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0,4 seconds: what automatic disconnection really requires

Table 41.1 in full, which circuits it applies to, the impedance condition behind it, and why an RCD needs several times its rated residual current to make the time.

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The slides, in words

  1. A time is a consequence, not a setting — Automatic disconnection of supply is a protective measure with three parts: an earthing arrangement, a device, and a loop impedance low enough to make that device operate in the required time.
  2. Table 41.1 - maximum disconnection times, in seconds — U0 is the nominal line-to-earth voltage. For d.c. the same table gives, for the three upper bands, TN 5 / 0,4 / 0,1 s and TT 0,4 / 0,2 / 0,1 s. In TT systems where an overcurrent device does the disconnecting and protective equipotential…
  3. Which circuits it applies to
  4. What has to be proved: the loop — The fault loop comprises the source, the line conductor up to the fault, and the protective conductor back from it.
  5. The conditions behind the time — I_a is the current causing the device to operate within the required time - so the time and the impedance are one calculation, not two. For the second fault in an IT system with non-distributed neutral the condition becomes 2 I_a Z_s ≤ U.
  6. The RCD detail that is usually missed
  7. The time is met by the loop, not by the label — A schedule that states "0,4 s" without the measured or calculated Z_s, the device characteristic and the I_a it implies, has recorded an intention. The verification is the loop impedance - and it is the number to ask for.