This is the multiplication factor for cross-bonded earthing.
When the lengths of the three cable sections are not known, $p_{cb}$ should be set to 1, and $q_{cb}$ to 1.2, giving a value for $f_{cb}$ slightly below 0.004.
For two single core cross-bonded cables, which is not covered in the IEC standard, $f_{cb}$ is set to 0.004.
When all the lengths of the three cable sections are identical, $f_{cb}$ becomes zero.
$\frac{1}{\left(p_{\mathrm{cb}} + q_{\mathrm{cb}} + 1\right)^{2}} \left(p_{\mathrm{cb}}^{2} - p_{\mathrm{cb}} q_{\mathrm{cb}} - p_{\mathrm{cb}} + q_{\mathrm{cb}}^{2} - q_{\mathrm{cb}} + 1\right)$ | three-phase systems |
$\frac{p_{\mathrm{cb}}^{2} - p_{\mathrm{cb}} + 1}{\left(p_{\mathrm{cb}} + 1\right)^{2}}$ | two-phase systems |
$0$ | single phase |