Using a temporarily long ring and high Zs

Discussion in 'Electricians' Talk' started by messtin, Oct 22, 2021.

  1. messtin

    messtin New Member

    Hello everyone,

    As part of an open plan kitchen diner project we have just finished, I extended the existing kitchen ring (2.5mm T&CPC) to accommodate extra sockets with a few switched fused spurs for extractor hood, dishwasher and washing machine.

    As we are doing a phased renovation of the property, the rooms between the kitchen and the CU are yet to be done, so the configuration of circuits will change moving forward, ending up in the installation of a new 18th edition CU in a few months time. Therefore, the kitchen ring circuit will be run differently, reducing its length by around 40m-45m.

    I have just carried out the testing of the ring and have run into a couple of things that I would appreciate some advice on...

    - End to end resistance of L and N both read ~1.09 ohms with cpc at 1.82ohms indicating ~ 147m of ring cabling, considerably more than the 106m max recommended by the OSG. Assuming 24A on the ring equal shared loading at furthest point gives around 6.6% voltage drop, is that too high to be left or justifiable taking into account we have shared the loading quite well on the circuit and given are appliances and usage we rarely exceed 20A at any given time?

    Secondly, and what is more of a concern, is the 0.56 ohm Ze reading I am getting on the supply side which is giving me a Zs of 1.26 ohms which is over the 1.1 ohms limit for the 32A breaker according to OSG (table B6 32A type B).

    We are on a TNC-S system but quite remote where we are, so it an overhead line down a fairly long road to us. Since I do not have experience with this side of things I wondered if this reading is acceptable for a more remote TNCS system and whether I should expect the DNO to provide better If I contact them?

    I also noticed that the main Earth coming from the cable splitting block seems quite small maybe 6mm, Is that a common size for this type of system?

    Last edited: Oct 22, 2021
  2. The Happy Builder

    The Happy Builder Screwfix Select

    If you have a 60 amp main fuse and no more than 3.000 metres of tails the Ze is okay.

    If the socket circuit is protected as it should be by a 30 mA RCD the Zs is okay.

    Regards voltage drop, you may be overthinking it, measure the voltage whilst boiling the kettle to make a mug of coffee whilst running a couple more appliances and I bet there’s not an issue, if there is tell us what the voltage is whilst drinking the coffee.
  3. MGW

    MGW Screwfix Select

    To ensure a MCB trips within the 0.1 second time frame the current for a B type needs to be between 3 and 5 times the thermal rating to trip the magnetic part, so it is really the prospective short circuit current we are interested in, so for 230 / (32 amp x 5) = 160 amp or 1.4375 ohms.

    The 106 meters is the volt drop limit and with a 0.35 ohm incoming the limit would be 0.94 ohms. That is line - neutral.

    The 1.09 + 0.35 I am assuming incoming using the correction factor Ct I get to 197.2 meters. But volt drop is not really the problem although using the standard 20 amp at centre as 12 amp even spread volt drop is 21 volts, there is nothing which says you must use that to get the design current for circuit Ib.

    So ((230/1.82)/5)*95% = 24 amp so that is the largest B type MCB which can be used, not using the 95% = 25.24 amp so maybe you can get away with 25 amp B type as temporary fix, but I would fit a 20 amp. Still well over volt drop at 16.45 volt, but likely you will get away with it. Even at 16 amp still exceeding the volt drop.
  4. MGW

    MGW Screwfix Select

    TN-C-S supply so not permitted to rely on the RCD, however in real terms yes you are right the RCD should protect OK.
  5. The Happy Builder

    The Happy Builder Screwfix Select

    How many rolls of cable did you use when you made the addition to the circuit?
  6. messtin

    messtin New Member

    Thank you both for your replies and advice.

    Main fuse is 60A,
    Circuit protected by 63A 30mA RCD.
    Supply sits at 240V at the CU DP switch.

    I will switch to a 20A until the alterations can be made to shorten the circuit at a later date to enable the use of the 32A.

    I ran a 50m drum and a bit during the alteration. The square area of the room is not that large (maybe 45m^2) but the number of drops in a vaulted section of roof space really added to the length. This plus the returns to the CU from the existing ring being at the opposite side of the house hasn’t helped.
  7. messtin

    messtin New Member

    Sorry to revive, but Just so I can confirm my understanding now that I’ve read through this again - with correction factor applied in theory 197.2 meters of ring will still have sufficiently low impedance in the event of an L-N short for the 32A MCB to trip in the required time (ignoring the voltage drop issue!) However, the problem occurs on the L-CPC short due to the increased impedance and so reducing the breaker rating brings the trip current within the expected fault current, but in reality although not permitted to rely on it, a correctly functioning RCD will catch the fault due to the current imbalance in L-CPC fault conditions?
  8. The Happy Builder

    The Happy Builder Screwfix Select

    So is the consumer unit in the west wing and the kitchen in the east wing or the other way around?

