Hi All,

I have recently encountered a problem where the DNO supply produces a Ze of 0.28ohms on L1, 0.16ohms on L2 & 0.15ohms on L3 for a 138kVA TPN supply with BS88-2 200A Fuses. The DNO maximum earth loop impedance value for a TPN 200A is 0.35 ohms, therefore have no obligation to provide lower values.

BS7671 states that the disconnection time for a BS788-2 200A fuse is to be 5 seconds which required  Zs =< 0.18 ohms, although, Zs = Ze + (R1 +R2) which will not comply with the required maximum of 0.18ohms and the EICs cannot be completed.

It appears to me that the DNO transformer has a fault or is very old, as the phases are so unbalanced. But as the measured figures are under 0.35 they will not look at it. 

How would the best solution be to resolve this, keeping in mind that the installation and switchgear have been installed?

Thanks,

J