# Termination of SWA on distant end of TT supply

Many thanks for comments and ideas on my new low readings. There is no bonding and testing was carefully carried out. The ground is very wet, but all water pipes are plastic. There may be an old iron water main across the adjacent field, but as the old house TT was fairly high it doesn't seem to be relevant. Maybe something I haven't spotted so will keep looking. Thanks again for interesting and useful words.

Many thanks for comments and ideas on my new low readings. There is no bonding and testing was carefully carried out. The ground is very wet, but all water pipes are plastic. There may be an old iron water main across the adjacent field, but as the old house TT was fairly high it doesn't seem to be relevant. Maybe something I haven't spotted so will keep looking. Thanks again for interesting and useful words.
Intriguing. I still don't believe that 1.3&#937; is down to your earth rod.

How are you making measurements - as loop impedance? If so, have you tried measuring just the earth rod (i.e. with it not connected to anything other than your tester? ...or is that the 1.3&#937; you've reported?

Kind Regards, John.

Per quick 'back of a fag packet calcs (hope they're right!), for the purpose of the adiabatic equation in BS7671:
• 4mm² 2-core SWA Armour CSA=21mm² Cu equivalent=9.3mm²
6mm² 2-core SWA Armour CSA=24mm² Cu equivalent=10.6mm²
10mm² 2-core SWA Armour CSA=41mm² Cu equivalent=18.2mm²
16mm² 2-core SWA Armour CSA=46mm² Cu equivalent=20.4mm²
... and that's even without counting the CPC core.

Kind Regards, John.

I think those figures are a bit out. The ratio of conductance between copper and steel for an equivalent size is about 9:1 so the CU equivalents above will be a lot smaller.

Per quick 'back of a fag packet calcs (hope they're right!), for the purpose of the adiabatic equation in BS7671:
• 4mm² 2-core SWA Armour CSA=21mm² Cu equivalent=9.3mm²
6mm² 2-core SWA Armour CSA=24mm² Cu equivalent=10.6mm²
10mm² 2-core SWA Armour CSA=41mm² Cu equivalent=18.2mm²
16mm² 2-core SWA Armour CSA=46mm² Cu equivalent=20.4mm²
... and that's even without counting the CPC core.
I think those figures are a bit out. The ratio of conductance between copper and steel for an equivalent size is about 9:1 so the CU equivalents above will be a lot smaller.
As I said when I originally posted those figures a week or two ago:
Note that if you try working out copper/steel CSA equivalents simply on the basis of resistivities, you'll get totally different answers from these.
As I say in the preamble to my figures (above) they are equivalents specifically for the purpose of the adiabatic approach to the calculation of minimum CSAs of Protective Conductors as described in 543.1 of the regs. The 'equivalence' is therefore determined by the 'k' factors (per Tables 54.3 & 54.4), the ones I used being 115 for copper and 51 for steel armour (both 70 degrees).

As I said, if you make the comparison simply in terms of resistivity/conductance, you will get very different answers - I presume because the thermal properties of the two materials (which are a part of what makes up 'k', which doesn't only relate to resistivity) are very different. However, if you are interested in equivalence in terms of the reg's requirement for protective conductor CSAs, you have to use 'k' values, not just resistivity/conductance.

Kind Regards, John

I think those figures are a bit out. The ratio of conductance between copper and steel for an equivalent size is about 9:1 so the CU equivalents above will be a lot smaller.
To add to and expand my recent post....

Firstly, I should mention that the context of my original calculation of the copper equivalent CSA of SWA armour was such that I took the conservative approach of using the 'k' value for copper from Table 54.3 ("protective conductor incorporated in cables or bunched with cables"), and that produces a copper:SWA armour ratio of 2.25. However, in other contexts (probably including the one of this thread) it may be more appropriate to use the 'k' value from Table 54.2 ("insulated protective conductor not incorporated in a cable and not bunched with cables"), which would result in a ratio of 2.8.

