user58687: I am in the process of implementing power down alerting system suing Arduino UNO and ZMPT101B Single phase relay.
My code as follows,
#include <Filters.h> //Easy library to do the calculations

float testFrequency = 50;                     // test signal frequency (Hz)
float windowLength = 40.0/testFrequency;     // how long to average the signal, for statistist

int Sensor = 0; //Sensor analog input, here it's A0

float intercept = -0.04; // to be adjusted based on calibration testing
float slope = 0.0405; // to be adjusted based on calibration testing
float current_Volts; // Voltage

unsigned long printPeriod = 1000; //Refresh rate
unsigned long previousMillis = 0;

void setup() {
    Serial.begin( 9600 );    // start the serial port
}

void loop() {

    RunningStatistics inputStats;                //Easy life lines, actual calculation of the RMS requires a load of coding
    inputStats.setWindowSecs( windowLength );

    while( true ) {   
        Sensor = analogRead(A0);  // read the analog in value:
        inputStats.input(Sensor);  // log to Stats function

        if((unsigned long)(millis() - previousMillis) >= printPeriod) {
            previousMillis = millis();   // update time every second

            Serial.print( "\n" );

            current_Volts = intercept + slope * inputStats.sigma(); //Calibartions for offset and amplitude
            current_Volts= current_Volts*(40.3231);                //Further calibrations for the amplitude

            Serial.print( "\tVoltage: " );
            Serial.print( current_Volts ); //Calculation and Value display is done the rest is if you're using an OLED display

            if (current_Volts <229) {
                test_loop();
            }
        }
    }
}

void test_loop(){
    Serial.print( "Warning!! Server Room Power Down" );
    delay(1000);
}

I am getting the correct voltage reading. When power down it says power down. But when power got up, it still shows power down with older reading. I think the reason for this because my code unable to stop the loop. And if I remove the delay (1000) in test_loop function it works, But my intention to use this sms alerting system and for that I need this delay to stop unnecessary alerts.
My code for SMS Alerting,
Serial.print( "\tVoltage: " );
Serial.print( current_Volts ); //Calculation and Value display is done the rest is if you're using an OLED display

if (current_Volts <229) {
    SendMessage();
}

void test_loop() {
    mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
    delay(1000);  // Delay of 1000 milli seconds or 1 second
    mySerial.println("AT+CMGS=\"xxxxxxxxx\"\r"); // Replace x with mobile number
    delay(1000);
    mySerial.println("Warning!! Server Room Power Down");
    delay(100);
    mySerial.println((char)26);// ASCII code of CTRL+Z
    delay(150000);
    mySerial.println("AT+CMGD=2");
} 

This code works for the very first outage but after power up also it send power down. First part of serial priniting in test_loop code of Serial.print is used for debugging.

user58687: Got a answer for the issue, 
Attaching code for anyone interested. 
/*
   Design by Vinod Amarathunga 26.08.2018
   FSSG R&D Team
*/


#include <OneWire.h>
#include <DallasTemperature.h>
#include <SoftwareSerial.h>
#define ONE_WIRE_BUS 2
SoftwareSerial mySerial(11, 10); //SIM800L Rx Tx
char msg;
String textMessage;

bool D7_alarmSent = false;
bool Temp_alarmSent = false;


OneWire oneWire(ONE_WIRE_BUS);

DallasTemperature sensors(&oneWire);

void setup()
{
  mySerial.begin(9600);   // Setting the baud rate of GSM Module
  Serial.begin(9600);    // Setting the baud rate of Serial Monitor (Arduino)
  delay(100);
}

void PowerSMS()
{
  Serial.println("Warning!! Server Room Power Down");
  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Power Down");
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");


  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Power Down");
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Power Down");
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



}

void SendMessage()
{

  Serial.println("Warning!! Server Room Temperature is Critical");

  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1\"\r"); // Replace x with mobile number
  delay(1000);
  mySerial.println("Warning!! Server Room Temperature is Critical");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Temperature is Critical");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Temperature is Critical");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");




