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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 Former Community Member over 4 years ago

It’s much clearer with the new pictures, thanks :)
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0 Former Community Member over 4 years ago

Sorry about the pics in my first post - not sure why they lost resolution on uploading. Here they are again, hopefully clearer;
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0 Alcomax over 4 years ago

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.
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0 keylevel over 4 years ago

DB MP1 is single phase and feeds DB FF1.

FF1 is three phase.

I'm guessing the entry for MP1 isn't correct!
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0 mapj1 over 4 years ago

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.
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0 Chris Pearson over 4 years ago

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. ?
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0 Former Community Member over 4 years ago

mapj1:

note 15 makes no sense.

Agreed :)
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0 tattyinengland over 4 years ago

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.
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0 Sparkingchip over 4 years ago

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.
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0 Sparkingchip over 4 years ago

Which begs the question, how was the existing installation assessed?
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Can Zs at DB ever be less than the Zs of the feeding circuit?

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