Twin and Earth CPC

[Cookie]

IEC 60364-5-54 states that phase conductors 16mm2 and smaller require a CPC of the same size, over 16 to 35mm2 a 16mm2 CPC and over 35mm2 a half size CPC is required.

My questions is why am I seeing catalogs with harmonized twin and earth with reduced size CPCs over 2.5mm2? And why doesn't the half size rule kick in when wire is over 4mm2 instead of 16mm2?

 

[davesparks]

Continuity but it would be a bit pointless proving it's of adequate size when we would already know it was.

Proving it is of adequate size is done by calculation before you start installing.

Zs proves continuity of the complete return path back to source using a higher current than is normally possible with a dead test.

 

[Mark Wright]

All I'm saying is if the CPC was the same size your concern that ADS could take place would be as much as your concern that a short Line to Neutral would trip the MCB - basically we don't even think about it because we know the Line and Neutral are of the same size so it doesn't even cross our minds we take it for granted.

 

[Ian1981]

All I'm saying is if the CPC was the same size your concern that ADS could take place would be as much as your concern that a short Line to Neutral would trip the MCB - basically we don't even think about it because we know the Line and Neutral are of the same size so it doesn't even cross our minds we take it for granted.

Unlike Line to neutral faults, line to earth faults increase the risk of dangerous mains voltage appearing on exposed conductive parts, which is why we are required to test that disconnection times are met and that earthing and bonding is correctly maintained through out the installation.
The regulations talk about that if a circuit downstream of an rcd is installed, then as long as continuity of the protective conductor is confirmed by adequate testing and that the rcd has been confirmed to provide ADS, then yes measuring earth fault loop impedance is not necessarily required on rcd protected circuits, regulation 643.7.1

 

[Mark Wright]

To me that's just all the more reason to use a CPC at the same size but anyway each to there own etc.

 

[Ian1981]

To me that's just all the more reason to use a CPC at the same size but anyway each to there own etc.

Which is fine if using any other cable than twin and cpc cable.
No ones going to wire domestic installations in conduit and singles tho are they?

 

[Risteard]

No ones going to wire domestic installations in conduit and singles tho are they?

I'd love to, but competing on price would definitely be an issue!

 

[Mark Wright]

I don't want to get bogged down but Ireland are now using T&E with full size CPC aren't they?

I mean I'm not bothered but from a safety point of view a full size CPC obviously makes sense that much I think we can all agree.

 

[Risteard]

I don't want to get bogged down but Ireland are now using T&E with full size CPC aren't they?

I mean I'm not bothered but from a safety point of view a full size CPC obviously makes sense that much I think we can all agree.

In the south, yes.

 

[davesparks]

I mean I'm not bothered but from a safety point of view a full size CPC obviously makes sense that much I think we can all agree.

From a safety point of view it doesn't make a blind bit of difference as long as the CPC is at least equal to the minimum size required by calculation.

And you still need to carry out a Zs test whatever size you use.

 

[Mark Wright]

Or course it makes a difference fault current should be carried away as quickly as possible.

and countrys that employee full CPC and adopted MCBs and RCDs earlier don't cry about sockets in bathrooms and carrying out Periodic Zs tests.

Skimping on CPC however you want to justify it, is cost over safety a practice that many countrys have left in the past where it bellongs.

 

[Cookie]

Yes, but if you are wiring standard circuits using T&E then you will meet the requirements given by the adiabatic method.

Your question was along the lines of 'why is this cable produced' (with smaller CPC) - well because it is perfectly acceptable to install if you size it correctly.

So I guess as long as you meet the 0.4 second disconnection times T&E will always fulfill the adiabatic method?

Question- how does the adiabatic method take into account that R goes up while the short circuit is occurring?

 

[Julie.]

So I guess as long as you meet the 0.4 second disconnection times T&E will always fulfill the adiabatic method?

Nope, depending upon the installation this may not be the case - especially as it could be up to 5 seconds disconnection time on some circuits; but sizing cable is part of the standard work on ensuring a safe installation, current carrying capacity, voltage drop, disconnection time, and checking against damage in the event of a fault is all part of the work involved in choosing the correct size and type of kit.

For a typical domestic installation however, following the guidelines etc. for 'standard circuits' then yes, there is no need to calculate as the 'standard circuit' is well proven/designed to comply with the usual fault levels in these cases.

Question- how does the adiabatic method take into account that R goes up while the short circuit is occurring?

