Why the difference in size of Main earth required for a TT compared to TNC-S?

[HappyHippyDad]

I am away from the regs book (but who needs it anyhow Lucien!) but I think the minimum size for the Main earth conductor for a TT is 2.5mm or perhaps 4mm as opposed to the minimum for a TNC-S being 16mm for new builds.

It is very likely that a TNC-S will have a greater PFC ( compared to a TT) as the Ze will be <0.35Ω but a TT can have a very low Ra (sometimes very low... as Trev would have confirmed ... RIP :sad_smile so its PFC could be as high as a TNC-S but it may have a main earth conductor size of only 2.5mm?

Why the difference in minimum sizes?

I'm afraid I'm away from home at the moment and would have liked to have read through some of my books to gauge an answer before posting.

 

[Guest77]

The cable doesn't need to be big to cope with MAs which will trip an RCD.

 

[HappyHippyDad]

The cable doesn't need to be big to cope with MAs which will trip an RCD.

Of course! RCD is the main protection not additional!

Cheers Mike.

.. bit bored actually, so trying to think of any questions that come to mind. I should have given that one a little more thought though.

 

[GEOFF S]

I am away from the regs book (but who needs it anyhow Lucien!) but I think the minimum size for the Main earth conductor for a TT is 2.5mm or perhaps 4mm as opposed to the minimum for a TNC-S being 16mm for new builds.

It is very likely that a TNC-S will have a greater PFC ( compared to a TT) as the Ze will be <0.35Ω but a TT can have a very low Ra (sometimes very low... as Trev would have confirmed ... RIP :sad_smile so its PFC could be as high as a TNC-S but it may have a main earth conductor size of only 2.5mm?

Why the difference in minimum sizes?

I'm afraid I'm away from home at the moment and would have liked to have read through some of my books to gauge an answer before posting.

Hi

Two points, first often withh TT the main earth will need to be of a size to conform with the requirements for mechanical and corrosion protection.

Secondly, not really relating to a TT, High PFC tend to be less damaging to a cable than a lower PFC, this is due to the energy limiting of the proterctive device at higher PFC.

Cheers

 

[Gardner]

Im still confused. What about a stable 1ohm? The PFC will be almost that of TN_C_S.

 

[telectrix]

a Ze of 1 ohm will give rise to a pefc of 240A. a typical TNC-S of 0.2 ohms will be 1200A. a bit of difference methinks.

 

[Gardner]

True, but still more then a few amps.

 

[davesparks]

I am away from the regs book (but who needs it anyhow Lucien!) but I think the minimum size for the Main earth conductor for a TT is 2.5mm or perhaps 4mm as opposed to the minimum for a TNC-S being 16mm for new builds.

It is very likely that a TNC-S will have a greater PFC ( compared to a TT) as the Ze will be <0.35Ω but a TT can have a very low Ra (sometimes very low... as Trev would have confirmed ... RIP :sad_smile so its PFC could be as high as a TNC-S but it may have a main earth conductor size of only 2.5mm?

Why the difference in minimum sizes?

I'm afraid I'm away from home at the moment and would have liked to have read through some of my books to gauge an answer before posting.

So which regulation specifies a 16mm minimum earthing conductor on tncs?

2.5mm is the minimum permitted size for all earthing types, not a recommendation on the size to use. You still calculate the required size in accordance with 7671.

The earthing conductor of a PME installation has to perform the function of a bonding conductor so will end up being bigger, but as far as I'm aware there isn't any stipulation as to it being 16mm.

 

[HandySparks]

The cable doesn't need to be big to cope with MAs which will trip an RCD.

Is there a requirement to use an RCD on a TT supply if the Ra is low enough to give required disconnection times on overcurrent devices only?

 

[Deleted member 9648]

Is there a requirement to use an RCD on a TT supply if the Ra is low enough to give required disconnection times on overcurrent devices only?

There is no requirement for disconnection times to be met by an RCD, only that an RCD is the preferred method.

 

[Hellmooth]

i am away from the regs book (but who needs it anyhow lucien!:d) but i think the minimum size for the main earth conductor for a tt is 2.5mm or perhaps 4mm as opposed to the minimum for a tnc-s being 16mm for new builds.

It is very likely that a tnc-s will have a greater pfc ( compared to a tt) as the ze will be <0.35Ω but a tt can have a very low ra (sometimes very low... As trev would have confirmed ... Rip :sad_smile so its pfc could be as high as a tnc-s but it may have a main earth conductor size of only 2.5mm?

