How did they calculate this ?

[Dylboy]

I always thought that was mostly for manufactures. But I then guess general purpose cable (GP) is considered flexible thus needing same Ecc as phase. So if that is the case then when we pull GP we need to use the length given in table 6.28 to then make sure we don't pull more cable than specified to no breach that table, then does that cable legth Ecc comply with table 8.1 for the maximum restance of the Ecc ?

So then how can we start to design a normal circuit such as the 2.5mm and 1.5mm flat twin cable to know how long we can run it as the Ecc is diffenrt size, what formula can be used or must we determine the length by using table 8.1 and the Recc = p x L / a ?

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[Dylboy]

Just thinking out loud...As for above with the cable stuff that is for a cable and not indicudule conductors such as GP so then can we run GP as 2.5mm phase and then 1.5mm Ecc ?

Edit: found the definition of a cable and i am still not sure if GP is a cable or a conductor as below speaks of " assembled together "

" 3.8
cable :
length of one or more insulated cores that has solid or stranded conductors
that are assembled together during manufacture and that may or may not have
an overall mechanical covering"

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[GCE]

Just thinking out loud...As for above with the cable stuff that is for a cable and not indicudule conductors such as GP so then can we run GP as 2.5mm phase and then 1.5mm Ecc ?

Edit: found the definition of a cable and i am still not sure if GP is a cable or a conductor as below speaks of " assembled together "

" 3.8
cable :
length of one or more insulated cores that has solid or stranded conductors
that are assembled together during manufacture and that may or may not have
an overall mechanical covering"

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When I refer to ECC I was talking Armour cable ECC - You are talking earth continuity conductor, general purpose house wire - There is a big difference - don't confuse the 2

When using house wire the reg 6.12.1.1 e is relevant

6.12.1.1 An earth continuity conductor shall
a) consist of compatible conductors,
b) if it forms part of a cable other than a flexible cable, comply with the relevant
requirements of the standard for the cable,
c) if it forms part of a flexible cable, be of the same material as, and have a
nominal cross-sectional area at least equal to, that of the largest phase
conductor,
d) be able to carry the prospective fault current without excessive temperature
rise of the conducor, within the disconnecting time
e) if it does not form part of a cable or flexible cable, have a nominal crosssectional
area at least equal to that determined in accordance with
table 6.28(a), as follows:

If you use that table you should be within the readings required by table 8.1

[Dave A]

Since they never ( hardly ever) run an earth wire from the sub to the premises ...

Just checking for clarity (because in Durban at least and as I understand, in the majority of metros in SA we are on TN-S supplies) - are you differentiating between cable armour being used instead of a dedicated earth conductor, or is it uncommon for you to have TN-S supplies in your part of the country?

[Dylboy]

When I refer to ECC I was talking Armour cable ECC - You are talking earth continuity conductor, general purpose house wire - There is a big difference - don't confuse the 2

When using house wire the reg 6.12.1.1 e is relevant

6.12.1.1 An earth continuity conductor shall
a) consist of compatible conductors,
b) if it forms part of a cable other than a flexible cable, comply with the relevant
requirements of the standard for the cable,
c) if it forms part of a flexible cable, be of the same material as, and have a
nominal cross-sectional area at least equal to, that of the largest phase
conductor,
d) be able to carry the prospective fault current without excessive temperature
rise of the conducor, within the disconnecting time
e) if it does not form part of a cable or flexible cable, have a nominal crosssectional
area at least equal to that determined in accordance with
table 6.28(a), as follows:

If you use that table you should be within the readings required by table 8.1

Now that is intresting and changes everything, thank you! I did not realise it was for the armouring as that is also not copper so then yes some different maths.

The bit in red I just saw as a supplementary earthing conductor of copper and didn't think of it being SWA.

Thanks GCE ! That does help a lot.

Edit: I read the heading of that table and it states Copper earth continuity conductor so then the SWA must be copper or it is then supplementary earthing conductor...

Either way very chuffed this conversation is happening as I have never been taught this on any course or even when doing my exams back in the day it never cropped up, only now when I start to dig deeper do I see all sorts of things and have more questions.



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[Lourens.dL]

3.9.2 flexible cable
cable of which the conductors consist of strands of diameter not exceeding 0,51 mm and of which the insulation and covering are such that they afford flexibility and in which the nominal cross-sectional area of each conductor exceeds 4 mm²

How do you understand the rule above?

