TT system

[Deleted member 9648]

Trying to be clever over a typo?? lol!! You stick to your 200 ohms Ra, your obviously a one, or single rod man.... good luck to you!!!

For your information, i am on a TT system here!! have brought Ra values down to below 20 ohms Iby the use of just 2 further rods to the system, on quite a few of my friends properties here. Mine is 3 ohms in winter and 5 to 8 ohms in the summer, but then i made the provision of having the steelwork of my house foundation to be available for connecting to my TT system, along with other structures steelwork on my property!! There is a hell of a lot more to TT systems than bunging a rod in the ground!!

Once you have a decent Ra value, you then have to maintain that value, ...Now how many electricians in the UK that quite happily create a so-called TT system, can instruct the home owner or client how to maintain those systems, ...not many at all, i fear!!!

Your right, i don't have the current BS7671 as yet, Does that mean i don't know what i'm talking about?? On this subject, i think i'll rely on my training, my working experience, and the knowledge i've gained over the 35 years i've been in this industry, ...after all, my employers are more than happy too!!!

If you actually had a copy of Bs 7671 you would realize that a value in the region of 20 ohms will result in a Zs far too high to operate an overcurrent device,even values of 5-8 ohms will be too high for most. Which means you are STILL reliant on the RCD to provide fault protection....which kind of negates your argument.

 

[Engineer54]

Whatever you say!! .... your obviously the expert on TT systems, ...as i say, you stick to your 200 ohm single rod installations and good luck to you!! ...lol!!!

 

[Deleted member 9648]

Good answer,a very helpful explanation of how a 20 ohm resistance is going to avoid the reliance on an RCD for fault protection.


FYI as an example a 16a type B mcb will require a max 2.87 ohms Zs to operate within the required disconnection time. There fore your efforts to achieve "a low Ra" of 20 ohms are entirely wasted as the circuit is still reliant on an RCD to achieve the required disconnection time.
The requirements for fault protection will be met with a Zs of 1667 ohms for a 30ma rcd,and 500 for a 100ma rcd. The reason a maximum 200 ohms Ra is specified is to allow for changing ground conditions,which will allow a considerable increase in ground resistance and hence Ra and still allow the fault protection device to meet the disconnection time.

Whether you like it or not that is the way it is.

 

[sparks1234]

200 Ohms is stated as the maximum value to be stable presumably because this will change between summer and winter
I really can't see why you have gone to the trouble of getting your own property to 5 ohms
Unless there is something you aren't telling us about your particular situation
The 200 ohm is there for a reason

 

[Engineer54]

I've consistently stated trying to get Ra values down below 20 ohms NOT to 20 ohms!!! Many of UK's old TT systems were around the 10 ohm mark some considerably lower, but then in those days, electricians knew how to make a half decent TT system, unlike today it seems!!!. I personally know of such systems in Essex that are comparable with TN-S values, year round!! Most of those older installations (before being converted to PME) had maximum fuse capacities of 15 amp, or in modernised installations of type 1 15A MCBs. No-one has ever stated that disconnection times will be achieved, but most of those MCBs will fall into the trip curves of protective devices, if an RCD should fail!! Many overseas countries have minimum standard Ra values of 20 ohms, the US being one of them i believe ...not this ridiculous figure of 200 ohms.... There is nothing funtionally stable about an Ra of 200 ohms, and you can take that to the bank!! ..lol!!!

So you re saying that a Ra of 200 ohms will allow what protection device to operate, ...a RCD?? ....As i have consistently stated, ...and if that device fails??? They certainly wouldn't allow any other protective device to operate, even if it was at 100 ohms!!
Zs is not Ra/Ze, TT systems will often have a decent Zs from the older metallic incoming services pipework, that will further improve disconnection times of protection devices..

Why did i bother to get my own TT system down to an Ra of 3 to 5 ohms?? I really don't know what to say here, never had someone ask such a daft question, and certainly not from any electrician!!! Save to say that my MCBs up to 16A meet disconnection times, and those that don't, will trip in tens of seconds if any of my RCD devices fail!!

Any earthing system, especially TT systems should be the best you can achieve. A TT system is not and never has been an ideal earthing system for most domestic/commercial installations. TT systems should Always be a considered a ''Last'' choice, and never a first choice, as it so often is these days in the UK, especially where out-buildings are concerned. With today's universal use of instantaneous shower units at crazy loading capacities you can't get away from total reliance on RCD devices, ...If anyone thinks that, that is a good place to be, then good luck to them, ...they'll need it!!! ...lol!!!

