Four RCBOs trip at the same time - IR is good on all circuit

[JohnW2]

I really must find out exactly what function the functional earth provides.

If you ever find a definitive answer, please do let us know!

The obvious one is as a source of "neutral" to power the trip mechanism if the real Neutral goes open circuit leaving only Live at the RCBO.

That's certainly the story that one usually hears - but, unless it's just intended as an 'additional feature', causing the device to operate if the N goes open-circuit (which I very much doubt), it seems to be addressing a 'vanishingly improbable' scenario - namely that a 'good reason' for the device to trip should arise at the very same time that the neutral is open-circuit! Or am I missing something?

But it may also be intended to enable a trip when Neutral is floating to far from ground potential.

Is that materially different from the above? If the supply neutral were to literally go o/c, in the presence of any connected loads at all, the potential of the neutral of the installation would rise to line potential, wouldn't it?

Kind Regards, John

 

[bernardgreen]

If the supply neutral were to literally go o/c, in the presence of any connected loads at all, the potential of the neutral of the installation would rise to line potential, wouldn't it?

Yes and in that situation it would be sensible to disconnect the Live at the RCBO(s)

The other situation is where the Neutral s still connected but assumes a voltage above true ground. ( un-balanced loads on the phases creating current in the network neutral conductors and thus voltage drop along the neutral conductors ). If the functional earth of the RCBO is connected to an "earth" derived from the Neutral then this will ( should ) not affect the RCBO if there is no volt drop ( or near zero drop ) along the Neutral between cut out and RCBO

 

[JohnW2]

If the supply neutral were to literally go o/c, in the presence of any connected loads at all, the potential of the neutral of the installation would rise to line potential, wouldn't it?

Yes and in that situation it would be sensible to disconnect the Live at the RCBO(s)

Indeed - but, as I said, I very much doubt that such 'additional functionality' (which has nothing to do with the primary functionality of an RCBO) will have been deliberately built in (and, even if it had, I would expect to see manufacturers shouting loudly about this extra 'not just an RCBO' safety feature!).

Kind Regards, John

 

[bernardgreen]

http://www.eaton.com/Eaton/OurCompany/NewsEvents/NewsReleases/PCT_255273 dated April 2010

Another often-made point is that, without a separate earth connection, an RCBO cannot detect an earth fault if the neutral becomes disconnected. This is true, but in reality it is not an issue worthy of consideration.

Losing the neutral connection is a rare event in itself, especially in the TN-C and TN-C-S systems that are almost invariably used today. The probability of losing the neutral and having an earth fault at the same time, a double fault condition, is vanishingly small – certainly so small that there is no reason to take it into account when specifying protection devices.

I guess metal theft was not an issue worthy of consideration in 2010

 

[martinxxxxxx]

At the origin (ie downstairs) it is TN-S then looped up.
And there are functional earths on the RCBOs

 

[JohnW2]

Losing the neutral connection is a rare event in itself, especially in the TN-C and TN-C-S systems that are almost invariably used today. The probability of losing the neutral and having an earth fault at the same time, a double fault condition, is vanishingly small – certainly so small that there is no reason to take it into account when specifying protection devices.

That's obviously exactly the point I just made.

I guess metal theft was not an issue worthy of consideration in 2010

I'm sure that the risk of those two, both still very rare, things coming together remains 'vanishingly small', even in 2014. For anyone who drives, let alone smokes, to be worried about such a level of risk is, IMO, just laughable!

Kind Regards, John

 

[JohnW2]

These higher frequencies mean that anything that is reactive has a much lower impedance

As you say capacitive reactance will be lower but inductive reactance will be higher so the current transformer that forms the sensor element will be higher impedance.

True, but would that not apply (significantly) only to the 'secondary' (toroid) of the sensor system, such that, since that's probably feeding electronics with a high input impedance, it therefore probably would not make a lot of difference? It's current through L-E capacitance in the cable/loads (which would obviously be greater at higher frequencies) which would potentially create the L-N imbalance that would cause an RCD/RCBO to operate.

