A new transformer unless there is miles of LV wiring in the way should do rather better (substation regulation == droop at full load  at the transformer terminals is typically about 4- 5 %, i.e. if the transformer was only supplying one 200A load then  more like 0.03ohms, mostly inductive rather than winding resistance ). I presume your 0.23 ohms  (only ~ 1000 amp PSSC then)  is Zs measured at the company incomer and there is more resistace in the way to get to  the point of the new load ? (and for sanity that L-N and L-E check out the same)

Note that 0. 2 ohms is rather more than 10m round loop path  (there and back to somewhere 5m away then) of 1mm2, or scaling to a more useful size,  350m of 35mm2  or 500m of 50mm2 etc. How far away is this upgraded transformer and how thin is the cable going to  it ? It is possible that something is a bit undersized on the DNO supply side.

How much of the cable route is under your control ?

M.

I believe the transformer is about 300m away on 185mm CNE cable. I measured that 0.23 at the cutout service head. Upgrading the cable was mentioned when the transformer was upgraded but my customer would be liable for the digging and cable costs. I think as 0.23 is still well in their max limit of 0.35 for TNCS I might be in for a hard time arguing that they need to upgrade the cable

300m of 185mm2 should not be anything like that.  - 0.06 to 0.08 ohms would be more like it.

It may be worth doing a volt drop-known load test to see if the supply impedance really is as high as you think. If it is 0.23 ohms, then a 10A kettle should dip you 2.3 volts- at the incomer, and more at the socket, you will see that on the meter.

M.

300m of 185mm2 should not be anything like that.  - 0.06 to 0.08 ohms would be more like it.

It may be worth doing a volt drop-known load test to see if the supply impedance really is as high as you think. If it is 0.23 ohms, then a 10A kettle should dip you 2.3 volts- at the incomer, and more at the socket, you will see that on the meter.

M.

That is more of the sort of figure I was expecting to see. I have inquired to Western Power to see what their readings were when they installed and commissioned the upgraded supply 

It may be if things are a bit corroded with thin layers of oxide etc , that the low current of the tester does not punch through and make a good contact - a high current test or a load VD test may reveal better the true impedance in a  high fault current situation.

Mike.

Yeah i agree. Th supply cable has been in there for 40+ years so thats a high possibility. I contacted Western Power this morning and they said it seemed higher than they would have expected for the site data so are sending an engineer to check it today

Dbat said:

so are sending an engineer to check it today

As they are obliged to do on request under R. 28 of ESQCR 2002.