RCBO Tripping

nerd mode

I wonder what the capacity is between the neutral and earth in a boiler or other item hard wired and only single pole switched at the FCU ?

The boiler in my daughter's first house was the main cause of the RCD tripping. If it was un-plugged ( 13 amp socket ) there were no trips.

It apparently tested as OK with the electrician's ( I assume ) standard IR testing.

Feeding it via an isolation transformer ( as temporary test over 3 months ) did reduce the number of nuscience trips.

end of nerd mode
 
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nerd modeI wonder what the capacity is between the neutral and earth in a boiler or other item hard wired and only single pole switched at the FCU ? end of nerd mode
That presumably depends totally on what 'the item' is. In the absence of an explicit capacitor (without series resistor) between N and E within the item, I would imagine that the capacitance between N and E would be incredibly low for most 'items', wouldn't you? When capacitors are used between N/L and E for filtering, I think there is usually a sizeable resistor in series, because of this very potential problem, isn't there?

BTW, although they once existed (and perhaps still do) it's a long time since I saw an FCU with a SP switch.

Kind Regards,
 
I was thinking of stray capacity in components such as wound transformers between winding and core ( or inter-winding earthed screen ). This was a problem when a batch of toroidal transformers were made with the Live end of the primary next to the screen instead of, as specified, the Neutral end being next to to the screen. Too be fair the equipment was used in locations where the supply was likely to be protected by a 10 mA RCD and the stray Live to Earth capacity was critical.
 
As you go on to say, sort-of. A..............<snip>...............efore that investigations might most profitably concentrate on (or, at least, start with) the downstairs circuit.
Kind Regards, John

Phew! I'm glad it wasn't all poppycock then which is what I feared for 4am with eyes like spanners. I wanted to add to all the other good stuff for when Martin went back out to it today.

Martin don't worry about becoming an 'outcast'. We've all been there I'm sure, even BAS :eek: And in the times that I have it's often been comments from others (sometimes people with little or no electrical/electronic knowledge :oops: ) that have broke the back of it.
 
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I was thinking of stray capacity in components such as wound transformers between winding and core ( or inter-winding earthed screen ). This was a problem when a batch of toroidal transformers were made with the Live end of the primary next to the screen instead of, as specified, the Neutral end being next to to the screen.
I haven't really got a clue what to expect of that - it's not a normal part of the spec of a power transformer - I guess the easiest way to find out would be to measure a few. My guess would be that the capacitance would be pretty low since, as you imply above, it's only those layers of the winding closest to the core/screen that are going to be major contributors to the capacitance.
Too be fair the equipment was used in locations where the supply was likely to be protected by a 10 mA RCD and the stray Live to Earth capacity was critical.
I realise that you would have wanted to keep well below that 10 mA figure but, by my calculations, to get 10 mA by capacitive coupling between 230V 50Hz L and E you would need something like 0.138 &#956;F - which is incredibly high for any sort of 'stray capacitance'. Even for 1 mA you would need about 0.0138 &#956;F - still a lot of 'stray capaciance'.

Kind Regards, John
 
Phew! I'm glad it wasn't all poppycock then which is what I feared for 4am with eyes like spanners. I wanted to add to all the other good stuff for when Martin went back out to it today.
As I said, I basically agree - at least that the problem is very likely to be 'downstairs'.
Martin don't worry about becoming an 'outcast'. We've all been there I'm sure, even BAS :eek: And in the times that I have it's often been comments from others (sometimes people with little or no electrical/electronic knowledge :oops: ) that have broke the back of it.
Indeed - as I wrote to Martin last night, we've probably all "been there" (some of us more often than we would like to admit :) ). It is, of course, probably good if we end up feeling at least a little stupid - since that makes sure that it's one of those lessons we're unlikely to forget!

Kind Regards, John
 
[nerd mode]
However, in terms of the maths of what you say, I don't think that it is quite right. The relationship between the capacitance between two wires and the distance between them is not quite as simple as ‘inverse natural log’. As can be seen in the equation below, it is the ‘inverse natural log’ of a function of the ratio of wire separation to wire radius. Interestingly, it’s only this ratio that matters, not the absolute dimensions – hence the capacitance between 1 metre lengths of two 1 mm radius conductors with their centres 4 mm apart will be the same as the capacitance between 1 metre lengths of 10 mm radius conductors with their centres 40 mm apart, or 100 mm radius ones 400 mm apart.

