Twin and Earth CPC

[Julie.]

So first of all an advanced warning of a rant, there may be scenes of a sexual nature, violence and scenes that some viewers may find distressing.

Well not really,

The thing about the UK electrical system is that it actually has been designed from the bottom - up, unlike the US for example where it has evolved and still exists in pretty much the original form in some places.

After the war, the whole system was effectively rebuilt to a new standard design, very coordinated - it really was a plug and play solution.

The utility would deliver power within set ranges, this means that individual homes could be built to standard forms - as long as there was a 60A fuse - normal ring in 7/029 on a 30A fuse, and so on.
No design work needed at all, even where the load was higher then the utility would ensure it was less than 16kA and provide a larger fuse up to 100A thus the let-through was within the capabilities of the installation.

For larger premises, and industrial sites, then yes the installation would have to be designed, but even then it was done via tables and standard building blocks rather than from first principles - which would only be required for the largest of sites.

Of course times move on, and things change, firstly close protection, which was an improvement, but then a push for mcb and so on - most certainly not an improvement! - In this case the let-through is so much bigger than a fuse, so the usual plug and play wiring is not so suitable, the regs have to expand as more 'normal jobs' now need to be checked or designed etc. And a regs book that is bigger than ever.

<RANT>
now we get to today, and politicians 'decide' that everyone can have local generation, or 'decide' that ev charge points can be installed everywhere.

Of course this means the fault levels are different with this new contribution, it also means that the systems have to be reinforced for the larger max demand...

The end result is that the system, and more importantly final installations become more involved to design, we are losing the ability to use standard designs and so on.

In itself this isn't an issue, you just coordinate the reinforcement to match the changes and all is well....

Errr NO!

It's a blanket allow the changes (it's vote winning) and blame the engineers when we start getting reliability issues and outages!!!!

</RANT>

 

[Cookie]

All equipment is designed not to shock you (I hope!) but the reversible polarity means everything has to be double pole safe.

Double pole switching is not needed. Everything conductive is either in a plastic case, enclosed in a metal case with a CPC or with the exclusion of light sockets, finger safe. Personally I think finger safe sockets should come into place considering they are 125 years behind in basic safety. GU 24 sockets or something similar should take hold IMO.

In the UK reversed polatiry is a C1 fault - up there with exposed live parts - because we have the fuse (and sometimes the switch) only in the line conductor. However, we had polarised plugs & sockets well before the 13A style were introduced and verifying polarity is one of the first and fundamental steps in checking any installation.
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Reverse polarity should not present a shock hazard to the user. Increase in fire yes as the neutral is not fused... but remember again copper shortages were what drove ring mains-along with other common practices like reduced sized CPCs.

The noise level is usually not a problem. In cases where it is you have a far bigger s*if-show with crappy SMPUSU to worry about...

With modern equipment it tends to be less of an issue, but you still have magnetic fields and potential differences to deal with.

It is a low risk, but equally it is among the "single point of failure" risks that we ought not to see in power distribution.

Now if only they use a ring circuit for the PME supply...
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This is a factor in design that seems to be overlooked a lot. Fuses are a very simple device, so simple that the behaviour is often not understood well, but when it comes to it they do a much better job of limiting fault energy than MCB or MCCB.

Agree. Perhaps you've seen this video, its gold here in the US:

View: https://www.youtube.com/watch?v=3dckmSgp1nw


Check out 4:02- that small wire evaporates lol.

The USA has a big thing about arc-flash, and part of that might be down to the approach to systems design. The UK has often adopted the solution of an HRC fuse up front and MCB/MCCB downstream so the peak fault current is often contained quite well.
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Do you use fuses or breakers more often above 200amps?

In the US short circuit currents tend to be higher- there are services which easily exceed 200,000 amps of fault current. Typically spot and secondary networks are behind these values however a 2,500kva 4% Z 277/480 volt transformer is not far behind.

Officially the max PFC is given at 16kA but in reality you are very unlikely to see more than 6kA. Also the incoming DNO fuse is probably going to limit it further. For example, a BS88 fuse at 100A rating has a peak fault current below 16kA even at 100kA symmetric RMS value of PFC.

