Electrical Technicians Question & Answers
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What type of Cable gland is used in Oil Field?
In explosion proof areas double compression glands are used to avoid the gasses entering in the electrical apparatus.
What is difference between fuse and circuit breaker?
Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple numbers of times.
Why we use AC system in India why not DC?
The output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. Thirdly the cost of plant associated with AC transmission (Circuit breakers, transformers etc) is much lower than the equivalent of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
What is the difference between MCB & MCCB, Where it can be used?
MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB molded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.
What is Automatic Voltage Regulator (AVR)?
AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
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.
What is SF6 Circuit Breaker?
SF6 is Sulpher Hexa Flouride gas.This gas is used as arc quenching medium in a Circuit breaker means SF6 CB.
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.
What is the definition of EExd IIB T5?
Ex ‘d’ II B T5 is Explosion Gas Group Temperature
Proof II B 1000C
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.
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.
Explain different Insulation classes and the corresponding temperature rise allowed?
Y – 0900C, A – 1050C, E – 1200C,
B – 1300C, F – 1550C, H – 1800C,
C – 1800C
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
What is the gas group classification?
Methane -I, Propane -IIA, Ethylene -IIB, Hydrogen, Carbon disulphide and Acetylene -IIC
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.
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.
What is the unit of Voltage, Current, Speed, and Frequency?
Voltage - Volt
Current - Ampere
Speed - Revolutions Per Minute (RPM)
Frequency - Hertz
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)
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
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.
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.
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
What type of Cable joint kits used?
Cold Compound joint kits, Hot Compound joint kits and Heat Shrinkable joint kits
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
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.
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.
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
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
What is UPS?
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
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
Why the battery room is normally considered for Hazardous area classification?
Because of Hydrogen evolution and it will come under gas group II C
How do you check the battery condition?
By conducting Discharge Test
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
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
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
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
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
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
What is vaccum currcuit breaker.define with cause and where be use it Device?
A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionized and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vaccum gas. Since the air in the CB is having vaccum pressure the arc formation is interrupted. VCB's can be used up to 11kv.
Why ELCB can't work if N input of ELCB do not connect to ground?
ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.
What is use of lockout relay in HT voltage?
A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.
What is reverse power relay?
Reverse Power flow relay are used in generating station’s protection. A generating station is supposed to feed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.
How tube light circuit is connected and how it works?
A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.
Why when birds sit on transmission lines or current wires doesn't get shock?
It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock. so if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded. so that means that human who touch the line closes the circuit between phase and neutral.
Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses?
Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.
What is an exciter and how does it work?
There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator.Rotory exciter is an additional small generator mounted on the shaft of main generator. If it is dc generator, it will supply dc to the rotory poles through slip ring and brushes (conventional alternator). if it is an ac exciter, out put of ac exciter is rectified by rotating diodes and supply dc to main fixed poles.AC exciter is the ac generator whose field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.
What happens if we connect a capacitor to a generator load?
Connecting a capacitor across a generator always improves powerfactor, but it will help depends up on the engine capacity of the alternator, other wise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.
Explain the working principal of the circuit breaker?
Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit Arc is extinguished by the Arc Quenching media like air, oil, vaccum etc.
What is CT and PT? Where it is used?
The term C.T means current transformer, and the term P.T means potential transformer. In circuit where measurements of high voltage and high current is involved they are used there. Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device. So CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagnetic flux produced by primary with secondary. They work on the ratio to they are designed.E.g if CT’s of ratio 5000\5A and it has to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter and after measuring 8A we can calculate the primary current. Same system of the operation followed in PT but it is for measuring voltage.
what is meant by insulation voltage in cables? Explain it?
It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.
Why Delta Star Transformers are used for Lighting Loads?
For lighting loads, neutral conductor is must and hence the secondary must be star winding.The lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.
How electrical power is generated by an A.C Generator?
For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines or hydro turbines .When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature...due to polarity change of rotor poles(i.e.) N-S-N-S.
What are the types of power in electrical power?
There are normally three types of power are counted in electrical power. They are,
· Apparent power
· Active power
· Reactive power
