Industrial Electrical System notes: Electrical System components

Industrial Electrical System: This content will be helpful SSC JE & All State AE/JE examand also for your B-Tech & Polytechnic semester exam.

Electrical Wiring

• It is defined as the process of connecting various accessories for distribution of electrical energy from meter board of supplier to home appliances such as bulb, fans, T.V, Refrigirator and other domestic appliances.

Switches

• Switch is used to make or break the electric circuit.
• At the instant of breaking the circuit, it should break the current to avoid the formation of arc between switch blades and contact terminals. Formation of arc can damage the switch contacts. Such an arc is avoided usually by means of providing a spring to Movable blade to perform the quick action.

Types of Switches 

Following are the types of switch:

1. Surface switches (Tumbler switch).
2. Flush switches.
3. Pull switches (Ceiling switches).
4. Grid switches.
5. Rotary switch.
6. Push button switch,
7. Iron-clad water-tight switch.
8. Knife switch

1.Surface Switch or Tumbler Switch

• This switch is designed for mounting on a plane surface and requires no enclosing parts.
• The surface switches are mounted on the mounting block or directly fixed over the surface of the wall
• such types of switches project out of the surface of the wall.

• It can be classified as-

Single-way or One-way Switch – Here switch is provided with a single pole to control one light point at a time.

Two-way Switch – These are used for wiring circuits which are to be controlled from two points independently. Ex- In case of stair case wiring, one lamp is controlled by two switches from two different positions.

2.Flush Switch

• It is a switch which is mounted in a flush wall box so that only its front face is visible.
• The flush switches are fixed in flush box with the wall and do not project out.
• this types of switches are used where high quality performance and appearance are desired.
• It is also known as Piano type switches.

3. Pull Switch or Ceiling Switches

• The pull switches are fixed at the ceiling and all the alive parts are out off reach of the operator.
• It has a strong mechanical action and is usually operated with a single pull on the cord for the ON and OFF position.
• The switch is widely used for wall lamps, table lamps, floor lamps, bedside lamps and other lamps etc.

4. Grid Switch

• The grid switches are similar to tumbler switches.
• But they are lighter and are portable type, so they are quite useful for the portable machines such as hand-drill, portable grinder etc. They are manufactured either in  single-way or two-way pattern.

5.Push Button Switches

• It is manually operated spring configured switch which is used for opening & closing aset of contact.
• It can either make or break an electrical contact.
• It is available in various colours like Black, Red, yellow, blue, etc.
• It may also have an internal light.
• These switches are used for dorr bell, refrigerator lighting circuit,Motor ON-OFF, Lift, etc.

6.Rotary Switches

• It consists of an insulated handle which is fixed to the blades. These blades move in steps by the movement of the handle and make contact with the terminals which are connected to the wires in the electric circuits.
• The handle motion is controlled by a cam or a spring.
• It provides the options to the operator to select a particular mode of operation like manual, semo-automatic, automatic.
• It is available in various colors but usually black color is used.

7. Iron-clad Water-tight Switches

• This switch is of cast iron and have very robust construction.
• Cork gasket is fitted between the case and the cover which makes it water-tight.
• These are used for outdoor lamp control.

8. Knife Switches

• The knife switch consists of blades(metal lever or knife) which are hinged at one end and are arranged to go into forked terminals or jaws at the other end.
• The current flows through the switch when the knife is pushed into the jaw.
• These are available as single throw switches in which the “knife” engages with only a single slot & double throw switches  in which the knife hinge is placed between two slots and can engage with either one.

Lamp Holders

• As the name suggest, It is used to hold the lamp required for lighting purposes.
• Earlier, brass lamp-holders were quite popular, but now-a-days these have been superseded by the all insulated pattern.
• It has either moulded or porcelain interior with a solid or spring plunger and  has wired terminals.