In effect this is taken into account via the factor k which takes the resistivity, start and final temperature, heat capacity and temperature coefficient in the adiabatic equation.

 

[Cookie]

Nope, depending upon the installation this may not be the case - especially as it could be up to 5 seconds disconnection time on some circuits; but sizing cable is part of the standard work on ensuring a safe installation, current carrying capacity, voltage drop, disconnection time, and checking against damage in the event of a fault is all part of the work involved in choosing the correct size and type of kit.

For a typical domestic installation however, following the guidelines etc. for 'standard circuits' then yes, there is no need to calculate as the 'standard circuit' is well proven/designed to comply with the usual fault levels in these cases.

I'm confused. How would you know a reduced size T&E CPC is sufficient if you do not calculate using the adiabatic equation?

In effect this is taken into account via the factor k which takes the resistivity, start and final temperature, heat capacity and temperature coefficient in the adiabatic equation.

Resistivity as it changes from start to finish, correct? Do they use the final resistivity or some averaged means in the math when deriving k?

 

[Julie.]

I'm confused. How would you know a reduced size T&E CPC is sufficient if you do not calculate using the adiabatic equation?

You don't need to if it's a standard circuit because the circuit has been well designed and proven to be suitable already!

That's the concept of using proven designs

Let's say your car has tyres of 255/55R18 as standard - the manufacturer approves this size of tyre.

If you now fit new 255/55R18 tyres (same speed rating etc etc) - do you recalculate what the speedo reads? Do you recalculate what the rotational speed of the rubber is to check that the tyre manufacturer has used suitable compounds etc?

Of course not - you have followed the approved sizes/ratings/type - so you do not.

If however you decide to choose 285/30/SR18 - will this work?

Well in this case you have departed from the approved design, so you would need to check and calculate everything.

Do they use the final resistivity or some averaged means in the math when deriving k?

It will be a varied value of resistivity (probably based on the Bloch–Grüneisen formula but of course there are many others) - integrated over the rise in temperature which is dependant on the resistivity etc. , however the value of k takes into account everything in itself you as a person applying the formula do not derive any further factors.

 

[Cookie]

You don't need to if it's a standard circuit because the circuit has been well designed and proven to be suitable already!

That's the concept of using proven designs

Let's say your car has tyres of 255/55R18 as standard - the manufacturer approves this size of tyre.

If you now fit new 255/55R18 tyres (same speed rating etc etc) - do you recalculate what the speedo reads? Do you recalculate what the rotational speed of the rubber is to check that the tyre manufacturer has used suitable compounds etc?

Of course not - you have followed the approved sizes/ratings/type - so you do not.

If however you decide to choose 285/30/SR18 - will this work?

Well in this case you have departed from the approved design, so you would need to check and calculate everything.




It will be a varied value of resistivity (probably based on the Bloch–Grüneisen formula but of course there are many others) - integrated over the rise in temperature which is dependant on the resistivity etc. , however the value of k takes into account everything in itself you as a person applying the formula do not derive any further factors.

Alright- but I still can't figure out why there is no exception to this table:

https://imgur.com/Gd51nvQ

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The tables I'm used to typically have an exception or there are over riding rules elsewhere in the code.

The way I'm reading it is that only two options exist- either calculate or use the table- similar to either using Tables in 310.15 or Neher-McGrath.

I'm imagining something like "exception: manufactured multi core none metallic sheathed cables shall be permitted to have a smaller cross sectional CPC than the largest current carrying conductor(s) as permitted in table --- or a factor not less than 25%"

 

[Risteard]

Alright- but I still can't figure out why there is no exception to this table:

There is. It's in 543.1.3 - although it isn't actually an exception. The Table is a means of verifying that a size is adequate without calculating. Calculation is not precluded and particularly in larger installations will be essential to ensure that cpcs aren't ridiculously oversized and uneconomic. It should be noted that it's not actually adiabatic for longer disconnection times and therefore even the calculation will give pessimistically large cross-sectional areas.

 

[Julie.]

Alright- but I still can't figure out why there is no exception to this table:

View: https://Upload the image directly to the thread.com/Gd51nvQ


The tables I'm used to typically have an exception or there are over riding rules elsewhere in the code.