That transients (probably high frequency) originating upstream of the device can sometimes cause an RCD/RCBO to operate is apparent from the fact that electrical storms can sometimes result in such trips - so I suppose that 'waggling a loose neutral connection might do the same. In the OP's case, his trips seem to be happening overnight, when there is minimal loading (hence minimal thermal effects at dodgy terminations) and, presumably, no 'waggling' - which makes it a bit more difficult to understand.

Kind Regards, John

 

[martinxxxxxx]

I have long realised there are up stream faults which can trip RCD's mine have been in around 25 years and every so often we get a batch of tripping then it just stops clearly something on the road causes it but never found out what. Also common if reset one without opening MCB's first the other will trip.

I know it shouldn't but it does.

Welder lives next door and I have wondered if it's his welder but unlikely as not the same phase.

This is a densely populated city and the flat below is fine (bear in mind that this supply is looped out to the flat below). So I am sure (as I can be) that there is not a fault from the supply to the incomer. That does leave a loose neutral about the house to look for. Which I will do. Thank you again Rob.
Another circuit has tripped (the lights and yes there was one on). I have asked the owner to reset it, ensuring all lights are switched off.
My thinking is even with a loose neutral there would only really be an effact if there was a current flowing in the circuit. I can see that RCBO unconnected to any load could flip in any condition (can you?)

Regs

Martin

 

[JohnW2]

[.... I have asked the owner to reset it, ensuring all lights are switched off. ... My thinking is even with a loose neutral there would only really be an effact if there was a current flowing in the circuit. I can see that RCBO unconnected to any load could flip in any condition (can you?)

One would certainly think so (and would really have to scrape the barrel to think of ways it could happen without any load at all). However, I thought you told us, at the start, that the shower circuit RCBO tripped even when the shower was switched off at its (presumably DP) isolator (and presumably no other loads on the circuit) - so who knows?!

It's all very odd. With tongue-in-cheek, I suppose I should ask whether the property is right next door to some high powered RF transmitter or industrial equipment which could create EMI

Kind Regards, John

 

[AndyPRK]

I assume the satisfactory flat downstairs has protec rcbos?

 

[martinxxxxxx]

No there is no radio station or welding going on and Andy you're correct that we did the board downstairs and that is fine.
The Shower switch was off and I never saw it trip (and what I did not see does not count to me) it has not tripped during this observation, so lets assume it was on with a neon at the very least as a load.

 

[JohnW2]

No there is no radio station or welding going on and Andy you're correct that we did the board downstairs and that is fine. The Shower switch was off and I never saw it trip (and what I did not see does not count to me) it has not tripped during this observation, so lets assume it was on with a neon at the very least as a load.

Ah, well! I imagine the neon will have a very high value resistor in series with it, so there's no way enough current could get through that to result in a trip, no matter where it were going! Nor do I imagine that the tiny current through the neon would be enough for a 'waggled loose neutral' to generate enough high frequency transients to do anything (even if there were people around at night to do the waggling)!

As for this 'loose upstream neutral' hypothesis in general, presumably there are very few connections to check between the RCBOs and the origin of the installation, so you ought to be able to confirm, or eliminate, that as the cause pretty quickly?

You are, of course, lucky that these trips are happen pretty frequently. Were than not the case, each of your 'experiments' might take weeks!

Kind Regards, John

 

[securespark]

In one of my conversations with Eaton a few years ago, I discussed functional earthing on their Memera 2000 range of RCBO's.

I was told it had a lot to do with detecting DC fault currents, ie type A and B devices.

 

[OwainDIYer]

Hi ya all
All the IR's are greater than 100MOhms.
Any Idea?

Have you checked IRs *between* circuits not just for each circuit. Could give the effect of a borrowed neutral.

 

[JohnW2]

Have you checked IRs *between* circuits not just for each circuit. Could give the effect of a borrowed neutral.

One of the interesting/pertinent things about martin's problem is that, as I understand it, the trips do not occur immediately circuits are energised, or immediately that a load is applied but, rather, 'spontaneously', often in the middle of the night, when nothing is (should be) changing, and no-one is around to change, or disturb, anything. Logically speaking, that largely rules out a lot of possible explanations about which one could speculate - and I would say that probably includes the sort of thing you're suggesting.

Kind Regards, John
Edit: messy formatting (only just noticed!) mended!