Earth bonding question.

yoshytoshy

Hello ,recently I was working in a house that needed it's earth cable changed from 6sq to 10sq ,to qualify for the seai grant.
My uncle who's a reci electrician told me that because the cable run was less than ten meters ,there wasn't any need to have 10sq.

Was he telling porkies ?

2011

I don't have enough information to answer fully.

However most bonding/earthing in a house is done with a 10 sq. mm cable. This would include the gas pipe, hot press and earth rod. Some supplementary bonding is done with a 2.5 as far as I can remember.

I have not carried out any work in domestic installations in years.

A 6 sq. mm cable would be used as a protective conductor for a circuit with a 10 sq. phase conductor.

I hope this helps

From me book:

" The equipotential conductor should be at least greater than half the size of the main earthing conductor. So if the main earthing conductor is 16mm2, the typical main bonding conductor should be 10mm2. This will always satisfy the requirements for PME supply where the supply neutral conductor is not greater than 35mm2

The minimum size permitted is 6mm2, but local electrical suppliers may have their own requirements."

yoshytoshy

Maybe I should ask a different question to get that an easier answer.

If the earth cable used for bonding is 150 meters long ,will 10sq be adequate ?
Will electricity pass through a cable ,regardless of it being connected to an earth rod ?

M cebee

If a main equipotential bond is a long way from MET
then it might have to be oversized.ive used 0.05ohms as a guide for max resistance

yoshytoshy

M cebee wrote: »

If a main equipotential bond is a long way from MET
then it might have to be oversized.ive used 0.05ohms as a guide for max resistance

Thanks M cebee ,I'm just trying to get to grips with the whole thing. I thought it was pretty simple at the beginning to be honest ,earth cable takes the power because it's the only real ground.

If an earth bond is two inches from an earth rod ,how small a cable would take the power ,hypothetically speaking.

EDIT : The reason I want to get to the bottom of this is because people don't appreciate that they have to replace a cable that involves ripping up floors etc. I want to give them a proper answer and not just because it has to meet some book rule.

M cebee

The connection to the rod is the earthing conductor.

Its not a main bond

It the same principle as gas pipe sizing and their is a calculation to match.

Off the top of my head you can run 10 mm2 for 27m and 16mm2 for 43m.
1mm2= 2.7m


L = 0.05Ωx10mm2 ÷ 0.0183 = 27.32 metres
27.32 is the maximum permissible length for a 10mm2 protective conductor

yoshytoshy

gary71 wrote: »

It the same principle as gas pipe sizing and their is a calculation to match.

Off the top of my head you can run 10 mm2 for 27m and 16mm2 for 43m.
1mm2= 2.7m

Thats what I was told by my uncle ,but a guy on the plumbing forum told me I was wrong posting that.

It's not nice telling people they have to change things in their homes ,when you think it's not necessary. I just want to be happy that I know why this has to be done and not be uncertain.

yoshytoshy

M cebee wrote: »

The connection to the rod is the earthing conductor.

Its not a main bond

Sorry ,I just symplified the example.

knighted_1

yoshytoshy wrote: »

Thats what I was told by my uncle ,but a guy on the plumbing forum told me I was wrong posting that.

It's not nice telling people they have to change things in their homes ,when you think it's not necessary. I just want to be happy that I know why this has to be done and not be uncertain.

if your doing gas work installs you should have a copy of IS: 813 to hand -

annex k covers electrical considerations

read k3

this is the only source of reference that is acceptable under rgi rules

yoshytoshy

knighted_1 wrote: »

if your doing gas work installs you should have a copy of IS: 813 to hand -

annex k covers electrical considerations

read k3

this is the only source of reference that is acceptable under rgi rules

There you are big man.

knighted_1

knighted_1 wrote: »

if your doing gas work installs you should have a copy of IS: 813 to hand -

annex k covers electrical considerations

read k3

this is the only source of reference that is acceptable under rgi rules

No, Annex K is for guidance only and directs you to look to ETCI for reference


let down again by the clarity:pac::pac: of our regs

yoshytoshy

I've done some googling myself and found that where an earth rod is used (TT) six square earth is adequate. All other earthing needs to be 10mm2

Only reason for posting this thread is to find out why it's necessary to replace a 6mm earth cable with a 10mm one. Of course running a 10mm earth cable from scratch is the way things are done ,but replacing a 6mm cable is the question.