Anyway, my point is that, far from being “inverse square like” (as you suggest), the relationship between capacitance and wire separation is actually a surprisingly ‘weak’ one, in fact significantly ‘weaker’ than even a ‘inverse linear’ relationship – as illustrated in the graphs below. For example, if one doubles the separation/radius ratio (e.g. double separation without changing wire diameter) from 4 to 8, the capacitance only falls to about 64% of its original value, whereas it would fall to 25% with an inverse square relationship or to 50% with an inverse linear relationship. Intuitively, I would not have expected an inverse square relationship (since the ‘lines of electrical force’ are all essentially in one dimension), but I would have expected it to be fairly close to inverse linear (which is the case for two, relatively close ‘plates’) – which I suppose it roughly is.

However, as I said, despite all this (which makes capacitances between different cables a bit higher than you were imagining), I certainly don’t think that ‘between-cables’ capacitance is even remotely significant in the context we’re discussing.

Kind Regards, John

My deep maths isn't brilliant, and in deep I survive by being able to 'see' what's going on. So your graph is most useful for that. I can be quite visual for problem solving. So I'd looked at the formula that looked like the closest approximation, and attempted to visually distill its general trend. And decided that it wasn't too far off the mark from what I've seen real world. Not seeing any other variables under the bar other than distance I left it at that. Thankyou for deeper visual insight.

John, which package did you use to graph it? Of late I've been on the hunt for a graphing package to trend various data but I'm still searching. Open office graphing works fine if its in auto mode for basic stuff, but hand driving it isn't the best. I've recently quickly looked at RRD but whilst its capable of decent output its setup doesn't seem to be easy. Others I know of that I have yet to explore would be MathCAD and Mathematica.

I do like the new nerd mode. I think this may be quite catching and I for one will adopt it. One of the problems in a flat thread is that these discussions often sidetrack from the main subject but into interesting discussions in their own right, where often new stuff is learned and especially when you've a decent sparring partner. But the extra thread content does make it hard to read from the start, something you'd have to do coming to it fresh in order to chip in. So nerd mode I think is a good idea. As they say....If you don't use it ......you'll lose it
[/nerd mode]
 
nerd mode
I was thinking of stray capacity in components such as wound transformers between winding and core ( or inter-winding earthed screen ). This was a problem when a batch of toroidal transformers were made with the Live end of the primary next to the screen instead of, as specified, the Neutral end being next to to the screen. Too be fair the equipment was used in locations where the supply was likely to be protected by a 10 mA RCD and the stray Live to Earth capacity was critical.

Years ago before RCDs saw domestic use I built an RCD from scratch using a donor toroidal transformer. The outer secondary was stripped off leaving the finer primary as the sense winding. Over the top was then wound a bifilar winding to carry line & neutral. It was crude compared to what I would do with it now but it was quite effective. One of the most interesting features of it was being to adjust the trip threshold from about 200mA down to nothing. Although its crude construction probably limited its performance due to winding mismatches and coupling (meaning that the mismatch had to be zeroed out in circutry rather than getting the windings more balanced, so losing sensitive and noise rejection) it was obvious that below about 10mA nuisance tripping became a problem. As the threshold decreased below around 5mA it became obvious which other piece of equipment was causing the homebuilt RCD to trip. So while it wasn't leakage on its own circuit that caused 'it' to trip, the much reduced threshold made it prone to electrical noise from other sources.

/nerd mode
 
[nerd mode]
My deep maths isn't brilliant, and in deep I survive by being able to 'see' what's going on. So your graph is most useful for that. I can be quite visual for problem solving. So I'd looked at the formula that looked like the closest approximation, and attempted to visually distill its general trend. And decided that it wasn't too far off the mark from what I've seen real world. Not seeing any other variables under the bar other than distance I left it at that. Thankyou for deeper visual insight.
You're welcome. In fact, if you look at the formula, you were actually very nearly right in saying that it was 'inverse natural log'. As R (ratio of spacing to radius) gets large, the bit 'square rooted' gets increasingly close to sqrt(R^2/4), which is obviously just R/2. The quantity being logged then becomes ({R/2} + {R/2})- in other words just R. Hence, for large R, capacitance per unit length is just &#960;&#949;/log(R). If the radius of the wire is fixed, then you can think of R as just being the distance between (centres of) the wires (that just introduces a constant) - so, as you said, the relationship between spacing and capacitance is 'inverse natural log'. Where you went a bit wrong was in thinking that this was similar to inverse square, as my graphs illustrated.
John, which package did you use to graph it? Of late I've been on the hunt for a graphing package to trend various data but I'm still searching. Open office graphing works fine if its in auto mode for basic stuff, but hand driving it isn't the best. I've recently quickly looked at RRD but whilst its capable of decent output its setup doesn't seem to be easy. Others I know of that I have yet to explore would be MathCAD and Mathematica.
That graph was just Excel - which is fine for simple things like that. For serious and/or complex graphics I use mega-expense software. MathCAD and Mathematica both have pretty good graphing facilities, but there are hundreds of pacakages out there. A lot depends on what sort of graphics you want to create and how much (if anything) you are prepared to pay.

