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I think I understand both E54 and wirepullers comments but it's a shame to see such a disagreement between such valuable members.

It's just a storm in a Tea Cup shortcircuit!! ...lol!!!
 
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I agree 1SC,

I can see both sides of this argument, and neither side are wrong per se.

E54's argument is correct in that stability is the main factor (IMO), I would rather have a rod that gives an Ra of a 100 ohms but was deep enough to be stable and not fluctuate than a shallow rod that read 50 ohms that is likely to increase in the dry or frozen seasons.

The other posters are correct too, in that it does not really matter whether the rod measured 20 ohms or 200ohms, in the sense that neither will operate the OCPD device directly, and both will rely on the RCD to achieve disconnection.

As I said stability is the key here.

Incidentally, the 16th ed amd.1 OSG gives 220 ohms as the max recommended Ra, this OSG uses 230V as the Uo for the calcs.
So which edition Regs gave the lower 10 ohms figure E54 ?, I have seen this figure given for generators and the like, but not for general TT systems that used RCDs to achieve the disconnection times.
 
I agree 1SC,

I can see both sides of this argument, and neither side are wrong per se.

E54's argument is correct in that stability is the main factor (IMO), I would rather have a rod that gives an Ra of a 100 ohms but was deep enough to be stable and not fluctuate than a shallow rod that read 50 ohms that is likely to increase in the dry or frozen seasons.

The other posters are correct too, in that it does not really matter whether the rod measured 20 ohms or 200ohms, in the sense that neither will operate the OCPD device directly, and both will rely on the RCD to achieve disconnection.

As I said stability is the key here.

Incidentally, the 16th ed amd.1 OSG gives 220 ohms as the max recommended Ra, this OSG uses 230V as the Uo for the calcs.
So which edition Regs gave the lower 10 ohms figure E54 ?, I have seen this figure given for generators and the like, but not for general TT systems that used RCDs to achieve the disconnection times.

Long before the 16th edition, more like in the 13th edition times. Not even sure if it was mentioned in the reg's per say, but the 10 ohm value was pretty much standard throughout the industry for ''All'' TT type systems at that time, including lightning protection systems. ...Now that hasn't changed any, that's still a Max 10 ohms too, if i'm not mistaken....
 
Long before the 16th edition, more like in the 13th edition times. Not even sure if it was mentioned in the reg's per say, but the 10 ohm value was pretty much standard throughout the industry for ''All'' TT type systems at that time, including lightning protection systems. ...Now that hasn't changed any, that's still a Max 10 ohms too, if i'm not mistaken....

Thanks E54, it was a genuine question, it is not actually mentioned in the 16th ed reg books I have (amd1 &2) either, but is mentioned in the respective OSG's, I cannot find my 16th ed GN3, and I have no other GN's other than the two 17th edition sets.

I was just intrigued to where the figure came from.
 
I think these discussions and disagreements are the strength of the forum-without them we could just produce an FAQ based on BS7671 and be done with it ;)

As has been stated many times above, stability is the key and on that front, I agree with E54 on the 'twigs' thing.
I keep asking for any documented evidence of any injury or incident concerning a TT system with a failed RCD but there simply isn't any available because the system actually works in it's present form and has done for many years now. I think part of the reason for that might be the fact that although an RCD might fail an instrument test at say 5X IΔn, how many faults are actually of that low magnitude?

I often ramp test failed 30mA RCD/RCBO devices and it's quite surprising to see how many still trip around the 200-500mA range and virtually all on the way up to 1000mA.

For the record, I'd never say 'no need to bother trying for a lower Ra value' and neither as I recall has Wirepuller ever implied that but his point about the futility of hitting 30, 40 or 200 Ohm values of Ra is absolutely correct!
 
Thanks to all for the constructive comments rather than a slagging match..:smiley2:...as this is probably the only subject on which I have a major dispute with anyone on here it's something perhaps this thread has cleared up for me, and I'll try and leave it alone now...:8:.(cue big cheers)
Re post 16....I suppose you'd have to ask yourself if the 200 ohms is ever likely to get to 1667 ohms.....as it would need to get above that before you might have a problem with the Ra value affecting a 30ma RCD....(not advocating such high values as ok before anyone asks!)
 
Technical issues aside, the thing the surprises me is the the UK regs are happy to put all their eggs in one basket and rely solely on the functionality of the RCD when it comes to the acceptable TT figures whereas pretty much everywhere else in the regs there's a belt and braces approach wherever possible. I can't help wonder why this is the exception to the rule...
 
Technical issues aside, the thing the surprises me is the the UK regs are happy to put all their eggs in one basket and rely solely on the functionality of the RCD when it comes to the acceptable TT figures whereas pretty much everywhere else in the regs there's a belt and braces approach wherever possible. I can't help wonder why this is the exception to the rule...

And like everywhere else in the world of any consequence, i'd be willing to wager a months salary, that your max Ra value isn't 200 ohms. ...lol!!! You also probably have a requirement for an up-front RCD, regardless of any downstream RCD devices being present??
 
Technical issues aside, the thing the surprises me is the the UK regs are happy to put all their eggs in one basket and rely solely on the functionality of the RCD when it comes to the acceptable TT figures whereas pretty much everywhere else in the regs there's a belt and braces approach wherever possible. I can't help wonder why this is the exception to the rule...

Agree....As you know I have no issues with the preferred means on disconnection being an RCD (which allows high Ra values) but it is a mistake to allow a single RCD.....it should be a requirement for more than one device,ideally a 100ma S type main switch with 30ma to final circuits.
 