M cebee

Theyre 3 different things

main bonding

protective conductors

earthing conductor

and theres different rules for each

yoshytoshy

M cebee wrote: »

Theyre 3 different things

main bonding

protective conductors

earthing conductor

and theres different rules for each

Is this the way it is ?

Main bonding is the joining of pipes together ,
Protective conductors are the connections between the fuse board and the main pipe
Earthing conductor is the earth rod.

Edit : Heres the link I found LINK

M cebee

No

yoshytoshy

M ceebee ,in your opinion ,is a 6mm earth cable adequate to bond piping where there is an earth rod as the earthing conductor ?

knighted_1

is 813 does not direct you anywhere for guidence
annex k is informative (for your information) (they are informing you what to do )

k3.2 gas pipework shall be bonded to the main earth terminal at the electrical consumer unit using green/yellow insulated 10mm2 cable

nothing else is accepable

you cant hook in to the existing earth in the house whether it be at the copper cylinder the sink a power socket etc -it states it should have its own earth cable 10mm terminating at the consumer unit - nowhere else

when you signed up to rgi you signed a dec to conform to is 813

yoshytoshy

knighted_1 wrote: »

you cant hook in to the existing earth in the house whether it be at the copper cylinder the sink a power socket etc -it states it should have its own earth cable 10mm terminating at the consumer unit - nowhere else

when you signed up to rgi you signed a dec to conform to is 813

I'm installing gas nearly twenthy years ,I didn't just sign up to rgi my friend.

knighted_1

knighted_1 wrote: »

is 813 does not direct you anywhere for guidence
annex k is informative (for your information) (they are informing you what to do )

From page 70 IS813:

K.1 General


This information is for guidance only and reference shall always be made to the currant edition of the ECTI rules

knighted_1

well rgi is only in existance since 2009 and the is 813 is dated 2002

i was trying to point out the regs ,you asked the original question but are trying to pick holes in the answers you recieved .

20 years ago ,no way were you installing to current regs so you will have to accept that things change . its all in the book nowadays for reference ,which is why on inspection from rgi you must have the is 813 with you .

knighted_1

knighted_1 wrote: »

well rgi is only in existance since 2009 and the is 813 is dated 2002

i was trying to point out the regs ,you asked the original question but are trying to pick holes in the answers you recieved .

20 years ago ,no way were you installing to current regs so you will have to accept that things change . its all in the book nowadays for reference ,which is why on inspection from rgi you must have the is 813 with you .

Lets not be arguing, the Sparkys are watching:D

knighted_1

gary71 wrote: »

From page 70 IS813:

K.1 General


This information is for guidance only and reference shall always be made to the currant edition of the ECTI rules

didnt see that my apologies

does etci tell you to use anything other than 10sq earth on gas pipe ?

knighted_1

knighted_1 wrote: »

didnt see that my apologies

does etci tell you to use anything other than 10sq earth on gas pipe ?

Not that i know of, but I'm the worst kind of gasfitter, one who done a electrical course a few years ago and only remembers bits.
I'm going to have to get my books out know because of bloody yoshytoshy because i can't remember if distance effects the size of cross bonding cable and if I'm getting main bonding and protective conductors mixed up.
But as a guess i would go with 10mm and not the 6mm as it could be a problem if 6mm didn't meet local suppliers regs




The equipotential conductor should be at least greater than half the size of the main earthing conductor. So if the main earthing conductor is 16mm2, the typical main bonding conductor should be 10mm2. This will always satisfy the requirements for PME supply where the supply neutral conductor is not greater than 35mm2

The minimum size permitted is 6mm2, but local electrical suppliers may have their own requirements

knighted_1

knighted_1 wrote: »

.
20 years ago ,no way were you installing to current regs so you will have to accept that things change . its all in the book nowadays for reference ,which is why on inspection from rgi you must have the is 813 with you .

I've been working under gas regs for nearly 25 years, you'd think i would have had it sussed by now:)

yoshytoshy

knighted_1 wrote: »

well rgi is only in existance since 2009 and the is 813 is dated 2002

i was trying to point out the regs ,you asked the original question but are trying to pick holes in the answers you recieved .

20 years ago ,no way were you installing to current regs so you will have to accept that things change . its all in the book nowadays for reference ,which is why on inspection from rgi you must have the is 813 with you .

I've done all my exams and have the book for the regs .Earth bonding was never a part of training , the only items shown were cross bonding at a gas meter in case of a spark when disconnecting.