 

[Deleted member 9648]

I totally disagree.
Do the maths for yourself....you will find at the kind of Ra values you can reasonably be expected to achieve ,the fault current will take a long time to operate a 16a mcb,and wont operate a 32a one at all,by which time the occupants of the caravans will be fried.The RCD is the only way to clear the fault.
While I would accept that more than one rod may be beneficial to achieve a more stable Ra...there is absolutely no point in getting the values down to what you describe as 'functional levels' as they still will not clear a fault.
By the way,if you have access to a copy,have a look at GN7,which shows exactly this type of caravan site TT system serving multiple pitches.
It shows ONE electrode at the point of separation at the DB serving ALL the pitches. Perhaps you should send some "lol!!!" messages to the IEE as clearly they have no idea on TT systems.

 

[sparks1234]

I wont argue with the ins and outs of a TT system as I along with many other sparks rely on the regs books for guidance. We do appreciate youre expert wisdom as you obviously know a lot about TT systems, When ever we install a TT system, the lower the resistance the better, I think we would all agree with that but you cant get away from the fact that 200 ohms is stated as the maximum that should be regarded as stable. Personally I would look to improve things if they were getting towards 100 ohms, the highest I think we have had is 63 whuich is well within the recommended guidlines.
As for saying "what if" the RCD fails, well perhaps it wont but lifes like that

 

[Engineer54]

Well unless the t/c curves have changed ...which i very much doubt, then My 3 ohms will give me over 80 amps fault current, so check your T/C curves for type B MCBs. I use actual voltage seen on my system ....not virtual or make believe voltage. We see between 240 to 245volts as the norm here, i've taken the average of those voltages...
Zs can improve disconnection times also, maybe not in the case of these caravans ..but certainly in domestic surroundings...

Getting back to the caravan installation, depending on the resistivity of the ground, the connecting together of multiple rods around this caravan site, Ra values can be significantly reduced. Competent electricians will provide at least 1 rod at each connection unit or at each caravan pitch, just as the OP suggested in his original plan...


Oh boy, you really are a Stick to the Reg's man, no matter what!!! Look if you want to follow a daft statement or picture made in your bible, ...fine!! Sensible electricians will not be that naive, especially on a caravan installation!! Don't suppose it ever occurred to you that , the nice little drawing your referring too, was for illustration purposes only??

Haha, ...i've been a member of the IEE for more years than i care to remember, and i can assure you, ...there not always as right s you may think, not by a long chock!!! So you've never even come across any conflictions within BS 7671??

Look, the bottom line is this, while there are electricians that will do the absolute minimum where TT systems are concerned because of there total reliance on an RCD device, then the general public and there customers/clients will be put at risk. If your one of those electricians, there's not much point arguing with you, is there!!! You Know Best Right !!!

 

[Deleted member 9648]

Well unless the t/c curves have changed ...which i very much doubt, then My 3 ohms will give me over 80 amps fault current, so check your T/C curves for type B MCBs. I use actual voltage seen on my system ....not virtual or make believe voltage. We see between 240 to 245volts as the norm here, i've taken the average of those voltages...
Zs can improve disconnection times also, maybe not in the case of these caravans ..but certainly in domestic surroundings...

Getting back to the caravan installation, depending on the resistivity of the ground, the connecting together of multiple rods around this caravan site, Ra values can be significantly reduced. Competent electricians will provide at least 1 rod at each connection unit or at each caravan pitch, just as the OP suggested in his original plan...


Oh boy, you really are a Stick to the Reg's man, no matter what!!! Look if you want to follow a daft statement or picture made in your bible, ...fine!! Sensible electricians will not be that naive, especially on a caravan installation!! Don't suppose it ever occurred to you that , the nice little drawing your referring too, was for illustration purposes only??

Haha, ...i've been a member of the IEE for more years than i care to remember, and i can assure you, ...there not always as right s you may think, not by a long chock!!! So you've never even come across any conflictions within BS 7671??

Look, the bottom line is this, while there are electricians that will do the absolute minimum where TT systems are concerned because of there total reliance on an RCD device, then the general public and there customers/clients will be put at risk. If your one of those electricians, there's not much point arguing with you, is there!!! You Know Best Right !!!

Ok...so what you are saying is that Electricians that deem Bs 7671 to be the guide to correct installation methods are incompetant,that it consists of daft statements,and those who use it are naive and not sensible...and worse, compliance with it will result in the general public being at risk!!
Well,it's a point of view I suppose,but a somewhat surprising one from someone claiming to be a member of the body responsible for producing Bs 7671.

I think the majority of Electricians on here are 'sensible' enough to make their own minds up on whether you or Bs 7671 are the guide of choice.
(oh,and by the way....you seem to be conveniently forgetting again that there are 3 rcd's to fail on this install before your low Ra has to start performing,and in the real world 3 ohms is totally unrealistic)

Edit....I would not argue with your point about RCD's failing,they do on occasion..but the way to deal with that is more than one rcd...such as here where the suggestion is a time delay 100ma at the origin,and 30ma rcd's protecting the finals,the chances of two,(or 3) devices failing are remote to say the least. Going to excessive legnths to achieve an extremely low Ra,which is likely to vary with the seasons anyway,is pointless.

