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The Sunday Salient..........Quiz.

Zoomup
Biblical references will instantly disqualify any entrant, so there, be warned, we don't want that nonsense on a Sunday do we?


The I.E.T. opening web site page with the rotating rectangular board has made me dizzy. Is the designer on something do you think? Clicking on it is like duck shooting at a fun fair side stall with a light cork gun and a wind blowing.


Anyway to the Sunday consideration. Where was I? Oh yes R.C.B.O.s, those residual current devices with inbuilt overload protection and a little functional tail coloured cream or green and yellow.


Q.1. Is there any safety advantage in having a separate connection to earth by an independent earth electrode to the main earth terminal for the functional earth tail of an R.C.B.O. for:


a, A TT supplied installation?


b, A TN-S supplied installation?


c, A TN-C-S supplied installation?


Z.




  • 0
    I think most manufacturers are ditching functional earth tails on RCBOs.


    That said I think there is a strong case for an earth electrode on all installations. The lower the earth resistance the better.


    My reasons are.


    1. The lower the earth resistance the better as is it lowers touch voltages and speeds up disconnection times.

    2. Very handy in the event of a lost PEN on a PME system if you have an EV charging point.

    3. Very handy on a PME installation to prevent fires in the event of a lost PEN. You will need a low earth resistance and a substantial earthing conductor.

    4. Essential in the future when we become Prosumers and we need to operate our own generation system in island mode.


    The Germans are big on Foundation earthing (FE) and have implemented it since 1966. House builders, and other buildings, build in foundation earthing at the construction stage and look to get the earth resistance down to less than 10 ohms. The foundation earthing is installed and tested for continuity by electricians before the concrete is poured. The Germans use a DIN standard for this work, an interesting read if you can get hold of a copy. And yes before you ask they have PME earthing systems.


    There is a first mention of FE in the 18th Edition that can be found at 542.2.3. It is used in large buildings where the re-bar is picked up to give an HV/LV global earthing system. Also in railway premises. 


    There is an issue with a lost PEN if you are the only one in the street with a low resistance FE and a PEN goes open circuit as you are going to get most of the diverted neutral current. If everyone around you has a FE then you will be OK as you will have "heard immunity". Note this is the first time anyone has used this term in the electrical field. I claim copyright on this along with my first use of the terms "doorstep competency" and "unicorn device".
  • 0
    The original question, you have to ask why the tail is there in a device that ostensibly only needs to know about live and neutral.

    There are 2 parts -

    one is an EMC consideration, there may be decoupling capacitors to earth from L and N. This is decoupling in the sense they decouple a source of interference from its victim, by lowering the source impedance at high frequencies,  (at 50Hz it may be milliohms, but at 1MHz it is  a lot higher ) and holding the local volts steady for a few microseconds during a short switching transient or whatever. In a more complex filter we make a divider with coils in series as well as the 'tide you over' capacitors in shunt, but there is no room for that in an RCBO.

    This may benefit from an electrode.

    2 Operate the internal trip if the neutral is missing - as live with no-where to go cannot pass current through the trip actuating coils.

    Again this may benefit from an electrode.


    However as John points out, and electrode that  is too good, can be a fire hazard, and while you do not really want a series fuse in a safety earth, it sort of has one in the form of the wiring to it.


    Only very large electrodes (pile driving comes to mind) are in this category, and there is a further problem if your earth is better than that at the transformer - maybe not the case in town, but certainly possible with rural pole-pig distribution, during a fault the line votage is shared between the 2 electrodes - if you can round a phase really well, then the neutral at the transformer end goes up instead.
  • 0

    John Peckham:

    I think most manufacturers are ditching functional earth tails on RCBOs.


    That said I think there is a strong case for an earth electrode on all installations. The lower the earth resistance the better.


    My reasons are.


    1. The lower the earth resistance the better as is it lowers touch voltages and speeds up disconnection times.

    2. Very handy in the event of a lost PEN on a PME system if you have an EV charging point.

    3. Very handy on a PME installation to prevent fires in the event of a lost PEN. You will need a low earth resistance and a substantial earthing conductor.

    4. Essential in the future when we become Prosumers and we need to operate our own generation system in island mode.


    The Germans are big on Foundation earthing (FE) and have implemented it since 1966. House builders, and other buildings, build in foundation earthing at the construction stage and look to get the earth resistance down to less than 10 ohms. The foundation earthing is installed and tested for continuity by electricians before the concrete is poured. The Germans use a DIN standard for this work, an interesting read if you can get hold of a copy. And yes before you ask they have PME earthing systems.


    There is a first mention of FE in the 18th Edition that can be found at 542.2.3. It is used in large buildings where the re-bar is picked up to give an HV/LV global earthing system. Also in railway premises. 


    There is an issue with a lost PEN if you are the only one in the street with a low resistance FE and a PEN goes open circuit as you are going to get most of the diverted neutral current. If everyone around you has a FE then you will be OK as you will have "heard immunity". Note this is the first time anyone has used this term in the electrical field. I claim copyright on this along with my first use of the terms "doorstep competency" and "unicorn device".  




    Thanks for your reply John. I like the idea of "bonding" the ground (soil) under an installation, it certainly suggests safety.


    I posted a question on here a while back as I was not happy about a TN-C-S supply to stainless steel class 1 potato peeling machines in an old building behind a fish and chip shop. The floor was made up of old quarry tiles, running wet. There were copper pips running through the room at a touchable height. The main bonding was sub-standard. I could not really TT the room. I think that my hesitancy and uncertainty were picked up by the business owners and I never did land the job of doing electrical work at that premises. I am not unhappy about that as I could not resolve my safety concerns. Foundation earthing would have helped in that situation I am sure. But it was impossible to install.


    Z.

  • 0
    Hmm, an intersting thought I`d never considered. It does give you a double edge that if your earth reference is lost say with a PEN break or MET loss then you still have an earth to oppose the L and hopefully give a trip.


    Ditto on the Bible refs too. It`s the kind of thing that spoils xmas - bringing religion into it
  • 0

    John Peckham:

    If everyone around you has a FE then you will be OK as you will have "heard immunity". Note this is the first time anyone has used this term in the electrical field. I claim copyright on this along with my first use of the terms "doorstep competency" and "unicorn device".  




    It has already been used in the public domain, although with the different spelling of herd, which is perhaps more relevant in the electric field. ?

    Clive

  • 0
    Surely the correct reply is "follow the manufacturer's instructions" ??? (I'll just get me coat).
  • 0
    Are you suggesting he might have mis-herd it?


    I posted to another topic this morning and had to double check where I had used metre and meter ?


     Andy Betteridge
  • 0

    wallywombat:

    Surely the correct reply is "follow the manufacturer's instructions" ??? (I'll just get me coat).




    Sometimes the manufacturer's instructions are simply wrong or even incomplete. I attended a house yesterday to wire up a new 6.0kW double oven and separate hob. The manufacturer's instructions had a diagram showing that the new 6.0kW double oven could be plugged into a socket at the rear of the appliance. After a bit of head scratching we concluded that the blanket instructions covered lesser rated ovens that could be plugged into a 13 Amp socket, but had been wrongly placed with the higher rated 6.0kW double ovens as well. I ignored the instructions.


    Z.

  • 0
    An old  John Ward  video on the subject of R.C.B.O.s.....

    https://www.youtube.com/watch?v=4pzFqj9-9Ic


    Z.
  • 0
    What makes you think that John Ward is Old ?

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