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TT supply and SPD
ARE 11001215607
29 Posts
Question
Greetings,

I have a TT installation, supplied by overhead lines; the DNO head with 100a fuse, an analogue meter and a DP isolator all housed in a BS8567:2012  external cabinet.  I propose a Lewden 4/55ENC plastic enclosure carrying a Fusebox RT1001002S 100a Time Delayed 100ma RCD with a Fusebox SPD2PT2 surge protection device (type 2) in parallel alongside, this lot fitted inside the external cabinet. The 25mm tails & 16mm earth will go 50cm through a solid stone wall to a Fusebox 20 way CU mounting a 100a DP isolator feeding 3 x 30ma RCDs and ultimately 14 MCBs.

 
Is there anything wrong with this basic design ~ ignoring the impropriety of fitting client equipment in the external cabinet.
 
Single pole RCBOs do not appeal to me because this is a TT installation.
 
And may I just be sure that the phase and neutral cables from the SPD should connect to the 25mm tails on the incoming (meter side) of the Time Delayed RCD ~ and not downstream (on the CU side).
 
Confirmation would be appreciated before I brief my electrician.
 
 Many thanks

 
119 Replies
AJJewsbury 77361768
1611 Posts
Where does the Earthing Conductor (to the electrode) run in all of this? It would be unusual for it pass through the meter cabinet in a TT installation, and running a protective conductor 'backwards' more than half a metre from the CU to the SPD isn't exactly ideal from a surge protection point of view.

You can get DP switching RCBOs if you prefer - although there's nothing wrong with single pole switching RCBOs provided you don't need to rely on them for isolation as well.

Some consider overhead supplies as warranting type 1 SPDs rather than type 2s (although BS 7671 doesn't seem to make it a requirement unless the building has a lightning protection system).

Yes, SPDs are normally better upstream of RCDs - but on a TT system they MUST then be of connection type 2  (the elements connected L-N and N-PE) rather than connection type 1 (connected L-PE and N-PE) as the latter can make the entire installation hazardous live if the L SPD fails to short.

I wouldn't ignore the DNO/supplier's requirement not to have consumer's equipment in the meter cabinet either - it would be a shame to do all that work only to have to remove it again.

  - Andy.
mapj1 80733779
2070 Posts
Looking at the data sheet,.it is not so clear but  the SPD2PT2  does not seem to be of the 'TT' compatible type so you will need an RCD, and presumably an MCB or fuse to gracefully handle the eventual moment when it goes dead short, as if we do not know we have to assume that could be either L_E or N_E, or L_N.
I  do suggest confirming those 2 points with the makers in email/writing however - they may after all  be internally fused and may or may not have a credible direct L-E failure mode.

FuseBox: Moorfield Industrial Estate, Kilmarnock KA2 0BA
t: 01563 533554 | e: sales@cpelectric.co.uk

If you could report back what they say, that would be useful to others in future as well.
If after all this  you need a larger unit and more space and there are problems with  the cabinet sharing with meters etc,  the usual trick is to spend £50 on a second meter cabinet to go alongside.
ARE 11001215607
29 Posts
Thank you Mike for your swift response. I will report back here once I have heard from Fusebox. The brand of SPD is not important to me but it is essential that it will fail safe; if that is impossible I will simply instruct my electrician not to include this device.
ARE 11001215607
29 Posts
Thank you Andy for your swift response. If I could clip a *.jpg image to this reply you would see my meter cabinet. As a new comer here I can't figure out how to do that because I don't know what the image URL is?

Rightly or wrongly the earthing conductor presently rises 3ft from the buried rod, passes unbroken through the meter cabinet & goes out alongside the existing phase and neutral tails. It would not be too difficult to reroute this earthing conductor around, so outside, the meter cabinet ~ with a Henley block used to splice in the short 6mm cable from the SPD. From what you say, I understand that it would be improper for the electrician to fit the Henley block inside existing cabinet.

