ELECTRICAL QUESTIONS AND ANSWERS

01. What is the classification of hazardous area?

Hazardous area classified as Zone - 0, Zone – 1 and Zone – 2.

Zone – 0 an explosive gas-air mixture is present continuously or for long period.

Zone – 1 an explosive gas-air mixture is present under normal operation.

Zone – 2 an explosive gas-air mixture is not present under normal operation and if it occurs it exist only for short time.

02. What type of protection is used in hazardous area?

  

 Zone – 0      Ex ‘s’ (Specifically certified for used in Zone ‘0’)

         

 Zone – 1      Any type of protection suitable for Zone ‘0’ and Ex ‘d’, Ex ‘I’,

                     Ex ‘p’, Ex ‘e’.

 Zone – 2      Any type of protection suitable for Zone ‘0’ and Zone ‘1’ and

                     Ex ‘N’, Ex ‘O’, Ex ‘q’.

                             Ex ‘d’                   -       Flame proof or Explosion proof

                             Ex ‘e’                   -       Increased safety

                             Ex ‘N’ or ‘n’        -       Non sparking

                             Ex ‘I’                            Intrinsic Safety

                             Ex ‘p’                   -       Pressurization

                             Ex ‘O’                  -       Oil immersion

                             Ex ‘q’                   -       Sand or Powder filled

                             Ex ‘s’                   -       Special protection.

03. What is the definition of EExd IIB T5?

           

                               Ex ‘d’          II B                  T5

         Explosion     Gas Group         Temperature

              Proof            II B                  100⁰C

04. What are the different type of work permits? What precaution you take

while doing maintenance in equipments in Hazardous area?

Cold Work, Hot Work, Vehicle Entry & Vessel Entry permit. Interinsic Safe meters only should be used such megger, vibration meter, temperature meter and other ordinary meters shall not used unless the Hot Work Permit is obtained.

05. Explain IP 54, IP55 and IP65?

IP means degree of protection of the enclosures for electrical equipment in accordance with the Electrical Standards. The first numeral: - protection against solid bodies and the second numeral: - protection against liquids.

IP54: - 5 (indicate) Protected against dust (no harmful deposit) and

4 (indicate) Protected against projections of water from all directions

IP55: - The second 5 (indicate) Protected against jets of water from all directions and IP65: - The first 6 (indicate) completely protected against dust.

           

06. Explain different Insulation classes and the corresponding temperature rise allowed?

Y       –       090⁰C,        A       –       105⁰C,        E       –       120⁰C,

B       –       130⁰C,        F       –       155⁰C,        H       –       180⁰C,

C       –       180⁰C

07. What are IR, WR, KWH, OLR, MCC, MCB, MCCB, VCB, BOCB and

SF6?

          IR               –        Insulation Resistance

          WR             -        Winding Resistance

          KWH          -        Kilo Watt Hour

          OLR           -        Over Load Relay

          MCC          -        Motor Control Center

          MCB          -        Miniature Circuit Breaker

          MCCB       -        Moulded Case Circuit Breaker

          VCB           -        Vacuum Circuit Breaker

          BOCB        -        Bulk OIL Circuit Breaker

          SF6             -        Sulfer-Hexa Floride

08. What is the gas group classification?

          Methane       -I,                Propane       -IIA,            Ethylene       -IIB,

          Hydrogen, Carbon disulphide and Acetylene -IIC

           

09. What is AVO meter?

A meter is used to measuring the electrical circuit and the value of current, voltage and resistance is called AVO meter.

10. What is Bridge Megger? Where it is used?

Bridge Megger is used to measure the lowest value of resistances accurately and it is used to measure the motor winding resistance and transformer winding resistance during preventive maintenance. The winding resistance of the three windings should be equal.

11. What is the unit of Voltage, Current, Speed, and Frequency?

          Voltage         -        Volt

          Current         -        Ampere

          Speed           -         Revolutions Per Minute (RPM)

          Frequency    -         Hertz

12. What are the different size of the cables?

0.75, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 600 (Dimensions are in Square mm)

13. What are the Cables used in Oil Field?

          PVC            -       Ploy Vinyl Chloride

          XLPE          -       Cross Linked Poly Ethylene

          LC              -        Lead covered

          SWA           -       Steel wire armoured

          PILC           -       Paper Insulated Lead Covered Cable

          MICC          -       Mineral Insulated Copper Conductor

           

14. What is MICC? Where it is used? What mineral used for this cable?

          Mineral Insulated Copper Conductor and it is used for fire alarm systems and

Lightings systems at Hazardous area without using conduits and

Magnesium oxide is used as mineral.

