Hi All,
Newbie time again!!
Ze is earth loop impedance entirely "external" to our installation. So, the loop impedance from the supply transformer, along the phase conductor and then back to star point by which ever means the nice DNO people might have provided, down the cable sheath [TNS] back along the neutral [TNCS] or, alternatively through the ground itself [TT], via the happy medium of a spike we have banged in ourselves.
So, Ze is completely external to our installation. Therefore, it follows that to measure Ze we have to do several things;
1, Isolate [in the usual sense of the term] our installation, as we are going to have to disconnect the earthing conductor [disconnected earthing conductor + "live" installation = lunacy!!!!]
2, Isolate our installation from a EARTHING point of view. In other words, disconnect the MET from the earthing terminal, [or our rod] by removing the earthing conductor.
The idea of this, is to remove any connections that may or may not exist within our installation itself [via bonding conductors] to any extraneous parts that would [due to the influence of "parallel paths" [say water or gas pipes [they would act as giant earth rods] give us an artificially low reading. This is obviously essential to obtain a true EXTERNAL loop impedance figure.
3, Measure the external loop impedance.
So, now we know a true figure for Ze...
But what about Zs??
Well, Zs is simply the sum of Ze + R1 + R2 for the particular circuit concerned.
So how you going to obtain a figure for Zs then, which is after all, what we are actually interested in ???
You could calculate R1 + R2 if you so desired, or you could measure it with your ohmmeter, which would obviously be a better bet, as it would be a lot easier than trying to figure out the exact circuit cable length [for calculation purposes], and furthermore, it would include the resistance of joints etc and simply add it to the figure you obtained previously for Ze..Yeh, Zs!!
Alternatively, you could however, instead of all this messing about isolating people's installations, and disconnecting earthing conductors, measuring R1 +R2 blah blah, simply measure Zs directly with your loop impedance tester...
Certainly would be easier!!! and would probably be a more sensible, "real world" test.
Only trouble is, i have a problem with this approach, a very big problem.... It includes the influence of parallel paths etc.
Now, why would i care about that then??? So long as Zs is low enough, happy days!!
Errr, not really. Say i go along to someone's house/factory, measure Zs at their CU/DB, work out R1 + R2 for the new circuit i am installing, add it to Zs/Zdb [whatever you like to call it] determine that the sum of the two combined gives a total Zs at the extremity of the circuit low enough to achieve the required disconnection times for the OCPD i have installed to protect the circuit, install it all, check it with the meter, and all is well.
Errrm, yes, it might be....for now....But what if in future, some of the parallel paths are removed???? In a house, metallic water or gas pipes might be removed, in a factory they might demolish half of it, complete with all the steel portal frames that were acting as earth rods.
Now, i might very well find that Zs has risen to such an extent, that my OCPD's no longer achieve required disconnection times....... and i end up in a coroners court...
Nope, for me, Ze, is loop impedance COMPLETELY EXTERNAL to the installation, and Zs, is the sum of Ze, plus R1 + R2, [calculated or measured] for the circuit [or circuits] involved.
If it involves a lot of hassle coming up with a TRUE figure for Ze and R1 + R2, and hence Zs, oh well, i would rather this than try to explain to the judge that; "How was i supposed to know that it was reasonably forseeable that they might have a new plastic water pipe installed instead, or knock half the factory down!!!!
Yes, i KNOW that a lot of this might be "idealistic" but it is right, isn't it????......
john....
being 0.54 ohms this is well within the required reading. now onto rcds? :slap :^O .
