External earth fault loop tt system

[brightspark2010]

Evening all could anyone please tell me the the maximum ze for tt systems the jobs entail Rec cut out fuse bs88 16a to feed 1x 30 ma rcd fuse spur for feeding socket outlet what should the max ze be please

 

[pete555]

remember you have to satisfy loop impeadance values to protect from over current

 

[Engineer54]

I don't agree with the statement that the ground consolidates especially when (very rarely) we have a long dry spell and the ground starts to crack and gaps appear. I realise the rod is 1200 mm long and the ground won;t dry out for the full depth but it could go down about 300mm thus reducing the contact with the rod.

This is fundamentally the problem, where most of you sparks go wrong right from the word go. You will never achieve any form of stability with a single 1.2m rod of any size, and that's a fact!! I have no idea why in fact earth rods were ever reduced from what they used to be, (8 and 10 foot X 5/8'' /3/4'') to these tiny 1.2m lengths you have now. You Need a good 8' of rod in the ground whenever possible, or there isn't going to be any stability in the system!! Fortunately depth also reduces a rod positions Ra value too, so it's the way to go!! So don't use a single 1.2m rod for a TT system, always use at least 2 X 5/8'' rods coupled together with a coupler as an absolute minimum....

So you don't believe that the ground/soil consolidates around an earth rod over time?? Perhaps you don't believe in ground settlement either?? It's a fact i'm afraid!!

 

[Engineer54]

This is where we disagree. I think they're perfectly suitable and defines the needs of a TT system quite clearly. The only bit I'm not quite sure on is the 200 ohms? Seems like a randomly generated value and in my opinion should be replaced with a calculation.

Really?? We must be looking at completely different versions then ...lol!!



And this type of remark helps no one. If I didn't like you and respect your knowledge I would be calling you an arrogant twit right about now

Really?? We must be looking at completely different versions then ...lol!!

Nothing arrogant in that statement at all, the whole section on TT systems is helpful to no-one, and why we get so many queries about TT systems on the Forum!! But i'll agree with you on the 200 ohms, it can only be a random figure pulled out of the air for what good it will ever do!! By the way, i may be many things, but what i'm NOT, is anybodies fool. I've been involved in the constructing of TT systems most of my working life, so i don't need a Reg book to tell me anything about what's needed and why. Now if that's being arrogant, then so be it!! lol!!

 

[itselectric]

This is fundamentally the problem, where most of you sparks go wrong right from the word go. You will never achieve any form of stability with a single 1.2m rod of any size, and that's a fact!! I have no idea why in fact earth rods were ever reduced from what they used to be, (8 and 10 foot X 5/8'' /3/4'') to these tiny 1.2m lengths you have now. You Need a good 8' of rod in the ground whenever possible, or there isn't going to be any stability in the system!! Fortunately depth also reduces a rod positions Ra value too, so it's the way to go!! So don't use a single 1.2m rod for a TT system, always use at least 2 X 5/8'' rods coupled together with a coupler as an absolute minimum....

So you don't believe that the ground/soil consolidates around an earth rod over time?? Perhaps you don't believe in ground settlement either?? It's a fact i'm afraid!!

I 've installed a few of these in my early days didn't put many 8ft ones in though about 3 I think.
they used to be terrible if you hit a bit of stubborn ground. the steel cap that was screwed on to help not deform the top of the rod didn't always work,
We were putting these in when voltage operated trips were the norm. That was because there was a higher current in the main earth when a fault occurred. Not long after they were outlawed(Voltage trips) it was decided the rods did'nt need to be that big because of the smaller current needed and the speed of the R.C.D.
As for the ground consolidating I've been on some rewires where I 've pulled the old rod out of the ground with little or no resistance. So I stand by what I say.
One thought though is why the pulled out easy in the first place. Could this have been the installers fault?

 

[Deleted member 9648]

I installed a rod a few weeks ago at a sewage plant serving a pub...(following on from an EICR). The reading from the existing spike was 700 odd ohms,no idea what the rod was as it was completely buried. Replaced it with a coupled rod beloved of E54 and got something like 15 ohms....happy days. But let me state now I'd have been just as happy with 115.
Why everyone keeps banging on about the importance of stability is another thing that defeats me.....yes,if you are going to rely on a low enough Ra to operate an OCPD the stability is vital....if your Ra means one or more RCD's are needed for earth fault protection why does stability matter?....it can be 10 ohms or 500....the RCD will still trip the same.
I think this is where the 'plucked out of the air' 200 ohms figure has been arrived at.....Ra values below that are not likely to exceed the 500/1667 value required for safe operation of RCD's even if ground conditions change.

