Testing Ze for a new consumer unit in a garage

[Sidewinder]

OK, first thing is I think that we all got too involved with debating with each other rather than the subject of the OP.

So two things first.

1. The definition of a system in 7671.

2. I'm gonna have to read the OP again, and then go through the whole thread again, this is gonna take some time! Oh & it won't be all tonight.

 

[Manator]

This thread, and the replies, has made me go back to my books, for that I am grateful. I do not read enough and rely very much on my memory, so it was good to get back to some basics. Now, I do installs that BS7671 forms a small part in all the regulations I have to follow. Many of the equipment installs require a 0.1 ohm bonding resistance, which often requires the need for 5 cores for 3ph supplies and 3 cores for 1ph supplies.(SWA).

Every installation is considered to be the same installation if it shares the common supply. Whilst BS7671 is ambiguous on this definition, the extra regulations I work to, clearly define this, which is why my defence was so robust.

However I will say that for most,not all, installations I do, TNC-S is not permitted. So I would seldom come across this problem.

Anyway I am going back to my books and having another read.

 

[steptoe]

can we all get ONE thing clear here,

everything I have read in this thread relates to this reg or other in BS7671,

do I have to change my sig line back again?

BS7671 IS NOT LAW ,

can we all understand this please,

oh, and as for the questioning knowledge bit, I dont think I have to, the ignorance about failing to understand why the N plays so large a part in this with a TNCS supply answers itself as far as I see,

go back to basics and first understand the principles of earthing arrangements before you start spouting about how you deciding to change a TNCS to a TT has no bearing and you may start to understand.

EDIT, couldnt spell thred....

 

[ADS]

I suppose with a membership this large, you'll always get someone who'll spoil the thread. :red card

 

[steptoe]

I suppose with a membership this large, you'll always get someone who'll spoil the thread. :red card

yes, you always will.

so then ADS, tell me what the bearing on the N is with tis kind of situation within a TNCS supply by the DNO is? or isnt? as you assert,

TNCS is at best a flawed system and until you understand its principles and as to why the N conductor has such a relevance then you will not understand why this has such a bearing on a fault condition.

imagine you have a TT install and your neighbour has a TNCS, you have a fault, your neighbour can pull that fault through his system,

even if you have TNCS/PME , it can still transpire into your neighbours install, this is why earth conductors are required to be so large in TNCS installs, so as to cope with your adjoining faults as well as your own.

does this help you in any way?

 

[ADS]

None of that was questioned during this thread.

What was stated, (and please read this carefully) was:

The Neutral conductor has no bearing on whether (or not) the outbuilding should be deemed a separate installation from the Main Installation.

Can you understand that statement?

Do you agree with that statement?

If not, please explain what it is about the Neutral conductor that determines that the outbuilding is (or isn't) a separate installation.

You see, my line was that it should be treated as a separate installation because it has it's own 'Earthing System' (TT).

This to me makes sense, to others it doesn't - but where does the neutral come into this argument??

Oh and by the way.....

can we all get ONE thing clear here,everything I have read in this thread relates to this reg or other in BS7671,

do I have to change my sig line back again?

BS7671 IS NOT LAW ,

can we all understand this please,

.....it's what most of us work to, so were going to reference to it in our discussions, aren't we?

Otherwise we might as well just quote pages from 'Gardeners Weekly'.

 

[steptoe]

well then ADS, perhaps you should start working to what is the LAW, and not what is a guide.!

and , yes, it makes a massive difference as to whether or not this is another installation in my mind to your fictional 'another installation'.

if you want to carry on your argument with people that dont understand earthing arrangements feel free, dont include me, and dont come crying back once you have killed someone cos you have earthed something badly.!

 

[ADS]

I give up - I'm out of here.

 

[Manator]

This is a good debate, and I must say I have learned from it. There is as, I suggested, a direct relevance to the neutral, but just as ADS pointed out, there is a need to fully understand, and more importantly design properly, from that statement I knew that ADS did see a relevance, however from the point of the OP it was not in question. Which< I think, is why ADS questioned the relevance. There are a number of reasons why TNC-S has problems, the broken neutral is possible, then the accumulative faults on the system can give rise to a constant flow of voltage to earth. When a TNC-S is at origin, and a TT is installed within the installation, we can have another problem, there will always be a difference in potential, and impedances, a (oh just got called out will complete this later) sorry.

