Tall vs compact RCBOs

[JohnW2]

Not on the neutral, only on the live. Double pole means both poles have thermal and magnetic protection ...

Is that really true of DP MCBs and RCBOs.? I thought that, although they had DP switching, they only had sensing on the L.

... which is necessary if you've got 230 V between two phases ...

If the circuit being protected is a single-phase one, I'm not sure that I understand - could you perhaps explain? In a single-phase situation any overload current or current due to an L-N fault will usually go through the L, as will the current due to an L-E fault - so I would have thought that sensing the L would be adequate.

and some places require the neutral to be fused in TT installations.

Unless I'm misunderstanding, this seems to be a different matter. Are you perhaps referring to the requirement for DP isolation in TT installations? If so, and if one wants to comply with that, the isolation doesn't have to be achieved with the circuit's MCD/RCBO - the 'Main Switch' would be adequate.

'Fusing of a neutral", which is literally what you have written is, of course not allowed ( I don't think 'ever) , at least in domestic installations.

 

[JohnW2]

1P+N usually means that the neutral is switched but doesn't have overload protection. At least that's what I've been told.

Yes, having been looking around a lot, that is clearly the case. However, the terminology can vary - I came across a SP one which was identified as such by describing it as having 'a solid neutral'.

 

[JohnW2]

Without smashing them open, was there actually any important bits within the tall skinny bit

Very good question - although one imagines that they did not start by making ones which were too tall to fit (easily or at all) into a good few of the contemporary CUs 'just for the hell of it", so one assumes that they must have needed the extra space at that time. There are plenty of photos, diagrams and videos out there (such as the one below) - and it does look as if every bit of the available space was being put to fairly good use.

All of this begs a question. If it's possible to put not only DP switching but also the bits and circuitry for residual-current functionality into a 'compact' RCBO package (essentially the same size as a single-module MCB), then why on earth are (DP) RCDs (RCCBs) still almost invariable 2-module width? One might even ask the same about Main switches, but the problem there might be the size of the incoming conductors.

 

[AndyPRK]

the double width Hagar could be an AFDD ?

I think I also heard that 45A ones were always tall. I think this applied to fusebox at the time

 

[JohnW2]

the double width Hagar could be an AFDD ?

That was my first thought, even though it was pretty expensive even for an AFDD. However, it was on the Hager website that I found it (and similar) and, despite a lot of trying, I found absolutely nothing the documentation which suggested that it was an AFDD.

Whatever, I remain more than a little confused about what RCBOs are available,and what all the suppliers are selling !

 

[Ragnar_AT]

Is that really true of DP MCBs and RCBOs.? I thought that, although they had DP switching, they only had sensing on the L.

If the circuit being protected is a single-phase one, I'm not sure that I understand - could you perhaps explain? In a single-phase situation any overload current or current due to an L-N fault will usually go through the L, as will the current due to an L-E fault - so I would have thought that sensing the L would be adequate.

Unless I'm misunderstanding, this seems to be a different matter. Are you perhaps referring to the requirement for DP isolation in TT installations? If so, and if one wants to comply with that, the isolation doesn't have to be achieved with the circuit's MCD/RCBO - the 'Main Switch' would be adequate.

'Fusing of a neutral", which is literally what you have written is, of course not allowed ( I don't think 'ever) , at least in domestic installations.

With MCBs that's definitely the difference between 1P+N and DP. As I said, there's countries that have 133/230 V supplies, e.g. Belgium.

I think the requirement for overcurrent protection of both live conductors is a matter of fault protection (i.e. L-E faults) rather than overload/short protection. Back when municipial metal water supply pipes were used as huge earth electrodes it was possible to have TT supplies without RCDs or ELCBs and I suspect the requirement for DP protection dates back to these times. As soon as there's RCD protection you wouldn't technically need that.
Fusing of the neutral has indeed very much been allowed but many countries in the world, including the UK, banned single-pole overcurrent protection of neutrals around a century ago. Double-pole MCBs on the other hand are still perfectly acceptable as they break both conductors of a single-phase circuit at the same time and that's the important difference.
Many countries consider the neutral of a TT supply to be earthed well enough not to require fault protection (because it can't assume a significant voltage to earth) but as far as I know some, e.g. Spain, do not and require fault protection, and double-pole MCBs are one way of achieving that, if a dubious one assuming your earth impedance isn't particularly low.

