Fusing a sub main

Discussion in 'Electricians' Talk' started by ajohn, Jul 8, 2019.

  1. ajohn

    ajohn Well-Known Member

    If I use 4mm twin core swa and the armour as a cpc and assume a length of 40m which in practice is likely to be much closer to 30 I get worst case earth faults in the cable of 200A accounting for temperature rise and Ze as mentioned in the guide. L to N faults comes out at 517A. At that the cable needs a disconnect time of 0.84sec. Not that time matters if it's bust.

    The far end is 25A for power and 6 for lighting with 30ma protection etc.

    Does this mean I need a fuse that will blow in some specific time at 200A? The 25A power will trip in <0.1sec at a bit over125A much faster than needed to protect the swa.

    Then comes fuse holders. There are DIN rail mounted holders for 2 types of fuse but I'm having problems tying these in with BS numbers for either type. Help.

    John
    -
     
  2. seneca

    seneca Screwfix Select

    I would fuse it at 40 amps.
     
  3. ajohn

    ajohn Well-Known Member

    Interesting area. Had another go as I would like to know the blow time. A search for BS88 fuses bring up fuses with :eek: bolt tags . A search for 40A fuse brings up 14x51 cartridge types. The BS fuses I found take 4 secs to blow at 200A. The 14x51s 2 secs which ties in with charts in the bible for a BS88-2 fuse.

    So the only question really is 2 secs ok? I've not seen anything on that so far in the bible.

    John
    -
     
  4. peter palmer

    peter palmer Well-Known Member

    5 seconds for distribution circuits like submains isn't it.
     
  5. ajohn

    ajohn Well-Known Member

    Thanks found it in the guide a seconds ago. Looked as I thought you might have all fallen out with me. I hate the blue book I'm afraid and will be glad to sell it when I've finished with it.

    This is for a garage come workshop etc so that it's completely independent. Thought 6 or 10mm initially and then noticed no calcs needed table for 4mm twin and earth. SWA is rather similar to 4mm with a 2.5mm cpc so thought worth looking at it. 25A power in real terms is plenty for even more extreme diy use.

    Just leaves one question really. Can I clip direct T&E forpart of the run? It would make installing the first 4m a lot easier. 6mm would keep the cpc resistance more or less the same as the swa armour. All of our cables are clip direct from the CU running up a wall for maybe 3m, then through holes in the wall, then rather high up for around 500mm up an internal wall into the ceiling. The guide suggest that I can do this.

    John
    -
     
  6. Comlec

    Comlec Well-Known Member

    Last garage I did I just split the tails and stuck in one of these with a 30A fuse (others are available)

    upload_2019-7-8_17-29-13.png
     
  7. ajohn

    ajohn Well-Known Member

    Trying to avoid splitting the tails as I feel that should only be done if needed. Space might turn out to be a problem as well. Plus a din rail fuse holder is cheaper also less time to fit. They can go to currents far higher than people are likely to want.

    John
    -
     
  8. Bazza

    Bazza Well-Known Member

    Maybe I misunderstand what you are trying to say, but the fuse at the supply end is selected according to the CCC of the cable.
    It has nothing to do with “what people want”.
     
    Pete Jones likes this.
  9. Tony Goddard

    Tony Goddard Active Member

    There are DIN rail fuse holders available for standard BS88 fuses, both those with no tags and offset tags (CEF make and sell them through their in house Proteus range) there are also holders for Diazed and Neozed fuses (bottle fuses) that are common in industrial plant.
    Personally I would split the tails off to a unit as shown in Comlecs post keeping the garage system entirely separate from the house and I would use 3 core SWA giving one core as earth, it is still permissable to use the armour as an earth, as it is to use conduit as an earth, but frowned upon these days. To be honest I'd be looking at using 10mm to alow for future additions or if someone purchased a welder!, my garage is hooked up with 10mm.
    As Bazza rightly says the fuse is there to protect the cable from exceeding its upper rated capacity, so should be no higher in rating than whatever that may be.
     
  10. Coloumb

    Coloumb Well-Known Member

    What if it's a PME?
     
  11. ajohn

    ajohn Well-Known Member

    Just pointing out that a din rail fuse holder is pretty flexible in terms of the current ratings that are available. So for instance if some nneded 10mm cable suitable fuses are there. They are also available for 22x58mm fuses, 100A holders. Also for 10x38, 32A holders. The 14x51mm size I mentioned goes up to 50A. Other than the 10x38 they need a 1/2 module blanking plate as they are 1 1/2 wide. The 22x58 seems to have the widest range of fuses available.

    If some one has a space in a cu it doesn't make much sense having a separate box or in my case is having a cu upgrade. Less work and lower cost.

    Space is a problem in my case thanks to how the smart meter and isolator was fitted. He put the isolator as far away as possible from the main supply outlet. It probably does leave space for a new cu, just.

    John
    -
     
  12. ajohn

    ajohn Well-Known Member

    I've used a welder on and off for most my adult life. What I went through is adequate. Future additions? No comment. Bottle fuses old hat. The fuses I mentioned are ok as far as BS88 is concerned. It is a BS88-2 fuse that appears to be read across from an IEC general purpose fuse.

    Frowned upon - why. That must be sparks not the standard and no reason for it. 10mm? if a spark wants to fit that I want another spark. I don't need it so wouldn't want to pay for it.

    Also what is the justification for putting it in a separate box?

    What is the justification for 3 cores?

    John
    -
     
    Last edited: Jul 9, 2019
  13. Tony Goddard

    Tony Goddard Active Member

    In which case you could either export the PME earth to the garage if it is close by, or separate the earth entirely and make the garage a separate TT install with a spike (in which case the 2 core SWA is just fine and dandy).
     
