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Relay Timing Characteristics | Instantaneous, Inverse Time and Definite Time Lag Relays



An important characteristic of a relay is its time of operation. By ‘the time of operation’ is meant length of the time from the instant when the actuating element is energised to the instant when the relay contacts are closed. 

Sometimes it is desirable and necessary to control the operating time of a relay. For this purpose, mechanical accessories are used with relays.

Instantaneous Relay

An instantaneous relay is one in which no intentional time delay is provided. 

In this case, the relay contacts are closed immediately after current in the relay coil exceeds
the minimum calibrated value. 

Figure shows an instantaneous solenoid type of relay. Although there will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added. 

The instantaneous relays have operating time less than 0·1 second. The instantaneous relay is effective only where the impedance between the relay and source is small compared to the protected section impedance.

The operating time of instantaneous relay is sometimes expressed in cycles based on the power system frequency.
e.g. one-cycle would be 1/50 second in a 50-cycle system.

Inverse Time Relay

An inverse-time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. 

Figure shows the time-current characteristics of an inverse current relay. At values of current less than pickup, the relay never operates. At higher values, the time of operation of the relay decreases steadily with the increase of current. 

The inverse-time delay can be achieved by associating mechanical accessories with relays.


  • In an induction relay, the inverse-time delay can be achieved by positioning a permanent magnet (known as a drag magnet) in such a way that relay disc cuts the flux between the poles of the magnet. When the disc moves, currents set up in it produce a drag on the disc which slows its motion.

  • In other types of relays, the inverse time delay can be introduced by oil dashpot or a timelimit fuse. Figure shows an inverse time solenoid relay using oil dashpot. The piston in the oil dashpot  attached to the moving plunger slows its upward motion. At a current value just equal to the pickup, the plunger moves slowly and time delay is at a maximum. At higher values of relay current, the delay time is shortened due to greater pull on the plunger.

The inverse-time characteristic can also be obtained by connecting a time-limit fuse in parallel with the trip coil terminals as shown in figure below. 

The shunt path formed by time-limit fuse is of negligible impedance as compared with the relatively high impedance of the trip coil. Therefore, so long as the fuse remains intact, it will divert practically the whole secondary current of CT from the trip coil. 

When the secondary current exceeds the current carrying capacity of the fuse, the fuse will blow and the whole current will pass through the trip coil, thus opening the circuit breaker. The timelag between the incidence of excess current and the tripping of the breaker is governed by the characteristics of the fuse. Careful selection of the fuse can give the desired inverse-time characteristics, although necessity for replacement after operation is a disadvantage.

Definite Time Lag Relay

In this type of relay, there is a definite time elapse between the instant of pickup and the closing of relay contacts. This particular time setting is independent of the amount of current through the relay coil ; being the same for all values of current in excess of the pickup value. 

It may be worthwhile to mention here that practically all inverse-time relays are also provided with definite minimum time feature in order that the relay may never become instantaneous in its action for very long overloads.

Substation Earthing System Design

We have already given an introduction to substation earthing system in an article published earlier. Today we are going to discuss about the design of substation earthing system.

Before 1960s the design criterion of substation earthing system was "low earth resistance.” (Earth Resistance< 0.5 ohms for High Voltage installations). 
During 1960s, the new criteria for the design and evaluation of Substation Earthing System were evolved particularly for EHV AC and HVDC Substations. The new criteria are :
  1. Low Step Potential 
  2. Low Touch Potential 
  3. Low Earth Resistance.
The conventional “Low earth resistance criterion” and Low Current Earth Resistance Measurement continues to be in practice for Substations and Power Station upto and including 220 kV.


The parts of the Earthing System include the entire solid metallic conductor system between various earthed points and the underground earth-mat. The earthed points are held near-earth potential by low resistance conductor connections with earth-mat.

Substation Earthing System Design

Underground Horizontal Earth Mesh (Mat/Grid)

The mesh is formed by placing mild steel bars placed in X and Y directions in mesh formation in the soil at a depth of about 0.5 m below the surface of substation floor in the entire substation area except the foundations. 