 

[JohnW2]

With MCBs that's definitely the difference between 1P+N and DP.

Thanks - so as I believed. I think this discussion may be getting a bit confused by the muddling up of DP sensing and DP switching.

I think the requirement for overcurrent protection of both live conductors is a matter of fault protection (i.e. L-E faults) rather than overload/short protection. Back when municipial metal water supply pipes were used as huge earth electrodes it was possible to have TT supplies without RCDs or ELCBs and I suspect the requirement for DP protection dates back to these times.

I don't really understand how that ever can ever have been used as an argument for having over-current sensing in the neutral, since none of the fault current flows through the neutral in the case of an L-E fault such as you mention.

As for achieving adequate OPD-based fault protection (i.e. no residual current devices) in a domestic installation, I suppose that, as you say, it might have just about been possible when TT earths consisted of networks of underground water pipes, but I don't think it is realistic when reliant on domestic TT electrodes (with no/little bonded underground metal pipes), since most domestic TT electrodes would not have a low enough impedance for it even to work with a 5A / 6A circuit, let alone anything larger!
However, as above, even 'back then' I don't see how it would have been an argument for neutral current sensing.

Fusing of the neutral has indeed very much been allowed but many countries in the world, including the UK, banned single-pole overcurrent protection of neutrals around a century ago. Double-pole MCBs on the other hand are still perfectly acceptable as they break both conductors of a single-phase circuit at the same time and that's the important difference..

Sure, there's no problem with any devices which have DP switching, since both poles inevitably get disconnected together. The potenbtial problem is obviously with DP fusing - since there is then the possibility that, in the case of an excessive current (through both L and N) due to overload or an L-N fault might result in just the neutral fuse blowing, leaving the L fuse intact.

Many countries consider the neutral of a TT supply to be earthed well enough not to require fault protection (because it can't assume a significant voltage to earth) but as far as I know some, e.g. Spain, do not and require fault protection, and double-pole MCBs are one way of achieving that, if a dubious one assuming your earth impedance isn't particularly low.

Again, we're not really talking about whether or not the neutral "needs fault protection" but, rather, whether over-current sensing in the neutral is needed. As I've said, other than in the presence of pretty bizarre wring error or faults, there is never going to be excessive current in the neutral but not in the corresponding line, is there?

Kind Regards, John

 

[Ragnar_AT]

The assumption is that a poorly earthed TT neutral can assume enough of a dangerous potential to require automatic disconnection in case of a neutral - earth fault, i.e. overcurrent sensing in the neutral path and/or RCD protection.
Fusing the neutral, as in using a wire fuse, is certainly not a good idea as you said. However, a DP MCB with current sensing in the neutral path will always disconnect both conductors, no matter which of the two poles senses an overcurrent. That's an essential difference and precisely the reason why DP and three-pole MCBs exist.

 

[JohnW2]

The assumption is that a poorly earthed TT neutral can assume enough of a dangerous potential to require automatic disconnection in case of a neutral - earth fault, i.e. overcurrent sensing in the neutral path and/or RCD protection.

I suppose that it is desirable that there should be disconnection in the case of a 'dangerously high N-E potential' although I'm not sure how 'dangerous' is could be, given that one doesn't usually have any opportunity to come into contact with 'neutral' in a TT installation. However, , as I wrote previously, I think that could only be achieved by current sensing (and even that only if there were a simultaneous N-E fault in the installation) if the loop impedance was extremely low - probably only achievable when the 'TT earth' (and/or bonded extraneous-c-ps) is/was an extensive network of underground water pipes.

