to draw excessive currents, which can damage the motors. What does it mean to tweet a low-voltage condition develops while the motor is running, the relay senses this condition and removes the motor from service.
A loss or deficiency in the generation supply, the transmission lines, or other components of the system, resulting primarily from faults, can leave the system with an excess of load. Solid-state and digital-type underfrequency relays are connected at various points in the system to detect this resulting decline in the normal system frequency. They operate to disconnect loads or to separate the system into areas so that the available generation equals the load until a balance is reestablished.
This is provided when a change in the normal direction of current indicates an abnormal condition in the system. This is actually a change in direction of power flow and can be directed by ac directional relays. This protection is used on feeders supplying motors where there is a possibility of one phase opening as a result of a fuse failure or a connector failure. One type of relay compares the current in one phase against the currents in the other phases. When the unbalance becomes too great, the relay operates.
Another type monitors the three-phase bus voltages for unbalance. Reverse phases will operate this relay. Where direction of rotation is important, electric motors must be protected against phase reversal. A reverse-phase-rotation relay is applied to sense the phase rotation. This relay is a miniature three-phase motor with the same desired direction of rotation as the motor it is protecting.
If the direction of rotation is correct, the relay will let the motor start. Protecttive incorrect, the sensing relay will prevent the motor starter from operating. Motors and generators are particularly subject to overheating due to overloading and mechanical friction. Excessive temperatures lead to deterioration of insulation and increased losses within the machine. Temperature-sensitive elements, located devvice the machine, form part of a bridge circuit used to supply current to a relay.
When a predetermined temperature is reached, the relay operates, initiating opening of a circuit breaker or sounding of an alarm. Electric protective devices Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions.
Protective relays These are compact analog or digital what mindless behavior phone number, connected to various points of an electrical system, to detect abnormal conditions occurring within their assigned areas. Zone protection For the purpose of applying protection, the electric power system is divided into five major protection zones: generators; transformers; buses; transmission and distribution lines; and motors see illustration.
Zones of protection on simple power system. Fault detection Fault detection is accomplished by a number of techniques, including the detection protecrive changes in electric current or deviice levels, power direction, ratio of voltage to current, temperature, and comparison of the electrical quantities flowing into a protected area with the quantities flowing out, also known as differential protection. Differential protection This is the most fundamental and widely used protection technique.
Overcurrent protection This must be provided on all systems to prevent abnormally high currents from overheating and causing mechanical stress on equipment. Distance protection Distance-type relays operate on the combination of reduced voltage and increased current occasioned protecfive faults.
Overvoltage protection Lightning in the area near the power lines can cause very short-time overvoltages in the system and possible breakdown of the insulation. Undervoltage protection This must be provided on circuits supplying power to motor what is a diabetic nurse. Underfrequency protection A loss or deficiency in the generation supply, the transmission lines, or other components of the system, resulting primarily from faults, can leave the system with an excess of load.
Reverse-current protection This is provided when a change in the normal direction of current indicates an abnormal condition in the system. Phase unbalance protection This protection is used on feeders supplying motors where there is a possibility of one phase opening as a result of a fuse failure or drvice connector failure.
Reverse-phase-rotation protection Where direction of rotation is important, electric motors must be protected against phase reversal. Thermal protection Motors and generators are particularly subject to overheating due to overloading and mechanical friction. Mentioned in? Encyclopedia browser? Full browser?
What is a Protection Device? The circuit protection device is an electrical device used for preventing an unnecessary amount of current otherwise a short circuit. To ensure the highest security, there are many protection devices available in the market which offers you a total range of protection devices for circuits such as a fuse, circuit breakers, RCCB, gas discharge tubes, thyristors, and more. Protective devices. Included in this category are fuses and fuseholders, LV and MV breakers, relaying equipment, line monitoring equipment, GFCI and GFEP interrupters, and neutral ground niceloveme.comzing damage to electrical equipment and prevention of injury to personnel are the principal reasons for applying protective devices. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system. Protective relays These are compact analog or digital networks, connected to various points of an electrical system, to detect abnormal conditions occurring within their assigned areas.
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