Testing Ze for a new consumer unit in a garage

[steptoe]

Denker, TT at the outbuilding fault goes to earth via the rod, the next rod this fault is likely to see is your PME rod at the house?(which is connected to, wait for it, the NEUTRAL)

or do reckon the fault will know this and decide to just go to the rod at star point instead?

this is why PME bonds have to be so big, we have had this all before about PME inheriting(pulling in) faults from external rods,

BTW, this TT fault could also manifest itself through any extraneous metalwork in the house too, down the MEBs to the MET where it will magically disappear into the ether, instead of going down the earthing conductor into the neutral.

 

[ADS]

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.

Until this paragraph, Green Hornet, you were getting a 'like' and a 'star' - then you blew it with this??

I haven't stated that it should be deemed as two installations.

I said that it could be - for clarity on forms, etc. - up to the designer/inspector/tester.

I also questioned how you would treat it as one installation - with two seperate earthing systems - on the forms etc.

I asked how the incomming neutral had a bearing on whether you called it two installations - the incomming neutral is shared with you and your neighbour - they could be TN-C-S and you could be TT - your house and theirs would still be two installations.......no one answered this question.

So, I don't like to 'diss' everything - and I only agree with 'Plumber' if I think he's right - the same with 'Specs', who I also think was right on this occasion......but I've had disagreements with them both in the past.

So, your final paragraph was uncalled for - you were doing well up until then.

 

[steptoe]

if you and your neighbour are both on PME with a shared N then the fault current back to star point should have the same resistance so will be negligible under normal circumstances, your cut out signifies the start of your installation, as that is where your earth conductor S from the N, that is why your earth needs to be half size of the N, unlike TNS where it can be much much smaller,

I suppose for TT you use 2.5 earthing conductors too?

 

[Sidewinder]

ADS,

The incoming neutral will not be shared between you and your neighbour though would it?

That would ONLY be IF you shared the same incoming main.

Even then it would be separated by the DNO head, thus as far as you are concerned the supply is completely separate and thus nothing to do with you.

So, your comment with regard to shared N being irrelevant is not true.

You are only concerned with any points downstream of the head.

IF the N is shared past this point then you are concerned with it.

 

[green-hornet]

I am sorry ADS, I do apologise, it was probably unfair of me.

I do think that if people had a full understanding of TT earthing, and the implications involved, then I think even you could grasp that whilst two separated installations sharing the same supply, could, and probably should be treated as one installation.I am thinking of step and touch variations, the possible neutral to earth faults, internal on one effecting the internal of another, and the fact that electrickery flows two ways. I do know that working on dead (shorted out) HV systems, the equipotential zone for safe working is 5m from the earth electrode, even though the next electrode could be within stepping distance of this, because of the differentials.

If you stated that it "could" then by definition you have to admit it may not, hence why I said what I said. No one least of all me is going to say you are wrong. I quite like your input, I like mine more, but that is because I am biased.

However I do apologise for my statement, it is unusual of me to single out any member, and even more unusual of me to make remarks about their judgement.

 

[ADS]

If you stated that it "could" then by definition you have to admit it may not, hence why I said what I said. No one least of all me is going to say you are wrong. I quite like your input, I like mine more, but that is because I am biased.However I do apologise for my statement, it is unusual of me to single out any member, and even more unusual of me to make remarks about their judgement.

I do.

But what everyone that is opposed to what I am suggesting seems to be missing is that, by the 'definition' of an 'electrical installation' in the BRB, I am suggesting that it can be treated as two seperate installations - especially as there are two seperate 'earthing systems' involved........this is purely clerical - for the certs.

For example, let's say Steptoe (we'll pick on him), was doing a job for me and he installed the main house 'installation' on a TN-C-S earthing system and then ran a supply to an outbuilding and gave it it's own TT earthing system - because that's how he believes it should be done.

If I then came along to inspect, test and certify this job and said - I'm going to treat it as two installations because of the 'earthing systems' - and then I issued an EIC for the house and a seperate EIC for the outbuilding.

