Twin and Earth CPC

[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

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.

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

View: https://Upload the image directly to the thread.com/9lRlWAY

https://imgur.com/a/tMdmTSg

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



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.