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Protective Devices Sticky

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  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Steve wrote: »
    Was the cooker wired through the RCD of not?

    Damp elements are notorious for leaking to earth.


    As a further update to my earlier post, if the cooker was through the RCD, it wouldnt have been any great unusual mystery, would it.....

    When the piece of twin and earth that was nailed between N and E was removed, no more tripping of RCD at all.

    With a N-E fault, there is an alternative path through the RCD to the N-E fault from the main Neutral bar for neutral current. Usually too little current will go that way to trip an RCD via such a fault.

    On non neutralised installations, this may be more likely to divert more than the 30ma required to trip an RCD, particularly with larger loads on, in certain circumstances.

    In the case here, there were overhead lines supplying houses along the road, suitable for loads in the 70s. So the N can end up at a higher potential than the earth bar when no neutralising is done in the house. So some current will take the path from the main neutral bar, to the N-E short on the RCD controlled circuit, and back to the MET in the DB, tripping the RCD.

    This was also the case in the house where the RCD was tripping even with power to the house off (overhead lines supplying houses).

    I would say it is unusual to see it.


  • Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    Bruthal wrote: »
    When the piece of twin and earth that was nailed between N and E was removed
    Was this on the T+E supplying the cooker or was it downstream of the RCD?

    Can't get my head around the scenario :)


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Steve wrote: »
    Was this on the T+E supplying the cooker or was it downstream of the RCD?

    Can't get my head around the scenario :)

    Downstream of the sockets RCD,

    Cooker was not on any RCD,

    Any N-E short on an RCD controlled circuit is providing another path for main neutral bar current to flow through. It will usually be very small.

    Basically, if you have 40 amps flowing through the main neutral bar, for 30ma to be diverted to a N-E short, the alternative (fault path) has to be greater than 1/1333th of the main neutral path ohms.

    This is unlikely in the case of modern underground cable feeding minipillars and neutralized installations, but is possible with old undersized overhead supplies, and probably more likely with non neutralized installations too, as neutralizing locks the N and E bars to almost identical potentials, but the alternative path is still there.

    So it would be unusual in newer installations.


  • Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    Is this what you are describing?

    z2PMcUE.png


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Yes except no neutralising link


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  • Moderators, Category Moderators, Entertainment Moderators, Sports Moderators Posts: 22,584 CMod ✭✭✭✭Steve


    Bruthal wrote: »
    Yes except no neutralising link

    So this then?

    N3DzJT0.png


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Yes thats about it.

    The first one wouldnt really be right, as there is no earth conductor from the house to the esb transformer, but that is academic.


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    So the brown is the cooker or any neutral return path, red is alternative path created by sockets N-E short. Current will flow that route, it is just a question of how much. Influences will be magnitude of the actual load current, and the ratio of N path ohms:Fault path ohms, which is governed by several factors including network supply line sizes, condition of earth rods in non neutralized houses etc.

    I used the first diagram, but the second is more accurate in terms of earth rod at house.

    sockets-short_orig.png


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    The esb transformer will have its neutral earthed. But there is no link to house earth apart from the neutral itself if the house is neutralised.

    The Neutral from the ESB sub is a combined NE.

    But again, not overly relative to the overall point.


  • Registered Users Posts: 3,456 ✭✭✭John.G


    I posted a version of this ~ 6 months ago but am doing so again in case there are different views on its cause:

    I had a very unusual RCD tripping problem a few years ago, I tried all the usual remidies but even with the RCD tripped I was still getting a full (o ohms resistance) between neutral and earth in any of the sockets in the house, the fault would disappear but over a 2 to 3 day period would re occur at random times, I even swapped the RCD for the shower RCBO and still no good, eventually the problem disappeared (neutral to earth resistance; infinity) and has never returned, original RCD still in use.
    I have a 1972 built house with fuses, TCNS system? with a earthing rod in the garage.
    I know it would seem to be impossible with a double pole RCD tripped to still measure a full neutral to earth fault except that it is on my (the consumer) side but is there any conceivable way get a apparent full short, neutral to earth, from a external source in the conditions described above?


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  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    John.G wrote: »
    I know it would seem to be impossible with a double pole RCD tripped to still measure a full neutral to earth fault except that it is on my (the consumer) side but is there any conceivable way get a apparent full short, neutral to earth, from a external source in the conditions described above?

    If a N-E short on sockets as example, trips an RCD, the N-E short will still be on the sockets with the RCD tripped.

    If a socket circuit with no faults is tested N-E at a socket while the RCD is on, a N-E short will be seen via the neutralising link, but with the RCD off, the N-E short would clear, unless the socket circuit has a N-E short (fault) on it , (downstream of the RCD)


  • Registered Users Posts: 3,456 ✭✭✭John.G


    Thanks Bruthal, that's the way I would see it myself but it was uncanny, at one time while watching the M.meter it was swinging from 0 to infinity every few seconds as if someone/something was switching the short in/out, you replied to my original post that it could be a nicked cable against the corner of a steel socket box which of course it could have been but one would think that it would have reappeared again at some stage.
    I sort of hoped that it might have as I would then have removed the neutrals from the neutral bar one at the time to track it down.


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    John.G wrote: »
    Thanks Bruthal, that's the way I would see it myself but it was uncanny, at one time while watching the M.meter it was swinging from 0 to infinity every few seconds as if someone/something was switching the short in/out, you replied to my original post that it could be a nicked cable against the corner of a steel socket box which of course it could have been but one would think that it would have reappeared again at some stage.
    I sort of hoped that it might have as I would then have removed the neutrals from the neutral bar one at the time to track it down.

    Yes thats why a meggar is better to find such faults than the multimeter, set to high voltage and test the individual neutrals at the neutral bar after disconnecting them.

    For an annoying fault like that, if it is not showing in testing, splitting the sockets over 2 RCDs can eliminate some circuits, and so on, to narrow it down.

    Also as mentioned earlier in the thread, some N-E faults will trip only as the load gets higher, especially if its on a circuit other than the faulty one, while it may be on the same RCD too, so it can seem even more intermittent then.

    These problems are made more tricky with multi circuits from 1 RCD.


  • Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,516 Mod ✭✭✭✭2011


    Posts such as the ones above make a strong argument for one RCBO per socket circuit.
    It is something I would strongly consider if ever wiring a domestic installation again.


  • Registered Users Posts: 3,456 ✭✭✭John.G


    How does one know which one of these RCD's is being purchased as RCD 1 with the test coil resistance taken from the load side would appear to be the better choice if one is doing a routine button test as all the test current flows through the sensing coil but only the neutral full test current passes through with RCD 2 which may not happen if the load side neutral has a earth fault. I just read somewhere that the (at least initial) button test always be done with the L&N load side disconnected, I now understand the reason I think.


  • Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Type 1 can have some test current flow through the N coil as there is a path to the N-E fault, thus also reducing the effective test current.

    N-E shorts reduce effectiveness of rcds if someone receives a shock also.

    Rcd will have a diagram on the side of it showing its internal setup.


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