How does this make the install any more 'dangerous' (or not), than if I'd came along and treated it all as one installation and issued one EIC??

The installation/s haven't altered in any way - the fault conditions you mention will still be there whether I issue one cert or two - , so what's the issue - and again.......what's the neutral got to do with what I have just explained??

That's all I'm asking.

I know the issues of Neutrals and PME and outbuildings etc etc - this is nothing to do with the question.

Can anyone give a valid reason why I can't choose to treat it as one installation or two installations - depending on how I see the complexity of the arrangements.

That's it - simple.

 

[steptoe]

OK ADS, I'll run on your theory for this one,

(BTW, I would probably have TT'd the house too mind )

if I have installed this and provided an EIC for this, as I should have done, then I, as the designer have taken the responsibility for the inherent dangers of this being recognised as 2 installations.

I have split the tails into ISCOs and decided that this in itself warrants 2 separate installations,

during your PIR you must decide if this is how you understand the regs to read and if you agree I have taken enough precautions to leave both installations completely isolated from each other during fault conditions.

sorry, just scanned again and you are coming in to sign off a job I designed and installed,

similar circumstances I suppose.

Now, notwithstanding that I agree with my post above, Im not in agreement it is 2 installations,

and another reason has just slapped me as I wrote the above,

and this one may be the deal clincher for the wobblers,

fault condition severe,

DNO fuse blows, does this take out one installation or 2 ?

main switch for isolation, either built into meter or DNO provided REC-2 ,

we switch off for isolation purposes, does it isolate 1 installation or 2 ?

does any of the above make sense to anyone other than me.?

 

[ADS]

OK ADS, I'll run on your theory for this one,(BTW, I would probably have TT'd the house too mind )

if I have installed this and provided an EIC for this, as I should have done, then I, as the designer have taken the responsibility for the inherent dangers of this being recognised as 2 installations.

I have split the tails into ISCOs and decided that this in itself warrants 2 separate installations,

during your PIR you must decide if this is how you understand the regs to read and if you agree I have taken enough precautions to leave both installations completely isolated from each other during fault conditions.

sorry, just scanned again and you are coming in to sign off a job I designed and installed,

similar circumstances I suppose.

Now, notwithstanding that I agree with my post above, Im not in agreement it is 2 installations,

and another reason has just slapped me as I wrote the above,

and this one may be the deal clincher for the wobblers,

fault condition severe,

DNO fuse blows, does this take out one installation or 2 ?

main switch for isolation, either built into meter or DNO provided REC-2 ,

we switch off for isolation purposes, does it isolate 1 installation or 2 ?

does any of the above make sense to anyone other than me.?

Now that is a more constructive answer.

Your question relating to the single overcurrent protection from the DNO fuse for two installations is a valid one.

As is the 'single isolation point' for two installations.

Both are valid points in what I'm proposing.

 

[steptoe]

OK ADS,

I sometimes get very wound up when it is suggested that a rod is banged in and we have another installation, or even the same one,

TT'ing something is much more than that,

and as much as I am a very strong supporter of TT it needs to be considered carefully for the normal domestic, not just have rods banged in 'cos thats how its done.

maybe now we can have some level ground we can work on. ?

 

[a1]

I have said it before and I will say it again.

Steps, by your own admission, you aren't very good at explaining things, so maybe work with someone who will type it all out both for TT and TNCS and I can make it a sticky that you can link to, rather than trying to explain all the time.

What do you think?

 

[kme]

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.

Steve:

Whilst I appreciate and (to a certain extent) agree with some of the points you raised; can I be "devil`s advocate" for a moment?

In your scenario above; the "boss" instructs the "labourer" to remove containment from an installation. Whilst the containment does not currently have any in-use circuits (and may be devoid of wiring altogether), surely it still forms part of the installation? The "boss" has therefore modified an in-use installation, without determining the possible risks associated with that modification.

If we were to include caveats against ALL possible future acts of idiocy on behalf of 3rd parties; the limitations box would be umpteen pages long, for the most straightforward install.

