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Can Zs at DB ever be less than the Zs of the feeding circuit?
Peter S3
25 Posts
Question
I am reviewing an EICR recently issued for a building with several Distribution Boards feeding sub-Distribution Boards.
I have noted that in some instances, the figure recorded for 'Zs at this board' is significantly less than the Maximum Measured Zs for the circuit recorded on the feeding DB.
e.g. DB FF4 is recorded as being fed from DB FF1.  The feeding circuit to DB FF4 is recorded as having a Maximum measured Zs of 0.4 Ohm, but the 'Zs at this board' for FF4 is recorded as 0.05 Ohm - which is less than the 'Zs at this board' recorded for FF1 (0.08 Ohm) - and which, is in fact, in turn itself less than the 'Maximum measured Zs' for the circuit feeding it.  Can this be true or are there errors in the report?  I thought that cascaded Zs can only get larger due to the added impedance of the feeding circuits? This is not my primary area of expertise, but I am concerned that the EICR is being used to justify the upgrade of several circuits which have passed previous inspections with no problem (hope the resolution of the extracts from the EICR below are sufficient resolution to read)...







Many thanks if anyone is able to confirm my concerns or otherwise put me straight...
75 Replies
gkenyon
1081 Posts
Were the measurements taken with all loads disconnected? Loads switching in and out can affect results.

I could also foresee theoretical situations in sprawling buildings, where what's locally bonded provides a low impedance earth path back to the MET, but perhaps not to that extent?

Of course, without knowing anything about this particular installation, I would certainly be asking the question you are asking.
Alcomax
194 Posts
Would expect mainswitch of tested DB to be off? Likely two differing inspectors per DB. Blurred image gives no clear detail, BUT, looks like R2 and Zs recorded on distn circuit to DB. Odd for EICR. Perhaps this Zs calculated, erroneously, simply from a wander lead origin to DB [R2], then inspector of sub db did live Zs test.

edit to add:  Mixed disconnection times one DB to the other.
Alcomax
194 Posts
but I am concerned that the EICR is being used to justify the upgrade of several circuits

But is there any observation for that dist'n circuit recorded suggesting "upgrade" ?

That will indicate if it was just a clerical error or not.
mapj1
2170 Posts
In a steel framed building you might get this, as in effect the later disboard, connected to the metal building frame, is better grounded than the board that supplies it if that one is say on a brick wall. However, that is not a common situation.

But, are any of the distribution circuits Zs so bad that a 5s disconnection would fail, and are any of the final circuits so bad as to fail 0.4 seconds - if not then it is nothing more than a comment. What 'upgrades' are being proposed, and why? -  it may be sensible - it may be just making work.

 
Peter S3
25 Posts
Many thanks for everyone's responses to date on this.
I do not know what they did regarding disconnecting loads etc when they did their tests.  This was actually a medium-size 2-storey church building built over 100 years ago, so not, for example, sprawling over a multi-building site, but has had various extensions and modifications over the years, so may have some structural steel members here and there.  The inspector has acknowledged multiple errors in the report elsewhere, but I was checking if these Zs values were also a bit of a red flag.  There is concern over one particular area of work that he has recommended, which is to replace/upgrade a circuit for reasons that the EICR does not support (a circuit which the inspector says in his observations is only protected by a 32A fuse, but which the EICR clearly says is protected by a 100A fuse). When pushed for clarity, he is now saying 'ah well, that circuit is actually fed from another circuit which is mislabelled'. The whole thing is just a bit fishy, but I am not sure whether to call him out on it or not.  He may be telling the truth, but the EICR (marked unsatisfactory) that he has issued seems to be full of factual/typo errors and I just cannot see that it clearly supports the conclusion that there is a circuit that need upgrading.  If you take the report at face value, as written, it is claiming that connections/circuits exist that just do not exist on the report and which were not recorded on the last inspection report 5 years ago by a different inspector/company. Either his report has failed to record the bad circuits that do exist, or he is recommending to replace circuits that the EICR does not indicate need replacing/upgrading.  On many of the circuits on the EICR, the 'Maximum measured Zs' for an individual circuit bears no resemblance at all to the 'Zs at this board' + the measured R2 for the circuit.
Peter S3
25 Posts
In the FF1 for example, the 'Zs at this board' is recorded as 0.08 Ohm.
Circuit 1/L1 records 'Continuity R2' as being 0.1 Ohm.
'Maximum measured Zs' for this circuit is recorded as 0.04 Ohm - less than the R2 for the circuit itself. Can this happen? 

