I have installed a new consumer unit in a garage and while filling out the certificates i came across something that made me think twice. I am issuing an installation certificate and schedule of inspection and test results but on the test results form it obviously asks for the Ze and PFC. Is this measured from the garage consumer unit or from the main consumer unit in the house. thanks.
Testing Ze for a new consumer unit in a garage
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Garage, under Zdb.
AndyGuinness
AndyGuinness
Ze is always external. your garage will have a Zs (submain)
As Scott and Andy have said.
You'd have trouble measuring Ze at the garage board anyway - after you've disconnected the 'Earthing Conductor' from the MET.
Oh, and don't get in this 'Zdb' habit either - it's the Zs of the 'Distribution Circuit' supplying the 'Distribution Board' and is noted on the cert as such. (Sorry Andy, just repeating your point)
You'd have trouble measuring Ze at the garage board anyway - after you've disconnected the 'Earthing Conductor' from the MET.
Oh, and don't get in this 'Zdb' habit either - it's the Zs of the 'Distribution Circuit' supplying the 'Distribution Board' and is noted on the cert as such. (Sorry Andy, just repeating your point)
The meaurement at the end of the cable coming from the origin DB will be the Zs for the Cct (Radial) coming from that board and should be with the earth connected, the sub main being a cct in its own right. Zdb for the next board must be taken with the incoming earth disconnected (GN3) as per Ze, this eliminates parrallel paths, although in practice you may well find that the Zs of the sub-main and the Zdb will be identical or very close.
as a note, it is now generally discouraged to take live Zs readings, preferable to calc using Ze + (r1+r2). i understand this is for two reasons 1. Less live working - ie is it neccessary under EAWR 89 as you can calc this easily from dead test measurements 2. Calcs give you safety margins and are as per design spec
as a note, it is now generally discouraged to take live Zs readings, preferable to calc using Ze + (r1+r2). i understand this is for two reasons 1. Less live working - ie is it neccessary under EAWR 89 as you can calc this easily from dead test measurements 2. Calcs give you safety margins and are as per design spec
In my opinion, the only reason you caculate ZE = R1 + R2 is to give you a figure for guidance as to what your actual Zs should be.
There is no avoiding 'live' testing if you are to do the job properly, so I don't know why they bother.
If you're doing a periodic there's a good chance you won't even measure R1 + R2 (as it's supposed to be minimum dismantling) - so what are you going to add your Ze to then?
Quote Originally Posted by SteveT View Post
The meaurement at the end of the cable coming from the origin DB will be the Zs for the Cct (Radial) coming from that board and should be with the earth connected, the sub main being a cct in its own right. Zdb for the next board must be taken with the incoming earth disconnected (GN3) as per Ze, this eliminates parrallel paths, although in practice you may well find that the Zs of the sub-main and the Zdb will be identical or very close.
You'll need to show me where you got that one from.
as a note, it is now generally discouraged to take live Zs readings, preferable to calc using Ze + (r1+r2). i understand this is for two reasons 1. Less live working - ie is it neccessary under EAWR 89 as you can calc this easily from dead test measurements 2. Calcs give you safety margins and are as per design spec
This is an NICEIC 'general discouragement' and is pathetic. (unquote)
Zdb is the Ze for the sub board. To measure Ze (The first of your live tests) You disconnect the main earth and measure with an earth loop impedeence tester, as per GN3. on susequent DB's you remove the incoming earth (external to the new DB) as if there were no previous board, ie Zdb is the external Z for that board
Pathetic in your opinion> i do think the combined wisdom of the NIC deserves some credit - are you bitter about something.
By the way I don't particularly hold the org in very high esteem, but that another story
Overall we tend to do more dismantling because there generally is very poor documentation - a point we have to remedy in our PIR's
The meaurement at the end of the cable coming from the origin DB will be the Zs for the Cct (Radial) coming from that board and should be with the earth connected, the sub main being a cct in its own right. Zdb for the next board must be taken with the incoming earth disconnected (GN3) as per Ze, this eliminates parrallel paths, although in practice you may well find that the Zs of the sub-main and the Zdb will be identical or very close.