I check polarity at a boiler to make sure it's wired properly and I'm capable of wiring a heating system ,but thats it. The regs in IS 813 don't make me a sparks ,no matter how well I know them.

M cebee

Minimum 10

at least half size of largest protective conductor

in general no bigger than 70 copper


dont see any mention of a max impedance

but as gary said up to 27m youre below 0.05anyhow

yoshytoshy

Thanks for all been so down to earth ,especially on a bank holiday sunday:)

Bruthal

Its sort of an irony with 10 square bonding, with the terminal on them bonding straps usually used are not up to that size cable.

yoshytoshy

These clips look the part.

Link

lucernarian

A quick question on earth bonding, people talking about a mininum 6mm2 being used for any earth bonding work, but what about light fittings and metal switches etc?! Surely you can't be shoving down 6 or 10mm2 wires and hoping they'll fit into the screw terminals of a back box or a metal faceplate?! If the rule is at least half the size of the conductor, there wouldn't be a problem there.

But anyhow, this conversation seems to be just about overall earth bonding for the house so I'll be off:p

Bruthal

To_be_confirmed wrote: »

A quick question on earth bonding, people talking about a mininum 6mm2 being used for any earth bonding work, but what about light fittings and metal switches etc?! Surely you can't be shoving down 6 or 10mm2 wires and hoping they'll fit into the screw terminals of a back box or a metal faceplate?! If the rule is at least half the size of the conductor, there wouldn't be a problem there.

But anyhow, this conversation seems to be just about overall earth bonding for the house so I'll be off:p

Bonding is the earthing of extraneous metal such as pipes and sinks which form no part of the electrical installation, so that in the event of a neutral failure on a neutralised installation, all exposed and extraneuos metal will be at the same voltage, even if thats 230v, which would reduce the chance of a fatal shock, which would be more dangerous if one pipe was at 230v and another was at ground potential.

Earthing of light fittings and metal back boxes is an earth to protect the box from becoming live due to a fault from contact with the circuit supplying the light or switch box, and so the earth to them only needs to be half the size of the circuit conductor. These items would not be considered extraneous or exposed metal like a sink or pipes etc would be.

lucernarian

I wonder if I should be concerned when I record 15-30V differences between copper water pipes and say a nearby socket's earth...:eek: This was in the family's house, in the attic which has a socket installed there on a ring main.

Bruthal

To_be_confirmed wrote: »

I wonder if I should be concerned when I record 15-30V differences between copper water pipes and say a nearby socket's earth...:eek: This was in the family's house, in the attic which has a socket installed there on a ring main.

Test between the socket neutral and its earth, and the socket neutral and the pipe. This should tell you which earth is above ground potential, assuming the socket neutral is ok. If its a neutralised installation the pipes should be at the same potential as all earths in the house.

lucernarian

It's definitely the pipes which are above ground potential. From what I've seen of the house, it appears to use TN-C. I've found nothing that suggests there's an earth connected to anything but the neutral bar near the meter.

Bruthal

Its not really possible when testing between 2 items with a voltmeter to tell which is the higher potential without using some known reference. The socket earth is known, but it may well not be a good socket earth, and a non connected (to earth bar) socket earth wire can show a few volts through induction as its in a live cable. But it could be the pipes above ground potential alright.

But anyway, all pipes should be bonded in a TN-CS setup.

M cebee

robbie7730 wrote: »

Bonding is the earthing of extraneous metal such as pipes and sinks which form no part of the electrical installation, so that in the event of a neutral failure on a neutralised installation, all exposed and extraneuos metal will be at the same voltage, even if thats 230v, which would reduce the chance of a fatal shock, which would be more dangerous if one pipe was at 230v and another was at ground potential.

Earthing of light fittings and metal back boxes is an earth to protect the box from becoming live due to a fault from contact with the circuit supplying the light or switch box, and so the earth to them only needs to be half the size of the circuit conductor. These items would not be considered extraneous or exposed metal like a sink or pipes etc would be.

- the sink is extraneous but the metal light fitting is exposed


- the 'protective conductor' has to be the same size as the phase/line(up to 16) except when using T&E

Bruthal

M cebee wrote: »

- the sink is extraneous but the metal light fitting is exposed

True, obviously enough:o

- the 'protective conductor' has to be the same size as the phase/line(up to 16) except when using T&E

This more or less happens automatically while wiring in a domestic situation anyway.