15. Why Lead Covered Cables are used in Oil Fields?

          Lead Covered Cables are used in Oil Fields due to avoid damages of

PVC insulation by hydro-carbon gases and chemical corrosion.

16. What is the use of cable gland?

Cable glands are used to protect from mechanical damage, to give earth continuity and for entering in junction box, MCC etc.

17. How to check the cable fault?

·       Isolate the cable

·       Megger the cable

·       Trace the cable fault through cable fault locator and cable route through cable route    locator

·        If the cable resistance is high use the burnout test kit

·        Before cutting the cable use spiking gun for safety

18. What type of Cable gland used in Oil Field? 

In explosion proof areas double compression glands are used to avoid the gasses entering in the electrical apparatus.

19. What type of Cable joint kits used?

          Cold Compound joint kits, Hot Compound joint kits and Heat

          Shrinkable joint kits

  

20. Explain about Batteries, Voltage and Specific gravity of Cell?

Nickel Cadmium Batteries: - It is used in Refineries for its reliability, long life and less maintenance. No need to deal with H2So4 dangerous.

Lead Acid Batteries: - It is also used but life is limited, maintenance costs are high and handling H2So4 is too dangerous for its chemical concentration.

Nickel Cadmium Cell - 1.2 Volts, Sp. Gravity is 1200 in all conditions and Lead Acid Cell - 2.1 Volts, Sp. Gravity is in Full charge – 1280, Half charge – 1240 and Discharge - 1180

21. What will effect if you are connecting the batteries in Series and Parallel?

If the Batteries are connected in Series the voltage will be increase and the Batteries are connected in Parallel the Current will be increase.

22. What is the maintenance procedure of Batteries?

·        Obtain the work permit

·        Clean the battery terminals and apply petroleum jelly

·        Check the voltage per cell

·        Check the specific gravity of electrolyte with hydrometer

·        Check the electrolyte level

·        Check the electrolyte temperature

·        Check the total cell voltage

·        Close the work permit.

23. What is constant current charging?

Charging method in which current does not appreciably change against battery demand. Suitable for applications where a battery needs to be fully recharged in the short time

24. How do you decide the boost charge current rate for given 100Ampere

-hour rating of battery?

C/5 for 7 Hours

C is Ampere Hour of battery

i.e. 100AH / 5 = 20 Amps.

So, Keep the battery for 7 hours charging at the charging current 20 Ampere

25. What is UPS? Draw its block diagram.

Uninterrupted Power Supply is very essential for petrochemical / processing industries as all the controls are governed by the computer which determines the operation under all condition.

·        UPS have Converter, Inverter, A V R, Battery Bank, Static switch and Bypass Switch

·        The converter is feeding supply to the inverter

·        The battery bank connected in between converter and inverter

·        The static switch have supply on both end i.e. Converter supply and

A V R supply (Static switch is nothing - the two SCR is connected in opposite direction)

·        The inverter is always taking the load, the converter fails to feed supply to the inverter, it will work with the help of battery bank

·        If the inverter is fails to take the load, the A V R supply will take the load without power interruption through the static switch

·        During the maintenance of UPS, changeover the load to standby supply by using Bypass switch

                                                                                                

26. What is constant potential charging?

Charging method where constant voltage is applied and charge current is allowed to vary against battery demand. Usually the initial current is limited this being classified as modified constant potential charging

27. Why the battery room is normally considered for Hazardous area

classification?

Because of Hydrogen evolution and it will come under gas group II C

28. How do you check the battery condition?

          By conducting Discharge Test

29. What are the parts of motor?

Parts of motor are Stator, Rotor, Winding, Shaft, End shields, Fan, Fan Cover, Terminal Box, Bearings, Bearing housings, Lifting hook and Pulley key

30. What is the maintenance procedure of motor?

To carry out preventive maintenance of motor, the following procedures should be adopted: -

·        Work Permit should be obtained

·        Tag Number of motor to be checked before power isolation

·        Starter feeder should be isolated and locked

·        Control circuit fuse and neutral link should be removed

·        Caution notice to be fixed on the feeder as well on the control station

31. How can you find a motor is good with using of Multimeter & Megger?

          For good motor, the following value should be satisfied: -

·       Insulation Resistance value should be acceptable by using the Megger between winding to earth

