Formulae Flashcards
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*.
[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
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?
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.
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
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
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
54A
5.0mm²
What is the formula for woking out the resistance of a circuit?
R = V / I
Resistance is equal to Voltage divided by Current.
What is the formula for working out the current of a circuit?
(Ohm’s law)
I = V / R
Current is equal to Voltage divided by Resistance.
What is the formula for working out the voltage of a circuit?
(Ohm’s law)
V = I x R
Voltage is equal to Current multiplied by Resistance.
What is the formula for working out the power in a circuit?
W = V x I
Wattage is equal to Voltage multiplied by Current.
What is the formula for working out the Voltage in a circuit?
(Watt’s law)
V = W / I
Voltage is equal to Wattage divided by Current.
What is the formula for working out the Current in a circuit?
(Watt’s law)
I = W / V
Current is equal to Current divided by Voltage.
What is the formula for calculating the resistance for a circuit comprising of multiple resistors connected in series?
R₁ + R₂ + Rₙ … = Rₜ
The total resistance of this circuit arrangement is equal to the sum of all the resistances on the circuit.
What is the formula for calculating the resistance for a circuit comprising two resistors connected in parallel?
Rₜ = R₁x R₂ / R₁ + R₂
The resistance of two resistors connected in paralell uses this formula.
What is the formula for calculating the resistance for a circuit comprising of more than two resistors connected in parallel?
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.
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.
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.
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?
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.