Circuit 3/L3 has a 'Continuity R2' recorded of 0(?), or 'R1 + R2' of 0.01, and yet a 'Maximum measured Zs' of 0.4 Ohm. I would have expected the measured Zs to be very close to 'Zs this board' + R2. (neglecting the fact that he has managed to find an impedance-less conductor linking two Distribution Boards.
Alcomax
194 Posts
If you can provide the following it may help someone give a more specific answer
1: Is there a steeple?
2: What is declared method of earthing... TT or a TN?
3: Protective bonding conductors to what, eg  Steel, LPS, gas water.

It all seems fairly self contained....but is the Origin/ metering actually inside this building?
Peter S3
25 Posts
1. No 'steeple', but there is a 'tower'.  Interested to know how that affects matters (I mean that genuinely - it's not intended to be a provocative question :-)
It has not had any floors 'added' on top, but it did have an internal floor 'inserted' in the middle some 10 years ago, and that would have required considerable new wiring added at the time plus, I would guess, numerous structural steel support beams.


2. TNC-C-S is declared on the EICR;


3. You will see from the above that 'N/A' has been recorded for all Main Bonding.

4. It is a self-contained building, and I believe the meters are all contained within the building.

My main concern is regarding feeder circuits to DB GF2.(not my original question, but the reason for my trying to check other details on the report);
The EICR records as follows;
DB GF2 is supplied from 'DB MP1 Circuit No 2TP' via a 25mm^2 live conductor, 16mm^2 earth and MCCB rating 160A;


DB MP1 circuit 2 records that GF2 is fed by 100A MCCB via 35mm^2 live conductor and 25mm^2 cpc;


and his observations in the EICR state;

and he has quoted to 'To supply and install new 25mm 3 Core Armoured cable from Main Electrical intake to
Distribution Board GF2. Disconnect and cut out where possible redundant feed to GF2'.

On being challenged that he has got his GF2's and GF3's mixed up, and what he says in his conclusions does not match what is recorded in his EICR, the inspecting contractor (who issued the EICR) is then saying;

"Sorry there was an error on my observations.
 
GF2 is fed from MP1 not GF1.
 
GF2 is labelled in MP1 as “Boiler Panel” but it feeds GF2 which the feeds Boiler Panel.
 
This is on a 32a MCB. GF3 is fed from a 62a MCB in GF2.
 
We  are not in the business of making recommendations for cables that are not necessary, our proposal is to give you a safe and fully compliant electrical installation.
 
Please confirm you still wish us to continue with these works."

This still does not match what his EICR says, raises all sorts of questions regarding how accurate the EICR is, and whether it is safe to rely on what he is recommending.  I am thinking his EICR is just too scrambled and full of inconsistencies to be taken as credible.
Or am I missing something?

Many thanks for your interest - it is much appreciated.

 
weirdbeard
161 Posts
Originally posted by “Peter S3:
(hope the resolution of the extracts from the EICR below are sufficient resolution “

Hi Peter welcome to the forum :) Not your fault but those pics are making my eyes hurt, how did you upload them? Pictures that appear blurry could be previously clicked on and viewed in more detail, I am not sure if this has changed recently.
Chris Pearson
1531 Posts
Peter S3:
I am thinking his EICR is just too scrambled and full of inconsistencies to be taken as credible.
Or am I missing something?

I cannot read your original charts, but there seem to be some basic errors in the later ones. Sections I and J: nominal voltage and frequency appear to be measured. If Ze = 0.14 Ω then at 243 V the PEFC is around 1.717 kA, but matey seems to have forgotten the possibility of a L-L fault. Why has supply polarity not been checked? The size of the MPBC is not given and there is no indication as to what is bonded.

weirdbeard
161 Posts
It’s much clearer with the new pictures, thanks :)
 
Peter S3
25 Posts
Sorry about the pics in my first post - not sure why they lost resolution on uploading.  Here they are again, hopefully clearer;








Alcomax
194 Posts
So the the murk vanishes with the new pictures.

Look like someone has gone all over "initial verification" style on an EICR.........................................................................good luck 😶

So a single phase sub DB.

I asked "steeple" due to being a Church type building. There is something pointy at the top of it?   All my questions were due to "other earthy bits" that may or may not influence things. You still have a mix and match disconnection time thing on some circuits that is odd or just clerical.

If your Inspector is part of a scheme that verified competency for EICR purposes, particularly commercial, you should follow that enquiry/ complaints procedure if the Report is not presented in a clear manner and they do not clearly substantiate their Observations. It does need to be fit for purpose otherwise there is just conjecture.

 
keylevel
17 Posts
DB MP1 is single phase and feeds DB FF1.

FF1 is three phase.