The crossings of the horizontal bars in X and Y directions are welded.

The earthing rods are also placed the border of the fence, surrounding building foundations, surrounding the transformer foundations, inside fenced areas etc. 

The mesh ensures uniform and zero potential distribution on horizontal surface of the floor of the substation hence low “step potential" in the event of flow of earth fault current.


Earthing Electrodes (Spikes)

Several identical earth electrode are driven vertically into the soil and are welded to the earthing rods of the underground Mesh. Larger number of earth electrodes gives lower earth resistance.

  • The number of Earth-Electrodes (Spikes) Ns for soil resistivity 500 ohm meter and earth fault current Is is :
Ns = Is / 250 Amperes
i.e., approximately 250 Amp per spike, for soil resistivity of 500 ohm-meter.


  • The number of Earth-Electrodes (Spikes) Ns for soil resistivity 5000 ohm meter is 
Ns = Is / 500 Amperes
i.e., approximately 500 Amp per spike, for soil resistivity of < 5000 ohm-meter.
Is = Short Circuit level of the substation, A
    Example:
    33 kV sub stations     : 25000 to 31000 A
    400 kV Substations   : 40000 A


    Earthing Risers

    These are generally mild steel rods bent in vertical and horizontal shapes and welded to the earthing mesh at one end and brought directly upto equipment / structure foundation.

    Earthing Connection

    Galvanized Steel Strips or Electrolytic Copper Flats or Strips/Stranded Wires (Cables) /Flexibles. 
    These are used for final connection  (bolted/welded/clamped) between the Earthing Riser and the points to be earthed. 

    For Transformer Neutral/High Current Discharge paths copper strips/stranded wires are preferred. 
    Galvanised Iron Strips/stranded wires are more common for all other earthing connections. The earthing strips are finally welded or bolted or clamped to the Earthed Point.

    Different Types of Capacitors and their Uses

    Basically a capacitor is formed from two conducting plates separated by a thin insulating layer. They are manufactured in many forms, styles, and from many materials. Capacitors are widely used in electrical and electronic circuits.

    In electronic circuits, small value capacitors are used
    • to couple signals between stages of amplifiers.
    • as components of electric filters and tuned circuits.
    • as parts of power supply systems to smooth rectified current.

    In electrical circuits, larger value capacitors are used
    • for energy storage in such applications as strobe lights.
    • as parts of some types of electric motors.
    • for power factor correction in AC power distribution systems

    Standard capacitors have a fixed value of capacitance, but adjustable capacitors are frequently used in tuned circuits.

    Types of Capacitors

    Capacitors are divided in to two common groups:
    1. Fixed Capacitor with fixed capacitance values
    2. Variable Capacitors with variable (trimmer) or adjustable (tunable) capacitance values.

    Out of these the most important group is fixed capacitors. Many capacitors got their names from the dielectric. But this is not true for all capacitors because some old electrolytic capacitors are named by its cathode construction. So the most used names are simply historical.

    Fixed capacitors include polarized and non-polarized. Ceramic and Film capacitors are examples of  non-polarized capacitors. Electrolytic and Super Capacitors are included in the group of polarized capacitors.

    The classification of fixed capacitors are shown in the figure below. Some of the important capacitors are listed here.

    • Ceramic capacitors
    • Film and paper capacitors 
    • Aluminum, tantalum and niobium electrolytic capacitors
    • Polymer capacitors
    • Supercapacitor
    • Silver mica, glass, silicon, air-gap and vacuum capacitors



    In addition to the above shown capacitor types, which derived their name from historical development, there are many individual capacitors that have been named based on their application. 

    They include:
    Power capacitors, motor capacitors, DC-link capacitors, suppression capacitors, audio crossover capacitors, lighting ballast capacitors, snubber capacitors, coupling, decoupling or bypassing capacitors.

