If both the current and voltage windings are used, the torque developed by the interaction between the fluxes is given by the equation

Where θ is the angle between V and I and the τ is the relay maximum torque angle.

If the relay has current, voltage and the torque angle, the torque will be developed, and it will be given as

where K₁, K₂, K₃  are the tap setting or constant of V and I. The K₄ is the mechanical restraint due to spring or gravity.

The operating characteristic of all types of relays is obtained by adding and subtracting all the other constants and letting others be zero or by adding other similar terms.

For example – In over current relay the K₂ = K₃= 0 because of the absence of the voltage windings. The torque equation becomes

The negative sign attributes to K₄ because the spring produces restraining torque.

Similarly, for directional relay K₁ = K₂ = 0 and the developed torque will be given as

The post Universal Relay Torque Equation appeared first on Circuit Globe.

It is believed that the current below 5mA are not dangerous. The current between 10 to 20 MA current is dangerous because the sufferer loses muscular control. The resistance of human body taken between two hands or between and legs ranges 500mA to 50kΩ. If the resistance of the human body has assumed as 20kΩ, then a contact with 230 volt supply can be potentially fatal, 230/20,000 = 11.5 mA.

The leakage current I = E / R, where E is the supply voltage and R the body resistance. The resistance of dry body varies from 70kΩ to 100 kΩ per square cm, but when the human body is wet, it reduces enormously to between 700Ω to1000Ω per square cm. (The skin resistance of the body is high, but external resistance is low).

To highlight the effect of the wet body, it can be said that 100v supply of the wet body, is as dangerous as 1000 volts when the body is dry.

Effect of Current Passing from Hand to Hand and Foot to Foot

The following are the effect of current passing from hand-to-hand and foot to foot.

1. The perception of electrical shock is about 1mA. At this level, a slight exciting sensation is felt by the person when there is contact with an electric field.
2. The maximum current at which the person is capable of releasing a conductor by using muscles directly affected by the current is called “Let Go Current “. This current is taken as 9mA for male and 6mA for female.
3. If the current level is higher than ” Let Go Current ” than a person loses ability to control his muscles and such currents are hard to bear. These currents are in the range of 20mA-100mA. These current causes physical injury, however, heart and respiratory function continue well.

If the current exceeds 100mA, then the accident could be deadly because the pumping action of the heart stops and pulse disappears. Once the heart stops pumping the brains began to die and after it is robbed of oxygenated blood.At the very high current of the order of 6mA and above, there is a danger of respiratory paralysis and burns.

The shock experienced through AC and DC may vary in their effects. The AC at reasonable frequencies (25-60c/s) is more dangerous than DC of the same RMS.

The increasing use of high-frequency equipment added danger arises from the passage of high-frequency current through the body. At a frequency of about 100c/s, the sensation of shock begins to disappear. The serious internal burns may prove dangerous. It is the current which kills.

An alteration voltage of 50v might produce a dangerous 50mA current. The people have survived much higher voltage due to various factors. The contact resistance can be significantly increased by dry skin, cleaning of clothes and the wearing of boots.

The post Effects of Electric Current on Human Body appeared first on Circuit Globe.

The main protection or primary protection is the first line protection which provides quick-acting and selective clearing of a fault within the boundary of the circuit section or element it protects. The main protection is provided in each section of an electrical installation.

Backup Protection

The backup protection provides the back up to the main protection whenever it fails in operation or its cut out for repairs. The backup protection is essential for the proper working of the electrical system. The backup protection is the second line of defence which isolates the faulty section of the system in case the main protection fail to function properly. The failure of the primary protection occurs because of the failure of the DC supply circuit, current or voltage supply to relay circuit, relay protective circuit or because of the circuit breaker.

The backup protection may be provided either on the same circuit breaker which would be normally opened by the main protection or in the different circuit breaker. The backup protection is mainly used where the main protection of the adjacent circuit is unable to backup the main protection of the given circuit. Sometimes for simplification, the backup protection has a low sensitivity and operated over a limited backup zone.

Example: Consider the remote backup protection is provided by a small time graded relay, as shown in the figure below. Let F be the fault occur on relay R₄. The relay R₄ operates the circuit breaker at D and isolate the faulty section. Now if the circuit breaker D fails to operate, the faulty section would be isolated by the operation of the relay R₃ at C.

The use of the backup protection depends on the economics and technical consideration. The backup protection usually for the economic reason not so fast as the main protection.

The post Primary & Backup Protection appeared first on Circuit Globe.