Types of Lamp Holders

Lamp holders can be classified as:

01. Batten Holder –

• This type of  holders are directly fixed to the wall or wooden board after that the lamp is forced in and slightly turned to left and the lamp will be fixed in holder.
• This type of holders is commonly used in bathrooms, testing board, and in laboratories etc..

02. Pendant Holder –

• This type of holder is used for a temporary purpose.
• It is hanged straight to the downwards or vertical to the ground.
• While fixing the lamp we must have to hold the holder because it is not fixed to the wall like a batten holder and then insert the lamp in it and sightly turn to the left to fix it.
• It is made of brass as well as bakelite.

03. Angle Holder –

• These types of holders are most commonly used nowadays.
• It will give light at an angle of 45 degrees.
• It is fixed only at sidewalls, not at ceilings.
• These types of holders are available in many designs and is used in rooms, hall, kitchen, etc.

04. Slanting Holder –

• These types of lamp holders are bayonet type, also it may be either of bakelite or brass.
• It is most commonly used for floodlight, the board of advertising, and for stages.
• It is used only for shades so that the lamp is concentrated on the material display and does not affect the viewers.

05. Bracket Holder –

• It is used to give direct light in the room or any place.
• They cannot be fixed on the ceiling or hanged but they are only fixed to the wall.
• It can also be used in table lamps.
• It is made up of brass.

06. Watertight Bracket Holders –

• It is used outside the house,
• They are used in street lights & is provided with a glass cover to avoid falling of water.

07. Miniature Screw Type Lamp Holders –

• It is used for decoration purposes.
• It is fixed to the wall or board just like a screw.
• It is made up of brass or bakelite.
• It is generally 100 watts, 230 volts.

08. Lamp Holder Adopter –

• The lamp holder adapters are used to tap temporary power for small appliances from the lamp holder.
• These methods are used for bathrooms or any damp places.

9. Fluorescent Lamp holder and starter holder-

The fluorescentholders are either of bi-pin type or of bayonet cap type, but pintype holders are generally used for ordinary fluorescent tubes.

10. Swivel Lamp-holder

The swivel lamp holders are designed forcontrolled wide angle directional lighting which are used for lightingof shop windows, show cases etc.

Ceiling Roses

It is used to provide a tapping to the pendant lamp-holders, fan or fluorescent tube,etc.

According to Indian slandered-

1. A ceiling rose of any other similar attachment shall not be used on a circuit where the voltage normally exceeds 250 volts.
2. Normally only one flexible cord shall be attached to a ceiling rose.
3. Specially designed ceiling roses shall be used for multiple pendants.
4. A ceiling rose shall not embody the fuse terminal as integral part of it.

Socket Outlet

• Sockets are an outlet where electrical appliances can be plugged in to complete a circuit
• The socket outlets have all insulated base with moulded or socket base having 3 terminal sleeves.
• The two thin terminal sleeves are meant for making connection to the cable,
• And the third terminal sleeve which is thicker in cross-section, is used for an earth connection.

Plugs

• 3-pin plugs are used for taking power from socket outlets.
• The thicker pin is used for an earth connection to the portable appliances.
• They are also made for 5 amps and 15amps load.

Terminal Blocks

• Terminal blocks used for termination and connection to the other circuits.
• A single-way terminal block is used for differentiating the live line from the neutral when the domestic connection is given and is called as Connector.

Main Switch

• The main switches are used to control the whole supply for a house or office.
• These are of following types:

A) ICDP(Iron Clad Double Pole)– It is used for single phase supply control (15 A, 30A, 60A, 100A, 250 V)

B) ICTP(Iron Clad Three Pole)– It isuse for three phase-three wire and three phase four wire supply control (15A, 30A, 60A, 100A, 150A, 200A, and 500V)

Distribution Box

• A distribution board is the main electrical supply system for any commercial or residential entity.
• The main cable comes into the distribution board and then get distributed via breakers to the secondary circuits such as lights and plugs.
• Distribution board is a set of MCBs, ELCBs/RCCBs which are installed in ametal box & is used to distribute distribute electrical electrical power to different loads.

