TT or TNCS

[La Poste]

Greetings.
I am working at a friend of the families house helping to make the place safe and will be shortly arriving at the decision of what earthing system to use.
The house allready has an earth rod although the readings on it are rubbish.
The main bonding is giving an earth fault loop impedance of 18 ohms, much better than the rod I might add.
The suppliers have said TNCS is available but after reading much on the subject of earthing I'm a little undecided.
I have read about losing a neutral on the siupply side that is pretty much catastrophic if you have TNCS.
Also the neightbours electrical standards are pretty low from what I have heard and with TNCS I might be importing a whole lot of aggro from next door.
The thing that puts me off the earth rod is the high earth fault loop impedance. If the gentleman in question had a faulty RCD then if he had an earth fault no MCB's would trip, the earth fault current wouldn't be high enough with an earth fault loop resistance of 18 Ohms.
In doing my research I have realised some people have very strong views on this subject so I am asking which earth method would you prefer?
Thanks.

 

[telectrix]

TT is OK as long as everything in the house is on RCD. The RCD then takes the place of the MCB as regards to fault current ,L/E, tripping. If your Ze is <200ohms, OK

 

[jmtisme]

Tncs as I don't have any option here really

 

[nickblake]

Much prefer TNCS but a TT reading of 18 ohms is very good max is 200 ohms , but you must have RCD protection covering the installation IE 17th edition dual RCD board ,as a main switch RCD no longer complies

sorry think i miss read did you do a Ze test with the ROD and get 18 ohms ???

 

[La Poste]

TT is OK as long as everything in the house is on RCD. The RCD then takes the place of the MCB as regards to fault current ,L/E, tripping. If your Ze is <200ohms, OK

Thanks mate.
I'm also thinking of the long term. I mean all is fine with a new board and new RCD's but how will it fair in a few years?
Just how reliable are RCD's in the long term?

 

[telectrix]

seems from what you say, you're getting a reading from main bonding of 18ohms. what does the actual Ze read with the rod dissed from the earth bar?

 

[nickblake]

Thanks mate.
I'm also thinking of the long term. I mean all is fine with a new board and new RCD's but how will it fair in a few years?
Just how reliable are RCD's in the long term?

they are pretty reliable ive tested some 20 year old RCD 's and they still work perfectly but a must if you want to comply with the regs

 

[La Poste]

seems from what you say, you're getting a reading from main bonding of 18ohms. what does the actual Ze read with the rod dissed from the earth bar?

Yes the bonding is 18 Ohms the earth rod is about 300 Ohms, it wasn't installed properly and is way too short that's why I started this thread. Do I go out and get a really long rod and dig a pit and do a proper job or just TNCS the place?
I'm really torn, I like the idea of independent earthing but I'm just not sure.
I've read some real horror stories of what could happen if you lose a supply neutral on TNCS.

 

[telectrix]

get a good 1meter long rod, wet the earth then test. Beauty is if the main bonding is in 6mm and is still good, it can stay.

 

[pushrod]

get a good 1meter long rod, wet the earth then test. Beauty is if the main bonding is in 6mm and is still good, it can stay.

For safety you really want a worst case scenario ie a reading in dry conditions.

If you are really worried about an rcd failing with a TT system i don't think there is anything stopping you putting an rcd in front of the new(?) consumer unit. There will be no discrimination between them but it will just mean that in the event of an earth fault there will probs be 2 rcds to reset.

 

[Amp David]

All this worry over loss of the supply neutral, isn't this one reason your main bonds should be good and in plce?

 

[La Poste]

Has anyone ever known a supply to lose a neutral, what were the consequences?

 

[malcolmsanford]

In all the years I've worked with PME have never lost a neutral personally but that isn't to say it can't happen. But if your bonded and earthed you should not import a fault from another system really.

I have to say I have no problems with either system, but if I had a choice I would have to say TNC-S this then puts the responsibilty of the earth on the DNO. Not sure about the other guys but I have lost count over the years the amount of TT systems that the earth had either rotted off the rod or even been removed.

You say your worried about RCD'S and how long they last, well to be honest your suppose to test them mechanically ever 3 months, and a recommended PIR should be done every 10 yrs. As it's your relative house then you most likely will do this. I always advised the customer about the mechanical test and showed them how to do it, but know full well who is going to reset all those timers every 3 months!!!

 

[La Poste]

I'm also a little concerned about the earth fault loop resistance on a TT and how with a phase to earth fault in an installation with a rod of say 100 Ohms non of the MCB's would trip on overcurrent making their ability to trip on a phase to earth fault effectively useless.
This is what concernes me the most, their will only be one level of protection on a TT and not the two of RCD's and MCB's acting against an earth fault on TNCS.
I would like to think the DNO's regularly inspect their own equipment to make sure their earthing system is fully sound but I can't see your average Joe inspecting their own in the back garden earth rod regularly at all.

 

[telectrix]

with a TT syatem, you rely on the |RCDs to trip in case of a fault current to earth. The Mcbs cover L/N overcurrent/short circuit