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

[a546345]

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

We are on a TT system. All circuits are protected by 30mA/30ms RCBO or RCD as well as 100mA/100ms Type S as the main switch on the CU.

He said that the cable going to the earthing point needed to be increased in CSA.

That got me thinking - Why is there a minimum size requirement for any of the earth conductors? What fault condition could cause more than 100mA to flow down any cable for more than a short period before the RCD or RCBO trips?

I read this, but it didn't answer my question.

https://www.voltimum.co.uk/articles/sizing-main-protective-bonding-0

I did ask the electrician but he couldn't really explain why it was needed. Just curious.

 

[Taylortwocities]

The problem is basically trying to equate the Wiring Regulations with common sense. Doing that leads to madness.

Indeed, as you surmise, the maximum earth fault current in a TT should not ever exceed the trip current of the RCDs employed. Unfortunately we must follow BS7671, and this says that - for copper - the minimum csa of a buried earthing conductor connected to an electrode is:
- 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.

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?

 

[EFLImpudence]

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.

 

[JohnD]

Luckily, no RCD has ever failed to operate.

has it?

 

[JohnW2]

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.

Quite. I doubt that many domestic TT electrodes are much less than 50Ω (mine gets roughly that low during very wet periods) and would suspect that virtually none are less than, say, 15Ω - which suggests that the most current would be expect would be roughly in the range 4.5 A - 15 A - so, in terms of electrical common sense a 2.5 mm² earthing conductor should, from the CCC point-of-view, be more than adequate, shouldn't it?

Kind Regards, John

 

[JohnW2]

Luckily, no RCD has ever failed to operate. has it?

That's obviously why one has to plan for the possibility that it doesn't operate when it should - but, as I've just written, that cannot really be used to justify the need for an earthing conductor bigger than 2.5mm² - at least, not in terms of CCC.

Kind Regards, John

 

[ericmark]

We have been in the last week or two been talking about types of RCD and how DC can cause them to fail to trip, so the current limit is down to the earth rod, I know when I was fitting earth rods at Point of Ayr gas terminal they had to be 8Ω or lower, so 230/8 = 28.75 amp, so would need at least 4 mm² to ensure it could take the current if the RCD failed.

OK maybe 8Ω is rather low, but I know of a radio ham who had a square of trenches dug and copper tape laid, he was on a TN-C-S supply and when the electrician wired up his shack, he took a 6 mm² earth wire and connected the radio earth bar to the house earth, during road works the PEN was lost, and the 6 mm² earth wire melted.

So I do know of one case where 6 mm could be considered not large enough, after this happened whole house was converter to TT, so the question is what is the minimum impedance permitted for an earth electrode? As far as I am aware there is no lower limit, and there is nothing to stop a home using re-bar in the foundations as an earth electrode so some homes will have a really good TT earth, the limit is how good the DNO earth is.

The DNO earth can be 15Ω so I am told, but often it is more like 0.2Ω. I have seen a resistor about the size of my car at Point of Ayr connecting the DNO earth to the site earth to limit the current which can flow between the two earth systems. Seem to remember 280 mm² cable was used for the main earth, but it was a long time ago I worked there. But site was well earthed, I spent around 4 months with a paddies motorcycle putting in earth rods, they all joked it was because I had zero potential.

So for a house I suppose 10 mm² would be reasonable. Remember the neutral is not fused, so the current is not even limited to the DNO fuse size.

 

[JohnW2]

... I know when I was fitting earth rods at Point of Ayr gas terminal they had to be 8Ω or lower, so 230/8 = 28.75 amp, so would need at least 4 mm² to ensure it could take the current if the RCD failed. ... OK maybe 8Ω is rather low, but I know of a radio ham who had a square of trenches dug and copper tape laid ... The DNO earth can be 15Ω so I am told, but often it is more like 0.2Ω.

Fair enough, but I was talking about ('typical') domestic TT electrodes - which is a totally different kettle of fish from gas terminals, 'DNO earths' or radio hams with 'trenches full of copper tape'!