    Often, more than one capacitor family is employed for these applications, e.g. interference suppression can use ceramic capacitors or film capacitors.

    Overview of Different Types of Capacitors

    As we explained above, there are many different types of capacitor that can be used. Some of the major types are outlined below:

    Ceramic capacitor:   

    The ceramic capacitor is a type of capacitor that is used in many applications from audio to RF. 

    Values range from a few picofarads to around 0.1 microfarads. Ceramic capacitor types are by far the most commonly used type of capacitor being cheap and reliable and their loss factor is particularly low although this is dependent on the exact dielectric in use. 

    In view of their constructional properties, these capacitors are widely used both in leaded and surface mount formats.

    Electrolytic capacitor:   

    Electrolytic capacitors are a type of capacitor that is polarised. 

    They are able to offer high capacitance values - typically above 1μF, and are most widely used for low frequency applications - power supplies, decoupling and audio coupling applications as they have a frequency limit if around 100 kHz. 

    Tantalum capacitor:  

    Like electrolytic capacitors, tantalum capacitors are also polarised and offer a very high
    capacitance level for their volume. 

    However this type of capacitor is very intolerant of being reverse biased, often exploding when placed under stress. This type of capacitor must also not be subject to high ripple currents or voltages above their working voltage. 

    They are available in both leaded and surface mount formats. 

    Silver Mica Capacitor:   

    Silver mica capacitors are not as widely used these days, but they still offer very high levels of stability, low loss and accuracy where space is not an issue. 

    They are primarily used for RF applications and and they are limited to maximum values of 1000 pF or so. 

    Polystyrene Film Capacitor:   

    Polystyrene capacitors are a relatively cheap form of capacitor but offer a close tolerance capacitor where needed. 

    They are tubular in shape resulting from the fact that the plate / dielectric sandwich is rolled together, but this adds inductance limiting their frequency response to a few hundred kHz. 

    They are generally only available as leaded electronics components.

    Polyester Film Capacitor:   

    Polyester film capacitors are used where cost is a consideration as they do not offer a high tolerance. 

    Many polyester film capacitors have a tolerance of 5% or 10%, which is adequate for many applications. 
    They are generally only available as leaded electronics components. 

    Metallised Polyester Film Capacitor:   

    This type of capacitor is a essentially a form of polyester film capacitor where the polyester films themselves are metallised.

    The advantage of using this process is that because their electrodes are thin, the overall capacitor can be contained within a relatively small package. 

    The metallised polyester film capacitors are generally only available as leaded electronics components.

    Polycarbonate capacitor:   

    The polycarbonate capacitors has been used in applications where reliability and performance are critical. 

    The polycarbonate film is very stable and enables high tolerance capacitors to be made which will hold their capacitance value over time. In addition they have a low dissipation factor, and they remain stable over a wide temperature range, many being specified from -55°C to +125°C. 

    However the manufacture of polycarbonate dielectric has ceased and their production is now very limited. 

    Polypropylene Capacitor:   

    The polypropylene capacitor is sometimes used when a higher tolerance type of capacitor is necessary than polyester capacitors offer. 

    As the name implies, this capacitor uses a polypropylene film for the dielectric. One of the advantages of the capacitor is that there is very little change of capacitance with time and voltage applied. 

    This type of capacitor is also used for low frequencies, with 100 kHz or so being the upper limit. They are generally only available as leaded electronics components. 

    Glass capacitors:   

    As the name implies, this capacitor type uses glass as the dielectric. 

    Although expensive, these capacitors offer very high levels or performance in terms of extremely low loss, high RF current capability, no piezo-electric noise and other features making them ideal for many performance RF applications. 

    Supercap:   

    Also known as a supercapacitor or ultracapacitor, as the name implies these capacitors have very large values of capacitance, of up to several thousand Farads. 

    They find uses for providing a memory hold-up supply and also within automotive applications. 