After reading all of these post it reminds me why I always try to get a PME supply put in by the DNO, then I've washed my hands of the responsibility, over to you DNO!
I do live in rural Wales though so have plenty of TT experience, and I've learnt that when it comes to solid slate they're best avoided!
There as always are difference's of opinion, and a lot of discussion, as to whether the BS7671 is correct in certain area's. But we have to remember that should somebody be hurt or killed by an electrical installation its the BS7671 that will be used to prosecute us. Whether we agree with it or not. And if its not followed then its porridge and water!
 
we have to remember that should somebody be hurt or killed by an electrical installation its the BS7671 that will be used to prosecute us. Whether we agree with it or not. And if its not followed then its porridge and water!
More like Playstations and university degrees mate:)
 
One has to wonder why, that old 10 ohm value was ever dropped, as that value had been the norm for years, long before i came into the industry, (and it was taken as the MAX and not the cut-off point) and long before RCD's were even thought about. One can only assume, mainly because of the common use of RCD devices. Which is fine to a point. But still foolish to then totally rely on a single RCD for your earth fault protection throughout a complete installation. Then you have the advent of these non extendable short thin twigs to replace 8 and 10 foot rods, and you can see straight away, that things have gone drastically down hill in a relatively short period of time in the UK...

I don't and can't accept that because people haven't read of instances of injury or accident or whatever, in publications it has never happened. That would be to assume that every accident or occurrence would automatically be reported and investigated, which is simply never going to happen in the real world. When accepted electrical theory tells us, what will or can happen in the absence of a fully functional RCD device and an earth rod resistance that is so high as to be totally useless. So the present system only really works when the RCD device is functional, it won't and can't work when it isn't. We know that the chances of a broken neutral on a PME/TNC-S as being, to say the least, remote, ...but we still take precautions against that possibility!! So why is there a reluctance to accept taking similar steps with TT systems??....

At the end of the day, the more effort and application put into creating a functional and working TT system, the better that TT system will be. You don't have to necessarily go overboard, just careful positioning of the rod, or made electrode, ensuring to the best of your ability the electrodes overall stability, a little knowledge on ground and/or rod enhancement where required, and you won't go too far wrong!! You will not achieve any of this using 3/8'' short twigs or being satisfied with a 200 ohm or thereabouts Ra level.... And you can take that to the Bank!!!
 
Another spin-off of a high impedance earth that hasn't been mentioned is that it sets the scene for accelerated corrosion of copper pipework and hot water cylinders or immersion heaters etc. Whilst this may not be a direct electrical safety issue it would certainly be a consideration when coming up with a regulated acceptable figure.
 
Another spin-off of a high impedance earth that hasn't been mentioned is that it sets the scene for accelerated corrosion of copper pipework and hot water cylinders or immersion heaters etc. Whilst this may not be a direct electrical safety issue it would certainly be a consideration when coming up with a regulated acceptable figure.

Marvo, Jessus, ...Don't go throwing them another spanner into their works!! lol!!! :rolleyes2:
 
Think i should have posted this on a TT system thread instead of crimping tool thread, where i came across this bit of useful kit...lol!! Anyway i've done so now.... lol!!

How about this little bit of kit. With a bit of adapting to extend the drill bit(s) this could be a god send for TT system installations. Perfect for Bentonite and conductive mortar applications, or even part bored, part driven earth rods. The bored section backfilled with Bentonite, keeping the conductive moisture content constant for long periods of time....

Well worth having this bit of kit, if you do a fair amount of TT systems in your area... Plus, as a bonus, you can even do your own fence posts etc, as well as a few other things...lol!!!!

NEW GROUND HOLE DRILL 52CC | eBay


 
I've seen one of those in use, well actually it was a lot larger but it was on that TV program (decked out or something) LETHAL bit of kit lol

No idea about UK TV channels and programmes these day's. lol!!

This bit of kit is not that too dissimilar, to those that are used professionally for drilling earth rod bores. Though certainly bigger and more powerful than this small hand unit... This units drill bit's just need extending by either welding or by threading modifications...
 
After reading all of these post it reminds me why I always try to get a PME supply put in by the DNO, then I've washed my hands of the responsibility, over to you DNO!
I do live in rural Wales though so have plenty of TT experience, and I've learnt that when it comes to solid slate they're best avoided!
There as always are difference's of opinion, and a lot of discussion, as to whether the BS7671 is correct in certain area's. But we have to remember that should somebody be hurt or killed by an electrical installation its the BS7671 that will be used to prosecute us. Whether we agree with it or not. And if its not followed then its porridge and water!
BS7671 will be used to prosecute us eh?....don`t think so love...BS7671 can be used to show compliance with the leccy at work regs 1989...but thats all....BS7671 is a guidance note lass......and non-statutary n all.....EAWR1989 will be the one thats used in court.....
 
BS7671 will be used to prosecute us eh?....don`t think so love...BS7671 can be used to show compliance with the leccy at work regs 1989...but thats all....BS7671 is a guidance note lass......and non-statutary n all.....EAWR1989 will be the one thats used in court.....

The problem being, whilst you may be correct... The prosecutor will appoint someone who can tie you up in knots with the BS7671. IF you've not worked to the regs then you will have to explain why and defend that it complies with or is better than what BS7671 states.

Also, IIRC Building Regs, which IS LAW, states that all work must conform with BS7671 so you're in the deep if you don't really and why chance it?

I personally would not like to EVER be in that situation:thumbsup
 
yeah well i know that working to one will/should demonstrate compliance with the other...but the point i was making is it will be the EAWR1989 that has the final say in court...not BS7671.....thats used to show compliance with EAWR.....
 

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