 

[SteveT]

Ok back to this after some research.

1. There is nothing in OSG/GN3 or BRB re Zdb

2. I asked 12 electricians if they understood the term Zdb - all did, most would cross out Ze and inserted Zdb on schedule of test results for that DB

3. You can of course measure (r1+r2) and add to Ze for Zs of the distribution Cct- conductors are disconnected for this.

4. You can mesure Zs - however and a caveat, you will be measuring any parrallel paths, herein lies a major problem

Scene.

A small industrial unit, 3P main board TNC-s feeding 3 of 1P DB's in SWA each dist Cct protected by BS 3871 63A Type C MCB

Zs of dist Cct at DB1 - measured at DB1 = 0.25ohms, this being within the max of 0.29ohms for a 5 second disconnection time

As we know overall measured resistance decreases when resistances are connected in parallel.

You measure Zs for dist Cct and it is low enough to meet the required disconnection time of 5s, you are advised in the publications that a measured Zs will likely include parallel paths and you should be aware of this.

In this scenario parallel paths exist via some redundant trunking, conduit and racking

The week after these parallel paths are removed by a labourer as instructed by the boss as no longer required, neither have any electrical knowledge but it was obvious no ccts utilised them.

Now your Zs is well above the Max.

A fault occurs, overheats dist cable, MCB fails to trip resulting in a fire and a fatality.

At the inquest you explain your actions as the last qualified person to check the installation and to have signed the inspection report.

An expert witness for the factory testifies that a strong body of opinion exists amongst him and his fellows - your peers - that a competant person should have forseen this possibility and should have taken steps to ensure no parallel paths were included in the measurement - ergo by disconnecting the incoming earth and measuring as if a Ze. This would have more than likely prevented the fire as the MCB would have tripped in the required time exonerating you from blame.

I discussed this with the C&G examiner(who is also an expert witness)- he said that he would expexct you to disconnect and measure or do it by calc for the reasons above.

If you do this as routine where you can, it will cover you in the above scenario - if not able to disconnect it should be noted on PIR and the client informed of the situation and the danger associated with it.

Of course in a garage at home you should either ensure that an earth with at least 1/2 the CSA of the incoming Neutral is fitted alongside the supply unless you are 100% sure that there are no other paths to earth (As the garage is outside the 'Faraday cage' of the house and its associated bonding), or terminate the garage supply in a plastic enclosure, ensuring if SWA the sheath is earthed at the origin, or T+E is earthed at the origin(to protect the cable) and then provide TT for the garage.

Otherwise as Steptoe says yuo may find you a carrying fault current for several house in the street if there is a problem witht the neutral.

Don't loose sight of the fact that earthing is a complete subject on its own and is more complexthat most believe.

; \

---------- Post Auto-Merged at 10:10 ---------- Previous post was made at 10:03 ----------

By the way passed 2391-10 practical - theory on 9th

 

[ADS]

OK, I'm going to have one more try.

I'm going to make this really easy for you, because neither you, Manator, nor Steptoe, have answered my question as to why the Neutral is relevant to this thread.

Here is my statement:

I believe that the outbuilding should be classed, and treated, as a seperate installation because it has it's own, seperate, earthing system(TT). The neutral conductor has no bearing on this classification as, at the outbuilding, it is just that - a neutral.........there is no connection with the outbuildings earthing and bonding.

Now, Steptoe and Manator, here is your statement - all you have to do is fill in the missing words - I can't make it any easier than that:

We believe that the outbuilding can't be classed as a seperate installation because the Neutral conductor _ _ _ _ _ _ _ _

If you can successfully complete the sentence, without prattling on about TN-C-S, PME, lost neutrals etc. etc.,we will have at least achieved something.