Classification of Distribution Box (DB)

1. Classification of DB according to usage –

LDB (Lighting DB) – It is used to distribute power to lighting circuits, 6A powerpoints, exhaust fans etc. It is used for maximum 6A/10A current loads.
PDB (Power DB) – It is used to distribute electrical power /supply to power loadssuch as Geyser, Fridge, AC etc. All these items draw high current. MCBs of currentrating 16A, 20A, 25A are used.
L&P DB – When DB is use for both lighting & power loads.
NDB (Normal DB) – It is used for normal AC supply where load is to be fed fromnormal power.
UPS DB – It is used for UPS supply where load is to be fed from UPS power.

2. Classification of DB according to supply –
Three phase DB – A 3 Phase DB consists of one no. three phase incoming MCB and a set of single phase outgoing MCBs. A 3 phase DB can be used for Lighting & Power load.
Single phase DB – A Single phase DB consists of one incoming and a set of outgoing MCBs.

IMPRTANT TERMS –
SP – Single Pole (only one point isolation contact where one phase is connected). Application–Switching & protection is done in one phase only.
SPN – Single Pole with Neutral. Application – same as above.
DP – Double Pole (Two point isolation contacts where Phase & Neutral are
connected). Application – Switching & protection is done in one phase & neutral only.
TP – Triple Pole (Three point isolation contacts where R,Y,B phase are connected). Application–Switching & protection is done in all 3 phases only.
TPN – Triple Pole with Neutral (Four point isolation contacts). R,Y,B phase & Neutral are connected but protection is given for 3 phases only, not for neutral.
4P – Four Pole (Four point isolation contacts. R,Y,B & Neutral are isolated & protection is given for 3 phases & neutral)
4/6/8 Way TPN DB – In this, each phase has 4/6/8/.. MCBs that is why 4 Way TPN DB has 12 outgoing MCBs while 6 Way TPN has 18 MCBs & so on.

Wires

• Whenever a circuit is extended or rewired, or when any new circuit isinstalled, it is critical that the new wiring is made with wire conductors that are properly sized for the amperage rating of the circuit, asdetermined by the size of the circuit breaker controlling it.
•  Manufacturers have labeled the outer coating of the wire with type sand size of wire.
• Wires are available with different sizes and their current carrying capacities are also varying according to their size.

Types of wires

1. V.I.R. (Vulcanized Indian rubber) wire
2. CTS/TRS(Cab tyre sheathed, tough rubber sheath) wires
3. Lead sheathes wire
4. PVC wires
5. Weather proof wires
6. Flexible wires

Selection of cables

Important factors that determine the selection of cables

Rated Voltage- the rated voltage of cable should always be equal to or grater than the system voltage.

Current Carrying Capacity– Each power cable is designed to operate under certain temperature conditions. Current carrying capacity of power cable is also dependent on conductor material (Copper / Aluminium) and insulation type. The current carrying capacity is also dependent on operating temperature. Higher the temperature, lower is the current carrying capacity of the cable and vice versa.

Derating Factor– A power cable designed with standard operating conditions may not operate so in practical. Therefore, the current carrying capacity may get impacted due to this.

For example: Cables installed deep under the ground will have reduced current carrying capacity than cables installed in air. This is impacted due to multiple factors like soil temperature, soil thermal resistivity etc.

Actual Current Carrying Capacity = Derating Factor x Cable current carrying capacity under std. conditions.

Voltage Drop– A voltage drop across the length of the power cable is very important. It is expressed as: mV / A-m.

The voltage drop per unit length of cable should as minimum as possible so as to get voltage at delivery end approximately same as supply side.

Short circuit Withstand – A power cable in case of short circuit event should be able to withstand the high current values without any damage to the cable and insulation.

