TT installation - Why is there a minimum size requirement for earth conductors?

[Taylortwocities]

The usual way is to run 16mm Main earth from CU to a separate MET. This makes disconnect and testing of the electrode easier. the conductor from the MET to the electrode is sized as per BS7671.
If/when supply type changes it’s easy then to run the appropriate earth (usually 16mm) from the MET to the DNO’s earth

 

[a546345]

Luckily, no RCD has ever failed to operate.

has it?

I have 2 RCD/RCBOs between the tails and the end of each circuit cable - a 30mA/ms and a 100mA/ms. So both would need to fail I think to get a current > 100mA ish for longer than it takes the RCD to trip.

 

[a546345]

Perhaps he judged that the size needed to be increased due to the environment and therefore comply with the good book.
What was the existing size and what size was installed?

Existing is 10mm2 from rod to MET and plan was to change it to 16mm2.

 

[JohnW2]

I guess the CPC size rules apply to PME as well as TT.

Assuming that you are talking about the 'earthing conductor' from MET to the DNO earth terminal, as has been said, the requirement is different (from TT), and potentially very different, for the reason explained by bernard, the minimum earthing conductor CSA with TN-C-S (PME) being 10mm.

As bernard has said, with TN-C-S that 'earthing conductor' is part of the same loop that could, under certain supply-side fault conditions, carry the same very high currents as the main bonding conductors - hence the requirement that the main earthing conductor have at least the same CSA as required for main bonding conductors (per Table 54.8 of regs). The same potential issue does not arise with TN-S, so the requirement is then no different from TT.

Kind Regards, John

 

[JohnW2]

The usual way is to run 16mm Main earth from CU to a separate MET. This makes disconnect and testing of the electrode easier. the conductor from the MET to the electrode is sized as per BS7671. If/when supply type changes it’s easy then to run the appropriate earth (usually 16mm) from the MET to the DNO’s earth

Thats what I have. In fact, my earthing conductor (from MET to earth electrode) is also 16mm² - totally unnecessary at present, but I did that because the path of that conductor goes past the 'cut-out' (such as it is with an overhead supply!), so that if anyone ever wanted to change to TN-C-S (which I have been offered), they merely have to snip that cable and connect it to the DNO's 'earth'.

Kind Regards, John

 

[JohnW2]

I have 2 RCD/RCBOs between the tails and the end of each circuit cable - a 30mA/ms and a 100mA/ms. So both would need to fail I think to get a current > 100mA ish for longer than it takes the RCD to trip.

Indeed - and, as I have pointed out, in determining the minimum size for 'protective conductors' in general (of which an 'earthing conductor' is one example) the calculations require that, for TN-S and TN-C-S systems, the usually single protective device (MCB, RCBO or fuse) does operate when it should - so having two RCDs/RCBOs in the path is, in many senses, better than that!

In any event, as has been said, unless you have an exceptionally low-resistance earth rod, just 2.5mm² would be adequate even if neither of your RCDs/RCBOs operated.

Kind Regards, John

 

[JohnW2]

Existing is 10mm2 from rod to MET and plan was to change it to 16mm2.

As has been said, provided that none of it is buried (in earth) (which is unlikley), without protection against corrosion and mechanical damage, 10mm² is more than enough.

Kind Regards, John

 

[EFLImpudence]

Also:

Discussing some work with an electrician including upgrading the tails from 16mm2 to 25mm2

Just because you do that, for no electrical reason, does not mean that the protective conductor sizes have to be increased.

The sizes are based on the "required" csa of the live conductors (for TN-S and TN-C-S).

 

[Taylortwocities]

@a546345 did he say why he felt it necessary to increase the size of conductor? That would help our understanding.

 

[a546345]

- 2.5mm if protected against corrosion and mechanical damage.
- 16mm if protected against corrosion but not mech damage and
- 25mm if not protected against corrosion or damage.

I will ask him why he wants to change the cable. Before I do, here is a pic of the rod. Would you say it was protected against mechanical damage and corrosion?

Cheers.

 

[JohnW2]

I will ask him why he wants to change the cable. Before I do, here is a pic of the rod. Would you say it was protected against mechanical damage and corrosion?

The figures that TTC quoted related only to buried (in the ground) earthing conductors (which yours isn't). If not buried, then the minimum for any 'protective conductor' (which is not part of a multi-core cable) is 2.5mm² if mechanically protected, otherwise 4mm² - both of which are clearly well under your 10mm².

I can but repeat that your 10mm² conductor is more than adequate.

Kind Regards, John

 

[Taylortwocities]

The cable is protected, it is inside the conduit. I would worry about damage to the rod though. I’d drive it further in so the terminal box is flat on the pavers.

 

[AdrianUK]

Back in those days..... was an earth electrode common?

 

[AdrianUK]

Forget the above..... I replied to wrong thread!

I meant to reply to the "With no functioning RCD, does/did a TT electrode achieve anything?"

 

[Levo1]

There is only a minimum size for conductors with or without mechanical protection.
2.5mm² is the minimum for protected conductor.

The fault current would not be limited to 100mA as in your example - or 30mA as it should be for personal protection.
RCDs limit only the duration of a fault.

Although still not much, the earth fault current will be limited by 240V/Earth Fault Loop Impedance of the installation.

Hi all, a newbie here.
I am trying to figure out the earthing theory and regulations and am a bit confused and at a loss with the discussion after the 2nd and 3rd posts here.
Basically my understanding based on post #2 is: the regulation is statıng that you need:
- 2.5mm2 for the actual earthing protection in terms of electrical current
- another 13.5 mm2 if someone is likely/can hit it with a pick axe = 16mm2 ; this extra bit has got nothing to do with currents
- and another 9 mm2 if the Cable is in a place with a potential to corrode over time = 25mm2. ; This bit too is just there to ensure after (say 25 years) of corrosion there is still at east 16mm2 thickness of copper there to stop the pick axe.