I will heed your advice concerning DNO/ supplier requirements.  Cutting an adjacent hole in the stone to accommodate a small plastic Lewden enclosure, outside the habitable space, is possible, just difficult and probably expensive. The SPD and 100ma Time Delayed RCD could be fitted inside the house ~ either in a plastic enclosure which, I understand, falls foul of one rule or in a metal enclose with no upstream protection so dangerous for a different reason. Almost a 'Catch 22' dilemma?

ARE 
AJJewsbury 77361768
1611 Posts
As a new comer here I can't figure out how to do that because I don't know what the image URL is?
It is a bit cryptic - Once you've hit the picture icon and got the Image Properties window, switch to the Upload tab, browse to find your picture and upload it to the server - that then fill in the URL of your uploaded picture for you automatically.
  - Andy.
ARE 11001215607
29 Posts
  

Thank you very much for your advice Andy.   I hope I have successfully clipped 3 images to this message.

My supply, since 2000, comes down the pole, underground about 30m and up a hockey-stick into the meter cabinet.which has sufficient vacant space to fit a Lewden enclosure supporting a 100ma Time Delayed RCD and an SPD2PT2 surge protection device. I am minded to ask Northern Powergrid whether, exceptionally, they will condone my mounting this Lewden enclosure in my meter cabinet part way along new 25mm tails which (like the 16mm tails which will be replaced) belong to me.




 
wallywombat 11001209453
225 Posts
As a minimum the DNO will want enough spare space in the cabinet to insert a test meter in addition to the main meter, should there
ever be a dispute about its accuracy.
ARE 11001215607
29 Posts
Thank you WW.  As you will see in my reply to Andy, just posted with images, I am minded to ask Northern Powergrid if, exceptionally, they will condone me fitting an enclosure in my cabinet. I don't doubt the present analogue meter is their property but I do not buy power from them.  SSE is my supplier and if I accept their offer  to substitute a Smart Meter, presumably SSE will retain ownership of this digital device?  Partly because of 50cm thick stone walls, I doubt that I would ever 'hear' the Smart Meter inside my house ~ so not much use to me. 
wallywombat 11001209453
225 Posts
Cutouts are owned by the DNO (Northern Powergrid) while meters are owned by the your supplier (SSE). Even if the smart meter can't be "heard" from inside your house, it can still communicate back to your supplier, allowing accurate daily or hourly or whatever meter readings, if that sort of thing appeals to you.
ARE 11001215607
29 Posts
I understand, from FuseBox, that their SPD2PT2 is not the best device to use in the environment I have described. Now I await a substitute device and I will advise this community of it once I receive it ~ but already I gather that supply through an MCB or fuse will be appropriate for the reasons that you explain.
ARE 11001215607
29 Posts
Thank you WW.  Does this 'reporting home' depend upon my internet connection or does it feed back through the grid?
wallywombat 11001209453
225 Posts
ARE:
Thank you WW.  Does this 'reporting home' depend upon my internet connection or does it feed back through the grid?

As far as I'm aware they use the mobile phone network. Nowt to do with your internet connection.

ARE 11001215607
29 Posts
Again many thanks WW.  I've just taken my mobile to the meter cabinet and it reports 'no signal' ~ much as I thought.  Will revisit 'smart meters' when the suppliers introduce variable - time of day dependent - tariffs, but I'm not holding my breath.
AJJewsbury 77361768
1611 Posts
ARE:
Thank you WW.  Does this 'reporting home' depend upon my internet connection or does it feed back through the grid?

It depends where in the country you are - down South it's the mobile phone network but up North there's supposed to be a separate 'long range' radio system (the dividing line looks to run roughly from the Mersey to a bit south of the Humber).
    - Andy.

ARE 11001215607
29 Posts
Rural North Yorkshire, so 50 miles above that dividing line.

I wonder how many folk who live in the north of England know that common or garden smart meters may not do all they are claimed to do, when installed at their home. 
mapj1 80733779
2070 Posts
The 'Arquiva' smart meter system is used for the larger area of the north of the UK, and the system uses UHF at 412-414 MHz for the up-link and 422-424 MHz fur the down-link and the frequencies are licensed from Ofcom . The network equipment is provided by a US company, Sensus and operates over a proprietary radio protocol, which has been customised for utilities use. I do wonder how well it will work near amateur radio installations - the '70cm' band is 430– 440MHz and I suspect that channel filtering in mass produced meters will be minimal to keep costs low.