Take a domestic environment. How many times have we been to a property where the plumbit has disconnected the earth clamps, and not bothered to refit? It therefore stands, by your reasoning above, that a plumber removing an earth clamp is a foreseeable eventuality; and we should guard against it somehow (suppose we could grind the head smooth on the screw?) ; except that WE need to be able to disconnect it for testing. So we need a BS941 type clamp, with a high-security screw head, otherwise we could be liable?

My return argument to the above would be that the "boss" interfered with the installation, and his interference; which could NOT have been foreseen, caused the resultant fire & fatality.

Your thoughts, please guys.

 

[Manator]

Plumber I do not think anyone is any longer debating if the installation is one or two, as I have stated I often work to other regulations that do not permit me to class it as two installations if it shares a common supply, however, that is not in debate. The reason why I would not diminish the effect of the origin, is for reasons of design and shared faults, if equipotential, touch voltages on any TT system will be low, but otherwise high.

If we take one fault, and use the available fault paths we would also see very significant current loop between the installed earth electrode, and the supply electrode, or, a shared electrode. Under normal conditions, albeit minor faults, there may also be present a voltage imbalance between the electrodes. If any faults within the boundary of the consumer occurs,the fault could be shared with any other earthing arrangement, and should be designed for.

I have already made an apology to you and ADS, and I feel that I need do no more.

 

[Sidewinder]

No one has yet commented on the status of a system?...

 

[green-hornet]

Manator, I do not think anything outside of BS7671 is in the argument.

I am not sure on what reasons anyone would have for either separating or including any installation, other than at design stage.

Another thing I would question, and this is not aimed at you Manator, is the relevance of The Energy Networks Association, they follow a totally separate definition of duty of care, and are only concerned with the presentation of supply, in a safe and coordinated fashion. They have even been given the ability to ignore certain possible faults. I may be wrong I often am, just ask the wife!

 

[Manator]

I respect your view green hornet, my initial response to the debate was based on the regulations that I am bound to work to, and I still have to.

I am not saying that any regulation other than BS7671 should be followed, just that I do not just have to follow them at the exclusion of other regulations.

 

[Manator]

Plumber ENA became business partners with the IET in 2007, they also replied to consultation on the updates for BS7671, because they form part of the regulations as the suppliers and distributors association, consultation at any level will involve their engineers, for instance the revised EMC directive with regard to fixed installations.

They are also very heavily into all the H&S awareness documents and practice guides, their main role is to help and assist, coordinate and maintain consultation, with all bodies, and interested parties, to ensure a stable power distribution network, setting standards for the renewal of outdated assets and liaise with government bodies, including Brussels on the implementation of legislation.

Their engineers are represented from all the distribution networks, and are highly skilled in their respective fields, but, apart from consultation with regard to the supply, they do not get too involved in the installations, it would be out of their remit.

 

[Manator]

I concur.

 

[hxsarge]

ze is always at the origin of suppy according to the am2 instructors. plus if its a fairly hefty run for the submain you would struggle to get the ze at the end of the submain under 0.35ohms in some more rural places.

 

[ADS]

ze is always at the origin of suppy according to the am2 instructors. plus if its a fairly hefty run for the submain you would struggle to get the ze at the end of the submain under 0.35ohms in some more rural places.

Correct...ish.

Ze is the 'Earth Fault Loop Impedance' external to the installation

The 'supply' is the power being fed to your installation.

'Origin' obviously means beginning

So, Ze is not at the 'origin of the supply' as you (and your am2 instructors) state - as the origin of the supply is the 'supply transformer.

Ze is measured at the Origin of the Installation - WHEREVER that may be.

That was the whole point of the latter part of this thread - what determines the 'origin' of an installation?

 

[steptoe]

well worded ADS,

so the origin of the Installation would be where the DNO provide you with their supply and OCPD,

you can have as many submains from this as you like, and Im not sure how all the other scams look at it, but NIC have a box for DBs NOT at the origin of the installation.