Availability of Cables–This needs to be checked with manufacturer or the dealer of a particular cable. Cables are manufactured in certain minimum length segments. Therefore, it will be difficult to procure a 30 meter length of 300 sq-mm cable than a 300 meter length of same cable. Also, the costing may vary largely between the two quantities.

Bending Radius– This can be a practical problem during installation. Large sized multi-core cables have bigger bending radius than small sized. Therefore a same size of multi-core XLPE cable has more bending radius than a PVC. In order to overcome this, a contractor might have to opt separate single core cables.

Single Line Diagram

a one-line diagram or single-line diagram is a simplified notation for representing an electrical system. The oneline diagram is similar to a block diagram except that electrical elements such as switches, circuit breakers, transformers, and capacitors are shown by standardized schematic symbols.

Importance of single line diagram

1. Identification of the problem location, in safety conformity and the staff safety can be benefited by the use of single line diagram.
2. If in any case the inaccuracy in the connection and the failure arises the updation of the single diagram becomes easy even on the regular basis.
3. The information from one line diagram can be widely used to enhance the performance of service activities.

Protective Devices

• When a failure occurs on any part of the electrical power system, it must be quickly detected and disconnected from the system.
• If the fault not cleared quickly , it may cause unnecessary interruption of the service to the customer.
• Rapid disconnection of faulty apparatus limit the amount of damage to it and prevent the effects of fault from spreading into the system.
• The detection of fault and disconnection of faulty part can be achieved by using fuses or relays in connection with circuit breakers.
• A fuse can perform both the function but for low voltage circuit only.
• For high voltage circuits, relays and circuit breakers are employed to serve the desire function for automatic protective gear.

Protective Relays

Protective relay is a device that detect the fault and initiate the operation of the circuit breaker to isolate the faulty element from the rest of the system.

The relay circuit can be divided in three parts:

1. First is primary winding of the CT which is connected in series with the line to be protected.
2. Second part is secondary of CT and the relay operating coil.
3. Third is the tripping circuit which may be either ac or dc. It consists of a source of supply, the trip coil of the CB and the relay stationary contacts.

Under normal load conditions, the e.m.f. of the secondary winding of C.T. is small and the current flowing in the relay operating coil is insufficient to close the relay contacts. This keeps the trip coil of the circuit breaker unenergized. Consequently, the contacts of the circuit breaker remain closed and it carries the normal load current. When a fault occurs, a large current flows through the primary of C.T. This increases the secondary e.m.f. and hence the current through the relay operating coil. The relay contacts are closed and the trip coil of the circuit breaker is energized to open the contacts of the circuit breaker.

Fundamental Requirement of Protective Relays

The main features of protective devices :

1. Selectivity: It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system
2. Speed: The relay system should disconnect the faulty section as fast as possible
3. Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity.
4. Reliability: It is the ability of the relay system to operate under the pre-determined conditions.
5. Simplicity: The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability.
6. Economy: The most important factor in the choice of a particular protection scheme is the economic aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise method has to be adopted. As a rule, the protective gear should not cost more than 5% of total cost. However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission line etc.), economic considerations are often subordinated to reliability.

Types of Protective Relays

According  to  construction   and principle    of operation:

1. Attracted armature type
2. Solenoid type
3. Electrodynamics type
4. Induction type
5. Thermal type
6. Moving coil type

According to applications:

1. Over Voltage/Over Current/Over Power Relay
2. Under Voltage /Under Current /Under Power relay
3. Directional or Reverse Power Relay
4. Directional or Reverse Current Relay
5. Differential Relay
6. Distance Relay

According to time characteristics

1. Instantaneous Relay
2. Definite Time Lag Relay
3. Inverse Time Lag-Relay

Fuses

• The fuse is a device which is used to protect the device from a current which is too high.
• It consist of a small piece of metal and is connected in series with the circuit
• When the flow of current through a fuse huge exceeds the predetermined value then the metal melts to interrupt the circuit current and protect the circuit.
• It is not able to make or break the circuit under normal condition
• The part of the fuse which is melt during the faulty condition is called fuse element or fuse wire.
• The most commonly used material for Fuse element are lead, copper, tin zinc, and silver.
• Fuge can be Connected in live wire or neutral wire but it should be connected in live wire.
• Fuse acts as a protective device but switch does not.

