Zs from board supplying another board

[Thomasthebeast]

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?

 

[Strima]

Ze is only at the origin, the rest are Zs.

Measure/calculate the R1+R2 for each distribution circuit and add the Ze. (Zs=Ze+(R1+R2))

 

[davesparks]

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?

Ze only exists at the origin, e stands for external.

In an existing installation you measure Zs at each board.

If this is at the design stage then you need to use the calculated impedance of the circuit and add it to the Ze quoted for the supply.

 

[Pete999]

Ze is only at the origin, the rest are Zs.

Measure/calculate the R1+R2 for each distribution circuit and add the Ze. (Zs=Ze+(R1+R2))

Does it not become Zs at DB? from board to board, I hear what you're saying, but does my statement not hold water in this case?

 

[davesparks]

Does it not become Zs at DB? from board to board, I hear what you're saying, but does my statement not hold water in this case?

Yes it does become the Zs at the DB, but obviously that is also the Zs of the distribution circuit because they are one and the same thing.

Strictly speaking 'Zs at DB' or 'Zdb' are not correct terminology, much like 'ring main' instead of 'ring final circuit', everyone knows what is meant by it, it's commonly used, but it's not correct terminology.

 

[Strima]

Does it not become Zs at DB? from board to board, I hear what you're saying, but does my statement not hold water in this case?

Each board would have its own Zs, sometimes referred to a Zdb. I think we're saying the same thing.

 

[westward10]

Doesn't matter what it supplies DB, light, socket etc.... it is Zs.

 

[Pete999]

Each board would have its own Zs, sometimes referred to a Zdb. I think we're saying the same thing.

Yes exactly nice one Mate

 

[ipf]

Yes it does become the Zs at the DB, but obviously that is also the Zs of the distribution circuit because they are one and the same thing.

Strictly speaking 'Zs at DB' or 'Zdb' are not correct terminology, much like 'ring main' instead of 'ring final circuit', everyone knows what is meant by it, it's commonly used, but it's not correct terminology.

But, 'Zs at DB1, 2, 3', whatever, is a statement...nothing wrong with the terminology, whatsoever.

 

[Mark Wright]

If it's the origin CUU does anything need to go into
"Zs at DB" on the "Schedule of test results" form ?

 

[davesparks]

If it's the origin CUU does anything need to go into
"Zs at DB" on the "Schedule of test results" form ?

Yes, the Zs at the DB goes in the "Zs at DB" box, the clue is in the name, it's not a trick question.

 

[ipf]

If it's the origin CUU does anything need to go into
"Zs at DB" on the "Schedule of test results" form ?

Nope, unless it's not connected to origin.

 

[Pete999]

But, 'Zs at DB1, 2, 3', whatever, is a statement...nothing wrong with the terminology, whatsoever.

I agree with ipf, but there will always be a Jobsworth that disagrees with this though.

 

[Mark Wright]

Yes, the Zs at the DB goes in the "Zs at DB" box, the clue is in the name, it's not a trick question.

Nope, unless it's not connected to origin.

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.

 

[Ian1981]

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.

You do your PEFC with all parallel paths connected at the origin not a ZE test .The Ze test determines the resistance of the earthing conductor external to the installation and confirms an earth connection to the installation hence it’s done with no parallel paths connected.

 

[Mark Wright]

If it's the origin CUU does anything need to go into
"Zs at DB" on the "Schedule of test results" form ?

You do your PEFC with all parallel paths connected at the origin not a ZE test .The Ze test determines the resistance of the earthing conductor external to the installation and confirms an earth connection to the installation hence it’s done with no parallel paths connected.

I'm talking about Schedule of test result - Zs at DB

 

[Ian1981]

I'm talking about Schedule of test result - Zs at DB

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
Source URL: Zs from board supplying another board - https://www.electriciansforums.net/threads/zs-from-board-supplying-another-board.186371/#post-1629157
Responding to your quote which stayed that someone had told you to test Ze with parallel paths connected at the origin

 

[Mark Wright]

ill just take it you don't have to do Zs at DB at origin DB, unless anyone says differently.