As I said before, I have personally never seen a ('typical') domestic TT electrode much less than 50Ω, so I think that assuming a credible minimum of 15Ω (or even 10Ω) is, electrically speaking, perfectly reasonable - and for that 2.5² mm would be adequate.

Kind Regards, John

 

[JohnW2]

This again!!! ...

 

[ericmark]

The question is why the regulations want such a thick cable, and my point is BS7671 is not just for the private house, it covers all, and there are situations where heavier cable is required, I would agree that as long as there is some mechanical protection and the earth rod is above 12Ω which in most cases it is, or ever 25Ω so making allowance for the dog peeing on it, then 2.5 mm² is ample, but the regulations would need to have this exception. And it does have exceptions like needing a key or tool except for a ceiling rose. But finding rules is hard enough now, without making the book thicker.

 

[JohnW2]

There is only a minimum size for conductors with or without mechanical protection.
2.5mm² is the minimum for protected conductor. ... Although still not much, the earth fault current will be limited by 240V/Earth Fault Loop Impedance of the installation.

Having thought about it a bit more, I'm not really sure what BS7671 really intends about the size of non-buried earthing conductors in a TT installation (TTC has already quoted the BS7671 minimum CSAs of buried earthing conductors with/without mechanical and/or corrosion protection).

Other than for buried ones (as above) BS7671 does not seem to make any distinction between earthing conductors and any other 'protective conductor'. That means that the primary method of determining minimum CSA is by adiabatic calculation (per 543.1.3). That calculation obviously assumes that the protective device does operate, and is really intended for situations in which that device is an OPD. If one attempts to apply such a calculation for an RCD-protected TT installation, then if one assumes:

maximum fault current ("due account being taken of the current limiting effect of the circuit impedances") = (pessimistically) 30A
Device operating time at 30A fault current = 150 ms (assuming worst case of TD RCD with >>5IΔn imbalance)
k = 143 (copper conductor, not in a {assumed they mean multicore} 'cable')

... then the calculated minimum CSA (i.e. that which would, electrically-speaking, be adequate) would be about 0.08 mm² - so that the over-riding minimum (given that it's not part of 'a cable') given in 543.1.1 (2.5mm² if mechanically protected, otherwise 4mm²) would apply. Do you agree?

The (I would say 'more sensible') approach we have taken of determining the minimum CSA if the RCD did not operate would give the same 2.5mm² answer.

Kind Regards, John

 

[JohnW2]

The question is why the regulations want such a thick cable, and my point is BS7671 is not just for the private house, it covers all, and there are situations where heavier cable is required ....

Sure, but this is a DIY forum, the question in the OP was clearly about a domestic dwelling and I have consistently said that I was talking about 'typical domestic TT electrodes'.

I would add that, in specifying minimum CSAs for 'protective conductors' in general, BS7671 assumes that the protective device does operate in response to a fault (otherwise cables would have to have ridiculously large CPCs - and, indeed, live conductors), so I don't see why that should be different if the device is an RCD. As I've just illustrated, if the RCD in a TT installation does operate, then it seems that an adiabatic calculation would indicate that the required CSA for the earthing conductor was less than 0.1Ω.

Kind Regards, John

 

[AndyPRK]

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

 

[ericmark]

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

Different rules if you look. My dad's house was changed to TN-C-S and I had to run new earth wires, however when I came to take over the job which resulted in this change, the electrician who had fitted the new consumer unit to replace the old wilex fuse box had connected up the earth used for the old GPO party line telephone, which was clearly imperial but around 2.5 mm² bare tinned copper clipped around the outside wall of the house to a steel earth spike.

Well he said he was an electrician, when I arrived he could not get the new RCD he had fitted to hold in, and his only bit of test kit was a multi-meter. He ended up with most of the house not on a RCD, he replaced it with an isolator, promising he would return, but was never seen again.

 

[bernardgreen]

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

The large cross sectional area is essential for PME systems where the Neutral is connected to extraneous metal in the ground. ( Via the Main Equi-potential Bonding ) In the event of a lost network Neutral the current through the bond can heat the bond cable to temperatures high enough to cause a fire. ( As Eric has already mentioned a 6mm² bond conductor melted when this happened )