However, as ricicle pointed out, both of these ratios (2.25 or 2.8 ) are much lower than the ratios of resistivity (or conductance) of copper and steel - which is somewhere between 6:1 and 10:1, depending on the type of steel. There seem to be at least three reasons for this, all of which are presumably taken into account in arriving at the tabulated values for 'k':
• 1...The tabulated value of k is that which limits the temperature rise to 160 degrees C for copper, but 200 degrees C for steel armour (see Tables 54.2-54.4). This allows steel armour CSA to be lower than it would be if it was limited to the same temperature as copper.
2...Steel is denser than copper, hence heavier per unit length of conductor than copper. For a given amount of energy dissipation, the temperature rise will therefore be lower for steel than for copper of the same CSA.
3...Not only (2), but the specific heat of steel is greater than that of copper, hence further reducing the temperature rise with steel as compared with copper for a given energy dissipation.
Those three things taken together (and maybe more) mean that the CSA of steel armour which is able to to cope with the let-through energy of the protective device (without the maximum permissible temperature being exceeded) is considerably lower than the CSA that would be needed to achieve the same resistance as the copper conductor that would be needed to handle the same let-through energy.

Sorry if that's a bit messy, but hope it makes sense.

Kind Regards, John

If the supply remains as a PME supply with a local TT conversion, then the TD RCD needs removing from the supply end of the cable,

I was wondering why the removal of the time delay RCD would be necessary. It would provide some protection for the cable and and descrimination would be acheived between the RCD at the workshop.

If the supply remains as a PME supply with a local TT conversion, then the TD RCD needs removing from the supply end of the cable,

I was wondering why the removal of the time delay RCD would be necessary. It would provide some protection for the cable and and descrimination would be acheived between the RCD at the workshop.

The SWA will be protected by ADS due to the PME earth being of low enough value so will not need RCD protection. Buried armoured cables are not usually required to have RCD protection (unless they are part of a TT system)

If the supply remains as a PME supply with a local TT conversion, then the TD RCD needs removing from the supply end of the cable,
I was wondering why the removal of the time delay RCD would be necessary. It would provide some protection for the cable and and descrimination would be acheived between the RCD at the workshop.
I see that ricicle has replied but, in case it'snot entirely clear to you, even though he used the wird 'need', I don't think (subject to him correcting me!!) he was meaning to suggest that the time-delayed RCD would do any harm - merely that it would become redundant. Because of the time delay, it would almost certainly never operate, because the main supply's devices would opearte first.

Kind Regards, John.

I think those figures are a bit out. The ratio of conductance between copper and steel for an equivalent size is about 9:1 so the CU equivalents above will be a lot smaller.
To add to and expand my recent post....
I've thought of what might be a far simpler/briefer bottom line summary of all this ....

The minimum CSA requirements for protective conductors (543.1) are nothing directly to do with the resistance/impedance of the cable. Rather, they are there to ensure that the conductor does not rise to an unacceptable temperature (for the material concerned) as a result of adiabatically dissipating the let-through energy of the protective device - regardless of the resistance/impedance of the conductor.

In opassing .... on might think that, particularly in relation to main bonding conductors, there might additionally be a requirement for resistance/impedance gto below some maximum value, but that does not seem to be the case in terms of the regs (although it seems that GN8 suggests a maximum of 0.05 for MPBs); neither 543.1 nor 544.1 say anything about the length of MPB conductors - so, provided one was using a conductor whose CSA satisfied those sections, it could theoretically be hundreds of metres long and hence of significant impedance.

Kind Regards, John

I think those figures are a bit out. The ratio of conductance between copper and steel for an equivalent size is about 9:1 so the CU equivalents above will be a lot smaller.
This could be important. Just when I thought I had cracked this one in my recent posts...

...I think that everything I've written in my recent posts remains true in relation to the required CSA of protective conductors in general - indeed, in relation to the requirements for any protective conductor other than a main protective bonding conductor with a PME system.