Characteristics of fuge element

• Low melting point
• High conductivity
• Free from detrodation due to oxidation
• Low cost

Advantages

1. It is the cheapest form of protection available.
2. It requires no maintenance.
3. Its operation is inherently completely automatic while a circuit breaker needs tripping circuit for automatic action.
4. It can break heavy short-circuit currents without noise or smoke.
5. The inverse time-current characteristic of a fuse makes it suitable for overcurrent protection.
6. The minimum time of operation can be made much shorter than with the circuit breakers.

Disadvantages

1. Considerable time is lost in rewiring or replacing a fuse after operation.
2. On heavy short-circuits, discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned.
3. The current-time characteristic of a fuse cannot always be co-related with that of the protected apparatus.

Fuse Element Materials

• The most commonly used materials for fuse element are lead, tin, copper, zinc and silver.
• For small currents upto 10 A, tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element.
• For larger currents, copper or silver is employed.
• Zinc (in strip form only) is good if a fuse with considerable time-lag is required i.e., one which does not melt very quickly with a small overload.

Important Terms

• Current rating of fuse element: It is the current which the fuse element can normally carry without overheating or melting. It depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse.
• Fusing current: It is the minimum current at which the fuse element melts. Its value will be more than the current rating of the fuse element.

• Fusing factor. It is the ratio of minimum fusing current to the current rating of the fuse element.

• Cut-off current- It is the maximum value of fault current actually reached before the fuse melts.
• Pre-arcing time. It is the time between the commencement of fault and the instant when cut off occurs.
• Arcing time. This is the time between the end of pre-arcing time and the instant when the arc is extinguished.
• Total operating time. It is the sum of pre-arcing and arcing times.
• Prospective Current- Before melting, the fuse element has to carry the short circuit current through it. The prospective current is defined as the value of current which would flow through the fuse immediately after a short circuit occurs in the network.

• Breaking capacity- The breaking capacity of a fuse is the maximum available current, at the rated voltage that the device can safely open without physically rupturing. The breaking capacity of the fuse must be greater than the potential fault (short circuit) current of the circuit.

Types of Fuses

• Low voltage fuses

a)Semi-enclosed rewireable fuse

b)High-Rupturing capacity (H.R.C.) cartridge fuse

c)H.R.C. fuse with tripping device

• High Voltage fuses

a)Cartridge type

b)Liquid type

c)Metal clad fuses

Miniature Circuit Breaker

MCB is a device that provides definite protection to the wiring installations and sophisticated equipment against over currents and short circuit faults.

Construction and Working:

Every MCB consists of the following part:
•External Casing: It holds all internal components firm and protects them from dust. It ismade of insulating materials such as plastic or ceramics.
•Contacts: A pair of contacts is present inside an MCB. One of them is fixed and the other ismovable.
•Knob: MCBs can be turned ON and OFF using this knob.
•Mechanical Latch: A latch arrangement is made inside MCBs to hold the contacts underspring tension tension at ON position position. •Bimetallic strip: The bimetallic strip offers delayed overload protection by sensing theprolonged flow of current greater than its rated current.
•Solenoid/Trip Coil: Solenoid offers instantaneous protection against short circuit byreleasing the mechanical latch. Solenoid gets activated when the current through the coilexceeds a particular value, normally more than 3 times of its rated current. This solenoid is notactivated by overloads.
•Arc Chutes: Arc chutes are used for splitting and quenching of arcs.