The connection of the summation current transformer is shown in the figure below. The line CTs are connected to the primary of the auxiliary CT. Each line CTs energise the different number of turns on the primary side of the auxiliary CTs. The resulting single-phase output appears across the secondary.

The summation transformers are used for comparing the relaying quantities derived from the current in the three phases of the primary circuit. It converts the three phase system into positive, negative and zero sequence components.

The simple arrangement of the phase sequence current segregating network is shown in the figure below. The summated output will be according to the fault condition. The summated transformer have their limitations because of loss of discrimination due to condensation of information.

The output amperes-turns from summation transformer may be expressed regarding symmetrical components in the form of MI₀ + NI₁ + PI₂ where M, N and P depend on the choice of summated quantities. The ratio of the compared quantities, for example, at the end of a protected feeder say A and B is

The values of M, N and P depends on the summation transformer and the phase affected by the fault. The ideal condition assumes that the current transducers are linear in response and so also the summation devices. The summation device has less burden and also very simple as compared to sequence network filters.

The post Summation Current Transformer appeared first on Circuit Globe.

The ungrounded three-phase system is shown in the figure below. The line conductors of the system have capacitance between one another and the earth. The former being delta connected and the next become star connected as shown in the figure below. The effect of the line capacitance on the ground is less as compared to the conductors. Therefore, they can neglects.

If the line has same capacitance to ground, then the charging current for each line to earth capacitor lead the phase voltage by 90º and are equal. The magnitude of the current is given by the ratio of the phase voltage V_P and the reactance due to capacitance X_C.

The charging current I_CR, I_CY and I_CB are balanced, and their resultant is zero and no current flows to the earth. When the system is in the balanced condition, then the potential of their neutral is held at the ground due to the presence of the shunt capacitance of the system. The phasor diagram of the balanced condition of the system is shown in the figure below.

Ungrounded System with Fault on One Phase

Consider a phase to earth fault in line Y. In this condition, the faulty line takes the earth potential, and the potential of the remaining two lines arise from the phase potential to the line value. The capacitance currents become unbalanced and the fault current flow through the faulty line into the earth and return through the capacitance C_R and C_B.

The fault current I_F has two components I_CR and I_CB, which flow through capacitance C_R and C_B under the potential difference of V_RY and V_BY respectively.The current I_CR and I_CB leads from their respective voltage by 90º, and their phasor sum is equal to fault current I_F.

Similarly,

The phase voltage of the line is equal to V_p, and the capacitance of the line is also equal to the X_C. The fault current is equal to the phasor sum of I_CR and I_CB as shown in the below

From the figures shown above the following points are concluded.

1. There is no zero sequence current, and because of this, there will be little interferences with the communication lines.
2. In the case of the fault in one phase the remaining two healthy phases of the line become raises from their phase value to the full line value. This cause the stress on the equipment of the three-phase ungrounded systems.
3. The capacitance currents in the two healthy phases increase from √3 times to their normal values.
4. In the ungrounded system, the voltage due to lightning surges does not find the path to the earth, thus raise the voltage of the system.

For the above mention reason, the ungrounded or isolated system is undesirable for the high voltage three phase system.

The post Ungrounded System appeared first on Circuit Globe.

The resistance of the earthing electrode is not concentrated at one point, but it is distributed over the soil around the electrode. Mathematically, the earth resistance is given as the ratio of the voltage and the current shown below.

Where V is a measured voltage between the voltage spike and I is the injected current during the earth resistance measurement through the electrode.

The value of the earth resistance for different power stations is shown below

Large Power Station – 0.5 ohms
Major Power Station – 1.0 ohms
Small Substation – 2.0 ohms
In all other cases – 8.0 ohms

The region around the earth in which the electrode is driven is known as the resistance area or potential area of the ground. The fault current which is injected from the earth electrode is passing away from the electrode in all directions shown below in the figure. The flow of current into the grounds depends on the resistivity of the soil in which the earth electrode is placed. The resistivity of the soil may vary from 1 to 1000 ohm-m depends on the nature of the soil.

The resistivity of the earth depends on its temperature. When the temperature is greater than 0ºC, then its effect on ground resistivity is negligible, But at 0ºC the water in the soil starts freezing which increase their resistivity. The resistivity of the earth is also affected by the composition of some soluble salts as shown in the figure below.

The resistance of the earth varies from layer to layer. The lower layer of soil has more moisture and lower resistivity. If the lower layer contains hard and rocky soil, then their resistivity increases with depth.

The post Earth Resistance appeared first on Circuit Globe.

DC Fuse

The DC fuse opens or breaks the circuit when the excessive current flow through it. The only difficulty with the DC fuse is that the arc produced by the direct current is very difficult to extinct because there are no zero current flows in the circuit. For reducing the DC fuse arcing the electrodes are placed more distance apart due to which the size of the fuse increases as compared to AC fuse.