If the only TT earth is a domestic earth rod (with no significant bonded extraneous-c-ps), that approach would presumably be a non-starter. Even under very wet conditions, my TT earth rod is rarely less than about 50 Ω - so only about 4.6 A, (far too little to trip a B6 or blow a 5 A fuse) even if full supply voltage were applied to it, let alone merely 'a high neutral potential'. If one wanted protection against 'dangerously high N-E potentials' in that situation, I think it would have to be achieved by voltage, not current, sensing.

I also wonder how the sort of situation such as you described (simultaneous high N-E potential and M-E fault) could/would arise.?

Fusing the neutral, as in using a wire fuse, is certainly not a good idea as you said. However, a DP MCB with current sensing in the neutral path will always disconnect both conductors, no matter which of the two poles senses an overcurrent. That's an essential difference and precisely the reason why DP and three-pole MCBs exist.

Yes, agreed. As I wrote, if the disconnection is DP, it doesn't matter what sensing brought it about - as agreed, it's the blowing of a 'wire fuse' in neutral (but not line) which is the potential worry.

Kind Regards, John

 

[Ragnar_AT]

Yes, current sensing on the neutral would only be necessary in case of both a dangerously high N-E voltage and a simultaneous N-E fault, along with sufficiently low loop impedance. I honestly have no idea what exactly was the reasoning back in the day, I only know I‘ve read historic regs that explicitly required fused neutrals in TT installations (in Austria as late as the 1970s and that was actually done with wire fuses, caused the occasional shock as DIYers only check that the lights are off before working on a circuit). I think Italy still requires DP sensing but I‘m not 100% sure.

 

[JohnW2]

Yes, current sensing on the neutral would only be necessary in case of both a dangerously high N-E voltage and a simultaneous N-E fault, along with sufficiently low loop impedance.

Glad you agree. However, as I asked (at least by implication) can you think of any remotely probable situation in which those two (both extremely rare) things would arise simultaneously?

I honestly have no idea what exactly was the reasoning back in the day,

Maybe there was not a lot of 'reasoning' - such that, without enough thought, the idea of a fused neutral 'sounded like a good idea'?

Back in the 60's, I constructed a lot of electronic things (mainly related to amateur radio), many of which I stikl have (even though they haven't been used for decades). It seemed totally standard practice back then to fuse both poles of an incoming mains supply,and widespred use was made of Belgin dual fuse holders for this purpose. The photo below is of one surviving example in a dusty corner of my cellar - two of those fuse holders can be seen (without their covers) - one for the incoming mains L & N and the other for two DC outputs of the power supply.

I only know I‘ve read historic regs that explicitly required fused neutrals in TT installations (in Austria as late as the 1970s and that was actually done with wire fuses, caused the occasional shock as DIYers only check that the lights are off before working on a circuit). I think Italy still requires DP sensing but I‘m not 100% sure.

As above, perhaps all these people/countries thought that 'it sounded like a good idea' and, perhaps more to the point, maybe worked on the basis that 'this is how it's always been done'?!

Kind Regards, John

 

[plugwash]

Glad you agree. However, as I asked (at least by implication) can you think of any remotely probable situation in which those two (both extremely rare) things would arise simultaneously?

The faults don't have to arise at the same time, you just have to have the second fault arise before the first fault is rectified.

In some scenarios you have an inspection and testing regime that detects and rectifies faults in a timely manner, but all too often you don't have that. All too often faults are only detected when either a protective device blows/trips, or when someone gets a shock. Even when a protective device does trip, it will often be reset with little to no investigation.

So I don't think a scenario like.

* Transformer earth bond fails (or is stolen by copper thieves)
* Installation 1 develops a live to earth fault.
* Installation 2 develops a neutral to earth fault.

Is at all implausible.

Nor do I think

* Lineman had a bad day and reconnected a property with reverse polarity
* Property later develops a "neutral" (actually live) to earth fault.