    Arduino based Automatic Light Controller

    Automatic light controller offers energy saving and convenience in the areas with a photo sensor (LDR). LDR senses the ambient light conditions in the surrounding area and switches ON-OFF the lighting load. The darkness level in the surrounding is settable.

    It is in-built with an additional PIR Sensor which TURNS ON Light in the presence of human and switches OFF after 10 seconds if no human detected for energy saving operation.

    Thus it provides artificial light only when it is needed. This reduces the large amount of energy wastage and helps in making the most energy efficient lighting.

    Components Needed

    The following components are required for making an automatic light bulb controller.

    1.  Arduino Uno
    2. PIR Sensor.
    3. LDR Sensor.
    4. 1k & 10k Resistors
    5. 12V Relay
    6. BC548 Transistor
    7. Switches.

    Circuit Diagram

    Circuit is constructed with PIR sensor, LDR and Arduino. Light Load is connected to Relay. Manual on off is possible with given switches.

    Program Code

    Day night Switch with Occupancy Sensor (Automatic Light Controller)
    const int RELAY =12;   //Lock Relay or motor
    //Key connections with arduino
    const int on_key =3;
    const int off_key =2;
    int counter =0, manual =0;
    //Sensor Connections
    const int LDR = A5 ;
    const int PIR =4;
    //=================================================================
    // SETUP
    //=================================================================
    void setup (){
    pinMode ( RELAY , OUTPUT );
    pinMode ( on_key , INPUT );
    pinMode ( off_key , INPUT );
    pinMode ( PIR , INPUT );
    //Pull up for setpoint keys
    digitalWrite ( on_key , HIGH );
    digitalWrite ( off_key , HIGH );
    digitalWrite ( PIR , HIGH );
    digitalWrite ( RELAY , LOW );        //Turn off Relay
    }   //
    =================================================================
    // LOOP
    //=================================================================
    void loop (){
    //Turn on Lights if Motion is detected and Light intensity is low
    if( digitalRead ( PIR )== HIGH )
    {
    counter =1000; //Set 10 Seconds time out counter
    if( counter >15) //Motion detected for 1.5 Seconds
    {
    if( analogRead ( LDR )>512) //Light intensity is low
    {
    digitalWrite ( RELAY , HIGH ); //Turn on Lights
    }
    }
    } counter --;
    if( counter ==0)
    {
    if( manual ==0) //Check that it is not manually turned on
    {
    digitalWrite ( RELAY , LOW );
    }
    }
    //Get user input for setpointsif( digitalRead ( on_key )== LOW )
    {
    digitalWrite ( RELAY , HIGH ); //Turn on Lights
    manual =1; //Manually it is turned on
    }     if(digitalRead ( off_key )== LOW )
    {
    digitalWrite ( RELAY , LOW ); //Turn off Lights
    manual =0;
    } delay (10); //Update at every 10mSeconds
    }//
    =================================================================

    Why Voltage Control is Important?


    In a modern power system, electrical energy  from the generating station is delivered to the ultimate consumers through a network of transmission and distribution. 

    For satisfactory operation of motors, lamps and other loads, it is desirable that consumers are supplied with substantially constant voltage. 

    Too wide variations of voltage may cause erratic operation or even malfunctioning of consumer's appliances. 

    To safeguard the interest of the consumers, the government in each country has enacted a law in this regard. The statutory limit of voltage variation is 6% of declared voltage at consumer's terminals.

    Causes of Voltage Variation

    The principal cause of voltage variation at consumer's premises is the change in load on the supply system. 

    When the load on the system increases, the voltage at the consumer's terminals falls due to the increased voltage drop in 

    (i) alternator synchronous impedance 
    (ii) transmission line 
    (iii) transformer impedance 
    (iv) feeders and
    (v) distributors. 

    The reverse would happen should the load on the system decrease. These voltage variations are undesirable and must be kept within the prescribed limits (i.e. i 6% of the declared voltage). 

    This is achieved by installing voltage regulating equipment at suitable places in the power system.

