Earth electrode tester or not?

[1Justin]

I did some work today (added a new circuit) to a TT where earth electrode/Ra (bonding disconnected) was 198 Ohms. Bit close to limit where advice is to "investigate".

Interestingly the figure I got is precisely the same as that measured in 2010 by previous electrician, so it does appear to be fairly stable. Maybe grounds to leave it all alone since it's well below 500 (Property has 100mA RCD also on it's PV system) . Despite this I advised householder that probably good idea to re-check frequently, so I put 2 years on my EIC.

Were I to install a new electrode to improve the figure (another reason to upgrade might be that the electrode is a thin mild steel rod, not great looking, and with rust). - Then I believe it's OK (according to GN3) to simply measure Ra for the new one? (We have 30mA RCD's).

Soil type is mostly a sandy area. Alongside the electrode is an old (actually rather nicer) galvanized rod (now disconnected). I get exactly the same figure if I check this one also, which implies it might just be the soil (or they are both equally bad).

Is it the case that if I were to re-install, I could measure only Ra? - Or would I need to get my hands on an Earth Electrode Resistance Tester and do it properly?

 

[Rockingit]

Not sure what your saying here, if an installation ''requires'' a TT system to be installed, then your going to have to make a mess of Mrs Jones dahlias anyway, if that's the only place to drive your rods.

No-ones making grand statements either, i've made the point several, several times over, that any typical domestic house TT system, should be a minimum of 2 X 5/8'' rods or preferably a single 2.4m 5/8'' rod. Are you saying this is OTT??

No, I'm not saying that is OTT at all - in fact I quite agree - what I am saying is that there is a point of diminishing return and assertions of being able to achieve TN figures de facto just don't stack up in real life. Hence....we end up falling back on our RCD as primary not secondary protection - and in which case - we end up full circle back with what does it therefore matter if I've got 10 or 100 ohms, both are equally useless.

 

[Engineer54]

Because having thought about Marvo's comments on reasons other than disconnection times for an Ra value I am convinced that such things are not relevant to the small scale TT installs I and most others work on. I am also convinced that if the potential issues Marvo highlighted were realistic the IEE would have been on it way before now,they hardly need any excuse to bring out an ammendment and fill their coffers.

Well, ...if were talking about a garden shed or garage with a light and a socket, you'd probably be somewhere near right. As far as a TT system for a complete house, i'd be taking a good deal more care!! We often never consider, that a TT system will be in place and serving that domestic installation for 30 years and more often than not, a good deal longer...

As far as the IET position goes, they don't seem to worry much too about the many contradictions contained in the BGB and the OSG's etc. Plenty of room for improvement and filling their coffers there alone!! A complete revision of the TT section wouldn't go amiss either!!

 

[Engineer54]

No, I'm not saying that is OTT at all - in fact I quite agree - what I am saying is that there is a point of diminishing return and assertions of being able to achieve TN figures de facto just don't stack up in real life. Hence....we end up falling back on our RCD as primary not secondary protection - and in which case - we end up full circle back with what does it therefore matter if I've got 10 or 100 ohms, both are equally useless.

I give up!!

 

[Rockingit]

I give up!!

LOL! I know the feeling!! SO...please explain to me, in specific blow by blow detail so that there is no ambiguity to dispute:

Scenario;
a) TT, BS88:2 @ 80A
b) Incoming services all plastic
c) measured U of 245V
d) Resistive load on dedicated radial, B32, R1R2 value of 0.3 ohms
e) load becomes fault, direct R1>R2

Applying the general rules for ADS,
1) what is the outcome with an Ra at 20 ohms?
2) what is the outcome with an Ra at 200 ohms?