Down south, indeed the units have a SIM card and use the O2 mobile phone  network to in effect text  encrypted messages.

Highlighted DNOs areas use  the UHF.
map of smart meter networks.


map link from this forum
ARE 11001215607
29 Posts
FuseBox have exchanged the Type 1 connected SPD originally supplied (stand-alone image) with a Type 2 connected SPD now shown in my Lewden enclosure.  I propose to connect the phase supply to the SPD through the 32amp fuse shown.  A 16mm earth cable from the TT earth rod, and a 16mm earth cable to the Consumer Unit will both attach to the small internal earth busbar illustrated top right. Total length of the L & N & PE cables (3), all 6mm, attaching to the SPD,  plus the fuse, is estimated to be 44cm. Is this arrangement technically respectable?
  
. 
ARE 11001215607
29 Posts
Thank you for this advice.  FuseBox exchanged the SPD originally supplied with a different one described in my reply to Andy Jewsbury, just posted.
The proposal now is simply to have a Lewden enclosure added along the 25mm tails from the DP isolator in my meter cabinet to the Consumer Unit.  No need now for the electrician to include a separate Henley block or anything else in the meter cabinet.  Any comment would be appreciated.
ARE  
AJJewsbury 77361768
1611 Posts
If you've got the SPD before the first RCD then you need to ensure that the live conductors have double or reinforced insulation - the result of a L-PE fault before the RCD will likely result in all the metalwork in your installation being hazardous live indefinitely (which counts as bad, very bad). That means you need something more than just basic insulation between things - which looks like it might be tricky given what you're trying to squeeze into there.

I can't quite make out the type of fuse you've got there - but it looks suspiciously like the kind more often seen inside appliances rather than a part of the fixed installation. The requirements for both fault breaking capacity and voltage withstand for fixed installations are normally more demanding than for inside appliances - so I'd have to raise a question mark on that. (The amount of bare metal is also unusual for fixed wiring devices these days, but may well comply as it's within a decent enclosure.)

   - Andy.
Sparkingchip 72796851
2474 Posts
ARE:
FuseBox have exchanged the Type 1 connected SPD originally supplied (stand-alone image) with a Type 2 connected SPD now shown in my Lewden enclosure.  I propose to connect the phase supply to the SPD through the 32amp fuse shown.  A 16mm earth cable from the TT earth rod, and a 16mm earth cable to the Consumer Unit will both attach to the small internal earth busbar illustrated top right. Total length of the L & N & PE cables (3), all 6mm, attaching to the SPD,  plus the fuse, is estimated to be 44cm. Is this arrangement technically respectable?
  
. 

Though they are not readily available at electrical wholesalers, if it needs a fuse why not installed a din rail fuse holder? 

I would refuse to install what you show in the photograph.

ARE 11001215607
29 Posts
Thank you Sparkingchip for your swift reply. I am most grateful.

The Lewden enclosure din rail is 4 usable modules wide, 2 taken by the RCD, 2 taken by the SPD, and it would be impractical to widen the aperture to accommodate a 5th MCB module.

The fuse shown is Ferraz Shawmut / Mersen  38mm x 10mm   FR10AM40V32 400V 32A - JPSF479  in a Bakelite fuseholder designed for that size of fuse. If I have chosen a fuse with inappropriate: specific current, voltage rating, breaking capacity and response time for the application that I have described, I would be most grateful if you will tell me what combination of characteristics would be more appropriate and what would be ideal, As I understand things, this fuse is only present to protect the DNO 100amp fuse in the event the SPD fails permanently L - N  or L - PE short circuit ~ and if that is not the case again I would appreciate your advice.

Bare metal at either end of the fuse will exist - I accept that - in an enclosure inaccessible without tools and downstream of a DP isolator..  Whether I like it or not I am stuck with a TT supply. I strongly suspect that there is no economically viable 'perfect solution', this design is a compromise. In all of 20 years, lightning has only cooked a couple of my dial-up (narrow band) modems and I'll never know whether the pulses came down the telephone or the power lines.