}


void loop()
{
  sensors.requestTemperatures(); // Send the command to get temperature readings
  delay(1000);
  Serial.println(String("Temp Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  Serial.println(String("Voltage Digital: ") + digitalRead(7));
  Serial.println(Temp_alarmSent ? "True" : "False" );
  Serial.println(D7_alarmSent ? "True" : "False" );
  Serial.println("***********");
  ReceiveMessage();


  if (digitalRead(7) == 1 || sensors.getTempCByIndex(0) > 29 || D7_alarmSent == true || Temp_alarmSent == true ) {
    if (digitalRead(7) == 1 ) {
      if (!D7_alarmSent) {
        D7_alarmSent = true;
        PowerSMS();
      }

    }

    else if (sensors.getTempCByIndex(0) > 29 ) {
      if (!Temp_alarmSent) {
        Temp_alarmSent = true;
        SendMessage();
      }


    }

    else {

      if (D7_alarmSent){                  //alarmSent == true means that previous state was "Power Down"
          SendMessage_After_Power_Down();          //Send message
          D7_alarmSent = false;
          Temp_alarmSent = false;             //Reset alarmstate
        }


    }
  }




}

void SendMessage_Normal()
{
  Serial.println("Temperature and Power is OK");

  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1\"\r");
  delay(1000);
  mySerial.println("Temperature and Power is OK");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Temperature and Power is OK");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);


  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Temperature and Power is OK");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);



}
void SendMessage_After_Power_Down(){

  Serial.println("Power Back Online");

  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1"\r");
  delay(1000);
  mySerial.println("Power Back Online");
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Power Back Online");
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);


  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Power Back Online");
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);






  }


void ReceiveMessage()
{
  if (mySerial.available() > 0)
  {
    mySerial.println("AT+CNMI=2,2,0,0,0"); // AT Command to recieve a live SMS
    delay(1000);
    textMessage = mySerial.readString();
    Serial.print(textMessage);
    delay(10);
  }
  if (textMessage.indexOf("Status") >= 0) {
    SendMessage_Normal();
    mySerial.println("AT+CMGD=2");

  }
}


58687: <p>I am in the process of implementing power down alerting system suing Arduino UNO and ZMPT101B Single phase relay.</p>

<p>My code as follows,</p>

<pre><code>#include &lt;Filters.h&gt; //Easy library to do the calculations

float testFrequency = 50;                     // test signal frequency (Hz)
float windowLength = 40.0/testFrequency;     // how long to average the signal, for statistist

int Sensor = 0; //Sensor analog input, here it's A0

float intercept = -0.04; // to be adjusted based on calibration testing
float slope = 0.0405; // to be adjusted based on calibration testing
float current_Volts; // Voltage

unsigned long printPeriod = 1000; //Refresh rate
unsigned long previousMillis = 0;

void setup() {
    Serial.begin( 9600 );    // start the serial port
}

void loop() {

    RunningStatistics inputStats;                //Easy life lines, actual calculation of the RMS requires a load of coding
    inputStats.setWindowSecs( windowLength );

    while( true ) {   
        Sensor = analogRead(A0);  // read the analog in value:
        inputStats.input(Sensor);  // log to Stats function

        if((unsigned long)(millis() - previousMillis) &gt;= printPeriod) {
            previousMillis = millis();   // update time every second

            Serial.print( "\n" );

            current_Volts = intercept + slope * inputStats.sigma(); //Calibartions for offset and amplitude
            current_Volts= current_Volts*(40.3231);                //Further calibrations for the amplitude

            Serial.print( "\tVoltage: " );
            Serial.print( current_Volts ); //Calculation and Value display is done the rest is if you're using an OLED display

            if (current_Volts &lt;229) {
                test_loop();
            }
        }
    }
}

void test_loop(){
    Serial.print( "Warning!! Server Room Power Down" );
    delay(1000);
}
</code></pre>

<p>I am getting the correct voltage reading. When power down it says power down. But when power got up, it still shows power down with older reading. I think the reason for this because my code unable to stop the loop. And if I remove the delay (1000) in test_loop function it works, But my intention to use this sms alerting system and for that I need this delay to stop unnecessary alerts.</p>

<p>My code for SMS Alerting,</p>

<pre><code>Serial.print( "\tVoltage: " );
Serial.print( current_Volts ); //Calculation and Value display is done the rest is if you're using an OLED display

if (current_Volts &lt;229) {
    SendMessage();
}

void test_loop() {
    mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
    delay(1000);  // Delay of 1000 milli seconds or 1 second
    mySerial.println("AT+CMGS=\"xxxxxxxxx\"\r"); // Replace x with mobile number
    delay(1000);
    mySerial.println("Warning!! Server Room Power Down");
    delay(100);
    mySerial.println((char)26);// ASCII code of CTRL+Z
    delay(150000);
    mySerial.println("AT+CMGD=2");
} 
</code></pre>