M cebee

ya:)

i find it's a good way to learn -checking the rules when a question comes up


if you were using pvc conduit and singles you'd have to use same size 'live conductors' and CPC


can you use a smaller size earth on final circuits in steel conduit as they're paralleled with the steel ?

(i know in theory you don't need them but industry practise is separate earth)
i usually do them same size-but copper is expensive now

Bruthal

M cebee wrote: »

ya:)

i find it's a good way to learn -checking the rules when a question comes up

Well the exposed metal one is sort of common sense, something which dissapears after a few early morning cans:D

if you were using pvc conduit and singles you'd have to use same size 'live conductors' and CPC


can you use a smaller size earth on final circuits in steel conduit as they're paralleled with the steel ?

(i know in theory you don't need them but industry practise is separate earth)
i usually do them same size-but copper is expensive now

In industrial wiring we used to use 4 square and 2.5 earth as far as i can remember in socket circuits alright, and 1.5 earth in the 2.5 lighting circuits. Its been a while since i did any final circuit wiring with individual singles now though, so the memory might be a bit dodgy. More switchroom terminating stuff on any industrial jobs i was at, even thats been a while now.

2011

Duplicate post deleted

2011

In industrial wiring we used to use 4 square and 2.5 earth as far as i can remember in socket circuits alright, and 1.5 earth in the 2.5 lighting circuits. Its been a while since i did any final circuit wiring with individual singles now though, so the memory might be a bit dodgy. More switchroom terminating stuff on any industrial jobs i was at, even thats been a while now.

+1

That was my experience up to the year 2000 on large industrial projects. Since then the CPC and phase have been sized the same on the jobs with steel conduit that I have worked on. This means that the prospective short circuit current will be higher and consequently disconnection time reduces.

M cebee

2011 wrote: »

+1

That was my experience up to the year 2000 on large industrial projects. Since then the CPC and phase have been sized the same on the jobs with steel conduit that I have worked on. This means that the prospective short circuit current will be higher and consequently disconnection time reduces.

larger cpc's will make little difference to PSCC or disconnections times i would say-as they're paralleled with the steel

steel takes 80-90% of fault current in a fault scenario afaik


they're sized same as live conductors where i work-i assume the reasoning is that if you're not depending on steel as a protective conductor then they should be regulation size evehn though they're paralleled with steel

Bruthal

M cebee wrote: »

larger cpc's will make little difference to PSCC or disconnections times i would say-as they're paralleled with the steel

steel takes 80-90% of fault current in a fault scenario afaik


they're sized same as live conductors where i work-i assume the reasoning is that if you're not depending on steel as a protective conductor then they should be regulation size evehn though they're paralleled with steel

It should not make very much difference between the half size earth and full size even without the conduit in parallel, if you take a loop impedence for example, at 2.5 ohms, thats nearly 100 amps, which shouldnt make much difference on a 20 amp circuit.

With the steel conduit there certainly would be no difference once its a continous path.

I think myself the main improvement about full sized earths is they have the better mechanical strength/strong connections, especially at connection terminals, for a given size circuit conductor.

2011

M cebee wrote: »

larger cpc's will make little difference to PSCC or disconnections times i would say-as they're paralleled with the steel

steel takes 80-90% of fault current in a fault scenario afaik

That is a very sweeping statement
Although I see your point and agree that in many cases you are correct. But how much fault current a steel conduit takes depends on the impedance of the conduit and the CPC. The impedance of the CPC is easy to calculate with a high degree of accuracy, but the impedance of the conduit will depend on many factors. These include:

1) How many joins there are in the conduit (couplers etc.)
2) How tight these joins are. A loose joint will present far more resistance.
3) If the joins corrode or loosen over time.
4) What diameter the conduit is (20, 25, 32mm) and how thick the walls of the conduit are (class 1,2,3,4,)
5) If trunking makes up part of the earth fault loop, what size is the trunking

etc.....

In fact there are so many variables that it is very hard to calculate form a design point of view. There is also a concern that in the future a modification will be made such as flexible conduit (like Kopex) is installed in a section and continuity will be lost.


Therefore it is often ignored and the calculation is done on the CPC instead.

