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

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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.




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  • 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.
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  • 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.
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