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.

 

[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.

 

[davesparks]

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.

Yes you've got me, there are other adiabatic equations.
But why else would you choose not to use the adiabatic option than being unable to use it? Following the selection method will result in unnecessarily large conductors and a waste of money.

2.5mm was not an assumption, it is a fact from the regulations. The 10mm effective minimum is only applicable for PME supplies and is a result of bonding requirements.

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

 

[Gardner]

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.

Im confused about what you mean regarding PME. Is this so if the PEN breaks, and something takes over like water pipes, no conductor overheating would occur?

 

[Deleted member 26818]

No, PEN faults are a different kettle of fish entirely.
Due to the nature of TN-C-S and PME, in normal operation, the neutral and any earth fault currents from all connected installations will be present in the earthing conductors of any individual installation (this is known as diverted neutral currents).
As most of the current from each installation will be taking the direct route back to the source, these diverted neutral currents won't be perticuarly large, which is why only 10mm is deemed adequate.

In the event of a break in the PEN conductor, the 10mm conductors will melt like butter, if any have to take the full current.

 

[mainline]

Yes you've got me, there are other adiabatic equations.
But why else would you choose not to use the adiabatic option than being unable to use it? Following the selection method will result in unnecessarily large conductors and a waste of money.

2.5mm was not an assumption, it is a fact from the regulations. The 10mm effective minimum is only applicable for PME supplies and is a result of bonding requirements.

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

DNO Requirements:

Main Earth Conductor:
The main earth conductor, which connects the Main Earthing Terminal (MET) to the DNO's earth connection (service head), is often required to be 16mm²