Now our domestic breakers are usually rated at 6kA so that is still not really acceptable, but it goes to show that an HRC fuse can go a long way to mitigate a very bad day in fault department...

Fuses are king in my world. But I see a lot of engineers falling for breakers because of all the features and constant excuse "you will single phase everything" I sense manufacturers have a rather good talent in persuasion.

 

[Cookie]

Well, what exactly is your point?

At the start you started off by saying that the cpc has to be the size as governed by the table (54.4/54.7)

If no calcs are done you must use the table which states Phase = CPC.

You also stated that you can't have reduced cross section

If the adiabatic equation results in a cross sectional area smaller than the phase conductor than the CPC is permitted to be reduced in size.

You then stated that you must calculate it.

Correct- if you choose not to use the table.

There are many ways of meeting the regs.

If it's a standard house with typical fault levels then pretty much no calculations are needed, use the standard 2.5mm^2 ring final circuits, normal 1 or 1.5mm^2 lighting on radials, cooker/shower etc on 6 or 10 depending on rating - it will all fall in line with the regulations. (that is why they are designed that way)

However, start getting bigger fault levels, or longer than usual runs, then you will have to check using the tables or by calculation and change the standard design accordingly. This is very common in industrial situations.

Go to really special situations and you pretty much have to calculate everything, it is very common that the cable is sized on minimum cross section for fault level rather than for load conditions - 2.5mm^2 is good for a 20A load, but because of the fault level the minimum cable has to be 3.1mm^2 - so 4mm^2 is specified etc.

Such differing scenarios tend to be fairly clear, so the different approaches are applied as and when, it is certainly not necessary to calculate every time.

Most electricians working on houses would rarely need to even refer to the tables as long as the fault level is less than 3ka - or incoming fuse less than 100A

Even when they do, it's usually in regard to longer cable runs.

It doesn't take too many jobs before you can write the max Zs by mcb from memory!

Makes sense, agreed.

But- I don't mean to be a hard head I only reason the way I do because I'm going by the way I was taught- if these rules were in NFPA 70 we would have to choose either the table or the calcs. So I am talking entirely from my own personal bias rather than doubting you. Please do not take this as me calling your experience into question. I know you are correct

As I understand it BS7671 isn't really "law" where as NFPA is treated like legal statute and must be followed word for word in its literal entirety.

No hard feelings I hope.

 

[pc1966]

Agree. Perhaps you've seen this video, its gold here in the US:

Check out 4:02- that small wire evaporates lol.

That is a fantastic video!

No, I had not seen it before but it was the sort of comparison I was looking for recently for a report I was writing. What actually is scary is how violent the breaker failure is on a x2 overload as, while I did not expect it to survive, I had assumed it would remain in one piece!

Do you use fuses or breakers more often above 200amps?

In the US short circuit currents tend to be higher- there are services which easily exceed 200,000 amps of fault current. Typically spot and secondary networks are behind these values however a 2,500kva 4% Z 277/480 volt transformer is not far behind.

I don't know enough to answer that, probably @Julie. would know.

Certainly I see fuses as a common primary LV supply protector in the 100A-1250A range, but most distribution systems now are MCCB / MCB based.

Seeing 200kA fault potential is really scary! The local 500kVA substation (415V) we will get power from has a PFC of just under 12kA, potentially a few kA more as there are some big motor in use on the site as well that would generate during a short-style fault, but it is all well within the 50kA rating of our 100A fused-switch feeding our stuff.

Fuses are king in my world. But I see a lot of engineers falling for breakers because of all the features and constant excuse "you will single phase everything" I sense manufacturers have a rather good talent in persuasion.

Breakers are very convenient operationally (though that video also shows that if big faults happen even within rating you may well be looking to replace it soon) and they have the advantage of opening all phases simultaneously. Also on lower fault currents, say up to a kA or so for MCBs, they have lower I2t due to the speed of the magnetic trip in that region.

But fuses have much better fault limiting as we have just seen, and they are easier to get selectivity as well. In many ways the main down-side is the need for phase-fault protection to avoid some motor limping along if one of the 3 phases goes, though of course your incoming supply might do that worst-case...

