How did they calculate this ?

[Dylboy]

I was then doing more reading on Touch Voltage and came across the link from many moons ago which once more throws a spanner.... hahaha

https://www.theforumsa.co.za/forums/...-Touch-Voltage

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[GCE]

Hi

I keep reading this thread and think you are confusing me or you are confusing yourselves.

You are taking 2 different regulations and test readings and combining them as one.

8.6.3 - Resistance of earth continuity conductor - Is the resistance of the earth wire used after the point of control and has been calculated to prevent touch voltages etc - The resistance you measure may not be higher than what is given in the table.
To make our lives easier all the clever people have completed the calculations and given us table 6.28 to ensure that earth cables are pulled in first time correctly

The values of the resistance given in Table 8.1 has become lower since they brought in the regulation that the phase conductor and earth conductor need to be the same size which is why it has changed from Ed2 to Ed 3 - Prior to that ED 2 is the same as ED1.1
You cannot use R= V/( 2*I) you need to stick to the tables otherwise your COC is incorrect

8.6.5 - Earth fault loop impedance at main switch - is measuring/calculating the supply authority earthing and ensuring that the neutral earth is bonded in the sub station - without the neutral bonded to earth there will be no reference to earth and so it cannot trip on an earth fault as there will be no current flow or very little.
Since they never ( hardly ever) run an earth wire from the sub to the premises we need to check that the earthing will be good enough to create a current flow in the event of an earth fault

Impedance is calculated using resistance , inductive reactance and capacitive reactance

[Dylboy]

Hey GCE

Yes ya I have been and still a tad confused, but what you said i agree with and makes sense as well.

Where is the regulation that the earth must be same size as lhase conductor ? Would that then not make FTE cable wrong?

One note for 8.6.3 it states "

In instances where it is not
practical to measure the resistance, suitability of the ECC can be confirmed
by doing an earth loop impedance test from the point of consumption as
prescribed in 8.6.5.1."

And in 8.6.5.1 it states

"8.6.5.1 At the main switch, the earth loop impedance shall be such that an
earth fault current double the rated current (or higher) of the main protective
device automatically disconnects the supply to the installation. Table 8.2
indicates the earth fault loop circuits for different distribution systems."

So that adds a bit more confusion now too as then the double the current formula is there but it does not meet TV?

Maybe I am now way over thinking all of this but I want to understand it properly rather than just follow a table for circuit design and CoC etc. For example a very old hosue is not to ED. 3 so does that mean if the Recc is less than the table it's a fail but Ed.2 had a higher value etc...

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[Derlyn]

Very interesting thread.

I have always been under the impression that the ECC can be downgraded one size from the phase conductor.

If this has been changed then it's not only FTE that does not comply, but also AIRDAC.

Should one pull in seperate ECC conductors when using the abovementioned cables ?

Peace out .. Derek Stuart.

[GCE]

Very interesting thread.

I have always been under the impression that the ECC can be downgraded one size from the phase conductor.

If this has been changed then it's not only FTE that does not comply, but also AIRDAC.

Should one pull in seperate ECC conductors when using the abovementioned cables ?

Peace out .. Derek Stuart.

The regs state

can carry earth fault currents, protective conductor currents and leakage currents to
earth without danger from thermal, thermomechanical and electromechanical
stresses and from electric shock that arise from these currents,

Ecc etc are designed to deal with thermal rise and electro mechanical stress and the calculation to work out the earth resistance and ability to get rid of a a fault current is complex hence the reason they refer us to table 6.28 - table D.2(b) and manufacturers spec

The regs allow for cables to be treated differently to bare copper

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:

[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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