First thing - you are using an outdated version of IEC60364-5-54 the latest is 2011 the one you have is 2002

Secondly, it is very clear it can be calculated; in later versions this is noted under 543.1.4 - 'where it is desired not to calculate using 543.1.2 (The adiabatic equation) then the cross sectional area may be determined in accordance with table 54.7'

(the reference is 543.1.3 in BS BTW)

In your older version the table is reference 54.3, so I assume the wording is slightly different perhaps 543.1.2 is another reference, or it is stated elsewhere - perhaps after the table.

The title of the table in the 2011 version is: Minimum cross-sectional area of protective conductors (where not calculated in accordance with 543.1.2)

If you have the standard, could you post the pages either side of this table?

 

[pc1966]

Or course it makes a difference fault current should be carried away as quickly as possible.

Most circuits are protected by MCB, and as long as you hit the magnetic trip point (which is the point of checking Zs at the desing stage) then it makes no significant difference to the disconnection time.

and countrys that employee full CPC and adopted MCBs and RCDs earlier don't cry about sockets in bathrooms and carrying out Periodic Zs tests.

Skimping on CPC however you want to justify it, is cost over safety a practice that many countrys have left in the past where it bellongs.

Considering the UK as often led the world on electrical safety that seems a bizarre statement to make.

Adopting MCBs and RCDs in the 80s and 90s was as much about cost and refresh cycles of installation than by standards, or would it be better if now nobody could do any electrical work unless they bought a new CU filled with AFDD? They are safer, so is the cost-benefit trade off of many homes not being repaired or upgraded better than allowing such technology to be adopted as it becomes affordable?

Many 3rd world countries (and the USA <cough>)do not test fully, or have such high standards of design, so are they better than the UK approach?

 

[Cookie]

First thing - you are using an outdated version of IEC60364-5-54 the latest is 2011 the one you have is 2002

Secondly, it is very clear it can be calculated; in later versions this is noted under 543.1.4 - 'where it is desired not to calculate using 543.1.2 (The adiabatic equation) then the cross sectional area may be determined in accordance with table 54.7'

(the reference is 543.1.3 in BS BTW)

In your older version the table is reference 54.3, so I assume the wording is slightly different perhaps 543.1.2 is another reference, or it is stated elsewhere - perhaps after the table.

The title of the table in the 2011 version is: Minimum cross-sectional area of protective conductors (where not calculated in accordance with 543.1.2)

If you have the standard, could you post the pages either side of this table?

Perhaps that is the issue... Here are the requested pages:

https://imgur.com/9lRlWAY

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https://imgur.com/a/tMdmTSg

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The numbers "skip" due to the copy being in both English and French.

I do not have any updated versions of my IEC documents. They get ridiculously expensive when ordering hundreds of standards. Wish they were free like NFPA-70.
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Considering the UK as often led the world on electrical safety that seems a bizarre statement to make.

My opinion? The UK is the first in the world. Although with things like PME I'd say you guys are going backward, not forward.

Adopting MCBs and RCDs in the 80s and 90s was as much about cost and refresh cycles of installation than by standards, or would it be better if now nobody could do any electrical work unless they bought a new CU filled with AFDD? They are safer, so is the cost-benefit trade off of many homes not being repaired or upgraded better than allowing such technology to be adopted as it becomes affordable?

AFDDs aren't safer. They came about for the exact reason the US does not test or have any restrictions on R1+R2.

Many 3rd world countries (and the USA <cough>)do not test fully, or have such high standards of design, so are they better than the UK approach?

I'd say its highly lopsided, major superiority mixed in with major deficits. The NEC is highly conservative typically resulting in services, feeders and branch circuits which are loaded to less than half of their already conservative rating. NEMA and UL equipment tends to be more robust than IEC equipment. Lug burn ups are less common in the US. Protection of wire is much more strict.

On the other hand earth wires are about 1/10th the size of current carrying wires, no testing, no loop impedance, no disconnection time requirements, no real de-rating for insulation, limited RCD use, no sleaving, no finger isolation in open equipment, less arc flash mitigation...

The thing is the NEC is more ligation driven. Under sized earth wires aren't making themselves well known due to steel framed buildings dominating universally wired in metal conduit which in of itself is recognized as an "effective ground fault current path". But with more large scale wooden buildings (like apartments and nursing homes) and I can see it making itself known... as such code will either change Table 250.122 or require GFP/GFCIs on all feeders and branch circuits.

I am not going to lie- article 250 of the NEC for years has been a hurriance of debate, revision, and misguided electrical theory.