---------- Post Auto-Merged at 10:28 ---------- Previous post was made at 10:13 ----------

............In this scenario parallel paths exist via some redundant trunking, conduit and rackingThe week after these parallel paths are removed by a labourer as instructed by the boss as no longer required, neither have any electrical knowledge but it was obvious no ccts utilised them.

Now your Zs is well above the Max.

A fault occurs, overheats dist cable, MCB fails to trip resulting in a fire and a fatality.

At the inquest you explain your actions as the last qualified person to check the installation and to have signed the inspection report.

An expert witness for the factory testifies that a strong body of opinion exists amongst him and his fellows - your peers - that a competant person should have forseen this possibility and should have taken steps to ensure no parallel paths were included in the measurement - ergo by disconnecting the incoming earth and measuring as if a Ze. This would have more than likely prevented the fire as the MCB would have tripped in the required time exonerating you from blame.

The above could be applied to any circuit on which you measure Zs - why pick on a distribution circuit.

Your measurement could be within limits with the parallel paths, and over the max when they are removed.

How do you suggest we deal with that?

If you do this as routine where you can, it will cover you in the above scenario - if not able to disconnect it should be noted on PIR and the client informed of the situation and the danger associated with it.

So we're going to disconnect parallel paths on every circuit from now on before we measure Zs - I can't see it.

My Blue.

 

[SteveT]

ADS - I am afraid you are the one missing the point, it is precisely because the garage is part of the installation, no mention of seperating by abandoning the TNC-s at the house and TTing the garage.

The poster implied that it was indeed part of the house, thus TNC-s - then missing neutrals certainly come into it, see my last post.

The reason we pick on a dist cct as you put it is because the likelyhood of the scenario occuring is much more likely (By the way this was an actual case the expert was involved in and the guy was convicted)

In a final cct with generally faster disconnection times a small rise in Zs wold not neccesarily cause a fire, the trip may operate more slowly but would often operate. if the trunking/conduit on a final were removed more than likely so would that cct.

Don't forget all the pubs you quote are non statuary - they are advisable, you do have to apply common sense and educated interpretation, and when filling in certs, reports make them understanable to the less technical, it is their report as well as info for the next guy testing after all.

Your last staement does sound like 'its my ball and i'm going home with it' sort of thing. Please this is a mature debate as you would be aware from the amount of responses generated.

 

[Manator]

Ads, please see my post above, quote "This is a good debate, and I must say I have learned from it. There is as, I suggested, a direct relevance to the neutral, but just as ADS pointed out, there is a need to fully understand, and more importantly design properly, from that statement I knew that ADS did see a relevance, however from the point of the OP it was not in question. Which< I think, is why ADS questioned the relevance." un quote.

If you now want another debate please feel free, but do not try and put words into my mouth.

 

[ADS]

Steve, the opening post was answered by about page 2 - if you've been away from the forum you need to re-read the whole thread - take particular note of the indepth post by 'Special Location.

OBVIOUSLY, if the earthing system remains TN-C-S throughout, then there is a neutral issue.

That wasn't the debate - it moved on to whether the outbuilding should be classed as a seperate installation if you give it it's own TT earthing system.

As for the 'my ball' comment - all I'm trying to do is get a straight answer off Manator or Steptoe - and Manator still hasn't answered even though he's posted again.

Every discussion we have on this forum ends up the same way - people making random statements and then unable to explain the relevance of their comment in detail.

Like Manators 'I've got one word to say, NEUTRAL'' - I'm still waiting for an explanation.

And then you get Steptoe insulting you and questioning your knowledge - and he doesn't answer any direct questions you put to him either......just rants on about the 'regs' not being law and PME!!

It's bloody frustrating - there are some on here you can have a good debate with - as for the rest.......

By the way, Steve, I think your scenario may have a flaw in it.

If it's TNC-S and the earth loop isn't sufficient (with parallel paths removed) to trip the MCB, what's going to happen under short circuit conditions?

Assuming 3 core SWA, both L-N loop and L-E loop will have near enough the same resistance along your sub-main.

If it won't trip under earth fault it's not going to trip under short circuit - so I'd say a design issue, not a 'Zs' issue.