AC Fuses

The AC fuses are categorised into two types they are the low voltage fuses and the high voltage fuses. The frequency of the AC fuses changes it amplitude from 0º to 60º in very one second. Thus, the arc extinction in the AC circuit can be done easily as compared to the DC circuit.

The low voltage fuses can be further divided into four classes shown below in the image Semi-enclosed or rewirable type and totally enclosed, or cartridge type switches are the most commonly used switches.

Rewirable Fuses

This type of circuit is mostly used in the small current circuit or for domestic wiring. The fuse case and the fuse carrier are the two main parts of the rewirable fuse. The base of the fuse is made up of porcelain, and it holds the wires which may be made up of lead, tinned copper, aluminium or alloy of tin-lead. The fuse carrier can be easily inserted or taken out in the base without opening the main switch.

Totally Enclosed or Cartridge Type Fuses

The fuse element is totally enclosed in an enclosed container, and it has metal contacts on both sides. These fuses are further classified as D-type cartridge fuses and the Link type cartridge fuses.

D-Type Cartridge Fuses

The main parts of the D-type fuse are the base, adapter ring, cartridge and a fuse cap. The cartridge is kept in the fuse cap, and the fuse cap is fixed to the fuse base. The cartridge tip touches the conductor when it is completely screwed to the base and thus completes the circuit through the fuse links.

Link Type Cartridge or High Rupturing Capacity

In such type of fuses, the fuse element carries the fault current for a long duration. If the fault is not clear, then the fuse element will melt and open the circuit. The major advantage of HRC fuse is that it clears the low as well as a high fault current.

HRC fuse has the high-speed operation and also does not require maintenance. But the fuse element of the HRC fuses needs to be replaced after each operation, and it also produced the heat during the faults which will affect the operations of the nearby switches.

The enclosure of the HRC fuse is filled with powdered pure quartz, which acts as an arc extinction medium. The silver and copper wire is used for making the fuse wire. The fuse wire has two or more sections which are joint by using tin-joint. The tin-joint reduces the temperature under overloaded condition.

For increasing the breaking capacity of the fuses two or more silver wire is joined in parallel with each other. These wires are adjusted in such a way so that only one wire will melt at a time. The HRC fuse is of two types

In knife blade type switches the fuse wire is replaced with a live circuit with the help of fuse puller.The bolted type HRC fuses have two conducting plates which are bolted to the fuse base. This fuse requires the additional circuit for taking out the switch without getting a shock.

Dropout Fuse

The melting of fuse causes the fuse element to drop out under gravity about its lower support. Such type of fuse is used for the protection of outdoor transformers.

Striker Fuse

It is a mechanical device having enough force and displacement which can be used for closing tripping/indicator circuits.

Switch Fuse

Such type of switches is used for low and medium voltages circuit. The rating of the fuse unit is in the range of 30, 60, 100, 200, 400, 600, and 800 amperes. The fuse unit is available as 3-pole and 4-pole unit. The making capacity of such type of fuses is up to 46 kA. They can safely break depending upon rating currents of the order of 3 times the load current.

High Voltage HRC Fuses

The main problem of the high voltage fuses is that of the corona. Therefore the high voltage fuses have the special design. They are mainly classified into three types.

Cartridge Type HV HRC Fuse

The fuse element of the HRC fuse is wound in the shape of the helix which avoids the corona effect at the higher voltages. It has two fused elements placed parallel with each other, one of low resistance and the other is of high resistance. The low resistance wire carries the normal current which is blown out and reducing the short circuit current during the fault condition.

Liquid Type HV HRC Fuse

Such type of fuses is filled with carbon tetrachloride and sealed at both the ends of the caps. When the fault occurs then the current, exceed beyond the permissible limit, and the fuse element is blown out. The liquid of the fuse acts as an arc extinguishing medium for the HRC fuses.They may be employed for the transformer protection and the backup protection to the circuit breaker.

Expulsion Type HV Fuse

Expulsion type fuses are widely used for the protection of feeders and transformer because of their low cost. It is developed for 11kV, and their rupturing capacity is up to 250 MVA. Such type of fuses comprises a hollow open-ended tube made of synthetic resin-bonded paper.

The fuse elements are placed in the tubes, and the ends of the tubes are connected to suitable fittings at each end. The arc producing is blown off in the inner coating of the tube, and the gases thus formed extinguish the arc.

The post Types of Fuses appeared first on Circuit Globe.