I do like the new nerd mode. I think this may be quite catching and I for one will adopt it. One of the problems in a flat thread is that these discussions often sidetrack from the main subject but into interesting discussions in their own right, where often new stuff is learned and especially when you've a decent sparring partner. But the extra thread content does make it hard to read from the start, something you'd have to do coming to it fresh in order to chip in. So nerd mode I think is a good idea. As they say....If you don't use it ......you'll lose it
I'm very conscious of having been party to Martin's interesting thread having being mucked up a bit, particularly with all this stuff about capacitive coupling - which I'm all-but-certain is totally irrelevant to Martin's problem. If a Mod who is able to 'see the wood for the trees' is reading it (s)he might consdier shifting all the 'capacitive' stuff into a separate spin-off thread.

Kind Regards, John
 
Anyway am back there Thursday, considering making the fault-find into a live webcast - but worried that I have missed something so fundamental that I will become a Diynot outcast :D

Plan of attack:-

Isolate supply
Remove bonding conductors and earthing conductor.
Remove all neutral and CPCs from busbars
Unplug EVERYTHING in house
IR at 250V between all conductors with all switches in off position (tabulate results)
If all good IR at 500V as above. If not seek and find low resistance point.
Perform R1+R2 tests on all circuits starting with crossed rings (check r1,r2,rn 1st)
Perform Crossed L-N on rings

Well did not get past

IR at 250V between all conductors with all switches in off position (tabulate results)
If all good IR at 500V as above. If not seek and find low resistance point.
Perform R1+R2 tests on all circuits starting with crossed rings (check r1,r2,rn 1st).

(Although I did carry on and repeat all the above steps and the full list of test required for a new EIC)

What was found was a short (n-e) on the down ring :oops: Sorry Everyone:cool:
Located to a nail in floor making a partial circuit.

My theory is that furniture was moved round for cleaning between my fitting the CU and getting the call to return due to tripping when they plugged in the vacuum cleaner. This may of made the connection because I did not make up my results for the EIC - When I returned to fault find the situation was very pressurised but as this was one of the fundamental things I was looking for as a cause of the fault I don't really understand how it was missed. But I was mighty relieved when I found it today.

Thanks again to everyone who contributed and I did enjoy exploring the subject and possible fault finding routes - I have always been worried about posting problems on here, but this one appeared so extraordinary and I needed to bounce ideas around to make sure I had a solid plan of attack today that I threw caution to the wind - but thank you forum for being gentle with me on this, now who can explain the reason for the upstair taking out the down circuit - I think peet is very close above.
 
What was found was a short (n-e) on the down ring :oops: Sorry Everyone:cool: Located to a nail in floor making a partial circuit.
Well done! So those who taught me that 'common things are common' was not wrong - and at least we correctly identified which circuit was at fault.
What Thanks again to everyone who contributed and I did enjoy exploring the subject and possible fault finding routes - I have always been worried about posting problems on here, but this one appeared so extraordinary and I needed to bounce ideas around to make sure I had a solid plan of attack today that I threw caution to the wind - but thank you forum for being gentle with me on this ...
As I and others have said, don't worry. What's the forum for if not posting questions/problems - and most people here are usually pretty gentle!
... now who can explain the reason for the upstair taking out the down circuit - I think peet is very close above.
If it's TN-S or TT, then Peet's explanation seems fine - just read N-CPC fault for N-GRND. If it's TN-C-S, the explanation is perhaps not so obvious, since CPCs and incoming N are effectively joined at the MET.

As per my exchange with Peet, the one thing which seemingly remains unexplained is why you only got an 'immediate trip' (even without load) when you connected both circuits to one RCBO. On the face of it, the dowstairs RCBO should have been permanently tripped. One can but assume that it was quite a high resistance N-CPC fault, not quite enough to trip the RCBO alone and that something (what?) about connecting both circuits to the same RCBO pushed it over the tripping threshold.

Kind Regards, John
 
I think it may of been possible that walking on the board could switch the fault on and of and there was a lot of traffic the other day, 3 in the landlords party and 5 student wandering around so some to the measurements and test may have been due to someone stepping in the wrong place.I think I am lucky to have been there with the fault "showing" today, it could have been o/c and given me hours and hours of chasing around, I put aside a half day and it took 2 hours = Result. Plus the landlords husband who was there today is a chartered m&e guy so completely understood the situation and has offered to pay for the fault find/ rectification time as an extra. Sometimes I find that trying to explain the the fact that the fault has arisen is not down to something one has done is not worth the aggro and would not charge rather than get a reputation for sticking incompressible (to non electrical types anyway) extras

So all good..