    Importance of Voltage Control

    When the load on the supply system changes, the voltage at the consumer's terminals also changes.

    The variations of voltage at the consumer’s terminals are undesirable and must be kept within prescribed limits for the following reasons:

    • In case of lighting load, the lamp characteristics are very sensitive to changes of voltage. For instance, if the supply voltage to an incandescent lamp decreases by 6% of rated value, then illuminating power may decrease by 20%. On the other hand, if the supply voltage is 6% above the rated value, the life of the lamp may be reduced by 50% due to rapid deterioration of the filament.

    • In case of power load consisting of induction motors, the voltage variations may cause erratic operation. If the supply voltage is above the normal, the motor may operate with a saturated magnetic circuit, with consequent large magnetizing current, heating and low power factor. On the other hand, if the voltage is too lo\v, it will reduce the starting torque of the motor considerably.

    • Too wide variations of voltage cause excessive heating of distribution transformers. This may reduce their ratings to a considerable extent.

    It is clear from the above discussion that voltage variations in a power system must be kept to minimum level in order to deliver good service to the consumers. 

    With the trend towards larger and larger interconnected system, it has become necessary to employ appropriate methods of voltage control.

    Difference b/w Microprocessor and Microcontroller

    The term microprocessor and microcontroller have always been confused with each other. Both of them have been designed for real time application. They share many common features and at the same time they have significant differences. 

    Both the IC’s i.e., the microprocessor and microcontroller cannot be distinguished by looking at them.  They are available in different version starting from 6 pin to as high as 80 to 100 pins or even higher depending on the features.

    Microprocessor is an IC which has only the CPU inside them i.e. only the processing powers such as Intel’s Pentium 1,2,3,4, core 2 duo, i3, i5 etc. These microprocessors don’t have RAM, ROM, and other peripheral on the chip. A system designer has to add them externally to make them functional. Application of microprocessor includes Desktop PC’s, Laptops, notepads etc.

    The microcontroller incorporates all the features that are found in microprocessor. The important thing is that microcontroller has built in ROM, RAM, Input Output ports, Serial Port, timers, interrupts and clock circuit. A microcontroller is an entire computer manufactured on a single chip. 
    For example, microcontrollers are used as engine controllers in automobiles and as exposure and focus controllers in cameras.

    Microprocessor V/S Microcontroller

    • It is very clear from figure above that in microprocessor we have to interface additional circuitry for providing the function of memory and ports. 
      • For example we have to interface external RAM for data storage, ROM for program storage, programmable peripheral interface (PPI) 8255 for the Input Output ports, 8253 for timers, USART for serial port. 

    • While in the microcontroller RAM, ROM, I/O ports, timers and serial communication ports are in built. Because of this it is called as “system on chip”. 
      • So in microcontroller there is no necessity of additional circuitry which is interfaced in the microprocessor because memory and input output ports are inbuilt in the microcontroller. 

      • Microcontroller gives the satisfactory performance for small applications. But for large applications the memory requirement is limited because only 64 KB memory is available for program storage.  
        • So for large applications we prefer microprocessor than microcontroller due to its high processing speed.

        Criteria for Selection of a Microcontroller in Embedded System

        Criteria for selection of microcontroller in any embedded system is as following:

        • Meeting the computing needs of task at hand efficiently and cost effectively
          • Speed of operation
          • Packing
          • Power consumption
          • Amount of RAM and ROM on chip
          • No. of I/O pins and timers on chip
          • Cost

        •  Availability of software development tools such as compiler, assembler and debugger.

        Transformer Oil Sampling / Testing : Video Training



        Oil testing in a transformer is just as a blood test in human body. A blood test provides a doctor with a wealth of information about the health of a patient. A sample of transformer oil, taken correctly, can tell service engineers a great deal about the condition of a transformer.

        Oil is used both to cool the transformer and to insulate internal components. Because it bathes every internal component, the oil contains a great deal of diagnostic information. So a laboratory analysis of a sample can provide advance warning of developing conditions such as tapchanger arcing.