How does this apply to Twin&Earth or GP wire?

[GCE]

Just checking for clarity (because in Durban at least and as I understand, in the majority of metros in SA we are on TN-S supplies) - are you differentiating between cable armour being used instead of a dedicated earth conductor, or is it uncommon for you to have TN-S supplies in your part of the country?

We are TN-C- S

Generally municipalities only run the last bit , +/- 70m of copper earth before the sub with earth spikes and on bigger supply's the same story before the premises

It will not be solid copper or even armour cable earth all the way

Some of the lead cables they do - It is hit and miss

[GCE]

3.9.2 flexible cable
cable of which the conductors consist of strands of diameter not exceeding 0,51 mm and of which the insulation and covering are such that they afford flexibility and in which the nominal cross-sectional area of each conductor exceeds 4 mm²

How do you understand the rule above?

How does this apply to Twin&Earth or GP wire?

I read it as the normal cabtyre being flexible cable or trailing cable

[GCE]

Now that is intresting and changes everything, thank you! I did not realise it was for the armouring as that is also not copper so then yes some different maths.

The bit in red I just saw as a supplementary earthing conductor of copper and didn't think of it being SWA.

on ECC armour cable there are stands of armouring that are replaced with tinned copper strands to accommodate the correct earth readings - It is then manufactured according to a standard and the amp rating of the copper phases in spec to accommodate the potential earth fault on the copper strands taking into account the surface area to dissipate the potential build up of heat on a fault

https://www.actomep.co.za/pvc-mains-ecc-copper-cable-50mm

[Lourens.dL]

I read it as the normal cabtyre being flexible cable or trailing cable

Note that flexible cable must exceed 4mm².

Would cabtire not fall under flexible cord?
3.9.3 flexible cord
cable of which
a) the nominal cross-sectional area of each conductor does not exceed 4 mm², and
b) each conductor consists of strands of diameter less than 0,31 mm


The point I'm trying to make is per definition neither GP wire or Twin&Earth or Airdac or Surfex etc can be classified as a flexible cable or flexible cord.

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[Dylboy]

on ECC armour cable there are stands of armouring that are replaced with tinned copper strands to accommodate the correct earth readings - It is then manufactured according to a standard and the amp rating of the copper phases in spec to accommodate the potential earth fault on the copper strands taking into account the surface area to dissipate the potential build up of heat on a fault

https://www.actomep.co.za/pvc-mains-...per-cable-50mm

Yes I can see that table working for the those but the everyday SWA we use is all metal so then we would have to work with the manufactures specs.

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[GCE]

Note that flexible cable must exceed 4mm².

Would cabtire not fall under flexible cord?
3.9.3 flexible cord
cable of which
a) the nominal cross-sectional area of each conductor does not exceed 4 mm², and
b) each conductor consists of strands of diameter less than 0,31 mm


The point I'm trying to make is per definition neither GP wire or Twin&Earth or Airdac or Surfex etc can be classified as a flexible cable or flexible cord.

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Apologies - Yes cabtyre would be flexible cord.

T&E , Airdac , surfix etc would be classified as cable and to strengthen the point I have pasted the headings that I see as surfix and T&E wherein they refer to cables - Then the earth would beclassified under 6.12.1.1b

6.3.6 PVC insulated multicore cables with a bare earthing conductor
and round cable with metal stiffening
6.3.6.1 The cables may be installed
a) on the surface,
b) under plaster,
c) under a raised floor,

6.12.1.1 An earth continuity conductor shall
a) consist of compatible conductors,
b) if it forms part of a cable other than a flexible cable, comply with the relevant
requirements of the standard for the cable,
c) if it forms part of a flexible cable, be of the same material as, and have a
nominal cross-sectional area at least equal to, that of the largest phase
conductor,
d) be able to carry the prospective fault current without excessive temperature
rise of the conducor, within the disconnecting time
e) if it does not form part of a cable or flexible cable, have a nominal crosssectional
area at least equal to that determined in accordance with
table 6.28(a), as follows:

[GCE]

Yes I can see that table working for the those but the everyday SWA we use is all metal so then we would have to work with the manufactures specs.

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I see normal armour cable as outside of 6.12.1.1 and I have highlighted in red the way I would read it - Normal armour cable does not have an earth continuity conductor and you need to look at the note under 6.12.1.4.

