Formulae

Question 1

Once you have obtained the values [R1+R2] {0.21Ω} and [R1+Rn] {0.16Ω}, what calculation should be used to determine the resistance of the CPC (R2)?

Explain how*.

Answer 1

[R1+Rn] / 2 = R1
[R1+R2] - R1= R2

What is happening here, is because the neutral and line conductors will have the same CSA and roughly the same length, the first calculation can be divided by 2 to provide the resistance of the line conductor on its own.

Using this, we can subtract this value from the second calculation to determine the individual value of the CPC (R2).

For eg:
[R1+Rn] / 2 = 0.16Ω / 2 = 0.08Ω
R1= 0.08Ω
[R1+R2] - R1 = 0.21Ω - 0.08Ω = 0.13Ω
R2= 0.13Ω

Question 2

If a ring final circuit is contstructed using twin & earth cable, how much higher will the R2 value be in resistance than R1 or Rn?

Answer 2

Approximately 1.67 times higher than the other two readings.
This is because of the smaller CSA of the CPC in twin and earth cables.
The CPC of twin and earth is 1.5mm compared to 2.5mm of the line and neutral.

Question 3

When testing the individual sockets of a final ring circuit…

What will the resistance be at each socket, if everything is correctly wired, as compared to the end-to-end readings?

When measuring the line and neutral conductors

Answer 3

The resistance will approximately be half the resistance of the end-to-end readings.

(R1 + Rn)/4
or
R1/2

For eg:
R1=0.67Ω
0.67 / 2 = 0.335Ω

Question 4

What is the current carrying capacity of a final ring circuit wired using 2.5mm² PVC/PVC?
What is the CSA?

Remember, FRC has two live conductors

Answer 4

54A

5.0mm²

Question 5

What is the formula for woking out the resistance of a circuit?

Answer 5

R = V / I
Resistance is equal to Voltage divided by Current.

Question 6

What is the formula for working out the current of a circuit?

(Ohm’s law)

Answer 6

I = V / R
Current is equal to Voltage divided by Resistance.

Question 7

What is the formula for working out the voltage of a circuit?

(Ohm’s law)

Answer 7

V = I x R
Voltage is equal to Current multiplied by Resistance.

Question 8

What is the formula for working out the power in a circuit?

Answer 8

W = V x I
Wattage is equal to Voltage multiplied by Current.

Question 9

What is the formula for working out the Voltage in a circuit?

(Watt’s law)

Answer 9

V = W / I
Voltage is equal to Wattage divided by Current.

Question 10

What is the formula for working out the Current in a circuit?

(Watt’s law)

Answer 10

I = W / V
Current is equal to Current divided by Voltage.

Question 11

What is the formula for calculating the resistance for a circuit comprising of multiple resistors connected in series?

Answer 11

R₁ + R₂ + Rₙ … = Rₜ
The total resistance of this circuit arrangement is equal to the sum of all the resistances on the circuit.

Question 12

What is the formula for calculating the resistance for a circuit comprising two resistors connected in parallel?

Answer 12

Rₜ = R₁x R₂ / R₁ + R₂
The resistance of two resistors connected in paralell uses this formula.

Question 13

What is the formula for calculating the resistance for a circuit comprising of more than two resistors connected in parallel?

Answer 13

Rₜ = 1/Rₜ = 1/R₁ + 1/R₂ + 1/R₂ + 1/Rₙ
To calculate the total resistance for more than two resistors in paralell, you must use the reciprocal to calculate the total resistance of the circuit.

Question 14

True or false:
When calculating the equivelant resistance or total resistance of a circuit (Rₜ) with paralell resistors, this figure will always be higher than the smallest individual resistor.

Answer 14

False.
The total resistance of the circuit (Rₜ) will always be lower than the smallest individual resistor value. This is because the calculation of this figure uses reciprocals, which makes the total resistance really small.

Question 15

What is the formula for calculating the current at each point of a circuit with resistors connected in parallel?

What is the name for the law which detemines this calculation?

Answer 15

Iₜ = I₁ + I₂ + I₃ … + Iₙ
The current split across the various resistors is added together.

This law is called Kirchoff’s Law and it states that the current going into a circuit is not lost at the end of a circuit.

Question 16

What formula would you use to calculate the Prospective Short Circuit Current (PSCC)?

Answer 16

Supply voltage / (R₁+R₂)

Question 17

What is the symbol which denotes the supply voltage?

Answer 17

Uₒ

Question 18

What is the equation used to work out the design current (Ib) for a single phase supply?

Answer 18

Ib = Power/Uₒ

Uₒ being the supply voltage

Question 19

What is the equation used to work out the design current (Ib) for a three phase supply?

Answer 19

Ib = Power / (√ 3 x Uₒ)

Question 20

Due to a highly inductive or capacitive load, AC circuits may have a poor Power Factor (PF), what alternate equation would be needed for ascertaining the design current (Ib) of a single phase supply?

Answer 20

Ib = Power / (Uₒ x PF)

Question 21

Due to a highly inductive or capacitive load, AC circuits may have a poor Power Factor (PF), what alternate equation would be needed for ascertaining the design current (Ib) of a three phase supply?

Answer 21

Ib = Power / (√3 x Uₒ x PF)

Question 22

What does the symbol Rₜ* denote?

* Imagine this is a capital T, because apparently unicode is annoying.

Answer 22

The symbol Rₜ denotes the total resistance of an installation/ circuit.

Question 23

What equation would you use to work out the PSSC of a circuit?

Answer 23

PSSC = Uₒ (or Vₛ) / Rₜ

* Vₛ can also be used to denote supply voltage.