By the arc extinction medium, the circuit breaker is categorised into four types. They are the air break circuit breaker, air blast circuit breaker, sulphur hexafluoride circuit breaker and vacuum circuit breaker. The classification of the circuit breaker is shown in the figure below.

The circuit breaker is mainly categorised into two types. They are the AC circuit breakers and the DC circuit breakers.

AC Circuit Breaker

The AC circuit breaker is sub-classified into two types, i.e., the low voltage circuit breaker and the high voltage circuit breaker. The circuit breaker whose value lies below the 1000V is known as the low voltage circuit breaker, and above 1000V it is known as a high voltage circuit breaker. The high voltage circuit breaker is further classified into two main categories; they are the oil circuit breakers and the oil-less circuit breaker.

Oil Circuit Breaker

The oil circuit breaker uses oil for an arc extinction. It is further sub-categorized into bulk oil type and the minimum oil type circuit breaker.

Bulk Oil Circuit Breaker – The bulk oil circuit breaker uses transformer oil as an arc extinction medium of the circuit breaker. The oil also acts as an insulator between the two conducting parts of the circuit breaker. The rating range of oil circuit breaker lies from 25MVA at 2.5KV to 5000 MVA at 230KV.

Minimum Oil Circuit Breaker – In the minimum oil circuit breaker, the oil is used for arc extinction by blast action. The main function of the oil in the minimum oil circuit breaker is to interrupt the arc formation, and it is not used for insulating the live parts of the earth.

The oil impulse circuit breaker is the other type of minimum oil circuit breaker. This circuit breaker used oil jet, which is produced by the piston pump for extinguishing the arc. The jet of the oil is placed between the gaps formed by the contacts of the circuit breaker

The four main types of oil circuit breaker are the air circuit breaker, air blast circuit breaker, Sulphur hexafluoride circuit breaker and the vacuum circuit breaker.

Air Circuit Breaker – In air circuit breaker the arc is initiated and extinct in the static air in which the arc moves. Such types of breaker are used in the range of low voltage up to 15KV, and the rupturing capacity of the breaker is 500 MVA.The classification of the air break circuit breaker depends on the types of air breaking methods. The types of the air break circuit breaker are shown below.

In the plain air break circuit breaker, the contacts are made in the shapes of the horns. The magnetic blow type breaker uses magnetic field as an arc interruption medium and in the arc-chute circuit breaker low and medium voltage circuit are used for arc interruption.

Air Blast Circuit Breaker – The air blast circuit breaker uses a blast of air to blow out the arc. In an air blast circuit breaker, compressed air is stored in the form of the tank and release through the nozzles to produce a high-velocity jet, which is used to extinguish the arc.

Such type of circuit breaker is used for indoor services which have a medium high voltage field. The air blast circuit breaker is used for the low voltage up to of 15 kV and rupturing capacities of 2500 MVA. Such types of breakers are also used in outdoor switchyards for 220 kV lines. The types of the air blast circuit breaker are shown below.

In axial blast circuit breaker, the air flows longitudinally in the direction of the arc while in the cross blast circuit breaker the air flows at the right angle of the arc.

Sulfur Hexa Flouride Circuit Breaker – The sulphur hexafluoride circuit breaker uses SF₆ gas for extinguishing the arc. The SF₆ gas has great arc extinguishing property, and it is also superior as compared to other arc quenching media such as the oil or air.

Vacuum Circuit Breaker – In such type of circuit breaker the contacts of the circuit are placed in the permanently sealed vacuum interrupter. The arc is quenched when the contacts are separated in the high vacuum. Such type of circuit breaker is less bulky, cheaper in cost, negligible maintenance and have a long life.

HVDC Circuit Breaker

The breaker which is used for the interruption of the high voltage direct current is known as the HVDC circuit breaker. The voltage breaking capacity of the HVDC circuit breaker is nearly 33KV, and for the current, it is 2KA.

The main problem of the HVDC circuit breaker is that the DC is unidirectional and hence there is no zero point in the DC system. The fault current in the HVDC circuit breaker should be reduced to zero by using some external methods. The arc quenching medium of the air break circuit breaker is either oil or air blast.

The post Types of Circuit Breaker appeared first on Circuit Globe.

Construction of Valve Type Lightning Arrester

The valve type arrester consists of a multiple spark gap assembly in series with the resistor of nonlinear element. The each spark gap has two elements. For non-uniform distribution between the gap, the non-linear resistors are connected in parallel across the each gap.

The resistor elements are made up of silicon carbide with inorganic binders. The whole arrangement is enclosed in a sealed porcelain housing filled with nitrogen gas or SF6 gas.