        The oil tests can uncover several potential problems within a transformer. The same sample can contain evidence of soluble contaminants, dielectric contaminants, and acid materials present in the oil. With so much riding on transformer maintenance, it's important to conduct and complete all of the recommended tests.

        This is a video training series on transformer oil sampling or testing done by TxMonitor.

        Transformer Oil Sampling - Part 1: Introduction

        As always, safety first. We start our Transformer Oil Sampling series with a number of highlights on health, environmental and safety considerations when performing these tasks.

        Please these are only guidelines and do not replace adequate competency and skills. Always follow the relevant company and legislative requirements for your particular work context.


        Transformer Oil Sampling - Part 2: Bottle Sample

        There are some sampling techniques better suited for certain types of oil tests than others. 

        The techniques described in this video should be sufficient to sample oil when common chemical and physical properties are analysed, such as Breakdown Voltage, Interfacial Tension, Acidity (or Neutralization Number), Colour and Visual Inspection.


        Transformer Oil Sampling - Part 3: Glass Syringe

        A sampling technique proven to produce repeatable results when testing the Dissolved Gas content or Moisture (Water) Content in the oil is by sampling with a Glass Syringe.

        In this video we'll learn how to execute this technique.


        Transformer Oil Sampling - Part 4 : The Buchholz Relay

        In this video, the fourth and final of the Oil Sampling series, you'll learn how to extract a gas and oil sample from a Buchholz gas accumulation relay through a sampling device.




        IEC 61850 - Features and Advantages


        Communication plays an important role in the real time operation of a power system. In the beginning, telephone was used to communicate line loadings back to the control center as well as to dispatch operators to perform switching operations at substations. 

        With the entry into a digital age, we needed the technology to cater to the hot requirements, which are;

        • High-speed IED to IED communication
        • Multi-vendor interoperability
        • Support for File Transfer
        • Auto-configurable / configuration support
        • Support for security

        Given these requirements, work on next generation communication architecture began with the development of the Utility Communication Architecture (UCA) in 1988.

        The result of this work was a profile of “recommended” protocols for the various layers of the International Standards Organization (ISO) Open System Interconnect (OSI) communication system model. 

        The concepts and fundamental work done in UCA became the foundation for the work done in the IEC Technical Committee Number 57 (TC57) Working Group 10 (WG10), which resulted in the International Standard – IEC 61850 – Communication Networks and Systems in Substations.

        Today, IEC 61850 is a standard for the design of electrical substation automation and it has been defined in cooperation with manufacturers and users to create a uniform, future-proof basis for the protection, communication and control of substations. 

        IEC 61850 meets the requirements for an integrated Information Management, providing the user with consistent Knowledge of the System on-line rather than just Gigabytes of raw data values. IEC 61850 defines standardized Information Models across vendors and a comprehensive configuration standard (SCL – System Configuration Language).

        Features of IEC 61850:

        Some of the features of IEC 61850 are given below.
        • Data Modeling: Primary process objects as well as protection and control functionality in the substation is modeled into different standard logical nodes which can be grouped under different logical devices. There are logical nodes for data/functions related to the logical device (LLN0) and physical device (LPHD).
        • Reporting Schemes: There are various reporting schemes (BRCB & URCB) for reporting data from server through a server-client relationship which can be triggered based on pre-defined trigger conditions
        • Fast Transfer of events: Generic Substation Events (GSE) are defined for fast transfer of event data for a peer-to-peer communication mode. This is again subdivided into GOOSE & GSSE.
        • Setting Groups: The setting group control Blocks (SGCB) is defined to handle the setting groups so that user can switch to any active group according to the requirement.
        • Sampled Data Transfer: Schemes are also defined to handle transfer of sampled values using Sampled Value Control blocks (SVCB)
        • Commands: Various command types are also supported by IEC 61850 which include direct & select before operate (SBO) commands with normal and enhanced securities.
        • Data Storage: Substation Configuration Language (SCL) is defined for complete storage of configured data of the substation in a specific format.

