Cable sizing

Question 1

What are the different cable de-rating factors and their names:

Answer 1

CG= grouping factor, CA = Ambient temperature higher than 30 Ci = insulation Ct = operating temperature of the conductor Cg = semi enclosed fuses

Question 2

Explain IZ, IT, Ib, In,

Answer 2

Iz = the current carrying capacity of the cable for continuous service under the conditions of a particular installation after correction factors have been applied. It = Tabulated current carrying capacity of a particular type of cable in a single circuit at 30 degrees. Ib = the design current of a circuit for continuous use. In = the nominal value of a circuit protective device. Iz=In/(Ca*Cg*Ci*Cc)

Question 3

What is the formula for converter three-phase power to current

Answer 3

I =Power/(sqrt3*400)

Question 4

What is the difference between star and delta three phase connections

Answer 4

Star is most commonly used. Delta is used for larger loads such as three phase motors

Star configuration has a neutral point that can be used to connect single phase loads.

Question 5

What is the formula for calculating line to line voltage versus line to neutral voltage in a delta system. Same with line to line current.

Answer 5

VL=VP because there is no neutral

The current in any individual line = total current/sqrt3

Delta is used when no neutral is needed like the distribution system.

Delta is useful for controlling harmonic currents.

Question 6

What are the advantages and disadvantages of star and delta connection. What are they both used for?

Answer 6

Star has a neutral so can be used for two different voltage levels.

Not having a neutral saves the cost of cable in delta.

Delta controls the harmonics. Star is used for low voltage transformers. Delta is used for high voltage transformers.

Question 7

How do you solve instanteus three phase voltage in a balanced three phase system if you know instantaneous voltage and resistance.

Answer 7

The voltage in any phase varies in a sinusidal pattern. The three phases are 120 degrees out of phase with each other. If you know the instantaneous voltage in one phase you can multiply it by sine120 & sine 240 to find the voltage in the other two phases. Then you calculate the current in each phase like a single phase circuit.

Question 8

Explain a star-delta motor starter

Answer 8

A star-delta motor starter initially connects a motor in star which results in 230V accross the coils to reduce starting current and starting torque. After the motor is moving it switches over delta to increase the voltage hence current accross the motor.

Question 9

Explain the power triangle

Answer 9

3 phase RLC circuits can be analysed like a right angle triangle. The X component is the real power, the y component is the reactive power and the hypotenuse is the apparent power.

Question 10

Explain leading and lagging power factor

Answer 10

An inductive circuit where the current is behind the voltage is a lagging power factor and a capacitive circuit where the power is ahead of the voltage is a leading power factor. Laggin is preferable

Question 11

What is the difference between line voltage and phase voltage

Answer 11

Line voltage is between two of the three phases. Phase voltage is between one phase and neutral.

Question 12

How do you calculate three phase power from line voltage and line current

Answer 12

Sqrt3 * Line voltage * line current * Sine Theta

(Theta is angle between current and voltage)

Question 13

What is an RLC circuit

Answer 13

An AC ciruit with a resistor, a capacitor and an inductor

Question 14

What is the formula for inductive reactance

Answer 14

2 pi F L

2 * 3.141 * frequency * inductance

Question 15

What is the formula for capacitive reactance

Answer 15

1/(2 * pi * frequency * capacitance)

Question 16

Explain how you use supposition theory to calculate currents in branches of a DC circuit with multiple voltage or current sources.

Answer 16

In a circuit with two voltage sources then draw two circuits each with only one voltage source. Calculate the current flows through each branch. Then super-impose the results on top of each other adding the currents in each branch.

Question 17

What is the difference between line current and phase current in a three phase system. What is the difference between the two in a three phase system

Answer 17

Line current is the current that is flowing through the supply cable to the load. Phase current is flowing through the load. In a star system line current = phase current. In a delta system line current = phase current/1.732