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Bit of Confusion there, Someone did tell me I have to do the Ze test with all Parallel paths even at origin seems a bit pointless.

No, Ze is measured with the earthing conductor disconnected to eliminate parallel paths.
Zs is measured with everything connected, you carry out this measurement to establish the prospective fault current, so you might as well write the number down since you are measuring it anyway.
 
Would it be the case that Ze may not always be at the first dB, for example if tails have been extended via fused switch and swa? you'd take ze at the switch, disconnecting earthing conductor, then your 'sub main zs would also be your zs at db1?
 
Would it be the case that Ze may not always be at the first dB, for example if tails have been extended via fused switch and swa? you'd take ze at the switch, disconnecting earthing conductor, then your 'sub main zs would also be your zs at db1?
Take Ze at the switch fuse
 
Or if, as is so many older domestics, there is a separate incoming earth bar. It's the earth bar in loads of industrial/commercial systems but, because it's included in the supply details with the certificates' it's still a case of it being recorded as your main DB.
 
I have a ccu with a Ze of 0.09 and it's supplying another ccu which that board supplies multiple other CCUs I was wondering how I would work out my zs and ze on the other boards?
As others have said Ze is only measured at intake.
So in simple terms, if you have a Zs figure at the first DB and then install a distribution circuit to a remote DB/CU the R1+R2 value of this circuit is added to the Zs value at the first DB irrespective of the Ze as the Zs at the first DB will already include/allow for the Ze value.
Subsequent distribution circuits are then added to (if in series) or calculated separately if in parallel.
 
I have another question, so... instead of starting another thread

Zs via measurement is a live test, so if I'm testing a plug socket Zs - so once the Live current reaches the MET all Earthed and Bonded metal work will become live - seems a bit dangerous - what is the current and voltage the meter uses for the test?
 
No. The MET will be, if all connections correctly made be at earth potential (0v with respect to the live of 230v).
Test voltage 230v.
 
Zs via measurement is a live test, so if I'm testing a plug socket Zs - so once the Live current reaches the MET all Earthed and Bonded metal work will become live - seems a bit dangerous - what is the current and voltage the meter uses for the test?
If the Earth is connected to the MET reliably then you get current flowing but the increase in local potential should be negligible in the test.

But clearly if you had a situation where the local MET was not well earthed, say missing wire or badly corroded terminals, or even with an earth rod for TT that could well have an Ra of 200 Ohms and be perfectly acceptable, then there is a very real risk of the local earth/CPC arrangement becoming live w.r.t. the true Earth to a dangerous potential.

It is possible that modern MFT have some safety features included, but I don't know for sure. For example, if you were diverting your Zs test current of a few A from L to E and you observe E going to many volts above N, this is not good, and you could reduce the test current and still get a measurable Zs result.

However, what I do know is the "no trip" type of Zs test only uses a small current (probably 15mA) so it won't trip an RCD and then correlates the cycling of that on/off over a long period to see the resulting mV (or less) change in supply voltage. In this case you are not at real risk (though getting a pulsing 15mA shock won't be nice) but the test is less accurate than a high current Zs that generates a more easily measured change in voltages to deduce the impedance.
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More likely a MFT would do a low-current test first, and if that caused very little change in potential difference, then up the test current.
 
Low current loop test is around 15mA to prevent RCDs from tripping. High current test for the Megger is 25A.

As live testing is in the final stages of testing the earth connections would have been proven during dead testing meaning the earth terminal voltage should not rise when testing. Also you shouldn't be touching the terminals when testing anyway.
 
Would it be the case that Ze may not always be at the first dB, for example if tails have been extended via fused switch and swa? you'd take ze at the switch, disconnecting earthing conductor, then your 'sub main zs would also be your zs at db1?

Ze is not at any DB, Ze is the earth fault loop impedance of the incoming supply.
The e stands for external, it is is the impedance external to the installation.

Ze is the earth fault loop impedance at the incoming supply terminals,
If the supplier has provided an isolator then it is measured between the outgoing L terminal of the isolator and the disconnected earthing conductor, if there is no isolator then strictly speaking it is from the outgoing L terminal of the meter. As meters are sealed we normally measure at the closest L terminal to the meter that is available
 
Ze is not at any DB, Ze is the earth fault loop impedance of the incoming supply.
The e stands for external, it is is the impedance external to the installation.

Ze is the earth fault loop impedance at the incoming supply terminals,
If the supplier has provided an isolator then it is measured between the outgoing L terminal of the isolator and the disconnected earthing conductor, if there is no isolator then strictly speaking it is from the outgoing L terminal of the meter. As meters are sealed we normally measure at the closest L terminal to the meter that is available

Yeah, I get that, so practically, often if no isolater etc is available ze will be taken from the top of the L N terminals of the main switch inside the first dB won't it? That's what I meant.
 
Thanks, Still interested in what voltage Loop Impedance testers use?

You could say you shouldn't be touching this and that but you know Zs tests take place without any preparation and probably R1+R2 is calculated from Zs tests etc.
 
Thanks, Still interested in what voltage Loop Impedance testers use?

You could say you shouldn't be touching this and that but you know Zs tests take place without any preparation and probably R1+R2 is calculated from Zs tests etc.

It is conducted at whatever voltage the tester is connected to, usually 230V nominal.
Any modern tester will monitor the voltage rise on the earth during the test and stop the test if it rises above a certain value.
 

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