Working of Valve Type Lightning Arrester

For low voltage, there is no spark-over across the gaps due to the effect of parallel resistor. The slow changes in applied voltage are not injurious to the system. But when the rapid changes in voltage occur across the terminal of the arrester the air gap spark of the current is discharged to ground through the non-linear resistor which offers very small resistance.

After the passage of the surge, the impressed voltage across the arrester falls, and the arrester resistance increases until the normal voltage restores. When the surge diverter disappears, a small current at low power frequency flow in the path produced by the flash over. This current is known as the power follow current.

The magnitude of the power follows current decreases to the value which can be interrupted by the spark gap as they recover their dielectric strength. The power follow current is extinguished at the first current and the supply remains uninterrupted. The arrester is ready for the normal operation.This is called resealing of the lightning arrester.

Stage of Valve Type Lightning Arrester

When the surge reaches the transformer, it meets the lightning arrester as shown in the figure below. For approximately 0.25μs the voltage attained the breakdown value of the series gap and the arrester discharge.

When the surge voltage increases, the resistance of non-linear element drops, thus allowing the further surge energy to discharge. So restricted the voltage transmitted to the terminal equipment as illustrated in the figure below.

When the voltage reduces, the current passes to the ground also decreases while the resistance increases. The lightning arrester attaining a stage when the current flow is interrupted by the spark gap and the arrester is sealed again.

The maximum voltage developed across the arrester terminal and transmitted to the terminal equipment is known as the discharge value of the arrester.

Types of Valve Type Lightning Arrester

The valve type lightning arrester may be station types, line types, arresters for the protection of the rotating machine distribution type or secondary type.

Station Type Valve Lightning Arrester – This type of valve is mainly employed for the protection of the critical power equipment in the circuit of 2.2kV to 400kV and higher. They have the high capacity of energy dissipation.

Line Type Lightning Arrester – The line type arresters are used for the protection of substation equipment. Their cross-sectional area is smaller, lighter in weight and cheaper in cost. They permit higher surge voltage across their terminal in comparison to station type and have lower surge carrying capacity.

Distribution arrester – Such type of arrester is usually mounted on the pole and are employed for the protection of the generators and motors.

Secondary arrester is meant for the protection of low voltage apparatus. The arrester for the protection of rotating machine is designed for the protection of generators and motors.

The post Valve Type Lightning Arrester appeared first on Circuit Globe.

Working Principle of Relay

It works on the principle of an electromagnetic attraction. When the circuit of the relay senses the fault current, it energises the electromagnetic field which produces the temporary magnetic field.

This magnetic field moves the relay armature for opening or closing the connections. The small power relay has only one contacts, and the high power relay has two contacts for opening the switch.

The inner section of the relay is shown in the figure below. It has an iron core which is wound by a control coil. The power supply is given to the coil through the contacts of the load and the control switch. The current flows through the coil produces the magnetic field around it.

Due to this magnetic field, the upper arm of the magnet attracts the lower arm. Hence close the circuit, which makes the current flow through the load. If the contact is already closed, then it moves oppositely and hence open the contacts.

Pole and Throw

The pole and throws are the configurations of the relay, where the pole is the switch, and the throw is the number of connections. The single pole, the single throw is the simplest type of relay which has only one switch and only one possible connection. Similarly, the single pole double throw relay has a one switch and two possible connections.

Construction of Relay

The relay operates both electrically and mechanically. It consists electromagnetic and sets of contacts which perform the operation of the switching. The construction of relay is mainly classified into four groups. They are the contacts, bearings, electromechanical design, terminations and housing.

Contacts – The contacts are the most important part of the relay that affects the reliability. The good contact gives limited contact resistance and reduced contact wear. The selection of the contact material depends upon the several factors like nature of the current to be interrupted, the magnitude of the current to be interrupted, frequency and voltage of operation.

Bearing – The bearing may be a single ball, multi-ball, pivot-ball and jewel bearing. The single ball bearing is used for high sensitivity and low friction. The multi-ball bearing provides low friction and greater resistance to shock.

Electromechanical design – The electromechanical design includes the design of the magnetic circuit and the mechanical attachment of core, yoke and armature. The reluctance of the magnetic path is kept minimum for making the circuit more efficient. The electromagnet is made up of soft iron, and the coil current is usually restricted to 5A and the coil voltage to 220V.

Terminations and Housing – The assembly of an armature with the magnet and the base is made with the help of spring. The spring is insulated from the armature by moulded blocks which provide dimensional stability. The fixed contacts are usually spot welded on the terminal link.

The post Relay appeared first on Circuit Globe.