No one near me wants this job - New ground rod

[ericmark]

Well if you are reying on an RCD

This is the argument against TT, and I would be testing the RCD far more often with a TT supply. With my TN-C-S supply, I am not really worried about type AC RCDs as they are secondary protection, but with a TT supply I would be changing to type A at least.

 

[JohnW2]

Well if you are reying on an RCD (which is almost always the case with an TT earth rod or plae etc) then an Earth Electrode tester and an Earth Loop tester is probably the best way to consider. ... You also need to consider the variation that might occour throughout the year (or its lifetime!).
Above 200 ohms should cause concern but above 100 ohms would be a better benchmark. ... Around here 50 to 60 ohms is usually achieveable ....

All true - and I think most of us would like to see less than 100 Ω - not the least because, as you imply, the figure might increase appreciably over time, and particularly during periods of very dry weather.

However, to put things into some perspective, even if supply voltage were as low as 216.2 V (the lowest 'permissible') the EFLI (hence the rod's resistance/impedance) would have to be above 7,000 Ω to prevent RCDs working as they should.

Kind Regards, John

 

[ebee]

Yes John.
But it`s like as me old grandad used to say when drinking pints with his pals in the pub - "Never trust a man who drinks halves!"

 

[ebee]

, I am not really worried about type AC RCDs as they are secondary protection, but with a TT supply I would be changing to type A at least.

with TT I`d be worried about the 7% possible failure rate and have two RCDs in tandem (hopefully different locations if one is a kitchen)

 

[ericmark]

with TT I`d be worried about the 7% possible failure rate and have two RCDs in tandem (hopefully different locations if one is a kitchen)

Yes, the idea of a 300 mA RCD before the CU doubling as an isolator does seem to make sense. But where do you stop?

 

[JohnW2]

with TT I`d be worried about the 7% possible failure rate and have two RCDs in tandem (hopefully different locations if one is a kitchen)

That "7%" figure has been knocking around for very many years and I think was based on a fairly limited survey (and, I believe, not even in UK, and probably related to 'slightly out-of-spec' as well as 'not working at all'). I would therefore suggest that we "don't really know".

It sounds as if you probably would not have similar worries with TN - does that mean that you have reason to believe that the 'failure' rate of ('essentially untestable') MCBs is less than 7% (or whatever)?

Having said all that, I cannot knock the concept of redundancy in 'safety' systems - so could not argue against having pairs of RCDs (or MCBs) in series. However, whether the risk of faults being un-cleared by faulty devices (RCDs or MCBs) is high enough to justify such a practice is, I would have thought, pretty debatable.

Kind Regards, John

 

[ebee]

That "7%" figure has been knocking around for very many years and I think was based on a fairly limited survey (and, I believe, not even in UK, and probably related to 'slightly out-of-spec' as well as 'not working at all'). I would therefore suggest that we "don't really know".

It sounds as if you probably would not have similar worries with TN - does that mean that you have reason to believe that the 'failure' rate of ('essentially untestable') MCBs is less than 7% (or whatever)?

Having said all that, I cannot knock the concept of redundancy in 'safety' systems - so could not argue against having pairs of RCDs (or MCBs) in series. However, whether the risk of faults being un-cleared by faulty devices (RCDs or MCBs) is high enough to justify such a practice is, I would have thought, pretty debatable.

Kind Regards, John

Yes John I agree with all of your points there.

Yes surveys are limited so it`s a guess at best.

RCD failure is not working to spec not just non functioning at all therefore could still save a life or worst effects anyways.

No we do not test MCBs out in the field, we rely on type testing - how accurate is that?