TN-S Earthing

[Gardner]

Are earth rods or earth electrodes required for TN-S supply?

 

[Gardner]

Trying to reconcile the NEC section 500 with the UK’s BS7671 is just going to give you a headache.

The NEC recognises a metal water pipe as an electrode, BS7671 doesn’t.

The last time I got into this discussion was on an American site, I’m not going along that path again, it dragged on for ages.
Some thoughts they held about TT systems would make them hero’s to our Electrical Trainee’s. A high resistance ground was to limit fault currents???

Electrical Trainee's?

 

[Des 56]

Electrical Trainee's?

Is Monday all the way through to Friday x 5 any help

 

[Gardner]

It's been on the books for decades now. The water supply system needs to be from a privately owned/derived system and permission granted from the owner. If the other criteria are met, the system can be used as a means of electrical earthing.
These systems though are as rare as hens teeth and would mainly be found in rural areas...

In all other situations, extraneous water pipes would be required to be ''main bonded'' to the installations MET and not relied on as a means of electrical earthing for the installation...!!

In a TN-C-S supply one earth electrode is needed, but TN_S does not need any, so we only bond gas, water and phone line shields?

Now, would a concrete foundation with rebar be considered an earth electrode or only a ground rod?

 

[Simonslimline]

Electrical Trainee's?

Someone who's trained for 5 weeks and believes they are an electrician.

Very common over here unfortunately.

 

[Gardner]

Someone who's trained for 5 weeks and believes they are an electrician.

Very common over here unfortunately.

I get it Kind of like Handyman here. They are good at fixing doors and painting, not so much other stuff, but do electrical anyways.

 

[Simonslimline]

I get it Kind of like Handyman here. They are good at fixing doors and painting, not so much other stuff, but do electrical anyways.

Over here we have people going on short courses and then paying a fee to a scheme/scam then let loose on the unsuspecting public.

It's become a big problem to be honest.

 

[Engineer54]

In a TN-C-S supply one earth electrode is needed, but TN_S does not need any, so we only bond gas, water and phone line shields?

Now, would a concrete foundation with rebar be considered an earth electrode or only a ground rod?

A Ufer earth, Dependant on the size and dimensions of the foundation would be considered as a very good earth electrode, but it will still need to be measured to confirm that it's providing a suitable value. It could after all be tanked in a non-conductive barrier sheeting....

 

[Knobhead]

One of the pitfalls is quite obvious but from what I read many fall in to the trap. The membrane so often laid as a requirement before the slab is cast.

 

[Engineer54]

One of the pitfalls is quite obvious but from what I read many fall in to the trap. The membrane so often laid as a requirement before the slab is cast.

Generally found either where the ground has a corrosive make-up (high in salts, sulphates and chlorides etc) or where the concrete grade/type used in the foundations are constructed of lower grade concrete (i think it's type 65 and above that have no need for membrane/tanking). Having said that highly water resistant concrete foundations don't make for good Ufer electrodes!!

 

[old_sparky_RE]

Hello Gardner are you on LOGCAP ?
The reason I ask it seems the questions you put to the forum, it sounds like you have been sent to look over BS system's and are putting NEC code to do it !

And has been asked before.... where in the world are you working.

 

[Engineer54]

Hello Gardner are you on LOGCAP ?
The reason I ask it seems the questions you put to the forum, it sounds like you have been sent to look over BS system's and are putting NEC code to do it !

And has been asked before.... where in the world are you working.

He is in America, his company produces portacabin type structures that are being exported overseas to the likes of UK and Europe. He has a PDF copy of BS7671 17th ed, although not the latest ed (not sure what European reg's he may be in possession of). He is not trying to apply NFPA 70 (NEC) to these structures, he's just trying to make sense of the differences, between the two set's of regulations, which can be rather daunting at the best of times.... lol!!