  14. Coloumb

    Coloumb Well-Known Member

    I thought steel could not be used as an earth? What if the shed has extraneous parts?
     
  15. Tony Goddard

    Tony Goddard Active Member

    The fuse you suggest is fine, nowt wrong with a BS88 fuse. The practice of using the armour as the earth has latterly been contentious, conduits used to be used as earth, both still can be but many including me won't do it (I don't know of anyone I work with who would). The argument would go the armour is the armour, and needs to be earthed as an extraneous part but you are far better off using a copper core as the CPC that is not at risk of corrosion and terminated directly. If as was suggested you are making the garage a separate TT system then of course the CPC doesn't apply anyway.

    As to a separate box, if there is not room, then there is not room!, it would be my preference purely as it entirely separates the garage from the house, splitting the tails is no problem and that way there is no risk of "nuisance" tripping from the garage if your fuse is on the back of an RCD protecting other areas of the house.

    I'm curious as to how the fuse holder is being fed and accessed within the CU, or are you using a separate DIN rail enclosure.
     
  16. ajohn

    ajohn Well-Known Member

    Same enclosure and totally separate from the house where all circuits will be on rcbo's. I wont be doing it anyway only suggesting it and there doesn't seem to be any problem doing it this way. In fact indications are that it has been done this way.

    One thing I have considered is main supply fuse rating. That seems to be 60A and could probably be upgraded to 80, maybe not 100 however 60 will do as will 25A power in the garage. 6A is too much for lighting.

    John
    -
     
  17. Tony Goddard

    Tony Goddard Active Member

    Good plan to use RCBo's throughout, I've done that method for years, saves much trouble when a fault develops.

    In that case, there is no reason not to use an MCB on a spare way to run the garage supply and forget fuses, take the SWA straight out of the board and across to the garage.

    In my experience most 60A service heads can be upgraded to 80A, the DNO will advise, many sparks I work with upgrade the fuse when they do a board change - always think that is rather cheeky!, the DNO here have no problems with us cutting the seals and re-sealing, but changing the fuse is a bit naughty!!

    Saying that I had a DNO engineer out to do a survey to convert a TT to PME, when he said it was OK I asked when they could do it, the response was along the lines of "your an (expletive) electrician, you do it" - so we did! - not all DNO's are so liberal!
     
  18. ajohn

    ajohn Well-Known Member

    Not keen on powering the garage that way because of what I do in there at times. :) Take one that will be done late in the year. A used espresso machine that works but was surprisingly cheap. The only certain way of checking for nasties long term is to plug it in and leave. I could check it cold for as long and any way I like. Garden stuff is plugged in their as well. Neither me or my son seem to be prone to cutting cables but it happens. It's a long walk to the CU as well. I just think if it can be done well then do it and in real terms the cost is pretty low.

    Also this way the garage can use rcbo's so the lights don't go out if a power problem. Don't suppose I can retain what is there at the moment. A separate light that just depends on the main cu fuse rather than the garage cu just for if. I used to have a static single to 440v 3 phase converter in there but metal working gear doesn't mix well with garages due to rust. All the garage has is the dirty stuff for that and I'll be adding a couple of woodworking saws at some point as it's a bit of a nuisance doing that in a spare bedroom on the top floor. None of it uses much power. :) The converter only fed a 1hp motor.

    I have my suspicion that the DNO I assume meter fitter I had was a bit of a comedian. ;) I am a very neat electrician. Can't do it any more but when one saw me finishing off my dad's cu after rewiring he said it's obvious that an electrician didn't do that - it's too neat. He didn't know that I could do it then. :) So you can imagine what I think when I see tails running all over the place.

    John
    -
     
  19. Tony Goddard

    Tony Goddard Active Member

    In my set up I have an MCB in an otherwise RCBo fuseboard in the house that feeds an SWA to the garage, that in turn feeds a small 4 way CU with RCBos in the garage running a lighting circuit, a ring circuit and a separate radial with the tumble drier and fridge on.

    Being a commercial spark I did go a bit overboard and wired it all in galvanised trunking round the top edge of the wall with 20mm drops in galv tube to sockets switches etc.(had some leftovers from doing a school workshop and was in the zone!!)
     
  20. ajohn

    ajohn Well-Known Member

    I wondered about MCB rather than fuse but they can't discriminate according to some. Maybe a D could against a B at the far end. Favourite for me which turns out to be a no no was a delayed RCD but after finally finding data on them they guarantee 40msec, max 150, even at 500A, too quick. On the other hand the comment about using them for protections if I remember correctly is primary protection. Not clear really like lots in the standard. ;) It reminds me of software that jumps all over the place. For a while they thought in terms of objects and later found a way of getting that method to jump all over the place while still calling it object orientated.

    One thing about 4mm and using the armour is that both to some extent in some cases keep fault currents down but I suspect an MCB wouldn't result in just one end opening. A fuse does.

    Given the cost of the bits compared with a split rcd cu there isn't all that much difference in price for rcbo's. Then some might use 2 of those on lights anyway. RCD's cost more than mcb's. Prices for rcbo's is rather variable.

    The electrician I'm going to approach is commercial and domestic. Only concern is that he seems to quote labour and not mention parts. I have seen him work. Wont happen for some time as a number of things need checking. ;) If the house needs rewiring it wont happen. I'm pretty sure that all of the wiring is modern metric but need to check a few that I haven't seen. If those can't be rated it would cost a stupid amount to update the CU due to rewire and redecoration. Some wire that I haven't seen - wall lights aren't needed so best remove them if there is a problem. It looks like we have 2 power feeds to 2 floors and something similar with lights. Pass where the same comes from on the 3rd floor. The previous owners used to rent it out and it's fitted with a coin operated meter. :eek: One room is anyway.

    John
    -
     

Share This Page