RCBO tripping mystery

My Eaton (MEM) RCBO instructions don't say anything about trimming the pigtail, but they do show a nice little loop in it.


Based on advice in here, I made it my practice to trim and bootlace ferrule the pigtails. However, the end of the Eaton pigtail is compressed into a square where it is exposed at the end, presumably to facilitate installation uncut.

(@Sparkingchip ? I wish I could say I haven't seen the same -- I did see the result of someone thinking they had to ferrule something but without a crimper had obviously used the "cutting" part of ordinary pliers to crush a single pair of lines into the ferrule!  High-resistance joint?)

@Chris Perhaps the instruction is only on "industrial" device instructions, but the guide has a wordless pretty picture showing that the exposed conductor should go into an uninsulated tube and then into a terminal.  "A picture speaks a thousand words" and yet sometimes just a few words can make an incomprehensible picture entirely make sense!  I suppose it removes the need for language-specific sheets, and yet this guide was otherwise written in English :-/

Most mfr's crimp the ends but what you get from EatonMEM is a "compression" weld (probably the wrong term, I'm no mechanical engineer) to virtually melt the strands together.  Exactly the same effect for the same purpose and treated in the same way: use it or crop-it-and-crimp-it according to circumstance.

The issue with multi strand conductors being compressed into a solid square section is that you then need to make sure the screw head is on a flat side.


I have seen burnt neutral bars in consumer units where the screw head has been tightened onto a corner of the square, which has then twisted around a bit loosing the connection.


Going back to the original post, if all the RCBOs were neatly installed with the neutrals trimmed to fit except for the two causing issues it begs the question I were they installed at different times by different installers?


Did the original installer know there was a problem with these two circuits and leave them on MCBs, only for someone else to come along and replace them with RCBOs which trip?


Disconnect both circuits and insulation test L+N to E on both separately, them insulation test between them to ensure there’s separation of circuits. There could be a borrowed neutral that only gets switched in at various times. I found a light in a airing cupboard that took a live from the lighting circuit and a neutral from the immersion heater circuit, anything like that could be an issue.


If all else fails, just try combining the two circuits into one and only using one RCBO. There’s no real reason why they cannot share an RCBO. That’s probably what you would end up doing with two lighting circuits and the classic issue of a borrowed neutral on the landing light.


Andy B.

Insulation testing between the circuits at 100 or 250 Vdc with all the lamps in and switches bring flicked on and off.


You are probably looking for a direct connection between the circuits, not just a high resistance fault.

But when can’t-be-arsed amateurs leave the strands 'parallel' (with only the slight construction lay) and doesn't think to bend over smaller conductors, then the force of the screw terminal will damage 2 or 3 of the precious 7 strands of a normal stranded conductor, and a great many fine extra-flexible strands of a tri-rated cable, which may well break at that time or will likely fracture as soon as it needs to be released for testing, alteration, etc.  The effective reduction in cross-sectional area can often create hazardously hot terminations making insulation creep back and softening the remaining strands.

(Sorry for that off-topic rant, as an inspector I just triggered one of my own hot-buttons…)


Nobody has run with what I started years ago, link below in case you missed it.

https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=56359&messageid=463705


Regards


BOD