As you say it is a shame that many electrical engineers dismiss fuses as old-fashioned or inconvenient as in many cases, especially as the feed protection to a set of MCBs or similar, they are a very good choice.

 

[Julie.]

If no calcs are done you must use the table which states Phase = CPC.




If the adiabatic equation results in a cross sectional area smaller than the phase conductor than the CPC is permitted to be reduced in size.




Correct- if you choose not to use the table.




Makes sense, agreed.

But- I don't mean to be a hard head I only reason the way I do because I'm going by the way I was taught- if these rules were in NFPA 70 we would have to choose either the table or the calcs. So I am talking entirely from my own personal bias rather than doubting you. Please do not take this as me calling your experience into question. I know you are correct

As I understand it BS7671 isn't really "law" where as NFPA is treated like legal statute and must be followed word for word in its literal entirety.

No hard feelings I hope.

No upset at all, sorry if I came across in that way.

But, you appear to be diving in at specific lines in the regs without gaining the context, see the regs like a jigsaw, look at one piece and it means nothing, put it in place with everything around it and now it presents a seamless picture, that piece fitting well into it.

A reg in itself only makes sense in light of the whole design, this is the same anywhere, if we took any particular part of say a US code - but viewed with our overall picture then it looks completely wrong, stupid in some cases. However viewed in light of the whole picture over there and it makes complete sense - in fact trying to slot the iec equivalent rule into the US design/picture then looks equally as wrong.

Also, I have rather misquoted the regs, with lines like "if you don't calculate then use table..." - for ease, but careful reading shows they never state this, instead it's "...shall be calculated, ...." type of thing.

This is subtly important - in my day job I am frequently calculating this nonsense and presenting designs that are installed by others - so the installer is not actually calculating anything - but this is compliant because it has been calculated - just in the background.

It is the same with using the osg - the installer is not actually doing the calculations, but they have all been performed in the background, and merely presented in a more usable format for the installer, in this way we are complying word for word.

The installer has installed cables of reduced cross section by reference to the tables in the osg and therefore the calculations have been performed (just not by the installer)

 

[Julie.]

That is a fantastic video!

No, I had not seen it before but it was the sort of comparison I was looking for recently for a report I was writing. What actually is scary is how violent the breaker failure is on a x2 overload as, while I did not expect it to survive, I had assumed it would remain in one piece!


I don't know enough to answer that, probably @Julie. would know.

Certainly I see fuses as a common primary LV supply protector in the 100A-1250A range, but most distribution systems now are MCCB / MCB based.

Seeing 200kA fault potential is really scary! The local 500kVA substation (415V) we will get power from has a PFC of just under 12kA, potentially a few kA more as there are some big motor in use on the site as well that would generate during a short-style fault, but it is all well within the 50kA rating of our 100A fused-switch feeding our stuff.


Breakers are very convenient operationally (though that video also shows that if big faults happen even within rating you may well be looking to replace it soon) and they have the advantage of opening all phases simultaneously. Also on lower fault currents, say up to a kA or so for MCBs, they have lower I2t due to the speed of the magnetic trip in that region.

But fuses have much better fault limiting as we have just seen, and they are easier to get selectivity as well. In many ways the main down-side is the need for phase-fault protection to avoid some motor limping along if one of the 3 phases goes, though of course your incoming supply might do that worst-case...

As you say it is a shame that many electrical engineers dismiss fuses as old-fashioned or inconvenient as in many cases, especially as the feed protection to a set of MCBs or similar, they are a very good choice.

Well I think you and I should be the founding members of the fuse appreciation society (FAS)!



For normal distribution the whole system tends to be broken into separate chunks so although most (inner city) MV networks tend to be in a ring main style or meshed they are actually run with normally open points forming smaller islands, this is the same at LV – take the example network attached although very meshed, you can see it’s actually small islands even the two pillars in Crosby Sub are NO between them, as is the link to Lonsdale sub along Lowther st near Warwick rd, and so on.

This keeps the fault level low, limits the scope/impact of faults and allows installations with lower rated protection equipment – eg 16kA through a 100A fuse.

However on the big industrial sites (and secure installations) where the subs are privately owned there can be high fault levels – the main issue is that to produce 100kA+ a number of transformers need to be in parallel and in the event of a fault the scope of outage could be huge.