Why the difference in minimum sizes?

I'm afraid i'm away from home at the moment and would have liked to have read through some of my books to gauge an answer before posting.

rip........?

 

[uksparks]

(sometimes very low... as Trev would have confirmed ... RIP :sad_smile

Oh my god, did Trev die?

 

[HappyHippyDad]

Oh my god, did Trev die?

Bloody ell what have I started!! No! No! He just hasn't been around on the site for a long time.. sorry for any confusion..

 

[HappyHippyDad]

So which regulation specifies a 16mm minimum earthing conductor on tncs?

2.5mm is the minimum permitted size for all earthing types, not a recommendation on the size to use. You still calculate the required size in accordance with 7671.

The earthing conductor of a PME installation has to perform the function of a bonding conductor so will end up being bigger, but as far as I'm aware there isn't any stipulation as to it being 16mm.

542.3.1 (table 54.1)
543.1.4 (table 54.7)

 

[davesparks]

542.3.1 (table 54.1)
543.1.4 (table 54.7)

The use of table 54.7 is an alternative to using the adiabatic equation for those people who can't cope with a simple calculation.
Table 54.1 is specific to buried earthing conductors only, but still lists 2.5mm as the minimum subject to protection.

543.1.1 gives a minimum size for a copper earthing conductor of 2.5mm subject to protection with no mention of being restricted to aparticular earthing system

For a PME supply the earthing conductor also has to fulfill the requirements of a main bond as per table 54.8 and subject to DNO requirements so effectively the minimum is 10mm

So I'll ask again, where did you get the minimum of 16mm from?

 

[GEOFF S]

Hi

Its often a requirement of the DNO, not that most would notice the differance between 10mm and 16mm.

Cheers

 

[HappyHippyDad]

The use of table 54.7 is an alternative to using the adiabatic equation for those people who can't cope with a simple calculation.
Table 54.1 is specific to buried earthing conductors only, but still lists 2.5mm as the minimum subject to protection.

543.1.1 gives a minimum size for a copper earthing conductor of 2.5mm subject to protection with no mention of being restricted to aparticular earthing system

For a PME supply the earthing conductor also has to fulfill the requirements of a main bond as per table 54.8 and subject to DNO requirements so effectively the minimum is 10mm

So I'll ask again, where did you get the minimum of 16mm from?

Some may choose to use the alternative and it is only your opinion that they cant cope with reg 543.1.3 (which unfortunately you have mistakenly labelled as 'the' adiabatic equation. I wouldn't normally make such a distinction with reg 543.1.3 and the term adiabatic, but I know you like the wording to be precisely correct Dave, so you really shouldn't just casually say 'the' adiabatic... that just doesn't mean anything as there are many adiabatic equations)

Ah... so your minimum is slowly rising from your first assumption of 2.5mm.

 

[Lucien Nunes]

Conceptually, a TT earthing conductor is only likely to carry current that has passed through the protective devices of its own installation, because is is not connected to either a live conductor or anyone else's earthing conductor. Thus, provided it is sized to suit the OPD, it will protect the system and the OPD will protect it.

A TN-C-S EC is connected to all sorts of naughty things via the CNE (like other peoples' neutrals!) that can drive currents through it if the bonding connects it to extraneous conductors of low impedance. These are not controlled by the OPD of the local system and therefore the cable cannot be sized the same way. The DNO's requirement for a particular cable size reflects the likely network conditions as regards pd and external impedance.

 

[GEOFF S]

Hi

Before we can utilise 54.7 we need to size the live conductors, part of that process is thermal constraints(adiabatic). So 54.7 not really for those who cant cope, more for those not needing to calculate twice.

Cheers

 

[Deleted member 26818]

In all earthing systems protective earth conductors are required to be sized so as to be able to conduct the PFC for as long as is required for ADS to operate.

This may be determined by use of the adiabatic equation.
Higher fault currents = shorter operating times.
Shorter operating times = smaller cables.

Other factors which determine the minimum CSA of earth cables, are whether the conductor is to be buried in the ground, whether the conductor is to be provided with protection against corrosion, whether the conductor is to be provided with mechanical protection and whether the conductor is part of a multi-core cable.

Where PME conditions apply, main protective earth and bonding conductors are also required to be sized so as to conduct diverted earth fault and neutral currents of an undetermined size and for an undetermined time.
As such the CSA of such conductors is guesstimated, and current thinking is 10mm minimum.

Lower fault currents = longer operating times.
Longer operating times = larger conductors.