 

[Manator]

Please read my last two posts ADS, I thought you agreed about some significance, because of a statement you included in one of your posts.

 

[Sidewinder]

Plumber,

You said you would post the reg numbers to reinforce or contradict my arguments for me, but I guess I'll have to kick this off?

OK, Part 2, Definitions, pg 30, System.

Can we agree on that?

Also are we still debating the OP, or are we just debating whether or not the TT'd outbuilding can be a separate installation?

This will have a big impact on how much research I have to do!!!

 

[SteveT]

I get your point about a design issue - on the curves a short cct - giving max fault current - would trip the MCB, dont forget this is a pir scenario something may have changed between initial design and install and the intervening years. It is still the last competent persons responsibility - if he has tested and signed up to say it is safe, he is responsible. whole point of PIR to moniter condition of installation and ensure it is safe for continued use.

I apologise if I missed the point where the thread went on to TT - a system far preferable for garages/outbuildings etc.

As for PME - it has limitations but is perfectly acceptable for origins. That is where the N comes into play and size of earth.

the initial arguement holds though, we as a group understand and refer to Zdb as an aid to dechipering Test Result sheets. You can obvioulsly enter Zs for the dist Cct on the sheet apertaining to the orign as it is merely another cct of off that, however ion the next sheet for DB1, DB2....etc we shoulkd enter something in the box for Ze - change this to Zdb, enter the figure and everyone knows what it means where it was measured, thus any deterioiation will be noted by the next guy refering to the previous.

 

[Des 56]

An earth fault in the outbuilding woud go via the CPC, MET, Earthing Conductor, 'Rod', General Mass of Earth, Next Rod, Star Point, Line Conductor, CPD, ......have I missed anything????

An earth fault in the house would go via CPC, MET, Earthing Conductor, Neutral, Star Point, Line Conductor....

If Steptoe cant understand the fault path and keeps avoiding the questions put to him,then you are flogging a dead horse

 

[Sidewinder]

Denker,

You have forgotten the PME rods in your second example but generally it is OK.

Steps does understand the fault paths mate, he is just carp at explaining things!

We'll slap him into line one day and then you'll all see his point of view, I understand him a little more being a fellow Celt you see!

 

[green-hornet]

My oh my.

Whats been going on here then? I think the definitions in the BRB are intentionally ambiguous, as with many laws or regulations, finite little interpretations can be taken from one to the other, and most regulations and laws are written this way, otherwise there would be no point in having a defence against anything.

Can one installation be split into two or more installations? Yes they can, can two installations be classed as one? yes it can.

Can Ze measurement be taken from each? Yes they can, Can you have two ze readings for one installation? No you can not, there should be no confusion between any earth reference point.

Can the neutral on one installation earthing system have an effect on another earthing system ? Yes it can, however there would have to be multiple faults to get to that stage.

There are regulations that do go a lot deeper into the definitions as they should be applied, I know of at least one definition that states that if any installation sharing a common supply should be treated as one installation, regardless of the earthing arrangements.

I have used this regulation, and each separate DB was designated as Zdb, no confusion, and the Zdb, whilst not really defined in the BRB, is used in most installations, and can be the Ze of the earth rod/mat or whatever, or the Zs, but is never confused.

My forms also allow me to mention where in the installation each DB is taken from, if not directly from supply, the model forms, just do not allow or cover for this.

The only wrong part of any debate is when one or more members of that debate are unwilling to accept two sides to every argument.

The only damaging part of any debate is to allow what you think you know, to influence what you think you should know.

Its very seldom that I get on the forum these days but perhaps I should, most will know I hate referring to a regulation to back up my argument, I rely more on common sense, and that is what this debate has needed.

For my part of the debate I will say that some people know what TT earthing is, but have little concept of the design required to establish a proper earthing arrangement, or the effect of two earthing arrangements can have on a single installation.

Now I know ADS will come back and tell me that it is two separate installations, but even he has to admit there is a direct relationship between the two, though I doubt he would ever admit that, he likes to diss everything, as long as plumber backs him up.

Oh well another spanner thrown into the works.