Operation of MCB

The operation of the MCB includes two stages, i.e., thermal operation and short circuit operation. With the use of bimetallic strip, thermal operation is achieved in case overload conditions. When overload current flows through the MCB, the bimetallic strip gets heated and causes to deflect. In doing so, it moves the trip lever and releases the latch mechanism and hence the contacts open under spring mechanism. During the short circuit conditions, the large fault current energizes the solenoid and the magnetic field of the solenoid attracts the plunger which in turn strikes the trip lever and hence the immediate release of the latch mechanism. When the moving contact separated from fixed contact, there may be a high chance of arc. This arc then goes up through the arc runner and enters into arc splitters and is finally quenched.

The MCB has some advantages compared to fuse.

1. It automatically switches off the electrical circuit during abnormal condition of the network means in over load condition as well as faulty condition. The fuse does not sense but miniature circuit breaker does it in more reliable way. MCB is much more sensitive to over current than fuse.
2. Another advantage is, as the switch operating knob comes at its off position during tripping, the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire should be checked by opening fuse grip or cutout from fuse base, for confirming the blow of fuse wire.
3. Quick restoration of supply can not be possible in case of fuse as because fuses have to be rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is possible by just switching on operation.
4. Handling MCB is more electrically safe than fuse. Because of to many advantages of MCB over fuse units, in modern low voltage electrical network, miniature circuit breaker is mostly used instead of backdated fuse unit.

Only one disadvantage of MCB over fuse is that this system is more costlier than fuse unit system.

Selection of MCB for different loads:

Choosing a particular MCB for a specific application is a careful task to ensure reliable protection against overloads and short circuits. If it is undersized (MCB rating less than the nominal load current), MCB causes frequent tripping and causes to interrupt the current to the load it is being connected, because the MCB nominal current less than nominal current value of the load.

Similarly, if it is oversized (MCB rating more than the nominal load current), the load to it is connected will not be protected efficiently. In such case, the MCB will not trip even though load is drawing over currents.

The following are the three factors to be considered for selecting an MCB for specific application.

1. Nominal rating of the circuit breaker
2. kA rating or breaking capacity
3. Type of MCB

MCCB

• A moulded case circuit breaker (MCCB) is a type of electrical protection device that is used to protect the electrical circuit from excessive current, which can cause overload or short circuit.
• The main difference between MCCB and MCB are that the MCCB can have current rating of up to 2500A with adjustable trip settings.
• These breakers are used instead of miniature circuit breakers (MCBs) in large scale systems for system isolation and protection purposes.
• The MCCB uses a temperature sensitive device (the thermal element) with a current sensitive electromagnetic device (the magnetic element) to provide the trip mechanism for protection and isolation purposes.

MCCB has main three functions

1. They offer protection in case of electrical faults by immediately interrupting acurrent that is extremely high due to a line fault or a short circuit.
2. They protect in the event of an overload where the current is higher than the rated value and lasts a longer time than normal.
3. The on and off function can be used to switch the circuit on or off for repairs and replacements.

ELCB

• An Earth-leakage circuit breaker (ELCB) is a safety device used in electrical installations with high Earth impedance to prevent shock. It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts the circuit if a dangerous voltage is detected.
• Also, the ELCB is used to protect the circuit from the electrical leakage. When someone gets an electric shock, then this circuit breaker cuts off the power at the time of 0.1 sec. for protecting the personal safety and avoiding the gear from the circuit against short circuit and overload.
• If any current leaks from any electrical installation, there must-be any insulation failure in the electrical circuit, it must be properly detected and prevented otherwise there may be a high chance of electrical shock if-anyone touches the installation. An earth leakage circuit breaker does it efficiently. Means it detects the earth leakage current and makes the power supply off by opening the associated circuit breaker. There are two types of earth leakage circuit breaker, Voltage ELCB and Current ELCB.