In reality I agree that in many cases increasing the CPC from say a 2.5 to a 4 sq. will make little difference if it is in steel conduit, but not in all! Design is generally done on a worst case sinario i.e steel conduit does not provide any connection to earth.

they're sized same as live conductors where i work-i assume the reasoning is that if you're not depending on steel as a protective conductor then they should be regulation size evehn though they're paralleled with steel

Regulation size is not always equal to the size of the phase conductor. If it can be shown that the CPC selected satisfies the "adiabatic equation" (section 543.1.2 in ET101:2008) the size can be reduced. For example standard 2.5 T & E has a 1.5 CPC.

I have worked on many large projects in Ireland where the CPC is 1/2 the size of the phase conductor for most circuits (offices, colleges, hospital, government buildings etc..).

Frequently on large projects the electrical designer will specify that the CPC must be the same size as the phase conductor (particularly in ATEX zoned areas) but this does not mean it is a regulation.

Bruthal

2011 wrote: »

That is a very sweeping statement
Although I see your point and agree that in many cases you are correct. But how much fault current a steel conduit takes depends on the impedance of the conduit and the CPC. The impedance of the CPC is easy to calculate with a high degree of accuracy, but the impedance of the conduit will depend on many factors. These include:

1) How many joins there are in the conduit (couplers etc.)
2) How tight these joins are. A loose joint will present far more resistance.
3) If the joins corrode or loosen over time.
4) What diameter the conduit is (20, 25, 32mm) and how thick the walls of the conduit are (class 1,2,3,4,)
5) If trunking makes up part of the earth fault loop, what size is the trunking

etc.....

In fact there are so many variables that it is very hard to calculate form a design point of view. There is also a concern that in the future a modification will be made such as flexible conduit (like Kopex) is installed in a section and continuity will be lost.


Therefore it is often ignored and the calculation is done on the CPC instead.

In reality I agree that in many cases increasing the CPC from say a 2.5 to a 4 sq. will make little difference if it is in steel conduit, but not in all! Design is generally done on a worst case sinario i.e steel conduit does not provide any connection to earth.




Regulation size is not always equal to the size of the phase conductor. If it can be shown that the CPC selected satisfies the "adiabatic equation" (section 543.1.2 in ET101:2008) the size can be reduced. For example standard 2.5 T & E has a 1.5 CPC.

I have worked on many large projects in Ireland where the CPC is 1/2 the size of the phase conductor for most circuits (offices, colleges, hospital, government buildings etc..).

Frequently on large projects the electrical designer will specify that the CPC must be the same size as the phase conductor (particularly in ATEX zoned areas) but this does not mean it is a regulation.

On a well installed counduit and trunking run, it will make little if any difference.

I would think the thickness of the steel conduit wall would be almost irrelevant compared to the effect of how well the conduits and trunking is installed. 1mm thick walled 20mm steel conduit has a CSA of about 60mm square, and its likely to be a little thicker than that, so that really is not much of a factor compared to the likely size of the earth conductor in such a conduit.

I think myself the steel conduit should be discounted anyway when deciding on the circuit size. Thats just my opinion though. As said here, a few corroded or loose couplers on the conduit, or loose bushings into trunking would not be unheard of, especially when flanged couplers are not used for the conduit into the trunking.

2011

robbie7730 wrote: »

On a well installed counduit and trunking run, it will make little if any difference.

Agreed. But well installed now may be in a very different condition in a year or two.
Also, conduit is not always well installed.

I would think the thickness of the steel conduit wall would be almost irrelevant compared to the effect of how well the conduits and trunking is installed.

It all matters. The thickness of the conduit is comparable to the CSA of a cable.

1mm thick walled 20mm steel conduit has a CSA of about 60mm square

60 sq. ??? Do you mean 60 sq. copper?? What size conduit?

I think myself the steel conduit should be discounted anyway when deciding on the circuit size. Thats just my opinion though.

In general it is. I agree with you.

As said here, a few corroded or loose couplers on the conduit, or loose bushings into trunking would not be unheard of, especially when flanged couplers are not used for the conduit into the trunking.

+ 1

Bruthal

2011 wrote: »

Agreed. But well installed now may be in a very different condition in a year or two.
Also, conduit is not always well installed.

True alright, that is why i said it should be discounted. I think the day where the conduit could actually be the earth is long gone now, or should be.

It all matters. The thickness of the conduit is comparable to the CSA of a cable.