2.5MVA is around 75kA fault level – outside the rating of MCBs but within MCCBs – if you put them in parallel then you need to use ACBs which go up to around 150kA.

So for example I would usually use ACBs on all the main switchboard – as even though it would be run split, you may parallel it during transition (or provide M&E interlocking to prevent it)

MCCBs and ACBs have two fault ratings – the usual one, say 100kA which can be safely and repeatedly interrupted by the MCCB/ACB – and another say 140kA – which can be safely interrupted – ONCE!

The big issue unfortunately with comparing kit is sales literature tends to be misleading, a typical MCCB or ACB may have a repeat breaking capacity of say 100kA and a one off as 145kA – however it can also make and break 220kA – sort of. The 100kA is the symmetrical capacity – the sort of value you get “long term” – steady state, in truth the current actually peaks much higher than this and decays down to a lower figure – as per the curve below.

In practice therefore when the breaker is interrupting the fault – it is actually breaking something like the 220kA - for a 100kA fault!

Of course sales people tend to like the higher figure although in truth they are not selling a 220kA breaker (i.e. 220kA/440kA) they are selling a 100kA breaker!

Going back to the size of cable thing, on these sites the cable sizing is certainly due to fault level and not running current – in fact I have had to fit 2000A breakers outgoing from a board – feeding directly into 630A breakers because the 630A can’t interrupt the fault level!

I also think we have had a previous thread on here – an outgoing cable damaged due to the fault – I think the cable was sized on load rather than fault level – but didn’t get a straight answer.

Personally, I would prefer fuses – they are available up to 6kA with fault levels of up to 200kA – much higher capacity than ACBs

However, every job I have done of this nature is the past 25 years or so has used MCCBs or ACBs, and in my opinion only one project (air traffic control centre in Southampton) has benefited from them!

And that was only because when the system was fed by the grid the protection settings needed to cater for high fault levels, however in the event of grid loss it was supplied via UPSs – fault levels much less than 2x FLC! – to actually get it to all coordinate we had to use MCCB with multiple settings – when they went to UPS the settings on the associated circuits switched to another set of settings!

 

[Cookie]

Fascinating! The thing is our networks are not run normally open. The secondary conductors are all connected together forming a giant grid or mesh. Transformers feed into this mesh at regular internals. See figure 1 as an example:

http://engineering.nyu.edu/power/sites/engineering.nyu.edu.power/files/uploads/Three–Phase Time–Domain Simulation of Very Large Distribution Networks.pdf

Simplified view:

https://imgur.com/oa8gcQF

View: https://Upload the image directly to the thread.com/oa8gcQF

https://imgur.com/a/nDfze2U

View: https://Upload the image directly to the thread.com/a/nDfze2U


Between 8 and 34 13.8kv, 27.6kv or 33kv circuits leave an area substation with each circuit supplying several dozen transformers each rated around 500kva. The secondary of each transformer feeds into a normally closed LV cable grid. High power customers like factories, malls, office buildings, ect will tap directly off the MV cables and feed their own "spot networks" typically consisting of 5-8 1,500kva - 2,500kva transformers with the secondaries electrically in parallel.

Here is is reference from NYC's power company (ignore the red arrows, they were from another discussion)

Reverse power for a failed primary cable is prevented via a "network protector"

Network Protector Basics: Applications, Operation, and Testing

This guide covers network protector basic operation and maintenance procedures. Photo: TestGuy. Network systems are commonly used in hospitals, high rise office buildings, and institutional facilities where a high degree of service reliability is required. In the network system arrangement...

testguy.net

Typically the design is such that you can loose two MV cables during the summer without overloading any equipment.

The result is exceptionally high service continuity. The down side being secondary LV short circuits typically make the 5'oclock news, relying on a concept of "burning clear"

https://imgur.com/oj398rR

View: https://Upload the image directly to the thread.com/oj398rR


Here are some examples of (similar) stuff I've grown up around:

View: https://youtu.be/pFPphf4qSOM?t=124

View: https://youtu.be/tJ1yEndX6ng?t=326


Multiple manhole fires, explosion in Midtown Manhattan - https://www.youtube.com/watch?v=ZU8TAJPeEAg

Manhole Fire In Queens Leaves Hundreds Without Power - https://www.youtube.com/watch?v=n7fomFjJs2o

If you search "manhole fire" and "manhole explosion" there is more where that came from.