However, in relation to MPB conductors with a PME system, I've just noticed the footnote to Table 54.8 which states that, in that situation, the equivalence to tabulated CSAs for copper conductors should be calculated on the basis of equivalent conductance, rather than just satisfying adiabatic equivalence in terms of 543.1.

In relation to that one specific situation, I may therefore owe ricicle an apology, and the consequences of this are fairly far-reaching. One has to assume that this footnote has to be taken at face value, and what it means (as ricicle was implying) that, for a MPB with PME, the minimum steel armour CSA which satisfies 543.1 in terms of adiabatic equivalence (i.e. temperature rise under fault conditions) would be pitifully inadequate in terms of satisfying the footnote to Table 54.8. In practice, this means that (particularly if one uses ricicle's 9:1 conductance ratio) unless one is talking about very large SWA sizes (95mm² minimum for 2-core), SWA armour alone is never going to be adequate as a MPB on a PME system. Of course, if one uses one of the SWA cores (plus armour) as a protective conductor, then 10mm² or larger will be adequate.

Whether the protective conductor going to an outbuilding counts as an MPB conductor is something we could debate, and the answer to that is clearly crucial in terms of non-copper conductor CSA requirements.

I have to wonder whether they really meant that footnote to Table 54.8 to say what it does. There is no mention of length, so a 10mm² copper MPB 500 metres long would theoretically satisfy 544.1.1, per se - which makes one wonder whether they really intended to effectively impose a conductance requirement for non-copper conductors.

Kind Regards, John.

The best I've seen on a TT was 2 ohms.

It was in a dairy farm, and consists of 3no. 2.4m long rods at various different locations, and a 2m x 20m metallic grid buried in concrete.

I see your 2ohms and raise you 1.41 ohms

Row of 18 small industrial units, Ze in each one was somewhere around 30 to 40 ohms, but the Zdb in them was much lower than that due to the fact that the steel framework was common to them all and also was securly bedded into the ground at tens of places

If the supply remains as a PME supply with a local TT conversion, then the TD RCD needs removing from the supply end of the cable,
I was wondering why the removal of the time delay RCD would be necessary. It would provide some protection for the cable and and descrimination would be acheived between the RCD at the workshop.
I see that ricicle has replied but, in case it'snot entirely clear to you, even though he used the wird 'need', I don't think (subject to him correcting me!!) he was meaning to suggest that the time-delayed RCD would do any harm - merely that it would become redundant. Because of the time delay, it would almost certainly never operate, because the main supply's devices would opearte first.

Kind Regards, John.

I suppose that need is probably the wrong word to use there but I would definately remove it if I was doing the install.

It serves no purpose as has already been mentioned the OCPD at the origin of the SWA will provide ADS in the event of a fault, and all the RCD does is provide a remote point of failure in the event of a fault.

At least it's time delayed, so it shouldn't ever trip, but I'd still do away with it anyway.

The best I've seen on a TT was 2 ohms. It was in a dairy farm, and consists of 3no. 2.4m long rods at various different locations, and a 2m x 20m metallic grid buried in concrete.
I see your 2ohms and raise you 1.41 ohms Row of 18 small industrial units, Ze in each one was somewhere around 30 to 40 ohms, but the Zdb in them was much lower than that due to the fact that the steel framework was common to them all and also was securly bedded into the ground at tens of places
I'm not sure that was a valid raise!! My interpretation was that all the metalwork RFL described was a deliberate part of an earth electrode system, such that I thought his 2&#937; was the Ze.

As for Zdb, I've already raised to 0.25&#937; - so, again, you may need to try harder

Kind Regards, John

I feel that I should referee here. Let's have a good clean fight with no Zdb-ing. Ze only please

My install was a milking parlour.

The 3 rods provided the main earth, and the metallic grid was bonded to keep the concrete at the same potential as all the parlour metalwork and milking equipment.

I can't remember the exact figures, but connecting the bonding to the grid did lower the figure a bit to 2 ohms, so i suppose my Ze was actually a bit higher than 2&#937; (But not a huge amount IIRC)

The install was supplied from the DNO as TN-C-S which we ditched and made TT.

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