Not really. A 20mm outer diameter hollow pipe with a wall thickness of 1mm will have a conducting CSA of 60 square mm, i.e. ten times what a 6 square copper wire has. So a 20mm steel conduit is the same. So it is a far bigger conductor than any earth thats likely to be in it, thats why i would think a slight difference in thickness would play very little part in the circuit earth path.

pipe CSA = 0.785x(outer diameter(20)^2-inside diameter(18)^2)
0.785(400-324) = 59.66

And for the same reason, once the conduit is in parallel with the earth, the earth size being changed will make little difference either, once the conduit/trunking path is sound, which as you say, is not highly reliable.

60 sq. ??? Do you mean 60 sq. copper?? What size conduit?

The conduit is 20mm outer diameter, and i am giving it as 18mm inside, but its usually about 17mm. 20mm outside minus the 18mm inside diameter gives a CSA of 60mm sq to conduct any current. So its quite a big conductor, so a slight thickness variation, while it will change the conduit resistance, will be insignificant in the scheme of things with a 2.5, 4, or 6sq earth in parallel with it.

2011

A 20mm outer diameter hollow pipe with a wall thickness of 1mm will have a conducting CSA of 60 square mm, i.e. ten times what a 6 square copper wire has

Apples and oranges. You have to compare like with like:

The resistivity of copper is very different to that of steel. That is why it is generally not used as a conductor.

ρ copper = 1.68×10^−8 [Ω/m]

ρ steel = 10 to 100 ×10^−8 [Ω/m]

Bruthal

2011 wrote: »

Apples and oranges. You have to compare like with like:

Your missing my point though. We are not comparing the conducting properties of the two.

Apples and oranges have no bearing, as i was talking about how the thickness of the wall of the conduit affects the parallel path of the conduit in parallel with the CPC. Varying the thickness of a conduit will only very marginally affect its resistance, and as its now in parallel with a CPC, the change is even smaller in the circuit.

We are not talking about how good a conductor the conduit is compared to copper, and that wont affect the percentage change in its own resistance for a thickness change.

Of course its thickness has to affect it, but im suggesting a slight variation in conduit thickness is insignificant in the overall scheme of things. After all, the proper CPC will be adequate on its own, so a conduit lowers this impedence of course, but a slight variation in thickness of conduit will have no major effect after that. Also, on just about any circuit, the conduit run will be much shorter than the circuit run, so its actual wall thickness again has little bearing.

My overall point was, the thickness will have much less bearing than the quality of fitting of the conduit, tight non corroded couplers etc.

The resistivity of copper is very different to that of steel. That is why it is generally not used as a conductor.

ρ copper = 1.68×10^−8 [Ω/m]

ρ steel = 10 to 100 ×10^−8 [Ω/m]

If we take a CPC of 2.5 Sq on a 50 meter circuit, its resistance is about 0.37 ohms, and 2 lengths of steel conduit coming from the trunking, and their impedence is also 0.37 ohms, even though it likely will be far lower as they only form a fraction of the circuit run, and are 24 times the size of the CPC, then the earth path is now about 0.185 ohms.

If the conduit thickness is increased by 10 percent, then the conduit impedence will now be 0.33 ohms, so the parallel path will now be 0.17 ohms, not a major difference(0.015). It wont matter what resistance you give for the conduit above, the variation in thickness will still have the same effect on its own % resistance, nothing to do with the fact its not a copper conductor. And in fact, the higher the resistance of the conduit, the less its variation will affect the parallel resistance with the CPC.

Its like saying put a 120 sq in parallel with a 2.5 CPC, then try a 50 Sq. For the overall 20 amp circuit, the difference between the 2 will be negligible. But either will be a lower reading than the 2.5 alone.

Dont forget, i am not saying the use of steel conduit will have little effect on the path, im suggesting a variation in thickness will have little bearing overall. Thats why the Ω of steel comared to the Ω of copper is not in question here. Any electrician would realise steel is not the conductor copper is.

Overall im not disagreeing with you, im just saying the variation in conduit thickness is of little bearing, and the major factors are its proper installation. But since the CPC will be adequate on its own, or should be, the conduit path variation will be of less importance anyway. Not that its an excuse for a sloppy installation though.

On a well installed installation, the conduit path back will not be exclusively the cinduit and trunking, other conduits from the trunking will be earthed at socket boxes, light switches etc, as well as click fittings on the trunking possibly, so the conduit path back does tend to be a low resistance path, once the conduits are well connected to the trunking, and the trunking joins are done properly.