Many in the EU are shocked at the lack of fusing in the US and to be honest so am I lol.

 

[Mark Wright]

Nice man hole you also need a good rod and ring.

sorry wrong forum
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There have been quite a few man hole explosions in the U.K

uk man hole explotion - Google Search

poor maintenance and aging electrical network.

but seriously while were roughly on the subject another maybe stupid question from me - how can the minimum recommend size of Earthing conductor for a TT be 2.5mm ? this is not the main earthing conductor? this is just an out building or something?

 

[Cookie]

Nice man hole you also need a good rod and ring.

sorry wrong forum
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There have been quite a few man hole explosions in the U.K

uk man hole explotion - Google Search

poor maintenance and aging electrical network.

but seriously while were roughly on the subject another maybe stupid question from me - how can the minimum recommend size of Earthing conductor for a TT be 2.5mm ? this is not the main earthing conductor? this is just an out building or something?

Right, but don't you fuse your conductors such that a fault in a man hole blows a fuse in under 1 second? I've read about "Lucy" pedestals and racks... the MV-LV transformer secondary is fused with any tap to a customer fused as well.

 

[freddo]

Agree. Perhaps you've seen this video, its gold here in the US:

I love that video, but I will still call BS on that third contactor 'till the day I die. I've watched it a number of times and the contactor certainly doesn't appear to move at all.

 

[Cookie]

I love that video, but I will still call BS on that third contactor 'till the day I die. I've watched it a number of times and the contactor certainly doesn't appear to move at all.

I'll have to watch it again. I love your avi btw!

 

[pc1966]

but seriously while were roughly on the subject another maybe stupid question from me - how can the minimum recommend size of Earthing conductor for a TT be 2.5mm ?

Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.

 

[davesparks]

how can the minimum recommend size of Earthing conductor for a TT be 2.5mm ? this is not the main earthing conductor? this is just an out building or something?

Where is this recommendation?

Unless I'm mistaken an earthing conductor is subject to a minimum size of 6mm. And if it is the conductor which connects to the earth rod it is also subject to a minimum size requirement if any part of it is buried.

 

[pc1966]

Where is this recommendation?

Table 4.4(iii) of the OSG has that for "Protected against corrosion and mechanical damage" cases.

I am not sure I would call it a recommendation though, more of an absolute minimum!

 

[Mark Wright]

Yeah I might be totally off but OSG 4.4 see table 4.4(iii) for TT "Protected against corrosion and mech damage" 2.5 - also iirc 54.1? BS7671.
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Table 4.4(iii) of the OSG has that for "Protected against corrosion and mechanical damage" cases.

I am not sure I would call it a recommendation though, more of an absolute minimum!

Yeah Can't really see this being adequate but maybe for a car charger or something TT it might be ok.

 

[davesparks]

Yeah I might be totally off but OSG 4.4 see table 4.4(iii) for TT "Protected against corrosion and mech damage" 2.5 - also iirc 54.1? BS7671.

That is not a recommendation, that is the minimum permitted.
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Table 4.4(iii) of the OSG has that for "Protected against corrosion and mechanical damage" cases.

I am not sure I would call it a recommendation though, more of an absolute minimum!

Which is my point exactly, it is not a recommendation, merely the minimum permitted.

 

[Cookie]

Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.

So what if metal water piping or CATV gets added latter lowering the fault loop path?

 

[Julie.]

So what if metal water piping or CATV gets added latter lowering the fault loop path?

That would be great, now the majority of the fault current would pass via the pipe rather than the 2.5mm^2 cable to the earth rod.

 

[davesparks]

So what if metal water piping or CATV gets added latter lowering the fault loop path?

The chances are pretty slim when all new underground pipes to a house will be plastic these days.

What is CATV?

 

[pc1966]

What is CATV?

Cable TV

But I can't imagine the coax cable has much current carrying capacity! Long term that will go fibre so isolated eventually, but existing infrastructure can be expected to be there for decades.