 

[Mark Wright]

Germany and other European country's were well ahead of us in adopting MCB and RCDs

We are now playing catch up, Europe is now writing the standards.

 

[Julie.]

Perhaps that is the issue... Here are the requested pages:

It is explained in 543.1.1, perform the calculation in 543.1.2 OR use the table 54.3

 

[Cookie]

Germany and other European country's were well ahead of us in adopting MCB and RCDs

We are now playing catch up, Europe is now writing the standards.

Yes, but I'd argue thats because over their hodge podge of plugs which do not always mate with an earth.

 

[pc1966]

Yes, but I'd argue thats because over their hodge podge of plugs which do not always mate with an earth.

It is not just the "will it or won't it" function of the earth pin or side strip, etc, but generally they have non-polarised plug so you can swap N & L, and also many EU supplies are TT so needed the RCD incomer at least for any chance for acceptable fault clearance.

But you are also right that the UK (and some other countries) biggest blind spot is using TN-C-S and the resulting risk from a PME fault. As mentioned before by @davesparks what they should have done is insist on every new property on PME having its own earth rod(s) as well. Sure one rod is not going to help much with a PME fault, but having many, many more rods would be more fault tolerant than a handful of supply rods.

 

[Ian1981]

They’ve used PME to save money on cable but ultimately it will backfire as the network breaks down and the money they supposedly saved , is spent on repairs, or shove the onus onto the consumer and make them come up with a solution to negate an open PEN fault.

 

[pc1966]

They’ve used PME to save money on cable but ultimately it will backfire as the network breaks down and the money they supposedly saved , is spent on repairs

I don't think TN-C-S is any more likely to break down than TN-S. I it just the consequences that are more serious!

 

[Ian1981]

I don't think TN-C-S is any more likely to break down than TN-S. I it just the consequences that are more serious!

If they stuck to TNS then the open PEN consequences never exist.
Protective bonding conductors could be smaller etc, EV chargers won’t require expensive devices to protect against open PEN faults , no earth electrodes to install and would be safer.
I’m not an engineer so I maybe looking at this one sided but that’s what I think anyway.

 

[Cookie]

It is explained in 543.1.1, perform the calculation in 543.1.2 OR use the table 54.3

The way I interpret it you must perform a calculation

It is not just the "will it or won't it" function of the earth pin or side strip, etc, but generally they have non-polarised plug so you can swap N & L, and also many EU supplies are TT so needed the RCD incomer at least for any chance for acceptable fault clearance.

But you are also right that the UK (and some other countries) biggest blind spot is using TN-C-S and the resulting risk from a PME fault. As mentioned before by @davesparks what they should have done is insist on every new property on PME having its own earth rod(s) as well. Sure one rod is not going to help much with a PME fault, but having many, many more rods would be more fault tolerant than a handful of supply rods.

I agree, but I don't think polarity makes much of a difference. My understanding is the EU sockets are designed such reverse polarity will not shock you.

Regarding earth rods- do you really want the earth becoming an even larger conductor? We are using the planet as an experiment, one with no "reset" or "stop" button if something goes wrong.

In the US stray voltage cases have resulted in a steady stream of law suits against utilities.

 

[Julie.]

The way I interpret it you must perform a calculation

No it's very clear you must EITHER perform the calculation in 543.1.2 OR select from the table 54.3

it is not ambigus

 

[Lucien Nunes]

From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S

From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.

But returning to the subject of CPC size, has anyone here done any practical experiments to satisfy themselves of the validity of the adiabatic limit? I have, years ago, using a very large battery bank, and the results were as expected and unremarkable.

 

[Cookie]

No it's very clear you must EITHER perform the calculation in 543.1.2 OR select from the table 54.3

it is not ambigus

Then how could a reduced size earth in T&E be compliant unless the installer calculates it?

 

[Cookie]

From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S

You. I like you

Audio noise is a big problem in the US in that not only is TN-C-S the only major earthing means, but also the fact older buildings are riddle with standing neutral to ground faults on top of 240 volt appliances which earth through the neutral. Lack of RCD means nothing will detect these faults or wiring errors.

The NEC allows for isolated grounding sockets which allow the user to run an insulated CPC all the way back to the service or origin of power supply. It works well until the metal chassis losses its isolation which is inevitable in something like a studio.

The other issue that few recognize are high magnetic fields which I personally do not believe people should be exposed to when they can entirely be mitigated through RCDs and TT/TN-S earthing.

