1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Mains power cable on surface

Discussion in 'Electrics UK' started by DaveOC, 18 Jun 2021.

  1. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I think this discussion may be getting rather confused/confusing ...

    Either a standard or TD RCD will only trip if the residual current exceeds its trip threshold, in both cases regardless of whether the residual current has reached that threshold very suddenly or after a long period of gradual increase. The only difference is that the tripping process will commence immediately after the threshold has been crossed in the case of a standard RCD, but will not commence unless/until the residual current (above the threshold) has persisted for a certain ('delay') period of time.

    I can't see how the 'history' of development of the fault/residual current (i.e. how long it was rising for prior to passing the trip threshold) can have any bearing on the behaviour of an RCD.

    Of course, if you want 'discrimination' in the sense that one device will operate earlier in response to a gradually increasing residual current, then the two devices obviously would have to have different trip thresholds, but that's a totally different matter - and not what we are discussing.

    That's how I see it, anyway!

    Kind Regards, John
     
    • Like Like x 1
  2. Sponsored Links
  3. plugwash

    plugwash

    Joined:
    28 Mar 2004
    Messages:
    9,528
    Thanks Received:
    374
    Country:
    United Kingdom
    Surely that is absolutely what we are discussing.

    The goal being considered is "how to provide the tails with RCD protection without subjecting the entire installation to trips if there is a fault on a downstream circuit". Surely that would include both faults where a large residual current suddenly appears and faults where leakage slowly rises as dampness/contamination slowly increases.
     
  4. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Indeed, give or take the rather important fact that I think we have decided and agreed that it could not be done as I suggested, since 30mA TD RCDs probably don't exist.
    Again indeed.

    Having reflected, I think I now understand the point you have been making, but I would say that it relates far more to the point you made earlier (that the actual trip thresholds of individual devices will vary, relative to the 'nominal' figure), rather than being directly related to how rapidly or slowly the residual current evolves.

    If, with two RCDs (both with the same nominal trip threshold) in series, 'it happens' that the downstream one has the lower trip threshold, then 'discrimination' (in the sense of the downstream one tripping first) will not only be present, but will actually 'improve' (become 'more certain') as the rate of rise of residual current becomes lower (i.e. as development of the fault became slower) - and that would remain true even if the upstream device (with higher trip threshold) were not time-delayed.

    However, I presume that you must be thinking of the opposite situation (which, statistically speaking, ought to happen in about 50% of cases) in which the upstream RCD had a lower trip threshold than the downstream one. In that situation, unless the rise of the residual current were fairly rapid, the upstream device would always operate first, even if it were time-delayed - hence failing to give the 'discrimination' that we wanted.

    However, as I said, the difference between those situations in terms of achievement/non-achievement of 'discrimination' (the desired way around) is primarily down to the direction of difference in the trip thresholds of the two devices, not the speed with which a fault develops.

    That's how I see it, anyway.

    Kind Regards, John
     
    • Like Like x 1
  5. Risteard

    Risteard

    Joined:
    18 Jun 2010
    Messages:
    2,177
    Thanks Received:
    157
    Location:
    Derry, Ireland
    Country:
    Ireland
    Time-delayed RCDs are wholly unsuitable and non-compliant for the provision of additional protection. That's why 30mA S types aren't available - they would serve no purpose.
     
  6. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I can't argue with the reasonableness of what you are saying (which is undoubtedly what most would say), and it certainly serves as an explanation for the apparent non-existence of 30mA Type S RCDs.

    However, as far as 'non-compliant' is concerned, as I implied in what I wrote (and you quoted), and much to my surprise, try as I may I can find no explicit prohibition in BS7671 of time-delayed RCDs for 'additional protection' - the only requirement is for a 30 mA trip threshold and compliance with BS EN 61008-1 (or BS EN 61009-1 for RCBOs). I don't have access to that Standard (which seemingly covers both standard and time-delayed RCDs), so I suppose it is possible that the Standard itself precludes 30 mA Type S ones - does anyone know?

    In passing, and moving from regulatory compliance, I would question the "wholly" of your "wholly unsuitable ... for the provision of additional protection', since I would suggest that it would only be in a tiny minority of cases (of RCD trips due to a residual current) that the 'protection' provided by a time-delayed RCD would be less than that provided by a non-time-delayed one.
     
  7. EFLImpudence

    EFLImpudence

    Joined:
    7 Jul 2010
    Messages:
    37,034
    Thanks Received:
    4,150
    Location:
    Retired to:
    Country:
    Portugal
    30mA RCDs for personal protection (not necessarily the same as additional protection) must trip within the prescribed times; presumably time-delayed ones, by definition, would not do that.
     
  8. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Very reasonable though that statement is, the phrase "personal protection" does not seem to appear anywhere in BS7671, "additional protection" being the only thing it ever talks about. Hence, BS7671 does not have any explicitly "prescribed times" specifically relating to 'personal protection. The nearest is gets is implicitly, in as much as Table 3A (in {informative} Appendix 3) gives "prescribed" trip times for both standard and TD RCDs but (probably because they don't exist) does not include figures for 30 mA TD ones.