VoltageEarth LeakageCircuit Breaker

• The working principle of voltage ELCB is quite simple. One terminal of the relay coil is connected to the metal body of the equipment to be protected against earth leakage and other terminal is connected to the earth directly.
• If any insulation failure occurs or live phase wire touches the metal body, of the equipment, there must be a voltage difference appears across the terminal of the coil connected to the equipment body and earth. This voltage difference produces a current to flow the relay coil.
• If the voltage difference crosses, a predetermined limit, the current through the relay becomes sufficient to actuate the relay for tripping   the      associated     circuit breaker   to disconnect the power supply to the equipment.
• The typicality of this device is, it can detect and protect only that equipment or installation with which it is attached. It cannot detect any leakage of insulation in other parts of the system.

Current ELCB or Residual Current Circuit Breaker

• A current ELCB has a toroidal iron core over which phase and neutral windings are wound. A search coil is also wound on the same iron core which in turn is connected to the trip coil. Figure below shows the constructional detail of Current ELCB.
• Under normal operating condition, the current through the phase winding and neutral winding are same but both the windings are wound in such a manner to oppose the mmfs of each other, therefore net mmf in the toroidal iron core will be zero.
• Consider a condition where earth leakage current exists in the load side. In this case the current through the phase and neutral will no longer be equal rather phase current will be more than the neutral current.
• Thus mmf produced by phase winding will be more than the mmf produced by neutral winding because of which a net mmf will exist in the toroidal iron core.
• This net mmf in the core will link with the Search Coil and an emf will be induced across the terminals of the Search Coil. This emf will in turn drive a current through the Trip Coil which will pull lever to open supply contacts to isolate the power supply.

MPCB

A Motor Protection Circuit Breaker is a simple electromechanical device that protects an individual electric motor against overload, fluctuations in input current or unscheduled interruptions to the main circuit. This includes line faults and phase loss or imbalance in three phase motors.

A Motor Protection Circuit Breaker saves on space and costs, providing fuseless protection that switches motors off instantly to prevent damage. It will also safely disconnect the current in the event of a short circuit.

Functions of MPCB

1. Protection against electrical faults such as short circuits, line-to-ground faults etc.
2. Motor overload protection
3. Protection against phase unbalances and phase loss
4. Thermal delay to prevent the motor from being turned back on immediately after an overload, giving the motor time to cool down.
5. Motor Circuit Switching
6. Fault Signaling
7. Automatic Reconnection

The main parts of a MPCB:

1. Thermal over current release
2. Electromagnetic over current release
3. Main contact system
4. Auxiliary switch position
5. Switch latch
6. Arcing chamber
7. Plunger armature
8. Differential trip slide
9. a) Thermal over current release b) Electromagnetic over current release

MPCB Working

• Thermal protection is used to guard the electric motor against overload
• Magnetic protection is used when there is a short circuit, line fault, or other high current electric fault
• The MPCB is capable of detecting these conditions by measuring the differences among phase voltages, and disconnects the motor immediately when they occur.

Isolator

It is essentially a knife switch and is designed to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Such switches are generally used on both sides of circuit breakers in order that repairs and replacement of circuit breakers can be made without any danger. They should never be opened until the circuit breaker in the same circuit has been opened and should always be closed before the circuit breaker is closed.

Types of Electrical Isolators

There are different types of isolators available depending uponsystemrequirement such as

1. Double Break Isolator
2. Single Break Isolator
3. Pantograph type Isolator.

Depending upon the position in the power system, the isolators can be categorized as

1. Bus side isolator – the isolator is directly connected with main bus
2. Line side isolator – the isolator is situated at line side of any feeder
3. Transfer bus side isolator- the isolator    is   directly connected with transfer bus.

Operation of Electrical Isolators

Opening Operation of Electrical Isolator

1. In the beginning, open the major circuit breaker.
2. Then divide the load from a system with isolator opening
3. Close the earth switch. Earth switch can become with an interlock system with isolator. That’s means when isolator is open only that time earth switch can be closed.