·   Winding Resistance value should be equal on three windings by using the Multimeter

33. What is the acceptable insulation value of motors?

                Motor Voltage        Testing Voltage          Accepted IR Value

                   250 V                             500 V

                   440 V                             500V                               1.5 Mega ohm

                   3.3 KV                           1000V                             4.5 Mega ohm

                   11 KV                            1000 V                            12 Mega ohm

34. What are the standard problems faced with motors?

Over load, Short circuit on motor winding, Single phasing, Loose connection of power cables, Bearing problem and Vibration on a running motor

35. What are the protections provided for motors?

440 Volts Motors: –

Over Load Relay, Single phasing preventer, Stalling Relay, Thermal Over Load Relay and  Earth Fault (Short-circuit) Relay

3300Volts and 11000Volts Motors: –

Over Current Relay (Instantaneous), Thermal Over Current Relay, Stalling Relay, Earth Fault Relay, Negative Sequence Relay and Temperature Relay

36. What are the parts of Transformer?

·        Primary and secondary terminal boxes

·        Transformer main tank

·        Primary winding

·        Secondary winding

·        Cooling radiator tubes

·        Conservator tank

·        Explosion vent and temperature sensor

·        Buccholz relay

37. What are the protections of Transformer?

·        Buccholz relay

·        Over current relay

·        Earth fault relay

·        Differential relay

·        Restricted earth fault relay

·        Temperature sensor

38. What is the maintenance procedure of Transformer?

·        Obtain the work permit

·        Transfer the load to other transformer (if provided) or isolate the load

·        Switch ‘OFF’ the incoming supply of the transformer

·        Switch ‘ON’ the Earth Switch on primary and secondary side

·        Physically clean the transformer

·        Check the insulation resistance of primary and secondary winding and check the winding resistance

·        Check the Terminal tightness

·        Check the Earth connections of Transformer and measure the earth value

·        Collect the sample of Transformer Oil and carry out B.D.V. test

·        Controls to be cleaned with contact cleaner

·        Check the silica gel condition if it is pink color, it must be replaced

·        Check the HT and LT bushings for cracks

·        Switch ‘OFF’ the Earth Switch on primary and secondary side

·        Switch ‘ON’ the incoming supply of the transformer and give load

·        Close the work permit

39. What are the light fittings used in Gathering Centers & Booster Stations?

Incandescent lamp, Mercury high discharge lamp, Fluorescent lamp and Halogen lamp fittings are used in Gathering Centers & Booster Stations and it certified by BASEEFA

40. What is stroboscopic effect? How can you eliminate it?

In twin tube light set  while glowing the lamp, ring will move one by one inside the florescent tube this is called stroboscopic effect due to two chokes are connected nearby. To eliminate this effect connect one capacitor in series with one light circuit

41. What is the use of AVR in Generator?

The automatic voltage regulators are usually provided to maintain the generator voltage reasonably constant in spite of the fluctuating load i.e. to increase the voltage when the load is high and reduce the voltage when the load comes down

42. What are reasons to produce low voltage in Generators?

          The following possible reasons to produce low voltage in Generators: -

·        Loss of residual magnetism

·        Open circuit or loose connection in the exciter circuit also short circuited turns in field windings

·        Reversed field windings

·        Open or short circuit in armature windings

·        Excessive contact resistance on commutator due to incorrect grade of brushes or insufficient pressure of dirty commutator

·        Incorrect position of brush

43. What is use of Clip-on-meter?

The Tong Tester is called in other words as clip on meter. In this we can measure the current flowing in the circuit in each phase for any unbalance if is noticed. Can be used without breaking the circuit.

44. What are the conditions of synchronous between two Generators?

          The following conditions should be satisfied before synchronous generators

·        The frequency of both should be identical

·        The voltage of both must be same

·        The phase rotation in both machine should be same

·        Identical phase wire R, Y, B should be connected together al the exact instant when there is zero phase difference between them

·        For satisfactory parallel operation the characteristics of the two machines should be similar

45. How can you parallel two Power Transformers?

         

 The following conditions to be checked for parallel operation of 2 transformers

·        Primary and Secondary Voltage must be same as check tap changer position

·        Frequency must be same

·        Phase sequence must be same

·        Impedance of both transformers must be same

·        Vector group of both transformers should be same