How would you solve riddle I'm faced with?

ARE 
Sparkingchip 72796851
2474 Posts
You have just designed your own consumer unit using components from several manufacturers without any regards to the British Standards or approvals.

FIt the SPD in the main consumer unit and ditch one of the 30 mA RCDs, then determine where to position the 100 mA upfront RCD, in the main consumer unit of in the meter box. If the tails are installed correctly the upfront RCD can go in the main consumer unit.

 
ARE 11001215607
29 Posts
Thank you for this advice Andy.

The proposal is to protect the phase input to the SPD with a Ferraz Shawmut / Mersen  38mm x 10mm   FR10AM40V32 400V 32A - JPSF479  fuse in a Bakelite fuseholder designed for that size of fuse.  Everything, the 100ma Time delayed 100amp RCD, the Type 2 connected (L-N & N-PE) SPD, the Ferraz Shawmut fuse, and a MET will be contained within the plastic Lewden enclosure in the BS8567 meter cabinet outside the habitable part of the building. This seems the only way to keep all three 6mm cables to the SPD less than 50cm total length.

Hopefully if either module of the SPD fails, short circuit, the Ferraz Shawmut 32amp fuse will blow before the 100amp DNO fuse.

If the characteristics of the fuse I have chosen are inapproriate I would be grateful if you can tell me what would be better and what would be ideal.

ARE
wallywombat 11001209453
225 Posts
ARE:
As I understand things, this fuse is only present to protect the DNO 100amp fuse in the event the SPD fails permanently L - N  or L - PE short circuit ~ and if that is not the case again I would appreciate your advice.
 

No, the 32A isn't to protect the 100A fuse, it's to stop the 6mm wiring and and a shorted SPD from setting on fire. During the initial stages of a short, very large currents can flow, in the region of 1000's of A. How much energy the upstream protection lets through before it operates must be less than the maximum energy the downstream wiring and equipment can absorb (by heating up) without overheating. Also, if this initial current is greater than the stated breaking capacity of the fuse, there is no guarantee that the fuse will open the circuit - the fuse wire may break, but an arc will continue that isn't extinguished, and everything sets on fire. Also, the fuse can explode, damaging the enclosure.

When you buy a CU from a manufacturer and only put in it devices approved by the manufacturer, you are getting an assurance from that manufacturer that the whole assembly has been tested such that as long as the upstream protection is a BS1361/BS88-3 fuse of up to 100A to provide backup breaking capacity, that any short-circuit will be successfully contained by the unit without setting on fire etc.

When you make your own enclosure with parts from multiple manufacturers, you are taking this design and testing responsibility upon yourself. Are you prepared to sign off that your assembly confirms to BS EN 61439-3:2012, "Low-voltage switchgear and controlgear assemblies. Distribution boards intended to be operated by ordinary persons (DBO)"? Are you able to to do a 6kV flash test of your enclosure, and be confident that you won't get an arc between your fuse holder and the neaby earth bar? Etc.

In short, you're creating a horrible DIY bodge job with that bare fuse carrier.

AJJewsbury 77361768
1611 Posts
I still see problems addressing the requirement for double or reinforced insulation with the proposed setup - the bare metal on the fuse holder right next to the PE bar makes that issue significantly worse.
 
How would you solve riddle I'm faced with?
I'd try to keep it simple - perhaps go for an SPD that doesn't need a lower rated fuse (avoids the need for an extra fuse) and put the 100mA RCD in a separate enclosure (avoids the need for crossing single insulated wires and protective conductors inside a small enclosure). Use enclosures that can accept insulated & sheathed tails L & N straight into the terminals so there's no chance of them coming into contact with the PE system.

I like Wylex REC2 enclosures for that kind of thing but there are other options.

Just by way of an example, if you'd like an isolator next to the SPD (handy for isolating before changing the replaceable SPD cartridges) then perhaps something like this: https://www.tlc-direct.co.uk/Products/WYREC2SPD.html 

Then one of these https://www.tlc-direct.co.uk/Products/WYREC2S.html but with the switch swapped for a compatible 100mA RCD.

  - Andy.

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