You'll need to show me where you got that one from.
as a note, it is now generally discouraged to take live Zs readings, preferable to calc using Ze + (r1+r2). i understand this is for two reasons 1. Less live working - ie is it neccessary under EAWR 89 as you can calc this easily from dead test measurements 2. Calcs give you safety margins and are as per design spec
This is an NICEIC 'general discouragement' and is pathetic. (unquote)
Zdb is the Ze for the sub board. To measure Ze (The first of your live tests) You disconnect the main earth and measure with an earth loop impedeence tester, as per GN3. on susequent DB's you remove the incoming earth (external to the new DB) as if there were no previous board, ie Zdb is the external Z for that board
Pathetic in your opinion> i do think the combined wisdom of the NIC deserves some credit - are you bitter about something.
By the way I don't particularly hold the org in very high esteem, but that another story
Overall we tend to do more dismantling because there generally is very poor documentation - a point we have to remedy in our PIR's
R1 + R2 is more of a requirement during 'Initial Verification' - it's a dead test to prove continuity of CPC within the circuit prior to energising.
The measurement gives the 'resistance' of the 'line' conductor and the CPC only.
I would still want to measure the Zs during 'Initial Verification' to ensure that I still have that 'earth loop' for each circuit once everything has been terminated etc.
R1 + R2 becomes less of a requirement during PIR as the installation has already been energised - the measurement itself becomes pointless. - unless you want to add it to your Ze.
Ze, by the way, is a 'live' test (uh oh, don't tell the NIC)- unless you intend getting the figure by 'enquiry' and adding that to your R1 + R2.
So, what do you think is better, isolate the CU and start pulling circuits out in order to measure R1 + R2 or isolate the CU and measure Zs in a no-more-dangerous manner than measuring Ze, PFC or RCDs.
I know which one I'd rather do.
It's not a case of bitterness, it's a case of having to listen to drivel from an organisation who think they make the rules.
By the way, Steve, you still haven't told me where it says to disconnect the 'earth' at a remote DB and take a measurement called Zdb.
The measurement gives the 'resistance' of the 'line' conductor and the CPC only.
I would still want to measure the Zs during 'Initial Verification' to ensure that I still have that 'earth loop' for each circuit once everything has been terminated etc.
R1 + R2 becomes less of a requirement during PIR as the installation has already been energised - the measurement itself becomes pointless. - unless you want to add it to your Ze.
Ze, by the way, is a 'live' test (uh oh, don't tell the NIC)- unless you intend getting the figure by 'enquiry' and adding that to your R1 + R2.
So, what do you think is better, isolate the CU and start pulling circuits out in order to measure R1 + R2 or isolate the CU and measure Zs in a no-more-dangerous manner than measuring Ze, PFC or RCDs.
I know which one I'd rather do.
It's not a case of bitterness, it's a case of having to listen to drivel from an organisation who think they make the rules.
By the way, Steve, you still haven't told me where it says to disconnect the 'earth' at a remote DB and take a measurement called Zdb.
ADS, you are correct in your assertion that GN3 does not instuct you specifically to disconnect the earth at the remote DB. However, you are required to note the external impedance (aka Ze) of the DB. Therfore you can treat the remote DB as though it were the only one, therefore the method of taking the measurement is the same. This is an interpretation and is a valid and safe method
The point about not measuring Zs is not my opinion but I do appreciate the arguements for not doing so, technically and safety wise , as I originally stated "its is generally discouraged" - many things we are encouraged/discouraged to do we may not always agree with, there are varying methods, although some do not always stand up to scrutiny!!
The point about not measuring Zs is not my opinion but I do appreciate the arguements for not doing so, technically and safety wise , as I originally stated "its is generally discouraged" - many things we are encouraged/discouraged to do we may not always agree with, there are varying methods, although some do not always stand up to scrutiny!!
Manator
©Honorary Essex Boy™
I am doing a little tutorial at the moment about PIR inspections, I will not get into the debate directly only to say I have some examples of some tests that can be carried out to determine what you are debating. There is also an interpretation in the amount of testing to be carried out during a PIR. Without any certification or previous test results I would always err on the side of caution and test as much as I can. I also listen to what our peers say, and, you have to remember that the NICEIC do have some very very good points. Anything contained in ANY electrical publication is guidance. I like to be guided!
Steve, I'm not being argumentative here (I'm really not)
Where do you get that it's a requirement to record Ze (as you say) of a remote (2nd) DB?
And if you are right, where do you record this?
(Please don't throw an NICEIC altered form at me - I'm basing my question on the model forms)
Where do you get that it's a requirement to record Ze (as you say) of a remote (2nd) DB?
And if you are right, where do you record this?