6.12.1 Earth continuity conductors
6.12.1.1 An earth continuity conductor shall
a) consist of compatible conductors,
b) if it forms part of a cable other than a flexible cable, comply with the relevant
requirements of the standard for the cable,
c) if it forms part of a flexible cable, be of the same material as, and have a
nominal cross-sectional area at least equal to, that of the largest phase
conductor,
d) be able to carry the prospective fault current without excessive temperature
rise of the conducor, within the disconnecting time
e) if it does not form part of a cable or flexible cable, have a nominal crosssectional
area at least equal to that determined in accordance with
table 6.28(a), as follows:

NOTE The information in 6.12.1.4 also applies to wire armouring if it is used as the
earth continuity path provided it meets the comparable resistance values of the
equivalent earthing continuity conductor and the armour wires are all correctly
terminated using an earthed metal captive coned gland; it may be necessary to replace
some of the steel wires with tinned copper ones or to use a supplementary earth
continuity conductor

[GCE]

Happy Friday all!

Anyone know the formula used for the attached screenshot ? (The table )

The numbers in Ed 2 are different to Ed 3.0

Also I was taught to do the touch voltage formula to get the Recc

Just want to dive deeper into this

To Answer the original question I would say that section 6.121.1.1 (3) gives you the calculation to get to maximum resistance values of the earth continuity conductor -

3) where the maximum length of the conductor may be determined using
the following equation
Lmax=
0,8×Uo×Sph
ρ(1+m)×Im
where
Uo is the phase voltage of 230 V,
ρ is the resistivity at normal working temperature in ohm.mm2/metre
which is 23 × 10-3 for copper which value increases above 120 mm2
with R+15 % (150 mm2), R+20 % (185mm2), R+25 % (240mm2) and
R+30 % (300 mm2),
Lmax is the maximum length of the conductor in metres (m);
Sph is the size of the phase conductor in square metres (mm2);
Spe is the size of the earth continuity conductor in square metres
(mm2);
Im is the instantaneous or short time tripping current in amperes (A).
For most general purpose circuit breakers it would suffice to use
10 multiply by the rated nominal current In for the instantaneous
tripping point of the circuit breaker, however the precise number could
be obtained from the manufacturer's information or from the
instantaneous current setting of adjustable circuit breakers, thus
𝐼𝑚 = 𝐼𝑛 × 10
Correction factor m =
𝑆𝑝ℎ
𝑆𝑝𝑒
= 1 [for table 6.28 (a)]
f) using the same formulae above where the earth continuity conductor
cross sectional area is 50 % of the phase conductor, thus m = 2,
table 6.28(b) for the maximum length of copper earth continuity
conductor for general purpose circuit breaker apply.

[Dylboy]

To Answer the original question I would say that section 6.121.1.1 (3) gives you the calculation to get to maximum resistance values of the earth continuity conductor -

3) where the maximum length of the conductor may be determined using
the following equation
Lmax=
0,8×Uo×Sph
ρ(1+m)×Im
where
Uo is the phase voltage of 230 V,
ρ is the resistivity at normal working temperature in ohm.mm2/metre
which is 23 × 10-3 for copper which value increases above 120 mm2
with R+15 % (150 mm2), R+20 % (185mm2), R+25 % (240mm2) and
R+30 % (300 mm2),
Lmax is the maximum length of the conductor in metres (m);
Sph is the size of the phase conductor in square metres (mm2);
Spe is the size of the earth continuity conductor in square metres
(mm2);
Im is the instantaneous or short time tripping current in amperes (A).
For most general purpose circuit breakers it would suffice to use
10 multiply by the rated nominal current In for the instantaneous
tripping point of the circuit breaker, however the precise number could
be obtained from the manufacturer's information or from the
instantaneous current setting of adjustable circuit breakers, thus
𝐼𝑚 = 𝐼𝑛 × 10
Correction factor m =
𝑆𝑝ℎ
𝑆𝑝𝑒
= 1 [for table 6.28 (a)]
f) using the same formulae above where the earth continuity conductor
cross sectional area is 50 % of the phase conductor, thus m = 2,
table 6.28(b) for the maximum length of copper earth continuity
conductor for general purpose circuit breaker apply.

Yes ya I saw that formula and did the calculations for it at the top and if I don't use the 0.8 factor (I assume to counter variances in tempreture) I get 48meters which works close with the Touch voltage formula which does not include tempreture variations.



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