Is implausible.

It seemed totally standard practice back then to fuse both poles of an incoming mains supply,and widespred use was made of Belgin dual fuse holders for this purpose.

In general, for a single phase portable appliance it is far from guaranteed that the nominally neutral conductor is actually a neutral. So if you want your protective devices to protect against faults to earth then you need them in both poles.

 

[JohnW2]

The faults don't have to arise at the same time, you just have to have the second fault arise before the first fault is rectified.

Agreed - but they obviously do need to be present at the same time ...

So I don't think a scenario like.
* Transformer earth bond fails (or is stolen by copper thieves)
* Installation 1 develops a live to earth fault.
* Installation 2 develops a neutral to earth fault.
Is at all implausible.

Nor do I think
* Lineman had a bad day and reconnected a property with reverse polarity
* Property later develops a "neutral" (actually live) to earth fault.
Is implausible.

I suppose we could debate what "implausible" means! Neither of those scenarios is impossible, but I would think that the probability of either is extremely low

In general, for a single phase portable appliance it is far from guaranteed that the nominally neutral conductor is actually a neutral. So if you want your protective devices to protect against faults to earth then you need them in both poles.

Agreed, but there is obviously another side to that sword. If both poles are fused but only the one in (the side which actually is) the neutral blows, then the L remains connected to the equipment, hence potentially very dangerous. Given that it is surely the case that "neutral actually is neutral" is the much more common situation, it would seem that that second side of the sword trumps the first, supporting the argument that only the pole which is (is meant to be) L should be fused.
Of course, that problem only exists with wire fuses or SP devices. With any sort of DP device, in the absence of total (and very rare) failure of the device, both poles will always get disconnected.

 

[plugwash]

Given that it is surely the case that "neutral actually is neutral" is the much more common situation

The tricky bit is that standards for appliances these days are harmonised. This differs from the situation for installations, where while an IEC standard exists that tries to harmonise some parts there is still significant national variation.

In the UK today, heck probably even the UK 50 years ago. I would agree that "neutral actually is neutral" is the situation in the overwhelming majority of cases. If we look across the channel though a rather different picture emerges.

* Italian plugs are unpolarised.
* German plugs are unpolarised
* French plugs are physically polarised, but my understanding is that there is no universal standard for what is the "correct" wiring.

These types are common, not just in France, Germany and Italy, but across most of Europe.

Going back in time, prior to the advent of earthing most plugs were unpolarised. While faults to the electrical earth are perhaps not a concern if there isn't one, faults from the mains supply to the radio system earth would certainly seem like a concern for radio gear.

If both poles are fused but only the one in (the side which actually is) the neutral blows, then the L remains connected to the equipment, hence potentially very dangerous.

Certainly less than ideal, but only really a massive issue if someone fails to disconnect the appliance from the mains before working on it.

 

[JohnW2]

The tricky bit is that standards for appliances these days are harmonised. This differs from the situation for installations, where while an IEC standard exists that tries to harmonise some parts there is still significant national variation.
..... If we look across the channel though a rather different picture emerges. ...

I agree with everything you go on to say, in relation to other countries and past points in time (in the UK). However, we are talking about the UK, and now, and I totally agree with you when you say:

In the UK today, heck probably even the UK 50 years ago. I would agree that "neutral actually is neutral" is the situation in the overwhelming majority of cases.

... and that is surely what matters to this discussion?

Certainly less than ideal, but only really a massive issue if someone fails to disconnect the appliance from the mains before working on it.

Many of the things we say, advise and do, as well as the regulations, would be less (if at all) necessary if everyone was sensible (and adequately informed) in what they did. Even 'bare live conductors' would be relatively OK if everyone were well educated not to touch them

Given that you seem to agree that, today in the UK, 'neutral is nearly always neutral', I would say that the agument for not fusing both L & N greatly outweighs (today in UK) the argue for doing that, doesn't it?