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Thread: Touch Voltage

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    Touch Voltage

    Guys I am finding it difficult to answer this question:

    "Calculate from the basic principles the maximum value of the protection device if the following is given:

    Length ECC is 69m with an area of 6mm2"

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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

    For this question and many more you may visit:
    http://www.installationrulesstudymate.co.za

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    Software contains sufficient questions which are presented in the same
    format used on the actual exam.

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    Thanks. I though the TV is 25V. I will check your site out

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