(Please don't throw an NICEIC altered form at me - I'm basing my question on the model forms)
ADS - This becomes part of the big Q - there are many variations on a theme - differing forms from diferent orgs.
On most all sets of forms (PIR EIC etc) the front sheet, which covers the whole installation, requires details of supply, earthing Ze etc and details of the Main Switch
each DB in the installation should have a seperate Schedule of Test Results. On these there is often a short section in the header where you insert the details of that board i.e.Its designation, Earthing, details of main switch etc, and Ze - this Ze is also sometimes shown as Zdb (Zdb being an accepted description which most electricians understand as the Ze - external impedance of the DB)
When we look at PIR's and the reason for doing them - To moniter any degradation and ensure continued safety of the installation.
It could be argued by merely entering a Zs for a remote cct, e.g. a ring off the remote DB, all the way back to the origin of the installation, it would not help in fault finding, however if we have split boards by measurement we can very quickly find where degradation is occurring. Certainly the 2391-10 (Lev 3 Test + Inspect) requires you to measure and record in this way. If this qualification is the industry accepted test of competence re testing and Inspection, backed by the IET , any arguement to the contrary would need to be very persuasive.
On most all sets of forms (PIR EIC etc) the front sheet, which covers the whole installation, requires details of supply, earthing Ze etc and details of the Main Switch
each DB in the installation should have a seperate Schedule of Test Results. On these there is often a short section in the header where you insert the details of that board i.e.Its designation, Earthing, details of main switch etc, and Ze - this Ze is also sometimes shown as Zdb (Zdb being an accepted description which most electricians understand as the Ze - external impedance of the DB)
When we look at PIR's and the reason for doing them - To moniter any degradation and ensure continued safety of the installation.
It could be argued by merely entering a Zs for a remote cct, e.g. a ring off the remote DB, all the way back to the origin of the installation, it would not help in fault finding, however if we have split boards by measurement we can very quickly find where degradation is occurring. Certainly the 2391-10 (Lev 3 Test + Inspect) requires you to measure and record in this way. If this qualification is the industry accepted test of competence re testing and Inspection, backed by the IET , any arguement to the contrary would need to be very persuasive.
Steve,
I have the 2391.
As you mention it, we'll use that as a reference.
If you recall the practical:
In the three phase board you had an MCB feeding a distribution circuit which fed a single phase board.
On the main 'Schedule of test results' this MCB was then listed as a circuit, with all the normal test results next to it (like any other circuit in the 3 phase board) - including Zs - taken at the main switch of the single phase board.
The circuits in the single phase board were then recorded on a seperate test sheet in a similar manner.
On this seperate test sheet there might have been a box at the top for 'Ze at origin' (like the model forms), or there might have been a box for 'Zs at this board', like some other forms have, BUT...
...there definitely wasn't a box for 'Ze at this board' OR 'Zdb at this board' and there definitely, definitely, definitely wasn't a requirement to remove the CPC from the single phase board to measure a 'Ze' or 'Zdb'
So I really don't know where you're getting this from.
The only time the means of earthing is disconnected from the installation is to measure the Ze at the origin of the installation.
I have the 2391.
As you mention it, we'll use that as a reference.
If you recall the practical:
In the three phase board you had an MCB feeding a distribution circuit which fed a single phase board.
On the main 'Schedule of test results' this MCB was then listed as a circuit, with all the normal test results next to it (like any other circuit in the 3 phase board) - including Zs - taken at the main switch of the single phase board.
The circuits in the single phase board were then recorded on a seperate test sheet in a similar manner.
On this seperate test sheet there might have been a box at the top for 'Ze at origin' (like the model forms), or there might have been a box for 'Zs at this board', like some other forms have, BUT...
...there definitely wasn't a box for 'Ze at this board' OR 'Zdb at this board' and there definitely, definitely, definitely wasn't a requirement to remove the CPC from the single phase board to measure a 'Ze' or 'Zdb'
So I really don't know where you're getting this from.
The only time the means of earthing is disconnected from the installation is to measure the Ze at the origin of the installation.
ADS - Me thinks you are now engaging in an exercise in symantics - as I said previously, Zs of that Cct is more than likely to be the same or very close to Zdb with the earth disconnected (Board isolated) It is the terminology that differs.
If you are correct about not disconnecting the incoming earth at DB and testing as a psuedo Ze, we are being wrongly guided by the C&G center where I am doing the 2391 this week, and we will all undoubtably fail the practical next week as the C&G external examiner will consider us to be dangerous.
