This discussion is locked.

You cannot post a reply to this discussion. If you have a question start a new discussion

Can Zs at DB ever be less than the Zs of the feeding circuit?

Former Community Member over 4 years ago

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...

0 Sparkingchip over 4 years ago

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
Cancel
Vote Up 0 Vote Down

Cancel
0 Former Community Member over 4 years ago

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.
Cancel
Vote Up 0 Vote Down

Cancel
0 Former Community Member over 4 years ago

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.
Cancel
Vote Up 0 Vote Down

Cancel
0 Former Community Member over 4 years ago

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.
Cancel
Vote Up 0 Vote Down

Cancel
0 Former Community Member over 4 years ago

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.
Cancel
Vote Up 0 Vote Down

Cancel
0 Former Community Member over 4 years ago

Zs at DB should be the value of Ze plus the total R1+R2 of the total circuit feeding the DB right? R2 might be reduced at a particular DB by local bonding, but there will always be some R1 which will generally act to increase the total Zs at DB as you add more DBs into the chain? Zs at DB can be measured by having all feeding circuits live so that you can measure the total Zs back to the feeding transformer (using a calibrated suitable multi-tester), or you can do it by adding your measured maximum R1+R2 values to a measured or known Ze? Maximum measured Zs for a circuit fed from a DB should, in theory (allowing for slight measurement tolerance), always be more than the Zs at DB + R1 for that circuit? (possibly also +R2, but local bonding in the circuit may reduce the effect of the measured R2). Have I understood that correctly? (Don't worry, I'm not looking for advice to help me design or approve electrical circuits without doing the exams - I just want to be sure that I am understanding what the inspector has put in the EICR I am looking at).
Cancel
Vote Up 0 Vote Down

Cancel
0 lyledunn over 4 years ago

Another one of those EICRs that has the insulation resistance of every circuit at +500Mohms! I suspect those straight away! However, when Zs is very low, accuracy is an issue and becomes problematic when trying to assess disconnection times. As others have pointed out, there are many issues in the Sections of the report you have posted. By the way, I have 2 really good guys who work for me, specifically carrying out I and T every day and I have often had to buttonhole them on some quite serious mistakes they have made. I hope that by calling their attention to them and requiring them to return to the installation they will learn and operate with a little more care. I may have years of experience under my belt but I do miss things occasionally. Only last week NICEIC inspecting engineer pulled me on a really stupid mistake I had made!

Do the industry a favour and get the guy back to address the issues.
Cancel
Vote Up 0 Vote Down

Cancel
0 wallywombat over 4 years ago

Well you would actually expect Zdb/Zs at a downstream DB to be equal to R1+R2+(Zs of upstream DB) rather than R1+R2+Ze, since downstream Zdb's are measured with the main bonding in place while Ze at the origin is measured with bonding disconnected. (Ignoring any deliberate or incidental local bonding of course.)
Cancel
Vote Up 0 Vote Down

Cancel
0 AJJewsbury over 4 years ago

Zs at DB should be the value of Ze plus the total R1+R2 of the total circuit feeding the DB right? R2 might be reduced at a particular DB by local bonding, but there will always be some R1 which will generally act to increase the total Zs at DB as you add more DBs into the chain? Zs at DB can be measured by having all feeding circuits live so that you can measure the total Zs back to the feeding transformer (using a calibrated suitable multi-tester), or you can do it by adding your measured maximum R1+R2 values to a measured or known Ze? Maximum measured Zs for a circuit fed from a DB should, in theory (allowing for slight measurement tolerance), always be more than the Zs at DB + R1 for that circuit? (possibly also +R2, but local bonding in the circuit may reduce the effect of the measured R2). Have I understood that correctly?

Zs will increase by R1 certainly, but parallel paths could possibly reduce the effective R2 - if they reduce it enough it is theoretically possible it might outweigh the increase provided by R1 and so give a lower overall measured Zs.

- Andy.
Cancel
Vote Up 0 Vote Down

Cancel
0 Former Community Member over 4 years ago

Yes agreed. I should have said Zs at source rather than Ze, as all/any supplementary bonding will also be in place.
Cancel
Vote Up 0 Vote Down

Cancel

Can Zs at DB ever be less than the Zs of the feeding circuit?

Community Rules & Guidelines