 

[sparks1234]

If you were to go back to the original question you might just realise that I was the OP of this thread and we are planning to put a rod per post.
I have not referred to any drawing anywhere in this thread.
It seems as though this has really got under your skin as you obviously carry out a lot of TT systems and cannot see anyone elses point of view and you see being sarcastic and trying to belittle myself and Wirepuller as the best option.
One other thing, we as electricians have to make decisions as to which way to wire something, perhaps the BRB may be wrong or right but if we do use it as a guideline then at least we are trying to do things as our "bible" suggests. I am not saying we should follow it like sheep but it is there for a purpose. That said, I do appreciate that we are on here looking for others opinions but I personally realise that we can all be mistaken and if for some reason something went wrong then the BRB is a lot better defence than "some bloke on a forum" said.

 

[Engineer54]

Sparks1234,

No-ones trying to be-little anyone... Yes your right, things like this DOE'S get under my skin. No-one seems to be interested anymore in getting such systems installed correctly. Like your friend, they take the minimalist route that's been mentioned in the Reg's, and think that's all that's needed !! The Reg's are ''minimum requirements'', they should be treated as a good guide ...Not as the all singing all dancing Bible ... they are not and never has been, or designed to be.

Where did i mention drawing above, i was referring to your plan as ''described'' by yourself in your posts on this thread. I'm also glad to hear that you plan is to provide rods at each location, are you intending on linking them too??

 

[sparks1234]

No, each position will be in excess of 30 mtrs apart and to link them will just be impracticle.
I appreciate your comments and thanks for your help, hopefully somebody can shut this thread down now, perhaps it has run long enough

 

[Engineer54]

Ok...so what you are saying is that Electricians that deem Bs 7671 to be the guide to correct installation methods are incompetant,that it consists of daft statements,and those who use it are naive and not sensible...and worse, compliance with it will result in the general public being at risk!!
Well,it's a point of view I suppose,but a somewhat surprising one from someone claiming to be a member of the body responsible for producing Bs 7671.

I think the majority of Electricians on here are 'sensible' enough to make their own minds up on whether you or Bs 7671 are the guide of choice.
(oh,and by the way....you seem to be conveniently forgetting again that there are 3 rcd's to fail on this install before your low Ra has to start performing,and in the real world 3 ohms is totally unrealistic)

Edit....I would not argue with your point about RCD's failing,they do on occasion..but the way to deal with that is more than one rcd...such as here where the suggestion is a time delay 100ma at the origin,and 30ma rcd's protecting the finals,the chances of two,(or 3) devices failing are remote to say the least. Going to excessive legnths to achieve an extremely low Ra,which is likely to vary with the seasons anyway,is pointless.

Your trying to be clever now, You know exactly what i was talking about in my post and it's nothing to do with what you've written here !!! I'm sure the electricians here ''Are'' sensible enough not to rely on a single rod as suggested by yourself on the basis of a picture in a IEE publication and statements in the Reg's that sometimes have no baring on the installation at hand ....


You just don't get it do you, Don't install anything that you can't get the best out of. If your just going to go through the motions to satisfy a minimal requirement in the regs that will in most cases have no baring to the installation your involved with, then your failing yourself and your client /customer....

You will never know if 3,5,10 ohms is unrealistic or not, as you'll never even try!! Your not even interested in how to go about getting a good TT system!!! Yes in this case there may well be 3 RCDs, but that wouldn't matter to you if there was only one, you would still be just providing the one rod, cause that's what the the Regs say ...Right?? And that is what i've been talking about all along!!! So as said before, you stick to your 200 ohms and hope for the best... fingers crossed and all that!!!

 

[sparks1234]

Your trying to be clever now, You know exactly what i was talking about in my post and it's nothing to do with what you've written here !!! I'm sure the electricians here ''Are'' sensible enough not to rely on a single rod as suggested by yourself on the basis of a picture in a IEE publication and statements in the Reg's that sometimes have no baring on the installation at hand ....


You just don't get it do you, Don't install anything that you can't get the best out of. If your just going to go through the motions to satisfy a minimal requirement in the regs that will in most cases have no baring to the installation your involved with, then your failing yourself and your client /customer....

You will never know if 3,5,10 ohms is unrealistic or not, as you'll never even try!! Your not even interested in how to go about getting a good TT system!!! Yes in this case there may well be 3 RCDs, but that wouldn't matter to you if there was only one, you would still be just providing the one rod, cause that's what the the Regs say ...Right?? And that is what i've been talking about all along!!! So as said before, you stick to your 200 ohms and hope for the best... fingers crossed and all that!!!

Are you talking to me or Wirepuller?

 

[Deleted member 9648]

The comments are aimed at me.
I'll bin my copy of the BRB in the morning.