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Calculate from basic principles the maximum value of the protection device, if the following is given:
the length of ECC (earth continuity conductor) is 69 meters of copper with an area of 6mm2.
For questions like this, remember two things: The first is that the maximum allowed touch voltage (T.V.)
according to the code is 30V. Use this formula:
Touch Voltage = 2 x I(CB) x R(ECC)
Where,
I(CB) is the value of the protective device (Circuit Breaker or Fuse) = the value to be calculated
R(ECC) is the resistance value of the Earth Continuity Conductor = we are given the size of the conductor
Touch Voltage (T.V) = 30V
The second thing: if you can recall from the previous post (voltage drop calculations) that the resistance in Ohms can also be
expressed in the following form:
R = (Value of R in Ω/Km x L in meters)/1000m [Ohms]
Please remember to use Table E1 for the value of R in Ω/Km for each given conductor size.
For this question:
ICB =? [That is what we are asked to calculate]
We can calculate: The value of R given in Ω/Km for 6mm2= 3.6 Ω/Km [From Table E1]
The length is L = 69m
Substituting values we have, R = (Value of R in Ω/Km x L in meters)/1000m [Ohms]
= (3.6 Ω/Km x 69m)/1000m = 0.2484 Ω
Substituting values in our touch voltage formula
we have, T. V. = 2 x I(CB) x R(ECC)
30V = 2 x I(CB) x 0.2484 Ω
I(CB) = 30V/0.4968Ω = 60.3865A
Therefore the value of the protective device is 60A
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