Testing Ze for a new consumer unit in a garage

[ADS]

The neutral conductor taken to the outbuilding CU, is still earthed as in it is effectively now a TNC. This is the reason why it is so important.

The neutral conductor is always earthed - whatever the system.

If you get a fresh supply off the DNO and you are installing a brand new TT system - the neutral conductor is still earthed - that's the reference.

And that doesn't make it a TN-C ...that's a ridiculous statement.

So what's your point for the outbuilding???

 

[steptoe]

T is always star point,

its the next part that is important,

whether it is N or T at the consumer side,

you are putting yourself in a position as to have your knowledge questioned if you are arguing that the neutral has NO bearing on this.

 

[kme]

Right!

Am I right in my understanding that the whole concept is hinging on the outbuilding being a SEPERATE installation? Because if so, how can you have a "seperate" installation, which is a submain of another installation.

You`ve said you would class them as seperate installations? And if I go to a property to do a PIR, and they`ve got a TT garage.

"Oh, Mr & Mrs blahdeblah, I`ll have to do TWO seperate PIR`s, as you`ve got two seperate installations at your house".

Are you saying that is what YOU do? Does ANYONE here do this?

I was at a commercial property last year, with 57 seperate outbuildings. The main property was TN-C-S (11KV TX on site, in the switch room). ALL the remote outbuildings (2 of which were annexed to the main building) had TT earthing. We were doing a PIR. The original installation cert. was available, as was the previous inspection.

The original designer had deemed it one installation. Subsequent testers had given ONE PIR for the whole thing. So did I.

Am I wrong to do so? Was the designer wrong to not have multiple installations?

I know we`re talking PIR not install, but I`m trying to get a grip on this "multiple-installation" business. WHO decides if an install is "seperate" Is this an arbitary decision, or are there rules to follow? Does the designer have the ultimate choice, or the testee?

I`m not being funny mate, I really want to understand how you envision this, or what book gives this info.

KME

Ooh - loads of pushy-shovey whilst I wrote my little "war & peace" :Neil

How could the neutral NOT have any relevance mate?

(I`m off to bed too - I`ve actually got work to do tomorrow - yaaaaaay. Get t`flags out lads Guinness )

 

[steptoe]

The neutral conductor is always earthed - whatever the system.If you get a fresh supply off the DNO and you are installing a brand new TT system - the neutral conductor is still earthed - that's the reference.

And that doesn't make it a TN-C ...that's a ridiculous statement.

So what's your point for the outbuilding???

headbang

Im not questioning your knowledge anymore,

I really dont know what to think after that statement,

and you claim to understand how electricity supplies and installations work.?

 

[Manator]

So can you now explain the fault path.

 

[ADS]

WHO decides if an install is "seperate" Is this an arbitary decision, or are there rules to follow? Does the designer have the ultimate choice, or the testee?I`m not being funny mate, I really want to understand how you envision this, or what book gives this info.

KME

KME, quite simply this....

Electrical Installation (abbrv. Installation) An assembly of associated electrical equipment having co-ordinated characteristics to fulfil specific purposes. (BS 7671)

I would say that having the same 'Earthing System' is a pretty big 'co-ordinated characteristic', wouldn't you??

So, by that definition, seperate installations.

 

[steptoe]

having the same LIVE conductors would be a mighty big characteristic for me.

earthing has NOTHING to do with a specific installation, are you insinuating that if the DNO dont supply an earth then you dont have the same installation?

 

[ADS]

Once again, you are all making your petty little comments, but no-one has yet told me the relevance of the neutral in this discussion.

I give up!!

And Manator, again, FAULT PATH.....RELEVANCE????

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....

What's your POINT????

---------- Post Auto-Merged at 00:45 ---------- Previous post was made at 00:43 ----------

having the same LIVE conductors would be a mighty big characteristic for me.earthing has NOTHING to do with a specific installation, are you insinuating that if the DNO dont supply an earth then you dont have the same installation?

You have the same 'live' conductors as half your street - but you haven't all got the one installation.

 

[steptoe]

you sure you have your fault paths in the right order?

can you explain the difference between T N C and T T , and do you know what part of each belongs where, this seems to be a sticky point for you as to determining the letters relating to the various parts of the supply chain.

---------- Post Auto-Merged at 00:49 ---------- Previous post was made at 00:47 ----------

Once again, you are all making your petty little comments, but no-one has yet told me the relevance of the neutral in this discussion.I give up!!

And Manator, again, FAULT PATH.....RELEVANCE????

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....

What's your POINT????

---------- Post Auto-Merged at 00:45 ---------- Previous post was made at 00:43 ----------

You have the same 'live' conductors as half your street - but you haven't all got the one installation.

NO you dont, they only become your LIVEs after the cut-out

 

[Manator]

In order to get you to understand, I would have to write a report that would take me at least a few days to write. I may do this one day, but not today.

I respect your view, as I do all views, the neutral in this instance is very important, because it is still connected to and shared with the house. The distribution cable will still be part of the fault route/path, for this reason we can not separate this installation from origin. The only reason why the outbuilding is TT is because of the potentials, for this reason we can not call the outbuilding a separate installation.