    Just how big is this house?
    messtin likes this.
  9. messtin

    messtin New Member

    No wings on this place unfortunately! It started out as an 1820s farm cottage but has had a few extensions over the years and so the circuits run all over the place. Kitchen diner spans full width of the property at the back and consumer is at the very front right hand side. Kitchen ring returns from the far back left currently where the original kitchen was, but this will be changed when the front rooms get stripped in coming months and the ring return legs can be moved to the right.
  10. Bob Rathbone

    Bob Rathbone Screwfix Select

    My concern over this ring final circuit is 'how large a floor area does it serve', remembering that a domestic ring final circuit may only serve a floor area of up to 100M2.
  11. MGW

    MGW Screwfix Select

    I would use a lollipop design so heavy cable CU to area served then 2.5 mm around area served and would use a double cooker connection unit to join supply cable to ring final. Other option is satellite CU in the area. However this was not the question, you say it is temporary and your looking to make it safe until you have done other work.

    You need a prospective short circuit current of 5 x MCB rating plus 5%, the PSCC and the loop impedance are two ways of measuring the same thing as the voltage is fixed, so 168 amp (B32) using ohms law 230/168 = 1.369 ohms. If the PSCC is not high enough, the magnetic part of the MCB will not work, so Fig 3.4 in BS 7671:2008 shows the curve, and straight line, so we jump from 0.01 seconds to just over 10 seconds once the PSCC drops below the figure required to trip the magnetic part of the MCB/RCBO.

    The RCD will trip in 0.04 seconds so slower than the magnetic part of MCB, but the main problem is we are told not to rely on the RCD however we clearly do with a TT system.

    In this house I have type AC RCBO's and a TN-C-S supply, since not relying on the RCD part of the RCBO this does not worry me, however if relying on the RCD part, then I know should be type A or better, as I have an inverter washing machine, freezer, fridge/freezer etc.

    So in essence doing a risk assessment. Now if it were my house I may take a chance, but some one else's house, I would be fitting a B16 MCB/RCBO two reasons, one is it will trip in the 0.01 seconds, and two one is never sure with temporary, I have done temporary repairs in the past then moved jobs, so the temporary becomes permanent.

    There are many things I have done in the past, like sending the electricians mate to do a job, which latter one reads how some one doing the same thing has landed up in court (Emma Shaw case). I thought my electricians mate could plug in a tester and write down readings without a problem, it would have never crossed my mind he may fudge up some results in the mess hut.

    So over time I have started to suck through teeth more and say jobs worth.
  12. The Happy Builder

    The Happy Builder Screwfix Select

    I worked in a cottage this week, I know it’s a cottage it says so on the name board fixed to the front gate.

    It has a three phase supply, a changeover switch for the generator, a big old silver Bill fused switch, and a rather large brand new three phase distribution board that supplies some circuits directly as well as feeding another three single phase dual RCD consumer units, one of which then feeds an old Wylex fuse board with BS3871 plug in MCBs.

    I would suggest you might aim for going somewhere between what you have and that, as you presumably just have a 60 amp single phase supply.

    A decent distribution circuit and another consumer unit closer to the kitchen might be a good idea, rather than just letting the existing installation becoming a rambling mess.
  13. messtin

    messtin New Member

    Thank you. Very true the point about temporary fixes often ending up permanent, or left a lot longer than intended… for this reason I have dropped to the 16B MCB.

    Since it’s our own property we can be sympathetic with our demands on the ring until further alterations are made so hopefully should avoid nuisance tripping.

    And having looked at the cable runs a bit more today and taken end to ends of the section of cabling serving the area (without the returns to the CU) there is around 80m of cable serving the floor space, so I’m confident that splitting and connecting back to the CU at rings closest point to the CU at a later date will resolve this

    Satellite CU and larger distribution cables are interesting alternatives I hadn’t considered, but given everything in the rest of the property is getting stripped back to bare brick and the ceilings are coming down at a later date we should have the luxury of a fairly straightforward rewire.

    Of course none of this would be an issue if we weren’t trying to live in the place whilst doing the works where I’m having to predict how things will end up being run in the end… lesson learned!

    this has highlighted the limit of my knowledge and understanding when circuits fall outside of OSG standard arrangements so thanks for your comments and advice… it has made interesting learning!
    Last edited: Oct 23, 2021
    Starslikedust and Bazza-spark like this.
  14. MGW

    MGW Screwfix Select

    That makes a huge difference. But most systems grow over time, so I would say planning is the way to go, order of events etc.

    My parents house build 1954 had rubber insulated cable and needed a rewire, however before this could be done, mainly due to dad refusing saying I'm not living in a building site you can do it when I'm gone, the kitchen needed re-doing so mother could use it in a wheel chair, so this involved new electrics in the kitchen.

    I told the electricians dad would not permit re-wire, and electrics in a state, so the work around was a steel wire armoured cable around side of house from main consumer unit with a large MCB to a 5 slot CU with 5 RCBO's doing only the kitchen.

    When my dad died, house re-wired except for kitchen and wet-room which had already been done.

    So you can sit back and work out stages, and what will be done at each stage, so when completed as little as possible done twice.

    I did this with my own house in about 2000, I worked out what was needed and made a plan, which was handy in 2004 when Part P came out, as it did not include pre-planned work.
    messtin likes this.

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