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.
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.
So I think that we are agreed that ZS = ZDB + (R1 + R2) and R1 + R2 = ZS - ZDB.
And this is what I'm trying to get to the bottom of, why is it frowned upon to back calculate r1r2 from ZS and ZDB.
In certain circumstances it can be the same,when I'm testing communal areas in small blocks of flats,wired in twin and earth it's very similar. My measured r1+r2 will often be very close to ZS-ZE,in a house though where they may be a gas boiler a water cylinder and maybe gas and water bonding it probably won't work unless the wiring is not connected to these metallic services.
Probably needs a proper circuit diagram that shows the spurious paths and components (e.g. RCD inductances; real resistances of the soil of Earth; which end has the test 'short'; etc. )
So I think that we are agreed that ZS = ZDB + (R1 + R2) and R1 + R2 = ZS - ZDB.
NO!
1. Parallel paths not taken into account (BS 7671 requires us to take into account worst-case, when fortuitous earthing is removed).
2. if, and ONLY if, there are no parallel paths, ZS = ZDB + (Z1 + Z2) and (Z1 + Z2) = ZS - ZDB ... to find out (R1+R2) you need to know the reactance in the circuit.
This makes me question why we measure r1+r2 with dc ? Why dont we use ac at 50 hz to give us a closer reading to real world conditions, i have also wondered this about ir why are they dc measured?
Why dont we use ac at 50 hz to give us a closer reading to real world conditions, i have also wondered this about ir why are they dc measured?
DC is OK for copper conductors up to at least 16 mm2. Only above this CSA are we concerned about reactance.
The problem with SWA, it its CSA is above 16 mm2 for all multicore armoured cables, with, if memory serves, the exception of 2c 1.0 mm2 SWA.
It also proves continuity (which is what BS 7671 requires for verification ... not an accurate resistance or impedance reading).
Loop testing is actually rather inaccurate, often out by 10 % or worse, and for low values of loop impedance (high values of prospective fault current), specialist loop test equipment is needed.
In fact, for initial verification and periodic inspection and testing, the loop impedance (calculated or otherwise ... BS 7671 does not require us to accurately measure it, although it's best to know you accurately have a fault path, once that's established, a calculation will satisfy the requirements of Part 6) is more of a "finger in the air" that everything is around about OK. ... the proof in the pudding is the original design (or use of standard circuits per Section 7 of the OSG).
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