TT earth rod need to be visually verified?

[Golden_Boy]

That's a good value Ra you have there for a single rod. What sort of soil type do you have there a clay type soil or a sort of marshy land soil?? I reckon if you supplement your existing rod with another you stand a very good chance of a single figure value...

THe rod is well away from the propery and the earth is almost all damp soggy clay.

 

[IQ Electrical]

The reasoning behind my philosophy (which i totally stand behind) on having as low a Ra value as possible, is that the lower you can get that value, the better chance you have of forcing a protective device to trip, even if it is outside the required disconnection times. Your not going to achieve that with a single 1.2 3/8'' rod bunged in the ground i might add!! lol!!

So yes, an Ra of 200 ohm is very good (tongue in cheek ...lol!!) , but ONLY while an RCD device is functional!!! Totally useless without that RCD protection. And we should all know, that RCDs aren't the most reliable bits of kit at the present time, you only have to look at the threads in these forums, ...almost every day there is at least one thread or another complaining about failing RCD devices... I'd hate to totally rely on a RCD for my families protection, well i just wouldn't, full stop!!

Another of my hang-ups with TT systems, is the lack of understanding of what makes a good, or stable TT system. Most TT systems these days rely on a single 1.2m 3/8'' rod, bunged in the ground where ever it's easiest to drive it, and normally right by the side of an outside wall where all the old building rubble is normally found!!!

There are many ways of getting a TT system in a ''stable'' state and keeping it that way, not always practicable in domestic installations i might add!!! Which is why they should be considered as a last choice of earthing an installation, it is always better to have a TT systems supply PME'd if it is available on your DNO's local network....

IQ, ...I personally know of domestic TT systems in parts of Essex that by careful placing, 2 x 3m 3/4'' rods (extended) achieve an Ra equal or bettering a standard TN-S earthing arrangement, and that is stable year round without any soil conditioning, i jest not!! I'm sure that there are other areas in the UK, where similar results can be attained..

Oh I believe it, soil resistivity varies enormously.

I hear what you're saying on the RCD reliability point but you have to remember that on faults of negligible impedance, RCD's have a virtually identical type tested failure rate to BS EN 60898 devices.

Imagine if we could test MCB's in the same way we can test an RCD, just at In rather than the usual fault values of a negligible impedance fault, I think we'd see a rather frightening figure then!

A TT system installed as per BS 7671:2008 is perfectly safe and after 25 years in the electrical business, I have yet to hear of a death or injury as a result of a failed RCD on a TT system.

 

[Engineer54]

IQ,

Not so sure at all, about MCBs having a similar failure rate as RCDs, the technology is much older and most of the earlier problems with MCCBs and MCBs have been well and truly sorted over the years. I'm sure in the course of time, RCDs of all types will also be just as reliable as the modern breakers. From my own experience they most certainly are Not at the present time!!!

Never been one to put much credence to reports and surveys, after reading many of them, i can normally pull them to pieces. You can make these things say, or steer them towards whatever you want them to say, very much in the same way creative accounting can make a poor set of figures say, ....All Is Good!! ...or Vicea Versa lol!!!

As for deaths and injuries from failed RCDs on TT systems, who knows, it's not news worthy news these days, these events (if they happen) just wouldn't attract media attention. It would probably take several days to officially determine cause one way or the other, which would further cloud the waters...lol!! Same as PME really, there are plenty of sparks that'll stand-up and tell you they have known of several cases of neutrals being lost, but have they, and have anyone been injured or killed in these so called known instances??? Again who knows ...lol!!

What i do know, is that i personally will always do my uppermost to get as lower Ra as possible. As far as i'm conserned total reliance on an RCD in a domestic situation is something to be avoided whenever and wherever possible. I know and am aware, that in the present economic climate, it can be hard to justify the time and expense needed for installing a decent TT system, especially on a domestic. But in some circumstances, such as these caravan site installations on recent threads, i would definitly be going that extra mile to get those lower Ra values....

 

[Engineer54]

THe rod is well away from the propery and the earth is almost all damp soggy clay.

Your luckier than most GoldenBoy, the electrician took the time to find a decent area to drive his rod, and well away from any builders rubble!! As i said mate, if you choose to drive another rod outside of the area of influence of the existing rod, you stand a very good chance of bringing your Ra down to a single figure.... well worth the effort in my opinion...

 

[IQ Electrical]

Your luckier than most GoldenBoy, the electrician took the time to find a decent area to drive his rod, and well away from any builders rubble!! As i said mate, if you choose to drive another rod outside of the area of influence of the existing rod, you stand a very good chance of bringing your Ra down to a single figure.... well worth the effort in my opinion...

And a waste of time in mine

We'll agree to differ on this one I feel!

 

[Chr!s]

Oh I believe it, soil resistivity varies enormously.

Imagine if we could test MCB's in the same way we can test an RCD, just at In rather than the usual fault values of a negligible impedance fault, I think we'd see a rather frightening figure then!

A TT system installed as per BS 7671:2008 is perfectly safe and after 25 years in the electrical business, I have yet to hear of a death or injury as a result of a failed RCD on a TT system.

From several reports ive seen, its been a case where the Tester has failed and not the RCD, many people seem to misunderstand how installation faults affect RCDS not only upon testing but also how they may be affected in operation.

If people tested the RCDs in isolation i think far fewer would complain of failures.

I hear what you're saying on the RCD reliability point but you have to remember that on faults of negligible impedance, RCD's have a virtually identical type tested failure rate to BS EN 60898 devices.

Its a very interesting and valid point IQ.

 

[Engineer54]

And a waste of time in mine

We'll agree to differ on this one I feel!

We will indeed!! lol!!

 

[Engineer54]

From several reports ive seen, its been a case where the Tester has failed and not the RCD, many people seem to misunderstand how installation faults affect RCDS not only upon testing but also how they may be affected in operation.

If people tested the RCDs in isolation i think far fewer would complain of failures.

Its a very interesting and valid point IQ.

The last 3 projects or so that i've been on, the contractor was sending boxes of these things back for replacement. Several different types, all top of the range MG stuff too, from Vigi units to RCBOs, RCCDs to S types, all from single pole to four pole. ....All or most of them went through the contractors test benches before being shipped back. MCB and MCCB failures were next to none in comparison... As i say, i prefer to go by personal experience, than anything in a report or survey...