Earth electrode tester or not?

[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?