Enabling EPS DIY?

[Oldgreybeard]

This information is not up to date.

The maximum acceptable earth electrode resistance for installations operating TN-S, or in TT systems where earth fault loop impedance is not restricted to a lower value, is 200 Ω.

Find me the clause in the regs that stipulates 200 ohms - big clue, there isn't one . . .

The 200 ohm figure only exists as guidance for best practice in the On Site Guide. The absolute limit is set by the RCD trip current of 30mA and the maximum safe touch voltage of 50V, hence the 1667 ohm figure. FWIW, the 200 ohm recommendation goes back at least 50 years - it was certainly in the guidance when I qualified in the 1970's, back when we had to use four terminal test sets to measure Ra.

Obviously the aim will always be to follow the guidance in the OSG and try to get the value of Ra down to 200 ohms or less, and in practice it's usually very easy to get well below that value - my single rod here has a value of Ra of about 25 ohms, for example (although that's pretty low - most electrodes tend to be around 50 ohms in my experience).

Anyway, it's irrelevant in the context of providing protection from the PEN fault risk, even if the supplementary parallel rod was 10 ohms it would make no difference to that risk. To mitigate that risk would require a very low Ra earth mat or grid, as used for intermediate earths and the earth at the transformer.

[Swwils]

IET Code of Practice for Electrical Energy Storage Systems - 2nd ed.

[Oldgreybeard]

IET Code of Practice for Electrical Energy Storage Systems - 2nd ed.

Indeed, NOT in the regs, as I said, it is only guidance and always has only been guidance. I've just double checked and there is no mention of the 200 ohm figure in BS7671 anywhere, apart from the guidance in that link the only other place the 200 ohm figure appears is the On Site Guide, and as I wrote it's been guidance (but not a regulatory requirement) for as long as I can remember.

[Swwils]

You'd immediately fall fowl of 551.4.3.2.1 in 7671 and low and behold, Table 41.5 aswell.

In any case you can just read BS 60364‑8‑2:2011+A11:2019.

[Oldgreybeard]

You'd immediately fall fowl of 551.4.3.2.1 in 7671 and low and behold, Table 41.5 aswell.

In any case you can just read BS 60364‑8‑2:2011+A11:2019.

This is getting tediously pointless in the context of a PEN fault (which is what is being specifically argued about at this instant - I was specifically replying to the post about PEN faults and the use of an additional earth rod in parallel with a TN installation PE). BS7671 is law in effect, it is a Statutory Instrument. None of the IET guidance or codes of practice are law, they are guidance, and failure to comply with guidance is perfectly acceptable, even if it may not be wise.

Can you point me to the section in BS7671 that stipulates that Ra must be lower than 200 ohms? I've just re-read every paragraph relating to Ra and cannot see that number mentioned anywhere. It is clearly stipulated in the On Site Guide, but again that is guidance, not a statute.

Just to be clear, I am deliberately being pedantic, because I know beyond any doubt what is written within the regs themselves (the damned book is sat here on my lap right now). I also know beyond any doubt that IET codes of practice are given a stiff ignoring in practice, with no repercussions on anyone that does work that doesn't comply with them. A good example of this would be all the car chargers installed before the advent of BS7671:2018, which were bound by an IET code of practice and guidance note that stipulated they be wired as TT islands with Type B RCD protection. In practice almost all installations from 2013 (when that IET code of practice was first published and when compliance with it was a condition of getting the grant) failed to take any heed of its recommendations, on the basis that they were impractical or too expensive (at the time Type B RCDs were around £400 or so).

[Swwils]

They are getting ready to recommend a £800 box that will fix it all.

[AGT]

I will have a look in BS7009 that used to have a figure.
I imagine the earth rod resistance for a generator set up would be a good area.

[Oldgreybeard]

The issue here is one of the real risk, and whether something is safe, versus an imaginary risk only ever described in guidance.

There are only two laws covering electrical work, the Building Regulations and the Wiring Regulations (BS7671). Both are statute law, so failure to comply with either is unlawful (and the situation regarding Building Regs is different in Scotland as they do not have Part P). Unlawful is not the same as illegal, these are not criminal laws, only statute laws.

Let's look at an unusual case of a TT installation where the earth electrode resistance (Ra) is 500 ohms. That's too high according to the published guidance (which is in the On Site Guide) but is it unlawful? The answer is no, because 200 ohm figure is guidance in the OSG and so not statute law.

Would an installation that has working RCD protection that trips at or below 30mA be unsafe if the value of Ra was 500 ohms? The answer to that depends on the values of R1 and R2. Modern teaching for electricians is for them to never measure Zs, on the basis that is is an "unnecessary live test", that carries a very slight risk of the electrician getting a shock from working on live parts. This means that Zs is now calculated, for a TT installation it is (R1 + R2) + Ra. R1 + R2 should be a low value (it's just the resistance of conductors) so, for the sake of this theoretical example lets assume that R1 + R2 is a slightly high value of 2 ohms. FWIW, I was taught to measure Zs and always do so, even now, and I never rely on the calculation method.

With the 500 ohm value of Ra, this makes Zs 502 ohms. Is that unsafe? Easy to determine. To be unsafe the voltage at the MET must not exceed the safe touch voltage of 50V in the event of an earth leakage fault. The highest earth leakage fault current that can flow is 30mA (anything over that will trip the RCD) so with Zs = 502 ohms that gives a potential at the MET of 15.06V (from Ohm's Law, V = I x R = 0.03A x 502 ohms). 15.06V is well below the 50V maximum so the installation is safe.

Just to be clear, I'm not advocating any relaxation of either the regulations or guidance, I'm just trying to put things into the context of whether a failure to comply with guidance poses a real risk to the end user. The answer is that it may not, as long as the proscribed safety devices within the installation are all working within their specified tolerances and the voltage at the MET under fault conditions doesn't exceed 50V.

FWIW, I've never ever seen a 30mA RCD or RCBO trip at 30mA when tested. If working they always seem to trip at around 25mA or thereabouts, so in reality even a value of Zs that is greater than the 1667 ohm limit may well not be unsafe (not suggesting this is a good thing to rely on at all).

As a final point, in the real world electricians are known to try and get Ra below the 200 ohm figure by devious means when testing. In the days when TT installations were very commonplace it wasn't at all unusual for Ra to be lowered when testing by wetting the ground around the electrode if it came out a bit high. This was such common practice that I rather suspect there are a lot of earth electrodes in difficult areas (rocky ground, or sand/gravel soil) that are above 200 ohms much of the time in dry weather.

[AGT]

Agree with all of the above.

New IET PV guidance comes out at the end of the month.

[AGT]

BS 7430 advises 20 ohms for generating sets.