 

[ADS]

In order to get you to understand, I would have to write a report that would take me at least a few days to write. I may do this one day, but not today.I respect your view, as I do all views, the neutral in this instance is very important, because it is still connected to and shared with the house. The distribution cable will still be part of the fault route/path, for this reason we can not separate this installation from origin. The only reason why the outbuilding is TT is because of the potentials, for this reason we can not call the outbuilding a separate installation.

Then on that basis, if you TT your house, you cannot call it a seperate installation from next doors TN-C-S/PME system, as you are still sharing the neutral and this neutral/distribution cable will still be part of the fault route/path.

Am I following the same logic as you???

 

[steptoe]

no, you are still digging.

 

[Manator]

Not exactly, the DNO have presented a type of supply, this supply is protected from next door via the cut out fuse, for the purpose of definition although the supply cable will be shared with houses on that street, they do not form the same installation.

 

[ADS]

Forget who is suplying what and what protective device is where - you were using fault paths and the shared neutral conductor as the reason why the outbuilding install cannot be deemed a seperate installation......you stated this -

....... the neutral in this instance is very important, because it is still connected to and shared with the house. The distribution cable will still be part of the fault route/path, for this reason we can not separate this installation from origin.

....which is exactly the same as sharing a neutral with your neighbour.

If the house is TN-C-S and the outbuilding is TT, then a fault at the house will have no bearing on the outbuilding installation - kind of the whole point in using a seperate earth. Additionally, a fault at the outbuilding will have no bearing on the house installation.

If you think you can prove otherwise....be my guest.

As I said, the neutral has no bearing on what is or isn't a seperate installation - the 'Earthing System' and the 'Means of Earthing', on the other hand, DOES.

 

[kme]

Electrical Installation (abbrv. Installation) An assembly of associated electrical equipment having co-ordinated characteristics to fulfil specific purposes.

Agreed mate. However, if the "outbuilding" was designed to be a sub-main of an installation; then surely its protective devices, and even the fact that we need to change the earthing system, are a direct result of co-ordination of characteristics between the sub-board and the "main" installation?

 

[ADS]

Agreed mate. However, if the "outbuilding" was designed to be a sub-main of an installation; then surely its protective devices, and even the fact that we need to change the earthing system, are a direct result of co-ordination of characteristics between the sub-board and the "main" installation?

I think maybe you're thinking like this by assuming the feed to the outbuilding comes from the main DB at the supply intake.

It doesn't really matter where we take the supply from, (or where the overload device is fitted), but let's say, for arguments sake, we split the tails, insert a switch fuse, and feed the outbuilding from this.

Now can I call the TT'd outbuilding a seperate installation - it has nothing in common with the main installation except the supply and suppliers overload device (which is only protecting the tails anyway)

The reason I am insisting on this distinction is because, if the outbuilding is treated as a seperate installation, then you can measure Ze at the outbuilding board - which will give you a figure that is relative to the installation (i.e. TT).

Otherwise, your Ze is measured at the intake, (e.g. 0.25 ohms) and has no relevance to the TT'd outbuilding whatsoever.

Can you see my problem?

 

[kme]

Sort of bud...........but that still leaves us with max.dem., (or would you have two of them?) ;

And the "switch fuse" we just hypothetically inserted - which installation does it belong to? Does the load on the "main" installation have to be taken into account, in order to calculate conductor (tails) sizing? I`d say yes - so we`ve STILL got co-ordination of characteristics.

Would we take it further, and say that a switch-fuse in the tails equates to a seperate installation at the other end; and an OCPD within a DB doesn`t? Can`t get my head around that; and that is without getting into the Neutral issues - Which ARE issues, AFAIAC.

I see the logic of some of the argument; but it feels more like individual bits of regs, taken out of context, to be honest - no offence intended bud. I`m not getting heated, and if the thread goes that way, I`ll just leave :coat

 

[Manator]

I would suggest that the comments above be noted, rubbish is not what I intended to write, however your lack of understanding prevented me from going through the design and coordination with the supply company so that we can calculate and find all expected faults (capped) and earthing. If you do treat this installation, as a seperate installation could I come and do your next assessment please. I would love to know why you ignored the characteristics of the supply. I could go a lot deeper if you wanted to, however as always some point in all this, is the reason why it should not be classed under your definition (not mine) a separate installtion. It still forms part of the same system, and the fault path will be shared.

 

[ADS]

I would love to know why you ignored the characteristics of the supply. Why would I ignore the characteristics of the supply??

I could go a lot deeper if you wanted to,

Please do!

It still forms part of the same system,

No it doesn't - One is a TN-C-S System and One is a TT System.

You do know what systems are, don't you??

and the fault path will be shared.

No they won't. - Draw it for us....PLEASE

MY Blue

If the Fault Paths would be shared, why the hell has Steptoe banged on about TT'ing sheds for so long??

 

[Manator]

I have now moved this thread to the student and learning zone, my apologies to the OP. The reason for the move is the above post by ADS.

If any first year student would like to draw a fault path showing why the supply cable will be at some point shared please feel free.