High Ze Causing Issues With Project

Question

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

gkenyon · Answer

OK, so is the issue that there's a distribution circuit of quite some length between the 200 A fuses and the first switchboard? Because for total load of 140 A, final circuit protective devices ought to be OK with the Ze we are talking about ... or is it a design condition for the switchboard? The latest version of P23 would seem to support the DNO's position, that the Ze is acceptable for the service rating. As others have said, though, that doesn't mean the voltage drop meets ESQCR. SOLUTION ... unless the switchboard manufacturer requires anything different ... is no different to that used in a 100 A single-phase domestic installation, where often the same conditions may apply (unable to operate the 100 A service fuse in 5 s where Ze > 0.38 ohms, e.g. TN-S, or 1 s for TT), i.e. upto the switchboard, all you need is a wiring system that is equivalent to double or reinforced insulation, and you are then ADS is not necessary, only protection against thermal effects (overcurrent and fault current protection via adiabatic). This is acceptable for protection against electric shock, because you are using Section 412, rather than Section 411. I'm guessing, though, that a design has been undertaken assuming something different?

High Ze Causing Issues With Project

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