<p>This code works for the very first outage but after power up also it send power down. First part of serial priniting in test_loop code of Serial.print is used for debugging.</p>
 58687: <p>Got a answer for the issue, </p>

<p>Attaching code for anyone interested. </p>

<pre><code>/*
   Design by Vinod Amarathunga 26.08.2018
   FSSG R&amp;D Team
*/


#include &lt;OneWire.h&gt;
#include &lt;DallasTemperature.h&gt;
#include &lt;SoftwareSerial.h&gt;
#define ONE_WIRE_BUS 2
SoftwareSerial mySerial(11, 10); //SIM800L Rx Tx
char msg;
String textMessage;

bool D7_alarmSent = false;
bool Temp_alarmSent = false;


OneWire oneWire(ONE_WIRE_BUS);

DallasTemperature sensors(&amp;oneWire);

void setup()
{
  mySerial.begin(9600);   // Setting the baud rate of GSM Module
  Serial.begin(9600);    // Setting the baud rate of Serial Monitor (Arduino)
  delay(100);
}

void PowerSMS()
{
  Serial.println("Warning!! Server Room Power Down");
  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Power Down");
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");


  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Power Down");
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Power Down");
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



}

void SendMessage()
{

  Serial.println("Warning!! Server Room Temperature is Critical");

  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1\"\r"); // Replace x with mobile number
  delay(1000);
  mySerial.println("Warning!! Server Room Temperature is Critical");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Temperature is Critical");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Warning!! Server Room Temperature is Critical");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);
  mySerial.println("AT+CMGD=2");




}


void loop()
{
  sensors.requestTemperatures(); // Send the command to get temperature readings
  delay(1000);
  Serial.println(String("Temp Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  Serial.println(String("Voltage Digital: ") + digitalRead(7));
  Serial.println(Temp_alarmSent ? "True" : "False" );
  Serial.println(D7_alarmSent ? "True" : "False" );
  Serial.println("***********");
  ReceiveMessage();


  if (digitalRead(7) == 1 || sensors.getTempCByIndex(0) &gt; 29 || D7_alarmSent == true || Temp_alarmSent == true ) {
    if (digitalRead(7) == 1 ) {
      if (!D7_alarmSent) {
        D7_alarmSent = true;
        PowerSMS();
      }

    }

    else if (sensors.getTempCByIndex(0) &gt; 29 ) {
      if (!Temp_alarmSent) {
        Temp_alarmSent = true;
        SendMessage();
      }


    }

    else {

      if (D7_alarmSent){                  //alarmSent == true means that previous state was "Power Down"
          SendMessage_After_Power_Down();          //Send message
          D7_alarmSent = false;
          Temp_alarmSent = false;             //Reset alarmstate
        }


    }
  }




}

void SendMessage_Normal()
{
  Serial.println("Temperature and Power is OK");

  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1\"\r");
  delay(1000);
  mySerial.println("Temperature and Power is OK");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Temperature and Power is OK");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);


  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Temperature and Power is OK");
  mySerial.println(String("Reading : ") + sensors.getTempCByIndex(0) + String("*C"));
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);



}
void SendMessage_After_Power_Down(){

  Serial.println("Power Back Online");

  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No1"\r");
  delay(1000);
  mySerial.println("Power Back Online");
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);



  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No2\"\r");
  delay(1000);
  mySerial.println("Power Back Online");
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);


  mySerial.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  delay(1000);  // Delay of 1000 milli seconds or 1 second
  mySerial.println("AT+CMGS=\"No3\"\r");
  delay(1000);
  mySerial.println("Power Back Online");
  delay(100);
  mySerial.println((char)26);// ASCII code of CTRL+Z
  delay(1000);






  }


void ReceiveMessage()
{
  if (mySerial.available() &gt; 0)
  {
    mySerial.println("AT+CNMI=2,2,0,0,0"); // AT Command to recieve a live SMS
    delay(1000);
    textMessage = mySerial.readString();
    Serial.print(textMessage);
    delay(10);
  }
  if (textMessage.indexOf("Status") &gt;= 0) {
    SendMessage_Normal();
    mySerial.println("AT+CMGD=2");

  }
}
</code></pre>