It is a TN-S arrangement

It might be worth editing out any testing procedures not suited for DIYers as this post is bound to come up on searches ;)
 
In Hindsight most things are easier. Don't apologise for anything Martin. Look at the positives here......

1) A successful outcome and its done & dusted.
2) Somebody (Martin) who was brave enough to put up a problem for the world to see and that's provided some interesting food for thought.

Chewing on the problem for a while, in those back-burner_staring_out_of_the_window_little_moments has been quite interesting as it's with a system that many are very familiar with, so many can join in. And yet the thoughts about it are varied. So becomes apparent the power of a diverse group focussed on one problem. It was also an interesting exercise in problem solving. What I did try to do was get some of the main known facts and not only look at what was happening, but also look at what was not happening. I'm sure that there may be a mathematical way to solve the problem, like looking for mutual exclusivity and causality but prior to the eureka moment I'm not sure it would be feasible as it's not easy to identify which variables are the important ones.
"If any socket on the Up circuit is used - the RCBO for down trips, but the up circuit stays on and continues to work"
So the inverse of that is

Everything does actually work apart from the down RCBO circuit.

If so what are the common or unknown connections between the two? Like John, and I think EFLI maybe on the other thread, have both stated, the probability of likely cause was that it would be something fairly straight forward. And yes with experience it usually is. While the actual physical cause was maybe harder to find, the actual cause (N-E fault) was fairly easy to find eventually.

Bernards suggestion of
Try the same but with the RCBO's live output disconnected. If it still trips then it can only be current on the neutral through the RCBO sense coil and that has to involve a neutral to ground current. That current could be flowing from a neutral to a CPC fault and then via a CPC to ground connection such as a bonded metallic service pipe.
Stuck in my mind as a very interesting point and something similar to what I was thinking of. Certainly a current from Neutral to something, although at that stage we weren't sure what.
If any socket on the down circuit is used the down RCBO trips.
The inverse of that is again the same as above. And that is 'everything stays in except the down RCBO'.

It was the 3rd bit
put everything onto one rcbo so that both circuits shared the same RCBO - and guess what - the RCBO tripped straight away.
I couldn't piece together from what had been said, so when you found today that there was actually a N-E fault on the down circuit that clears that up mostly.

It does clarify that we had a fault that wasn't dependent on there being a load plugged in and that in itself is an interesting troubleshooting technique maybe. ie When one circuit trips another, then ratings/situation etc permitting sticking them on one RCBO could tell you something important. It may not tell you what's right but it may confirm what isn't wrong.

The anomaly it doesn't fully clear up is that at some point when you first visited, the down circuit was capable of staying in until a load was applied and it tripped. I put down to the vagarities of the fault caused by a nail giving a partial connection. Having come across many like this (not specifically nails but partial connections) and similar especially on circuit boards I know that this uncertainty could well of been due to factors such as movement and vibration, caused by factors such as people or the building moving and maybe even something so subtle as temperature. The nail may have been there for a long time only being disturbed by the current work. Plus the nature of the connection could well be voltage dependent. Some dirty connections can be diode like in their nature.

There's been a few threads of late that have made me make a mental note to do some further research with a bit of time, and this is one of them. Specifically related to how circuits may effect one another when it comes to RCD protection. I can simulate quite a bit of it on the bench but the chance to test some of it out on a representative installation may come up soon.

One of my theories is that with a neutral short or partial fault to GND/CPC is that the returning neutral current will divide. Like Bernards earlier thoughts on PME potential shifts etc, deciding where the divide point comes could be tricky. I think this might also happen in PME but at higher currents.

Consider this - With a leak of current on one circuit from N to say the CPC, the load current on all circuits will contribute to this, as the neutral busbar is common to everything. This leads to a situation where say 30A of load current in the neutral may then divide, and in an unloaded RCBO protected circuit (ie ANY neutral current in the RCBO heads towards tripping it as line current is zero) with a N-E or N-GND fault it only needs a small portion of the 30A to trip. So if we need 30mA tops to trip it, the ratio of 30A needed is at most 30A/30mA so 1000:1 and that, at a guess, would be the ratio of impedances at the divide point needed with a N-E fault in place. Okay on PME the two are connected locally but it may still be possible that enough current goes through the RCBO to do the job. If the impedance of the path through the RCBO is not at least 1000 times less than the impendance of the path straight to where the CPC joins the neutral then would it do it?
 
I personally would have zapped 1000 volt through it to make sure before dismissing a circuit as clear.

Sounds like a way of maybe cajouling those extra-weird faults into action which may save a lot of time. But elevated votage testing of insulation can lead to the deterioration of the insulation if you're approaching its limits. Anybody know what kind of limits T&E has? And would 1kV testing present a problem to it?
 

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