 

[Gardner]

Ok so far my understanding is this, but I could be wrong:

1. Where the DNO supplies a TN-S system, no earth electrodes are required, even if the plumbing is plastic, no metal in the foundation and no gas lines. If gas, foundation or plumbing is present within the structure they are to be bonded to the MET (and a sticker saying "safety earth, do not remove")

2. Where the DNO is TN-C-S, at least one earth electrode is required. This may a be an earth rod, or underground water pipe coming into the building. Gas, plumbing and foundation are bonded to the main earth terminal.

3. Where the DNO is TT (usually encountered in an overhead supply) earth electrodes are essential and must be included in earth fault loop impedance calculations. The earth electrodes must pass current well over 30 ma to initiate RCD tripping. All plumbing, gas and foundation is bonded to the MET.

All correct?

 

[Richard Burns]

Ok so far my understanding is this, but I could be wrong:

1. Where the DNO supplies a TN-S system, no earth electrodes are required, even if the plumbing is plastic, no metal in the foundation and no gas lines. If gas, foundation or plumbing is present within the structure they are to be bonded to the MET (and a sticker saying "safety earth, do not remove")

2. Where the DNO is TN-C-S, at least one earth electrode is required. This may a be an earth rod, or underground water pipe coming into the building. Gas, plumbing and foundation are bonded to the main earth terminal.

3. Where the DNO is TT (usually encountered in an overhead supply) earth electrodes are essential and must be included in earth fault loop impedance calculations. The earth electrodes must pass current well over 30 ma to initiate RCD tripping. All plumbing, gas and foundation is bonded to the MET.

All correct?

The only time it is required to have an earth electrode is for a TT system to provide an earth path for the installation, the resistance of this should be stable and as low as possible. The resistance must be low enough to cause disconnection of an RCD.
You are correct that you also need to have protection for all circuits by means of an RCD to allow Automatic Disconnection of Supply to work. (The RCD does not need to be 30mA in this case, but because there are a lot of requirements for 30mA RCDs for other reasons this is generally easiest. I have a customer who happens to have a 500mA RCD upfront and does not want to change this, it is still compliant for the earthing system).

For TNS and TNCS systems no earth rod is required.
In all earthing systems any extraneous conductive parts (incoming metal services) must be bonding back to the main earth terminal.
A public water supply pipe (or a gas pipe!) are not permitted to provide the function of an earth rod.

 

[Gardner]

The only time it is required to have an earth electrode is for a TT system to provide an earth path for the installation, the resistance of this should be stable and as low as possible. The resistance must be low enough to cause disconnection of an RCD.
You are correct that you also need to have protection for all circuits by means of an RCD to allow Automatic Disconnection of Supply to work. (The RCD does not need to be 30mA in this case, but because there are a lot of requirements for 30mA RCDs for other reasons this is generally easiest. I have a customer who happens to have a 500mA RCD upfront and does not want to change this, it is still compliant for the earthing system).

For TNS and TNCS systems no earth rod is required.
In all earthing systems any extraneous conductive parts (incoming metal services) must be bonding back to the main earth terminal.
A public water supply pipe (or a gas pipe!) are not permitted to provide the function of an earth rod.

So if lets say I had a home with no rebar in the foundation, and the water line coming in was plastic with no gas on the property, I can just run a jumper from the neutral to the main earthing terminal and be code complaint?

An earthing electrode only serves the function of fault clearing as in a TT network?

 

[Richard Burns]

If there is an earthing system of TNCS or TNS then the supply earth is provided by the distribution network operator, it is not permitted for us to create a TNCS system by making a N-E link.
The earth must either be supplied from the public supply system by the DNO or, if an earth is not provided by the DNO, then we can install an earth electrode.

If there is no earth provided at the installation then one can request an earth provision from the DNO, but they do not need to provide one, e.g. if the network conditions are not suitable or the installation may present danger with a TNCS system.

So if lets say I had a home with no rebar in the foundation, and the water line coming in was plastic with no gas on the property, I can just run a jumper from the neutral to the main earthing terminal and be code complaint?

An earthing electrode only serves the function of fault clearing as in a TT network?