I have been looking at the implications of using an RCD to provide ADS on a TN system under Regulation 411.4.204, rather than relying on the overcurrent characteristic of the MCB/RCBO.
Historically, where the overcurrent device provides ADS, Table 41.3 gives a relatively low maximum Zs. As well as demonstrating the required earth-fault disconnection time, this also provides a useful practical check on the integrity of the installed circuit.
Where an RCD is instead relied upon for ADS, Table 41.5 can permit a dramatically higher Zs. Previous EngX discussions confirm that this is a legitimate design approach on TN systems, although several contributors have also commented that measured Zs should still be reasonable for the circuit concerned. [engx.theiet.org], [engx.theiet.org]
My question concerns verification of the line-neutral fault path.
At design stage, software such as ProDesign performs separate line and earth fault adiabatic checks. It therefore determines minimum phase fault current and verifies that the overcurrent protective device will clear an L-N fault before the conductor's thermal withstand is exceeded. Trimble describes the software as performing both line and earth fault adiabatic checks. [engx.theiet.org], [trimble.com]
However, at initial verification the principal impedance value being recorded is Zs.
Consider a TN circuit where the design calculation assumes a low L-N loop impedance, but an installation defect introduces significant resistance into the line or neutral conductor.
If the RCD is relied upon for ADS, the measured L-E Zs could remain well within the Table 41.5 limit, because very little residual current is required to operate the RCD. Yet the actual L-N fault current could now be considerably lower than that assumed in the designer's Chapter 43 calculation.
Historically, where Table 41.3 governed, there was an indirect safeguard: if the L-CPC loop, often containing a CPC smaller than the neutral, had sufficiently low impedance to operate the MCB within the required time, there was considerable confidence in the L-N fault path as well.
When Table 41.5 is used for ADS on a TN system, what verification replaces that safeguard?
More specifically:
How does initial verification demonstrate that the as-installed L-N circuit impedance remains sufficiently close to the designer's calculated value for the minimum short-circuit current and Chapter 43 thermal withstand calculation to remain valid?
I appreciate that continuity measurements provide useful information and that an unexpectedly high Zs should be investigated rather than simply accepted because it is below the RCD-derived limit. But I am struggling to identify the explicit acceptance benchmark against which the verifier assesses this where the declared maximum Zs has been derived from the RCD rather than the overcurrent device.
Am I missing another requirement in BS 7671 or Guidance Note 3 that closes this loop?