2013 ~ EEE Interview Tips

September 11, 2013

Draw the block diagram of a mobile communication system.

10:16 PM Digital Electronics, Telecommunication Engineering 1 comment

block diagram of a mobile communication system

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Useful Electrical Equations!

2:41 PM Useful Equations No comments

Useful Electrical Equations:

· For Sinusoidal Current: Form Factor = RMS Value/Average Value = 1.11

· For Sinusoidal Current: Peak Factor = Max Value/RMS Value = 1.414

· Average Value of Sinusoidal Current (Iav) = 0.637 x Im (Im = Max.Value)

· RMS Value of Sinusoidal Current (Irms) = 0.707 x Im (Im = Max.Value)

· A.C Current = D.C Current/0.636.

· Phase Difference between Phase = 360/ No of Phase (1 Phase=230/1=360°, 2 Phase=360/2=180°)

· Short Circuit Level of Cable in KA (Isc) = (0.094 x Cable Dia in Sq.mm) /√ Short Circuit Time (Sec)

· Max.Cross Section Area of Earthing Strip (mm2) = √(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K

K = Material Factor, K for Cu = 159, K for Al = 105, K for steel = 58 , K for GI = 80

· Most Economical Voltage at given Distance = 5.5 x √ ((km/1.6) + (kw/100))

· Cable Voltage Drop (%) =

(1.732 x current x (RcosǾ+jsinǾ) x 1.732 x Length (km) x 100) / (Volt(L-L) x Cable Run.

· Spacing of Conductor in Transmission Line (mm) = 500 + 18 x (P – P Volt) + (2 x (Span in Length)/50).

· Protection radius of Lighting Arrestor = √h x (2D-h) + (2D+L).

Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=V x t (V=1m/ms, t=Discharge Time).

· Size of Lighting Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (System Voltage/1.732).

· Maximum Voltage of the System = 1.1xRated Voltage (Ex. 66KV = 1.1 × 66 = 72.6KV)

· Load Factor = Average Power/Peak Power

· If Load Factor is 1 or 100% = This is best situation for System and Consumer both.

· If Load Factor is Low (0 or 25%) = you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.

· Demand Factor = Maximum Demand / Total Connected Load (Demand Factor <1)

· Demand factor should be applied for Group Load

· Diversity Factor =

Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)

Diversity factor should be consider for individual Load

· Plant Factor (Plant Capacity) = Average Load / Capacity of Plant

· Fusing Factor = Minimum Fusing Current / Current Rating (Fusing Factor>1).

· Voltage Variation (1 to 1.5%) = ((Average Voltage – Min Voltage) x 100)/Average Voltage

Ex: 462V, 463V, 455V, Voltage Variation= ((460 – 455) x 100)/455 = 1.1%.

· Current Variation (10%) = ((Average Current – Min Current) x 100)/Average Current

Ex: 30A,35A,30A, Current Variation = ((35-31.7) x 100)/31.7 = 10.4%

· Fault Level at TC Secondary

= TC (VA) x 100 / Transformer Secondary (V) x Impedance (%)

Motor Full Load Current = Kw /1.732 x KV x P.F x Efficiency

Sometimes we have seen that a ceiling fan suddenly starts moving in opposite direction with slow speed, what could be the defect?

10:17 AM Motor 1 comment

The starting winding circuit is open-circuited may be due to burnt winding or damaged capacitor or the running winding is damaged. The fan is running only on one winding. The blades and thrust of air in the room accelerated the blades in opposite direction. As we know that the single phase motors are not self-starting, they will continue to move in the direction in which these are initially accelerated.

What are the Functions of Substation Equipment?

11:25 PM Sub Station, Switchgear and Protection No comments

Functions of Substation Equipments:

Sl. No.	Equipment	Function
	Bus-Bar	Incoming & outgoing circuits Connected to bus-bar
	Circuit Breaker	Automatic switching during normal or abnormal conditions
	Isolators	Disconnection under no-load condition for safety, isolation and maintenance.
	Earthing switch	To discharge the voltage on deadlines to earth
	Current Transformer	To step-down currents for measurement, control & protection
	Voltage Transformer	To step-down voltages for measurement, control & protection
	Lightning Arrester	To discharge lightning over voltages and switching over voltages to earth
	Shunt reactor	To control over voltages by providing reactive power compensation
	Neutral-Grounding resistor	To limit earth fault current
	Coupling capacitor	To provide connection between high voltage line & PLCC equipment
	Line –Trap	To prevent high frequency signals from entering other zones.
	Shunt capacitors	To provide compensations to reactive loads of lagging power factors
	Power Transformer	To step-up or step-down the voltage and transfer power from one a.c. voltage another a.c. voltage at the same frequency.
	Series Capacitor	Compensation of long lines.

What is REF relay?

10:20 PM Switchgear and Protection No comments

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

What are the errors in Current Transformer?

10:18 PM Switchgear and Protection No comments

(a) Ratio error
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.

(b) Phase angle error:
Phase angle =180/π[(ImCos δ-I1Sin δ)/nIs]

What is an arc? Give the two methods of arc interruption?

10:16 PM Switchgear and Protection No comments

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition.

Methods of arc interruption:

High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished
Current zero method:-The arc is interrupted at current zero position that occurs 100 times a second in case of 50Hz power system frequency in ac.

What do you mean by current chopping?

10:12 PM Switchgear and Protection No comments

When interrupting low inductive currents such as magnetizing currents of transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

How to calculate voltage drop?

9:40 PM Basic Electricity, Power System No comments

How to calculate voltage drop:

In situations where the circuit conductors span large distances, the voltage drop is calculated. If the voltage drop is too great, the circuit conductor must be increased to maintain the current between the points. The calculations for a single-phase circuit and a three-phase circuit differ slightly.

Single-phase voltage drop calculation:

Three-phase voltage drop calculation:

OR 3øVD = (SQRT(3)*L*R*I)/1000

VD = Voltage drop (conductor temp of 75°C) in volts

VD% = Percentage of voltage drop (VD ÷ source voltage x 100). It is this value that is commonly called "voltage drop" and is cited in the NEC 215.2(A)(4) and throughout the NEC.

L = One-way length of the circuit's feeder (in feet)

R = Resistance factor per NEC Chapter 9, Table 8, in ohm/kft

I = Load current (in amperes)

Source voltage = The voltage of the branch circuit at the source of power. Typically the source voltage is either 120, 208, 240, 277, or 480 V.

Related Link: How much voltage drop is Acceptable?

How much voltage drop is Acceptable?

9:39 PM Basic Electricity, Power System No comments

The national Electrical Code (NEC) recommends that the combined voltage drop of the electrical system (brunch circuit and feeders) not exceed 5% for optimum efficiency. This recommendation not only can improve safety but can ensure proper equipment operation and power efficiency. Foe Example, In a 220 volt 5 ampere circuit, this means that there should be no more than a 11 V drop (109 volts) at the furthest outlet when the circuit is fully loaded.

Related Link: How to calculate voltage drop?