Is the only reason you do not calculate r1 r2 by deducting Ze/Zdb from Zs is that it is down to parallel paths. The reason I am asking is, when the apprentices are doing their trade test, they are encouraged to short the circuit they are working on to the earth bar via a crocodile clip to get r1 r2. Surely this is picking up parallel paths also? Or is there is another reason?
Sorry, I was meaning what is the difference between live testing and dead testing if you are keeping all earths connected. I understand there will be a small difference live to dead testing but I'm asking why can you calculate ZDB plus r1 r2 to get ZS but you can't do it the other way ZS - ZDB to get r1 r2? Is it only due to parallel paths and I understand to get a proper r1r2 you should have it disconnected but they are getting taught to do it on to the earth bar so you will still be getting the parallel paths?
why can you calculate ZDB plus r1 r2 to get ZS but you can't do it the other way ZS - ZDB to get r1 r2?
Well, of course, a mathermatician would tell you the approach is perfectly sound.
So where are these parallel paths? If the utilities come in plastic pipes, so there is no main bonding, let alone supplementary, there should not be any.
I get all that and sometimes you will or will not get parallel paths. My main question is why you can calculate it one way but not the other? And I get dead testing is the preferred method but if you are doing a test that can't be switched off and you have ZDB and ZS why do they say you can't calculate r1r2, that is really what I am wanting to know.
Well if you are confident that know what you are measuring, you can of course deduce what one measurement might have been, from knowledge of the other, and if it is an inspection on a running system, then a series of live tests and some sums may well be faster and more reliable. Some meters even allow you to zero from the ZS at the meter and take difference readings at various points with the subtraction pre-done. Its not necessarily wrong, just 'cos its not the book method.
Going back a long way, the method used to be to place the LE short at the far end, and dead test the free ends from the board end just prior to hook up. I think enough folk then powered up into the dead short that this is no longer the recommended advice , but my point is only that the recommended methods do change slightly over time, as problems arise and are corrected.
Note that with some types of loop test meter, a test that draws a small AC current through an RCD can fool you due to the impedance of the sense coils within ... see this link
https://engx.theiet.org/f/wiring-and-regulations/32034/large-rcd-uplift
Mike.
Well, of course, a mathermatician would tell you the approach is perfectly sound.
Even ignoring 'parallel paths', that would be true only in cases where you can rely on resistance measurements alone.
Practically every armoured cable installation, in which the armour is used as a cpc, would give a different reading by a few % (up to 10 %), because of the impedance of the steel armour. The mathematician could, of course, use relevant formulas to work this out too, though.
Well, of course, a mathermatician would tell you the approach is perfectly sound.
Even ignoring 'parallel paths', that would be true only in cases where you can rely on resistance measurements alone.
Practically every armoured cable installation, in which the armour is used as a cpc, would give a different reading by a few % (up to 10 %), because of the impedance of the steel armour. The mathematician could, of course, use relevant formulas to work this out too, though.
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