Personally if I had a choice I would use a 138/240Y system. Connect everything phase to phase and have the neutral point just for earthing purposes. Any fault will trip a breaker. Half the difficulty would disappear. Lots of debates spared.

https://imgur.com/CfBnL39

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From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.

That is until a fire starts:

Open Neutral

But returning to the subject of CPC size, has anyone here done any practical experiments to satisfy themselves of the validity of the adiabatic limit? I have, years ago, using a very large battery bank, and the results were as expected and unremarkable.

The NEC's Table 250.122 is living proof the adiabatic method might actually be ultra conservative.

 

[Julie.]

Then how could a reduced size earth in T&E be compliant unless the installer calculates it?

Because it is a standard circuit and it has already been calculated - the whole point of standard circuits is that all the factors have been calculated!

You could calculate it again, but why?

If I have a circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Knowing this - I now calculate it again - Why???

OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)

This works out as SQRT(1980x1980x0.003) / 115 = 0.94mm^2

Isn't that odd, you use the figures provided by the IET as acceptable - and when you calculate it - it actually works out!

Now I go to a different site, and need another circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Do I calculate it again?

 

[Cookie]

Because it is a standard circuit and it has already been calculated - the whole point of standard circuits is that all the factors have been calculated!

You could calculate it again, but why?

If I have a circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Knowing this - I now calculate it again - Why???

OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)

This works out as SQRT(1980x1980x0.003) / 115 = 0.94mm^2

Isn't that odd, you use the figures provided by the IET as acceptable - and when you calculate it - it actually works out!

Now I go to a different site, and need another circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Do I calculate it again?

Ill take your word for it. That if it means if means loop impedance requirements are met than the CPC will always be greater than what the adiabatic method would calculate out to be when dealing with T&E.

 

[Cookie]

In reality few electricians have to use the adiabatic equation (though they should know how to) as the IET's On Site Guide has some useful tables that incorporate the information for circuit design. For example this table is for BS88 fuses and shows the maximum Zs values for different CPC sizes:
View attachment 58753
For example, if you have 10mm T&E cable with a 4mm CPC used as a sub-main feed so you could allow 5s disconnection, you might have a 63A fuse for short circuit protection only, and then the downstream DB can use a mix of MCBs up to 32A in order to provide overload protection with a reasonable chance of selectivity. Looking at the above table you see you max measured Zs is 0.49 ohms, so your final test at the nice new DB would be to confirm this is met.

Also you see the value is 0.62 ohms in all the larger CPC sizes - they are time-limited for the fuse action, where as at 4mm it is adiabatically limited (hence lower Zs for a shorter fault disconnection time).

This helps- thank you!

OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)

On a side note. Instantaneous in current liming like a fuse? As I understand it a fuse begins to melt as soon as it gets hot, while a solenoid must saturate, pull in, and then wait to unlatch with an arc which takes time to extinguish.

I know that RK low peak fuses tend to reduce arc flash to a big degree relative to instantaneous tripping of molded case circuit breakers and power circuit breakers .

 

[Ian1981]

Here are some printed tables from Hager and MK which may help with regards to the requirements of the cpc at different fault levels using class 3 current limiting mcb’s.

 

[Julie.]

On a side note. Instantaneous in current liming like a fuse? As I understand it a fuse begins to melt as soon as it gets hot, while a solenoid must saturate, pull in, and then wait to unlatch with an arc which takes time to extinguish.

I know that RK low peak fuses tend to reduce arc flash to a big degree relative to instantaneous tripping of molded case circuit breakers and power circuit breakers .

Yes, although the I^2t let through is very much more than a fuse will give; for the very reasons you state

I do not like MCB - fuses are so much better for protecting circuits, coordinating with each other and providing an overall better system.

Unfortunately fuses are inconvieniant - so MCB end up being king!
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Ill take your word for it. That if it means if means loop impedance requirements are met than the CPC will always be greater than what the adiabatic method would calculate out to be when dealing with T&E.

Slightly more to it than that, firstly you have to ensure the protection operates within the time limits (which is via selection of correct MCB and the Zs {loop impedance} requirements)

Then, secondly that the CPC is larger than the minimum size given the MCB type, class, and fault level.

