Hi All,
Here is a good one regarding "High Ze"
First off, as Steptoe says, there is NO rule that says Ze has to be anything. Like Steps said, the quoted figures are only a guideline, the DNO can do as they like....
The only thing they HAVE to do, is to make sure that the voltage they supply you with is not less than Minus 6% from nominal.
If you have a high supply impedance then, [due to being miles from a substation] if you moan too much, they are perfectly likely to come along and tell you to TT it all, [to take care of the earthing] and also fit smaller cutout fuses to make sure that you cannot load up the system enough so as to be able to complain about the volt drop.. [Crafty lot eh!]
Anyway,
About a year ago i had a three phase PME supply connected to the shed round the back of my house. About two months ago, having just taken delivery of my new Megger 1730 i thought i would have a play with it.
I checked Ze on each phase and discovered that L1 and L3 were both about 0.3 but that L2 was more like 0.44
I did not know what to do. I wondered if the DNO would even care....
My main concern was nothing to do with earthing, but that as i loaded up the system, that i would have more volt drop on one phase than the others, and that the resulting unbalance could damage my equipment.
What made things even worse, was that when i checked in the house, [that is also on L2] i got a figure of about 0.28
0.28 in the house. 0.44 in the shed....It was obvious that there was something wrong with the new service cable......
Anyway, this morning, i was driving up the road when i noticed that the DNO were digging large holes just around the corner from me. I went home and took some more readings, saved them and printed them out.
I got
House; 0.23 [L2]
Shed
L1 0.29
L2 0.51 !!!!!!
L3 0.23
I went to see the DNO chaps, and showed them my figures and asked what should i do, and added that i did not think that the DNO would be too happy to have to dig the road up again...
They said; "might just be loose terminals in the cutout" so i said "Think it might be?? Shall i go and take it to bits and check?"
They said, "give us five minutes to drink our tea and we will come round and have a look!!!!"
Around they came, and guess what!! the screws in the cut out were UNBELIEVABLY slack. I mean to say, they all did up about 4 turns!!!!!
They explained that they are instructed to do them up using this "T" handled "flexy" allen screwdriver thingy that flexes when they get to the right torque. [Far too slack] but that "I just do them up!!!"
So all screws tightened and my "New" readings are;
L1 0.28
L2 0.29
L3 0.25
Well pleased!!!!!
So if you find high Ze, even though you are not meant to, you could always check the screws in the cutout, especially if it is a newish supply!!!!!
john..