 

[Strima]

ill just take it you don't have to do Zs at DB at origin DB, unless anyone says differently.

The test carried out at the origin DB is the Ze not Zs. This is done with parallel paths removed to confirm the continuity and loop impedance of the DNO supply.

Zs is done with parallel paths in place on distribution and final circuits to ensure the installation disconnection times can be met.

 

[Mark Wright]

I know that - Top left corner of Schedule of test results - "Zs at DB"

 

[davesparks]

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.

 

[Mark Wright]

Ok so it's just like prospective Earth fault impedance - just Ze with all Parallel paths and it is done even at the origin ccu - is that correct ?

 

[pc1966]

No, Ze is measured with the earthing conductor disconnected to eliminate parallel paths.
Zs is measured with everything connected

That was my understanding.

 

[JAWS]

Dont forget to include your volt drop Thomas from each board when calculating your final circuits.

 

[ipf]

I wonder if the OP's fathomed it out. yet?

 

[goasis]

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?

 

[Ian1981]

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

 

[ipf]

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.

 

[DefyG]

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.

 

[Mark Wright]

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?

 

[DefyG]

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.

 

[Mark Wright]

Even so very briefly things will be live on the path to the MET.

Is the test actually at 230v seems unlikely and what's the Amps?

 

[pc1966]

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.
[automerge]1589706640[/automerge]
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.

 

[Mark Wright]

Ok the potential to shock as the earthing Should be the path of least resistance etc

Ok 15mA seems right what about voltage 50v?

 

[Strima]

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.

 

[davesparks]

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

 

[goasis]

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.

 

[Mark Wright]

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.

 

[ipf]

Thanks, Still interested in what voltage Loop Impedance testers use?

It's a live test...….no operation until the voltage is available....indicated by the instrument prior to test.

 

[davesparks]

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.

 

[Mark Wright]

Ok thanks, surprised if it actually uses 230v for the test and doesn't use a transformer.

but maybe people should think a bit more before just plugging there tester into a socket and letting whatever A.C voltage 15mA flow through the earth etc.

 

[Ian1981]

Ok thanks, surprised if it actually uses 230v for the test and doesn't use a transformer.

but maybe people should think a bit more before just plugging there tester into a socket and letting whatever A.C voltage 15mA flow through the earth etc.

There is no danger,If the meter detects that the voltage rise is above the meters setting, typically 50v to earth then your meter will abort the tests, so the danger is removed.

 

[Mark Wright]

There is no danger,If the meter detects that the voltage rise is above the meters setting, typically 50v to earth then your meter will abort the tests, so the danger is removed.

Even a low voltage it might not be a "Danger"
but a child touching a metal faceplate in a residential property while a break in the CPC during such test it's obviously not very good, you can say you shouldn't be touching this and that but like I say some people probably don't take any precautions and work out R1+R2 from Zs test without even thinking a Live Current is flowing through the CPC.

 

[Ian1981]

Even a low voltage it might not be a "Danger"
but a child touching a metal faceplate in a residential property while a break in the CPC during such test it's obviously not very good.

The meter will not let you carry out the test if the voltage to earth rises above 50v by a break in the continuity of a cpc connecting to any exposed conductive parts
If we are talking initial verification, then continuity of the cpc and the connection to earth of any exposed conductive parts should have been verified before any live testing as prescribed in part 6 of bs7671

 

[davesparks]

Ok thanks, surprised if it actually uses 230v for the test and doesn't use a transformer.

but maybe people should think a bit more before just plugging there tester into a socket and letting whatever A.C voltage 15mA flow through the earth etc.

How would you connect a transformer to be able to test the loop impedance of the supply at anything other than the supply voltage?