 

[D Skelton]

Nothing arrogant in that statement at all

What's arrogant is this notion that just because you've got loads of experience dealing with TT systems you are right and everybody else is inexperienced, misguided and wrong lol (I don't mind so much coz I know you know I'm not a complete ignoramus). I however see things differently, I see your knowledge as good and I don't see your opinion as being wrong. I also think you'll find that when it comes to TT systems we actually agree on more than we disagree on, in the same way that some scientists are convinced by quantum theory and some are convinced by string theory. Both groups have an excellent fundamental understanding of the universe but both choose to describe it's minute operation in slightly different ways. Neither is right, and neither is wrong, and by the same token, I believe that people who understand TT systems the way you do add just as much valuable knowledge to the pool as people who understand TT systems like myself and wirepuller.

I love a good debate but the minute anyone starts to try and devalue an opponents viewpoint instead of continuing to bolster their own, in my eyes their opinion starts to lose credibility. Now I'm not saying this is the case here, I've known for a long time how strongly you feel on some matters regarding the installation of TT systems. My point is, you can't accuse someone of being outright wrong when you can clearly see that they too have a fundamental understanding of what a TT system is and how it operates but just chooses to disagree on one or two points

 

[Engineer54]

I installed a rod a few weeks ago at a sewage plant serving a pub...(following on from an EICR). The reading from the existing spike was 700 odd ohms,no idea what the rod was as it was completely buried. Replaced it with a coupled rod beloved of E54 and got something like 15 ohms....happy days. But let me state now I'd have been just as happy with 115.
Why everyone keeps banging on about the importance of stability is another thing that defeats me.....yes,if you are going to rely on a low enough Ra to operate an OCPD the stability is vital....if your Ra means one or more RCD's are needed for earth fault protection why does stability matter?....it can be 10 ohms or 500....the RCD will still trip the same.
I think this is where the 'plucked out of the air' 200 ohms figure has been arrived at.....Ra values below that are not likely to exceed the 500/1667 value required for safe operation of RCD's even if ground conditions change.​

​

You see that's where we completely differ, your all too willing to put your faith in RCD's instead of making the effort to have a TT system work as it should!! You may well install an S type up-front, to cover any down stream RCD's, but lets be honest now, ...how many UK electricians, let alone so-called DI's would even think about installing one?? Most of them are convinced that RCD's are as reliable as MCB's so why would they bother!! Again, having so easily obtained a 15 ohm reading, means that your ground conditions are pretty good, (and that the existing rod is probably a twig that hasn't been installed properly) so i'd of put a little more effort into the system. Who knows, that TN value could well have easily been realised!! lol!!

To be totally honest, i don't really know why you bother installing a TT system if that's your honest view. If your not bothered about Ra values and the importance of stability, why not just add your RCD's and let any bonding to the installation be your reference earthing. Using your argument the 700 ohm Ra from the existing rod is as good as any, it's under your magic 1667 ohms ...lol!! All in all, it's a sad day, when other international earthing standards can teach the UK a thing or two, especially when in many cases, their standards are actually based or taken on/from past British standards!!

On all the projects i've ever been involved on, i can't think of a single one where the specifications didn't call for any earthing system to be higher than 1 ohm. That value covered TX installations, as well as other made earthing systems... and your trying to defend your Reg's and it's 200 ohms!!!! ....Quite amazing!!

As the old saying goes, ...you can lead a horse to water but..... lol!!

 

[Engineer54]

I 've installed a few of these in my early days didn't put many 8ft ones in though about 3 I think.
they used to be terrible if you hit a bit of stubborn ground. the steel cap that was screwed on to help not deform the top of the rod didn't always work,
We were putting these in when voltage operated trips were the norm. That was because there was a higher current in the main earth when a fault occurred. Not long after they were outlawed(Voltage trips) it was decided the rods did'nt need to be that big because of the smaller current needed and the speed of the R.C.D.
As for the ground consolidating I've been on some rewires where I 've pulled the old rod out of the ground with little or no resistance. So I stand by what I say.
One thought though is why the pulled out easy in the first place. Could this have been the installers fault?​

​

A few of what?? I can't imagine those thin twigs were about in the day's of VOELB's. I've personally still never seen one, ...until recently i'd never seen a 1.2m 5/8'' rod either!! lol!! That was in a wholesalers in Cyprus, who told me they are only used as an extension to their standard 2.4m rods!!