Closing Operation of Electrical Isolator a) Detach the earth switch.

1. Shut the isolator.
2. Shut the circuit breaker.

Contactors

A contactor is an electrical device which is used for switching an electrical circuit on or off. It is considered to be a special type of relay. However, the basic difference between the relay and contactor is that the contactor is used in applications with higher current carrying capacity, whereas the relay is used for lower current applications. Contactors can be field mounted easily and are compact in size. Generally, these electrical devices feature multiple contacts. These contacts are in most cases normally open and provide operating power to the load when the contactor coil is energized. Contactors are most commonly used for controlling electric motors.

The following three are crucial components of the contactor:

1. Coil or Electromagnet: This is the most crucial component of a contactor. The driving force that is required to close the contacts is provided by the coil or electromagnet of the contactor. The coil or electromagnet and contacts are protected by an enclosure.
2. Enclosure: Just like the enclosures used in any other application, contactors also feature an enclosure, which provides insulation and protection from personnel touching the contacts. The protective enclosure is made from different materials, such as polycarbonate, polyester, Nylon 6, Bakelite, thermosetting plastics, and others.
3. Contacts: This is yet another important component of this electrical device. The current carrying task of the contactor is done by the contacts. There are different types of contacts in a contactor namely, contact springs, auxiliary contacts, and power contacts. Each type of contact has an individual role to play.

Operating Principle of a Contactor:

The current passing through the contactor excites the electromagnet. The excited electromagnet produces a magnetic field, causing the contactor core to move the armature. A normally closed (NC) contact completes the circuit between the fixed contacts and the moving contacts. This permits the current to pass through these contacts to the load. When current is removed, the coil is de-energized and opens the circuit. The contacts of the contactors are known for their rapid open and close action.

Electric Shock

• An electric shock occurs when a person comes into contact with an electrical energy source.
• Electrical energy flows through a portion of the body causing a shock.

Factors determining the severity of electric shock

• The type of current (AC/DC)
• The amount of current
• Duration of contact
• Surface area of contact or Electrical field strength
• The pathway the electricity takes through the body
• Overall health of person

What happens when you get an electric shock

1. An electric shock can have no injury at all
2. Burns (which are the most common)
3. Cardiac arrest due to the electrical effect on the heart
4. Muscle, nerve, and tissue destruction from a current passing through the body
5. Death (electrocution)

Causes of Electrical Accidents

Accidents and injuries with electricity are caused by one or a combination of the following:

• Unsafe equipment and/or installation.
• Unsafe workplaces caused by environmental.
• Unsafe work practices.

Prevention to be taken to prevent electric shock

• Avoid water at all times when working with electricity. Never touch or try repairing any electrical equipment or circuits with wet hands. It increases the conductivity of electric current.
• Never use equipment with frayed cords, damaged insulation or broken plugs.
• Always use insulated tools while working.
• Always use appropriate insulated rubber gloves and goggles while working on any branch circuit or any other electrical circuit.
• Never try repairing energized equipment. Check all the wires, the outer metallic covering of the service panel and any other hanging wires with an electrical tester before proceeding with your work.
• Never use an aluminum or steel ladder if you are working on any receptacle at height in your home. An electrical surge will ground you and the whole electric current will pass through your body. Use a wooden or a fiberglass ladder instead.
• Always use a circuit breaker or fuse with the appropriate current rating.
• Working outside with underground cabling can be dangerous. The damp soil around the cable is a good conductor of electricity and ground faults are quite common in the case of underground cabling. Using a spade to dig at the cable can damage the wiring easily so it is better to dig at the cable by hand while wearing insulated gloves.
• Replace immediately broken switches and plugs etc.
• Check that all metallic parts of electrical equipment are effectively earthed.
• Always turn off the mains when perform electric work at your home.
• Never place bare wire of leads in plugs. Fit a plug top.

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