I'm guessing the entry for MP1 isn't correct!
 
mapj1
2170 Posts
Are you able to look at the offending breaker yourself and determine it's rating, and eyeball the size of cable coming off it?
160A and 100A should be easy to tell the difference, as should 35mm vs 25mm, unless the cable really does change size halfway along.

To err is human, but really with modern phones able to take pics and so on, there is no excuse for muddling up which DB and cct is under discussion, unless of course the report was written after the event.
Before getting too critical perhaps he'd like to correct the erroneous report - for free of course (I wonder how many customers do read the numbers and how many sign a cheque - not many I suspect)?
incendentally If those notes are supposed to justify rework, well
Note 13, lack of discrimination, is not a safety issue - just it may trip something at both ends if you get a big fault
note 15 makes no sense.
Note 14 can be checked by sticking your head in the cupboard - has someone started storing mops and brooms leaning against the cable run ? If so some boxing in trunking may be needed.
 
Chris Pearson
1531 Posts
Somebody has got in a right old muddle - that's being polite; or is just sloppy.

If MP1 is the main board, it seems odd that the supply is only 25 mm² with a 16 mm² earthing conductor.

Then we have a distribution circuit to GF2 with 35/25 mm² conductors and a Zs of 0.16 Ω; yet by the time that they get to GF2 they have shrunk to 25/16 mm² and the Zs has fallen to 0.06 Ω. Similarly, the supply to FF1 is 50/35 mm² with a Zs of 0.19 Ω and once again they have changed by the time that we get there to 25/16 mm² and 0.08 Ω. All the max permitted Zs for the 60898 MCBs are for the next size up in Table 41.3.

I could go on, but I am bored now. 🙄
weirdbeard
161 Posts
mapj1:

note 15 makes no sense.
 

Agreed :)

And a 315Amp main fuse supply to a main switch in 95mm apparently- did the inspector bring this up somewhere in the codes? It may be OK depending on installation method, but I'd double check the installation method----- perhaps the inspector mistook 120mm for 95mm?

I'd certainly get the inspectors governing body involved. If he's NICEIC he'll have an NICEIC area engineer (That ostensibly checks his work quality once a year) that may be willing to swing by?

There's certainly enough there to justify dis-believing his recommendations and recommended work as a result. 
Sparkingchip
2675 Posts
If in doubt I would code FI-Further investigation required.

There is no mention of anyone doing calculations and assessments to determine what protective devices and cables are required to make comparisons with what is already in place to determine if any upgrades are actually required.

There is probably hours of design work to do before anyone starts upgrading the installation.
 
Sparkingchip
2675 Posts
Which begs the question, how was the existing installation assessed?
Sparkingchip
2675 Posts
Loop testing throws up odd readings, so yes you can get what is apparently “the wrong result” when you are inspecting and testing.

So you try to determine why and if there is no enough time and it’s a potentially an issue record it as FI.

That means have another look at it before ripping it out.

Andy Betteridge 
Peter S3
25 Posts
Well spotted!  You are absolutely correct.,  MP1 is in fact a 3-phase DB.  The inspector simply has not made any care in filling out the individual DB details.  The previous report (different inspector, 5 years ago) noted whether each circuit was 3ph or 1ph, but in this report, other than occasional mention (sometimes incorrect) that the DB is 1 or 3ph, the individual circuits are not clarified at all.  I took a quick visual look at the installation myself last night; MP1 is a 3ph DB, fitted with a mix of 3ph and 1ph MCCB, depending on which circuit is being fed.  The 32A MCCB that he claims (in his recommendations but not in his EICR) is feeding 3-phase DB GF2 is, in fact, a single phase MCB and definitely not feeding the 3ph DB GF2!  I am only advising technically as a lay person, but I think I will have to tell those authorising the work that the whole inspection and therefore the entirety of all recommended works have to be seriously held in question, even if some of them do appear to be justified.
Peter S3
25 Posts
Many thanks mapj1 - Note 15 (14) did not make any sense to me either - I'm glad to have that verified.  Note 13, it turns out, is complete nonsense in any case, and does not reflect the reality of the installed circuit.
Peter S3
25 Posts
Thanks Chris - since this is not my day job, I was not 100% sure if I had understood correctly what information should be recorded in those boxes - I was sure it should be what was actually installed, but wondered if it might have been what the rated capacity of the DB was (which still would have raised many red flags anyway).  I am glad to have my understanding confirmed that it should record what is actually installed.
Peter S3
25 Posts
Many thanks to everyone for all your posts - I wasn't sure if I had exceeded my knowledge base regarding EICR and was completely misinterpreting the information recorded and what it meant.  It has been extremely helpful to have it confirmed that there are indeed many aspects in the EICR to be concerned about, and particular regarding the recommended works/observations.

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