    I presume that the distinction which you (but, seemingly, not BS7671) are making is that "personal protection" relates to a situation in which the 'fault current" (which, if of adequate magnitude, will cause an RCD to trip) passes through the body of a human being - in which case it is clearly the case that the longer time taken by a time-delayed RCD to operate would be highly undesirable.

    However, you presumably understand that my comment in response to the suggestion that TD RCDs are "wholly unsuitable" (for protectuion against electric shock) stems from the fact that, as you are aware, my personal belief is that the vast majority of RCD trips are not due to current passing through a human being and that, in all other cases, an RCD will clear a fault before anyone gets a shock - for which purpose a TD device (which will operate in well under 1 second, even with a residual current of only IΔn) would be essentially as 'protective' (against electric shock) as would a standard RCD.

    Kind Regards, John
     
  9. EFLImpudence

    EFLImpudence

    Joined:
    7 Jul 2010
    Messages:
    37,034
    Thanks Received:
    4,150
    Location:
    Retired to:
    Country:
    Portugal
    Yes, all true, of course.

    However, where RCDs are now required, e.g. for concealed cables, it is obviously for personal protection although no distinction is made when calling it additional protection as it must not be the sole protection.

    "415.1.1 The use of RCDs with a rated residual operating current not exceeding 30 mA is recognized in
    AC systems as additional protection in the event of failure of the provision for basic protection and/or the provision
    for fault protection or carelessness by users."

    It would be silly to demand 30mA devices, itself a compromise; 10mA would be better, if people might or could fit time delayed ones which operated less quickly.
     
  10. Sponsored Links
  11. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Agreed - but that is all common sense, despite, rather than because of, the regs.
    I agree with the spirit of what you're saying, but you perhaps make it sound more straightforward than it actually is. For example, if the residual current is at least 30 mA, then a 30 mA RCD should limit the duration of that current to the same extent as would a 10 mA one (and, in either case, the current might, for its duration, be much more than 30 mA).

    Furthermore, the difference between ('requirements of') a standard and time-delayed RCD are not necessarily as great as one might think. In the 'worst case' (residual current = IΔn), the maximum permissible trip time for a standard RCD is 300 ms, whilst for a TD one it is only 500 ms.

    In passing, briefly returning to the 'discrimination' issue, in that 'worst case' scenario (residual current = IΔn), the maximum permissible trip time of a standard RCD (of any available IΔn) is 300 ms, but the minimum for a time-delayed one is only 130 ms - so there is theoretically plenty of scope for the TD one to trip first.

    Kind Regards, John
     
  12. studentspark

    studentspark

    Joined:
    13 Mar 2010
    Messages:
    589
    Thanks Received:
    15
    Location:
    London
    Country:
    United Kingdom
    Switch fuse in the meter cupboard. Run SWA to the CU
     
    • Like Like x 1
  13. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    I imagine that the switchfuse would probably not be necessary, but I got the impression that the OP wanted to bury the existing cables, rather than to have them replaced with SWA.

    Kind Regards, John
     
  14. studentspark

    studentspark

    Joined:
    13 Mar 2010
    Messages:
    589
    Thanks Received:
    15
    Location:
    London
    Country:
    United Kingdom
    The SWA could be chased in
     
  15. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Indeed it could. However, as I said, the impression I got was that the OP just wanted to bury the existing cables, rather than to get an electrician (and probably also DNO) involved to replace them with SWA - but it's obviously the OP's choice/decision.

    However, as I said, if a switch-fuse is not currently required (because of length of the tails) it presumably wouldn't be required if the cable were changed to SWA. Having said that, the SWA would have to be terminated into some sort of 'box' (not directly into the meter), so I suppose a switch-fuse would be as good as anything else - just 'a box' would do or, perhaps more usefully, maybe an isolator.

    Kind Regards, John
     
  16. studentspark

    studentspark

    Joined:
    13 Mar 2010
    Messages:
    589
    Thanks Received:
    15
    Location:
    London
    Country:
    United Kingdom
    As far as I can see if the OP wants rid of the surface tails he will need an electrician, and SWA is the simplest option. Half a day, all done :)
     
  17. JohnW2

    JohnW2

    Joined:
    28 Jan 2011
    Messages:
    48,120
    Thanks Received:
    3,137
    Location:
    Buckinghamshire
    Country:
    United Kingdom
    Indeed - I think we are agreed that either that (SWA) or metal conduit etc. would be the only way of avoiding the surface (in trunking) tails. I was merely pointing out that the switch-fuse you mentioned probably wouldn't be necessary.

    I have to say that, if it were me, I don't think I would be too worried about the plastic trunking next to my froint door (or, if I were worried, would find a way of aesthetically improving the situation - but that's obviously a matter of personal opinion/taste.

    Another approach which occurred to me, which would avoid less work/disruption/mess, would be to run ('surface') SWA (from external meter cabinet) on the outside of the wall, then straight through the wall into the CU (above the front door) - but the OP might well not like that, aesthetically, either.

    Kind Regards, John
     
Loading...

Share This Page