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adiabatic equation in both 434 and 543

wallywombat
Why does the adiabatic equation (and associated tables of k values) appear in both sections 434 and 543? Okay, so one is referring to line conductors and the other to protective conductors (and so will have different initial temperatures), but it's all much of a muchness, and surely could be combined. It also seems a bit strange that one is in part 4 while the other is in part 5. It all seems a bit arbitrary.


PS - fun fact: none of the following appear in the BS 7671 index: 'adiabatic equation', 'equation, adiabatic', 'I2t', 'let-through', 'current, let-through'. Although to be fair, 'k values' does.
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    Because the regulations are NOT for the non-competent or not instructed persons who should know all that theory backwards and forwards! It is not a textbook.
  • 0
    The real answer is that a cpc - or other types of protective conductor - may be common to two or more circuits (example is where metallic conduit is used as a cpc, or a multicore cable serves more than one circuit), and therefore the single-circuit approach in 434 is not entirely appropriate.


    I agree that where a cpc is considered to be part of the circuit concerned (another rabbit hole to go down), and serves only that circuit, then sure it's a doubling up or repeat of an earlier requirement. However, in other cases, it's not that clear-cut.
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    I think that it is significant that the equation is arranged differently in the two paragraphs.


    In 543, the aim is to find an appropriate value of S, which may be obtained either by calculation or selection. Then we know what size of protective conductor to install.


    In 434, the aim seems to be to find a value for t. For e.g. a distribution circuit, this could be up to 5 s (411.3.2.3). However, by then, the cables may be overheating so it is necessary to calculate t. If that is less than 5 s, it becomes necessary to ensure that the protective device will open within that shorter interval (434.5.2). In other words, here the calculation assists in choosing the correct protective device.

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