NB. I must add I do not expect this to be the case and we should all pass - God willing
If you are correct about not disconnecting the incoming earth at DB and testing as a psuedo Ze, we are being wrongly guided by the C&G center where I am doing the 2391 this week, and we will all undoubtably fail the practical next week as the C&G external examiner will consider us to be dangerous.
NB. I must add I do not expect this to be the case and we should all pass - God willing
All I have been saying all along is what Andy said in post number 4 - There is only one Ze measurement (with 'Earthing Conductor' removed from MET) and that is at the origin of the installation.
Any other Earth Loop Impedance measurement is Zs - even at remote boards.
If your 2391 lecturers are telling you different, they are wrong.
Think about why you are measuring Zs at the board - that should tell you how to take the measurement.
You are ensuring that a fault at the furtherest point on the 'distribution circuit' feeding the board (as with all circuits), will automatically disconnect in the required time (i.e. the MCB or RCD protecting the distribution circuit will trip under earth fault conditions)
What possible benefit would a measurement of the same 'loop impedance' be, with the CPC disconnected? - and why would you want to record it?
No Zdb in the 'Regs' by the way.
Any other Earth Loop Impedance measurement is Zs - even at remote boards.
If your 2391 lecturers are telling you different, they are wrong.
Think about why you are measuring Zs at the board - that should tell you how to take the measurement.
You are ensuring that a fault at the furtherest point on the 'distribution circuit' feeding the board (as with all circuits), will automatically disconnect in the required time (i.e. the MCB or RCD protecting the distribution circuit will trip under earth fault conditions)
What possible benefit would a measurement of the same 'loop impedance' be, with the CPC disconnected? - and why would you want to record it?
No resumation required mate: If a C&G course are telling you to take some modified form of test at sub boards, involving removal of an earth cable, they are incorrect.
You are currently being taught "how to" pass 2391.
ADS has ( I understand) already passed his. I know I have.
Not only is this NOT a requirement ( the sched`s ask for Zs at the board - that is what you give `em.)
What would you do (real world situation) if the sub board is supplied via M.I, SWA, SY, steel conduit or trunking? You`re going to disconnect them for testing? What if you aren`t allowed to isolate all the outgoing circuits from that board?
IF they are indeed teaching you to do this, I`d be really concerned! Disturbing all these earth cables leads to a likelihood of a connection being disturbed, and left in a worse state than it was ( the whole reason for limiting "dismantling").
As ADS has stated. You disconnect the earthing conductor (IF you can de-energise the whole install) for Ze. That is it. Sub -boards are tested as they are found, WITH parallel paths intact. Hope that helps to clear things up a little?
KME
You are currently being taught "how to" pass 2391.
ADS has ( I understand) already passed his. I know I have.
Not only is this NOT a requirement ( the sched`s ask for Zs at the board - that is what you give `em.)
What would you do (real world situation) if the sub board is supplied via M.I, SWA, SY, steel conduit or trunking? You`re going to disconnect them for testing? What if you aren`t allowed to isolate all the outgoing circuits from that board?
IF they are indeed teaching you to do this, I`d be really concerned! Disturbing all these earth cables leads to a likelihood of a connection being disturbed, and left in a worse state than it was ( the whole reason for limiting "dismantling").
As ADS has stated. You disconnect the earthing conductor (IF you can de-energise the whole install) for Ze. That is it. Sub -boards are tested as they are found, WITH parallel paths intact. Hope that helps to clear things up a little?
KME
I will 2nd the comments by KME & ADS, so would that be 3rd ing?
Anyway, you cannot measure Ze anywhere other than at the origin, by the BS7671 definition.
Any sub boards cannot have a Ze.
During a PIR there are so many "things" that come into play it is not as easy as it seems.
Even 2391 does not prepare you for them all, by any means!
2191-10 is a course as ADS suggests as a revision for qualified, experienced and competent testers to prove that they can undertake their works in compliance with current legislaiton really.
It is more CPD than training IMHO.
Anyway, you cannot measure Ze anywhere other than at the origin, by the BS7671 definition.
Any sub boards cannot have a Ze.
During a PIR there are so many "things" that come into play it is not as easy as it seems.
Even 2391 does not prepare you for them all, by any means!
2191-10 is a course as ADS suggests as a revision for qualified, experienced and competent testers to prove that they can undertake their works in compliance with current legislaiton really.
It is more CPD than training IMHO.
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