        Advantages of IEC 61850

        The main advantages of using IEC61850 include:
        • Offering a complete set of specifications covering all communication issues inside a substation.
        • Meeting the requirement for an integrated information management providing the user with consistent knowledge of the system on line, rather than just gigabytes of raw data.
        • Inter-operability between various manufacturers’ IED’s, thus forming an integrated system.
        • Substation Configuration Language(SCL)
        • Lowering installation and maintenance costs, with self-describing devices that reduce manual configuration.
        • Support for functions difficult to implement otherwise.

        How to Make a Digital Lock Using Arduino?

        Digital code locks are most common on security systems. An electronic lock or digital lock is a device which has an electronic control assembly attached to it. They are provided with an access control system. This system allows the user to unlock the device with a password. The password is entered by making use of a keypad. The user can also set his password to ensure better protection.

        In this project major components include a keypad, LCD and the controller Arduino. This article describes the making of an electronic code lock using arduino.

        Components Required

        The following components are required for making the digital code lock
        1. Arduino Uno
        2. 16x2 LCD Display
        3. 4x4 keypad
        4. Relay
        5. 1K Resistors Qty. 3
        6. BC548
        7. LEDs

        Digital Code Lock Circuit

        Code lock circuit is constructed around Arduino Uno, using LCD and keypad. 

        LCD and keypad forms the user interface for entering the password and displaying related messages such as “Invalid password”, “Door open”, etc. 

        Two LEDs are provided to indicate the status of door whether it is locked or open. To operate latch/lock we are using Relay which can be connected to the electronic actuator or solenoid.


        Program Code for Arduino Digital Code Lock

        Program is constructed using two libraries “LiquidCrystal” and “Keypad”. 

        Program have different modules, Setup, Loop, Lock. 

        In setup we initialize all the IO connections and LCD, Keypad. 

        In main loop we are taking pressed keys in array “code[]”, Once the four digits are entered we stop accepting keys. 

        We are using numeric keys and ‘C’ , “=” key. ‘C’ key is used to lock or clear the display incase wrong password is entered. 

        We can hide the entered password by putting Star character ‘*’.

        After entering password ‘=’ key acts as ok. If password is correct door is kept unlocked for few seconds. If it is incorrect message will be displayed.

