RCBO WITH FUNCTIONAL EARTH LEAD CAUSING SMALL EARTH LEAKAGE

I suspect that if the device is 'passive', and in a PME (Protective Multiple Earth) style arrangement (incoming side), then it is not the RCBO that creates the leakage, rather it is the natural leakage of all those RFI filters on all the electronic equipment connected to the down stream load.


The RFI devices have the Y capacitor connected such that there are two equal capacitances between Live and Neutral (hence half live volts at the split point, with a Thevenin equivalent capacitance of 2.C [or is that half C]) and, that half live volts, fed through the equivalent capacitance connected to the Earth, creates the leakage (<1.6mA if I remember correctly), and all the 50Hz contributions (all those RFIs) are in phase at the RCBO.


Any filtered harmonics at 3/6/9 (150Hz etc), when spread across 3 phases will then be additive in the neutral, making for a large neutral current (hence all the regulations about level of harmonics in modern electronics..)


If the RCBO itself has active circuitry (rather than just a sensitive relay), then it is also likely to have it's own RFI (depending on how it's powered) to filter the internal power supply. But it's likely to be negligible relative to the external load RFI filters.

The R.C.B.O's neutral blue wire will be at the same potential as the small cream functional earth wire, as the neutral and earth are connected together at the head in a P.M.E. supply, which is the dominant supply type these days in the U.K. I believe. So any leakage will be L to P.E.N.


The N and functional connections are in effect shorted together in a P.M.E. system. This is acceptable.

https://www.electriciansforums.net/threads/what-is-an-rcbos-functional-earth-connection-really-for.51405/


Z.

So, if the cream coloured R.C.B.O. wire is to allow correct operation of the R.C.B.O in case of a N. disconnection, should we run a separate cable to a dedicated earth electrode independent of the P.M.E. system, and connect all of the installation's cream wires to that?


Z.

I have seen two variations on the use of the functional earth on RCDs and  RCBOs, one indeed to carry the current required to fire the RCD in the event of a lost neutral, and the other as an EMC earth, with small capacitors from it to the L and N incoming terminals.


In both cases there is potential for some small current to flow in the functional earth, during the normal operation of the device. It is not clear how many such devices woudl cause a nuisence tripping probem if down stream of another  30mA device, but I'd hazard somewhere between 10 and 30.

I am happier with designs that do not need it and can trip from an L-N voltage less than 50 without assistance.

Watch out for any loose RCBO flying earth leads, they have enough voltage on them to make you pay attention.


Andy Betteridge

A cautionary tail gained from experience Andy? ?