Size of meter tails
- Thread starter Ray Tay
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kai wrote: "In simple terms it means that a single rcd must never be used at the origin of the installation, to avoid inconvenience when a device trips out - this new requirement was introduced in 1992, before then the regs actually permitted a single main rcd at the origin of an installation."
Are you sure that is the case? I have seen many fitted after 1992.
Are you sure that is the case? I have seen many fitted after 1992.
I've seen people driving at 100 miles an hour on the motorway. What's your point?
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Interesting:
http://www.modbs.co.uk/news/archivestory.php/aid/6397/__65279;Consumer_units_and__the_17th_Edition_of_the_Wiring_Regulations.html
http://www.modbs.co.uk/news/archivestory.php/aid/6397/__65279;Consumer_units_and__the_17th_Edition_of_the_Wiring_Regulations.html
4. To prevent nuisance tripping, unnecessary hazards and to minimise inconvenience, circuits should not be connected to a single upstream RCD (Reg. 314.1).
5. Separate circuits must not be affected by the failure of other circuits (Reg. 314.2).
Although these conditions may at first seem very precise, the regulations are in practice open to a degree of interpretation, particularly regarding the division of installations, where acceptable levels of inconvenience resulting from a fault can be somewhat subjective.
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A dual RCD board (Fig. 1) is often promoted as a fully compliant 17th Edition solution. In reality, however, it can only be a partially compliant assembly (PCA) at best. This arrangement satisfies conditions 1, 2 and 3 in the list above, but it is most unlikely to satisfy conditions 4 and 5. This is because, in the event of a fault on either set of MCBs, the associated RCD may also trip. This creates an unwanted disconnection of the MCBs where no fault exists, which is contrary to conditions 4 and 5.
So, it comes down to the installer/householder what circuits can be off in case of an RCD trip. Fitting one RCD for the whole place, a small place, is OK if the householder think so. Only RCBOs on all circuits comply with the 17th then.5. Separate circuits must not be affected by the failure of other circuits (Reg. 314.2).
Although these conditions may at first seem very precise, the regulations are in practice open to a degree of interpretation, particularly regarding the division of installations, where acceptable levels of inconvenience resulting from a fault can be somewhat subjective.
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A dual RCD board (Fig. 1) is often promoted as a fully compliant 17th Edition solution. In reality, however, it can only be a partially compliant assembly (PCA) at best. This arrangement satisfies conditions 1, 2 and 3 in the list above, but it is most unlikely to satisfy conditions 4 and 5. This is because, in the event of a fault on either set of MCBs, the associated RCD may also trip. This creates an unwanted disconnection of the MCBs where no fault exists, which is contrary to conditions 4 and 5.
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I think I'll settle down with a nice glass of merlot and wait to see how long it is before another of his threads is closed.
A single RCD does not comply.
A plastic box does not comply.
The DNO can change or replace it, no one else can.
No, it's entirely due to what is stated in BS7671.
That random bits of information obtained from searches of Google or similar is not the way to design an electrical installation.
Except that there's no obligation to follow BS7671. And even many of those who profess to do so due to scheme membership etc. seem to ignore the part about a fault on one circuit not taking out another to some degree.
And does the BS7671 wording in 314.2 actually refer to a "final circuit" or merely a "circuit" without further qualification? If the latter, that could open up another whole can of worms regarding sub-feeds......
K
kai
Oops, Had too much to drink today - 0.0225 square inches is the intended measurement!
What happened to sizing cables to the load drawn within 3M of the origin/busbar chamber? Minimum CSA 4.0mm²? There is no way you could get 400mm² cables into a 32A switchfuse when taking power from a 400A busbar chamber for example. I had no issues the other day reconnecting 6.0mm² tails from various small lighting switchfuses/ 2way fuse boxes etc. into connecting blocks fed by 25mm² tails from a replaced 100A supply. The switchfuses would probably only accept 10.0mm² cables maximum.
PBC_1966 wrote: 'Except that there's no obligation to follow BS7671.'
Thank you. Just as I thought. The article I linked to and cut & pasted implies what you confirm. A single RCD can do it all.
http://www.modbs.co.uk/news/archivestory.php/aid/6397/__65279;Consumer_units_and__the_17th_Edition_of_the_Wiring_Regulations.html This article is also used in the The Engineer's web site.
Flameport, you write the DNO can only change or replace a main fuse to a lower rating, and not even a Part P Spark with the sealing machine. So a Spark cannot change a £5 fuse type for type or a lower rating? As PCB_1966 put you wrong on a major issue, I would need conformation of that.
Flameport, A plastic box with an RCD in like this does not comply? Are you serious?
Thank you. Just as I thought. The article I linked to and cut & pasted implies what you confirm. A single RCD can do it all.
http://www.modbs.co.uk/news/archivestory.php/aid/6397/__65279;Consumer_units_and__the_17th_Edition_of_the_Wiring_Regulations.html This article is also used in the The Engineer's web site.
Flameport, you write the DNO can only change or replace a main fuse to a lower rating, and not even a Part P Spark with the sealing machine. So a Spark cannot change a £5 fuse type for type or a lower rating? As PCB_1966 put you wrong on a major issue, I would need conformation of that.
Flameport, A plastic box with an RCD in like this does not comply? Are you serious?
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Nothing, it can still be done, see 433.2
However only applicable for switchfuses and similar containing one or a small number of final circuits, overload protection being provided by the switchfuse or sum of the protective devices if it contains more than one.
A consumer unit for a whole dwelling would usually contain enough outgoing circuits so that the total of all would be greater than the cable capacity, making overload a possibility.
Calculations may be required in order to confirm that the reduced size cable is still protected against fault current.
Have I missed something?
He doesn't have a switch/fuse; he has an upfront RCD.
He doesn't have a switch/fuse; he has an upfront RCD.
It's of no real help, but last week I was at a pub to 'check' a mains supply. It was 100A single phase (with 100A fuse as the cutout was replaced 2 weeks ago and that was what the DNO said was fitted). Due to prolonged overload of connecting a commercial kitchen to the same single phase supply as the rest of the pub, the insulation on the new 25mm² tails had started to melt. Cutout fuse still good though. Tails were around 1M long.
No I did not.
The question whether that style of plastic enclosure is combustible or not was answered here:
//www.diynot.com/diy/threads/cu-change-pricing.438309/page-6#post-3425527
BS7671 is not mandatory, but in reality you either use it or the equivalent regulations for some other country, all of which are substantially similar.
The discussion over using it or not has been done to death many times before.
If you want to install non compliant electrical circuits or anything else into your own home, crack on.
However if you ask for advice here or anywhere else, or have people quote for work, they will suggest and quote in compliance with those regulations.
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