So if I have a fault level less than 3kA, a 20A class 3 B type MCB then:

1) The Zs must be less than 1.1ohm (at the time of install - i.e. when cold)
2) The CPC must be bigger than 1.5mm^2

1) can be found from table B6 in the OSG
2) can be found from table B7 in the OSG

In which case standard 2.5mm^2 T&E can be used

Change the fault level to between 3kA and 6kA then again from B6 & B7:

1) The Zs must be less than 1.1ohm (at the time of install - i.e. when cold)
2) The CPC must be bigger than 2.5mm^2

- In this case you can't used standard T&E , and is often the case for industrial sites where the CPC has to be the same size as the live conductors

(of course you would also check for volt drop, cable rating due to installation method etc.)

Above 6kA fault level then you need the manufacturers data and have to calculate manually (although there are other tables available - just not in the standard stuff)
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Yes, although the I^2t let through is very much more than a fuse will give; for the very reasons you state

I do not like MCB - fuses are so much better for protecting circuits, coordinating with each other and providing an overall better system.

Unfortunately fuses are inconvieniant - so MCB end up being king!
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Slightly more to it than that, firstly you have to ensure the protection operates within the time limits (which is via selection of correct MCB and the Zs {loop impedance} requirements)

Then, secondly that the CPC is larger than the minimum size given the MCB type, class, and fault level.

So if I have a fault level less than 3kA, a 20A class 3 B type MCB then:

1) The Zs must be less than 1.75ohm (at the time of install - i.e. when cold)
2) The CPC must be bigger than 1.5mm^2

1) can be found from table B6 in the OSG
2) can be found from table B7 in the OSG

In which case standard 2.5mm^2 T&E can be used

Change the fault level to between 3kA and 6kA then again from B6 & B7:

1) The Zs must be less than 1.75ohm (at the time of install - i.e. when cold)
2) The CPC must be bigger than 2.5mm^2

- In this case you can't used standard T&E , and is often the case for industrial sites where the CPC has to be the same size as the live conductors

(of course you would also check for volt drop, cable rating due to installation method etc.)

Above 6kA fault level then you need the manufacturers data and have to calculate manually (although there are other tables available - just not in the standard stuff)

So unfortunately it timed out and I can't edit it!

I changed the MCB to 20A so I could use the 2.5mm^2 T&E to illustrate the differance, but couldn't change the Zs to 1.75 from 1.1 ohm as it timed out!!

 

[Cookie]

In this case you can't used standard T&E , and is often the case for industrial sites where the CPC has to be the same size as the live conductors (of course you would also check for volt drop, cable rating due to installation method etc.) Above 6kA fault level then you need the manufacturers data and have to calculate manually (although there are other tables available - just not in the standard stuff)

And thats my point right there- you do indeed have to check. I'm not sure what the max PFC is in UK supplies but in large cities like Manhattan, Brooklyn, Queens and Bronx where underground secondary networks are used 22,000 amps or more at a residential service is not unheard of.

Of course the NEC is silent on this issue- just that 2.08mm2, 3.31mm2, 5.26mm2 circuits must have an equal size CPC.

 

[Julie.]

And thats my point right there- you do indeed have to check. I'm not sure what the max PFC is in UK supplies but in large cities like Manhattan, Brooklyn, Queens and Bronx where underground secondary networks are used 22,000 amps or more at a residential service is not unheard of.

Of course the NEC is silent on this issue- just that 2.08mm2, 3.31mm2, 5.26mm2 circuits must have an equal size CPC.

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)

- No it doesn't

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

-Yes you can

You then stated that you must calculate it.

- No you don't


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!

 

[pc1966]

I agree, but I don't think polarity makes much of a difference. My understanding is the EU sockets are designed such reverse polarity will not shock you.

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

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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From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S

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...

From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.

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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I do not like MCB - fuses are so much better for protecting circuits, coordinating with each other and providing an overall better system.

Unfortunately fuses are inconvenient - so MCB end up being king!

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.

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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And thats my point right there- you do indeed have to check. I'm not sure what the max PFC is in UK supplies but in large cities like Manhattan, Brooklyn, Queens and Bronx where underground secondary networks are used 22,000 amps or more at a residential service is not unheard of.

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...

 

[Ian1981]

As stated in appendix 14 for domestic or similar premises , it is not necessary to measure or calculate prospective fault current at the origin of the supply where a CU is installed to BS EN 61439-3 and the DNO declare a max PFC of 16Ka, So 6 Ka rated MCB’s are fine whatever.
We still measure tho through habit.