If you could pull an earth rod out, then it was never installed properly in the first place and/or more likely installed close to the building external wall, where all the builders rubble would have been dumped!! Sorry, but ground consolidation is a known fact of life whether you believe it or not!! lol!!

 

[Engineer54]

Firstly not all TN circuits will have RCD protection,and secondly there are maximum Ze values for TN systems,therefore if a Zs reading was considerably higher than the maximum for a standard OCPD something is clearly not right.That said I would have no problem relying on an RCD to meet disconnection times wher a type B needed to be changed for a type C/D and Zs slightly exceeded the max permitted value.

As I've already stated...if you achieve a TN value of Ra I'm with you all the way......but thinking 10...20...30 ohms is any better than 100-200 ohms ,I'm not.

I must have missed this post!!

Most, if not ALL will be, on a domestic installation if i'm not mistaken!!

Clearly not right, but still protected by a RCD with it's magic 1667 ohm Zs, so complies completely with your analogy!!

All depends what you mean by ''slightly exceeded'' replacing a type B with a type D MCB and the difference can be substantial!! And if Zs values don't comply, then disconnections times will be extended!! But again, no problem we have a RCD to cover it!! lol!!

 

[telectrix]

much more of this thread and i will be tempted to bang a scaffold pole in the garden, hopefully will miss the incoming cable, gas main, water main , and drains. aiming for a Ra comparable to the Ze i've already got from a 951 on the lead sheath. :ack2:

 

[itselectric]

engineer 54, from day 1 of my apprenticeship it was drummed into me that the regs are the electricians bible and even though they are not a legal document they can be used in a court of law.
So if the regs recommend that a circuit be installed in a certain way, then it is probably advisable to follow them.
We have been treat like sheep from day 1 and to deviate from this path would be a tad risky, in thatif something went wrong through no fault of your own you could find yourself up s...t creek without a paddle

 

[Engineer54]

engineer 54, from day 1 of my apprenticeship it was drummed into me that the regs are the electricians bible and even though they are not a legal document they can be used in a court of law.
So if the regs recommend that a circuit be installed in a certain way, then it is probably advisable to follow them.
We have been treat like sheep from day 1 and to deviate from this path would be a tad risky, in thatif something went wrong through no fault of your own you could find yourself up s...t creek without a paddle

It doesn't take much to improve on anything it states in BS7671!! As far as the section on TT systems, you would be hard pressed to actually NOT conform! In fact, i would go as far as saying you would have to go out of your way to do so!! lol!!


BS 7671 is basically minimal requirements to be met, there is nothing wrong in deviating from the path, as you put it, if you can substantiate or improve on what is being called for!! There are for example many non-standard circuits that have no mention or reference in either the Reg's or it's various OSG's but conform to all aspects of basic circuit requirements...

 

[richy3333]

I have no idea why in fact earth rods were ever reduced from what they used to be, (8 and 10 foot X 5/8'' /3/4'') to these tiny 1.2m lengths you have now.

Rumour has it that the rod length was shortened so all those DI's could get the shorter rods into their ex-company cars

 

[Stroppy]

I reckon TT systems should be outlawed and everything made TN.

To me it seems archaic and unreliable expecting something as important as fault current to somehow dribble its way through soil, rock, sand, concrete, and whatever other crap lies between an installation and its transformer, when there is a perfectly good neutral connection going back to it anyway, which doesn't get worse or disappear in hot weather.

 

[Silly Sausage]

much more of this thread and i will be tempted to bang a scaffold pole in the garden, hopefully will miss the incoming cable, gas main, water main , and drains. aiming for a Ra comparable to the Ze i've already got from a 951 on the lead sheath. :ack2:

I'd have to be careful banging a rod in at my house.
The Manchester Water Ring Main runs alongside it.
It'd be fun puncturing that!