        /* Digital Code Lock Demo */
        #include <Keypad.h>
        #include <LiquidCrystal.h>
        // initialize the library with the numbers of the interface pins
        LiquidCrystal lcd (9, 8, 7, 6, 5, 4);
        const byte ROWS = 4; //four rows
        const byte COLS = 4; //four columns
        //define the cymbols on the buttons of the keypads
        char hexaKeys [ ROWS ][ COLS ] = {
        {'7','8','9','/'},
        {'4','5','6','*'},
        {'1','2','3','-'},
        {'C','0','=','+'}
        };
        byte rowPins [ ROWS ] = {3, 2, 19, 18}; //connect to the row pinouts of the keypad
        byte colPins [ COLS ] = {17, 16, 15, 14}; //connect to the column pinouts of the keypad
        //initialize an instance of class NewKeypad
        Keypad customKeypad = Keypad ( makeKeymap ( hexaKeys ), rowPins , colPins , ROWS ,
        COLS );
        const int LED_RED =10; //Red LED
        const int LED_GREEN =11; //Green LED
        const int RELAY =12; //Lock Relay or motor
        char keycount =0;
        char code [4]; //Hold pressed keys
        //=================================================================
        // SETUP
        //=================================================================
        void setup (){
        pinMode ( LED_RED , OUTPUT );
        pinMode ( LED_GREEN , OUTPUT );
        pinMode ( RELAY , OUTPUT );
        // set up the LCD's number of columns and rows:
        lcd . begin (16, 2);
        // Print a message to the LCD.
        lcd . print ("Password Access:");
        lcd . setCursor (0,1); //Move coursor to second Line
        // Turn on the cursor
        lcd . cursor ();
        digitalWrite ( LED_GREEN , HIGH ); //Green LED Off
        digitalWrite ( LED_RED , LOW ); //Red LED On
        digitalWrite ( RELAY , LOW ); //Turn off Relay (Locked)
        }//
        =================================================================
        // LOOP
        //=================================================================
        void loop (){
        char customKey = customKeypad . getKey ();
        if ( customKey && ( keycount <4) && ( customKey !='=') && ( customKey !='C')){
        //lcd.print(customKey); //To display entered keys
        lcd . print ('*'); //Do not display entered keys
        code [ keycount ]= customKey ;
        keycount ++;
        }
        if(customKey == 'C') //Cancel/Lock Key is pressed clear display and lock
        {
        Lock (); //Lock and clear display
        }
        if(customKey == '=') //Check Password and Unlock
        {
        if(( code [0]=='1') && ( code [1]=='2') && ( code [2]=='3') && ( code [3]=='4')) //Match the
        password. Default password is “1234”
        {
        digitalWrite ( LED_GREEN , LOW ); //Green LED Off
        digitalWrite ( LED_RED , HIGH ); //Red LED On
        digitalWrite ( RELAY , HIGH ); //Turn on Relay (Unlocked)
        lcd . setCursor (0,1);
        lcd . print ("Door Open ");
        delay (4000); //Keep Door open for 4 Seconds
        Lock ();
        }
        else
        {
        lcd . setCursor (0,1);
        lcd . print ("Invalid Password"); //Display Error Message
        delay (1500); //Message delay
        Lock ();
        }
        }
        }
        //=================================================================
        // LOCK and Update Display
        //=================================================================
        void Lock ()
        {
        lcd . setCursor (0,1);
        lcd . print ("Door Locked ");
        delay (1500);
        lcd . setCursor (0,1);
        lcd . print (" "); //Clear Password
        lcd . setCursor (0,1);
        keycount =0;
        digitalWrite ( LED_GREEN , HIGH ); //Green LED Off
        digitalWrite ( LED_RED , LOW ); //Red LED On
        digitalWrite ( RELAY , LOW ); //Turn off Relay (Locked)
        }

        Basic Relays - Electromagnetic Attraction and Induction Relays

        Most of the relays used in the power system operate by virtue of the current and/or voltage supplied by current and voltage transformers connected in various combinations to the system element that is to be protected. 

        Through the individual or relative changes in these two quantities, faults signal their presence, type and location to the protective relays. 

        Having detected the fault, the relay operates the trip circuit which results in the opening of the circuit breaker and hence in the disconnection of the faulty circuit.

        Most of the relays in service on electric power system today are of electro-mechanical type. They work on the following two main operating principles :

        1. Electromagnetic attraction 
        2. Electromagnetic induction

        Practically, a relay is an amplifier, since it is able to control a high power output through a low power input. They were first invented in 1835 by Joseph Henry, the man who also discovered electromagnetic induction and whose name is currently being used for the SI unit of inductance.

        Electromagnetic Attraction Relays


        Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by DC or AC quantities. 

        The important types of electromagnetic attraction relays are:
        • Attracted armature type relay
        • Solenoid type relay
        • Balanced beam type relay

        Induction Relays

        Electromagnetic induction relays operate on the principle of induction motor and are widely used forprotective relaying purposes involving a.c. quantities. 

        They are not used with d.c. quantities owing to the principle of operation. 

        An induction relay essentially consists of a pivoted aluminium disc placed in two alternating magnetic fields of the same frequency but displaced in time and space. 

        The torque is produced in the disc by the interaction of one of the magnetic fields with the currents induced in the disc by the other.

        The following three types of structures are commonly used for obtaining the phase difference in the fluxes and hence the operating torque in induction relays :
        • shaded-pole structure
        • watthour-meter or double winding structure
        • induction cup structure
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