What earthing system is this?

[Engineer54]

The LV main is the cable originating in the substation that all supplies will be breached from. it is run as a radial configuration sometimes with the option to be backfed or backfeed other LV mains via link disconnector boxes.
Any joint or repair carried out on a paper lead distribution cable will have neutral and earth connected together in that joint. They will already be connected at the substation and every main will have been PME'd at some point in the past. When a paper lead main or service is repaired wavecon CNE cable or hybrid will be used (3 core and copper neutral/earth 3 phase and 1 core and copper neutral/earth for single phase. On new supplies a rod must be installed every 40m (if there are joints) or before the last 4 properties on that main. This is to reduce the amount of customers affected by a neutral fault.
TN-S can be supplied from a PME'd main as previously mentioned, this is current practice on new installs with multiple supplies to steel framed buildings. This is to stop circulating currents in the frame. Alternatively no earth will be supplied and TT will most likely be used.
All the above is based on my knowledge of the DNO in my area and other areas may work to different practices.

Thanks for the reply mechelec,

Have read you post several times, and can't see how a neutral fault on the PME side can't be transmitted to any of the TN-S customers earthing terminals. That S in TN-S stands for ''separate'' which it clearly isn't in your explanation/description, if both joints and PME'd service heads are N-E linked throughout the system. That especially goes for supplying a TN-S from a CNE cable. Can you describe how that is achieved, as i just can't get mind straight on that one....

I understand the process of PME'ing new underground supplies, they will generally be done to the book, it's the converting of the older TN-S supply cables that sometimes leaves a lot to be desired, and that is almost always down to cost implications.

Now i maybe missing something here in all this, by not letting my thinking have some rope so to speak, so any clarification on your part, or misguidance on my part that you can see, will be gratefully welcomed.

 

[Deleted member 26818]

I must admit, that I'm a bit confused here.
With TN-C-S, the supply is TN-C, which is presented at installations as TN-S.
To my mind, whether the TN-C is taken right up to the installation, or only taken to a point some distance away from the installation, It is still TN-C-S.
In either case, PME conditions will apply.
To my mind, the only way that a TN-C-S sytem can become TN-S, is for an isolating transformer to be used.
The TN-C would be fed to the transformer, and all feeds from the transformer would be TN-S.
Earthing the neutral at any point after the transformer, will effectivly re-establish the TN-C-S.

 

[Engineer54]

I must admit, that I'm a bit confused here.
With TN-C-S, the supply is TN-C, which is presented at installations as TN-S.
To my mind, whether the TN-C is taken right up to the installation, or only taken to a point some distance away from the installation, It is still TN-C-S.
In either case, PME conditions will apply.
To my mind, the only way that a TN-C-S sytem can become TN-S, is for an isolating transformer to be used.
The TN-C would be fed to the transformer, and all feeds from the transformer would be TN-S.
Earthing the neutral at any point after the transformer, will effectivly re-establish the TN-C-S.

All TN-S supplies are in effect TNC-S at the origin of the supply (from distribution TX) via earthing of the neutral point. Thereafter the earthing conductor remains continuous and wholly separate throughout the DNO distribution system, through to the service heads and beyond.

The problem comes when you mix the TNC-S with TN-S as that combined N-E can't be isolated from that separate earth conductor (typically the lead sheath) of a converted TN-S cable, if all the rodded N-E joints incorporate the lead sheath, and/or the sheath being physically connected to the N-E connection at the service heads...

The other potential problem that i can see, is that any customers still on a presented TN-S earth will be subject to a neutral fault on the system. And it won't be limited to a certain number of houses, as it would be in a new full underground PME system, as the sheath will be carrying that fault to every installation where it is used as either a full earthing point or in-part to the installation. In other words that lead sheath will carry that fault to all installations on the network distribution cable, not just to the houses/installations after the neutral break, as would be generally expected... That can't happen on an overhead PME system, no ex TN-S system to worry about. ...lol!!

As i stated, i may be missing something here, but i'm dammed if i can see where... lol!!!

 

[mechelec]

TN-S from CNE cable is achieved either by using split concentric service from main to cutout or CNE cable and a separate earth from the main joint to the cutout. This gives a TN-S from the cutout to the main only ie separate neutral earth. This is only used on multiple supplies to a steel famed building for the sole reason of stopping circulating currents in the frame. A neutral fault on one installation service will not result in current flowing through the metal frame to another service neutral/earth terminal as would happen with multiple PME supplies.
Neutral faults on the network are always a problem and the results can vary depending on system balance etc regardless of earthing arrangement.

 

[Engineer54]

TN-S from CNE cable is achieved either by using split concentric service from main to cutout or CNE cable and a separate earth from the main joint to the cutout.

If the earth is derived from the neutral, in either of your examples, then both the neutral and the earth conductor at the cut-out will be at the same potential, and effectively be exactly the same as a service head PME connection. The earth bonded to steel frame is still effectively a neutral connection....

Neutral faults on the network are always a problem and the results can vary depending on system balance etc regardless of earthing arrangement.

Agreed, but when you have a a mixed TN earthing system, where the lead sheath is going to be a PME exposed common conductor throughout that cables length and at every service cut-out then your going to be in trouble.

In a new PME cable installation, (for simplicity let's talk about a single phase main cable) the neutral is regularly rodded/grounded along it's length, from the sub-station. There is no common earth in the cable construction. So to give an installation an earth, the earth is derived from the neutral conductor at the service head. Any neutral fault will/should then only affect service heads beyond the fault. That is Not So with a converted TN-S cable, where it's sheath has been included in the N-E connections at joints and/or at the service head cut-outs. Acting as a common conductor, any neutral fault will be transmitted to installations that shouldn't be affected by the fault, the length of the cable in fact. Which is why TN-S sheaths should NOT be connected to a PME cut-out, and underground joints should not include the lead sheath in N-E rodded conversion connections...

Thinking about it, my comment in para 4 of post 24 about the OP's photograph is ''wrong'' it's not fine!! lol!!! I've mainly been considering TN-S installations supplied from old converted cables, but the sheath scenario will affect both TNC-S and TN-S installations, if it is included in the PME conversion...

I'd love to be proved wrong on this one, so if i'm now over thinking things, reel me in quick and show me the error of my way's. lol!!!!

 

[mechelec]

If the earth is derived from the neutral, in either of your examples, then both the neutral and the earth conductor at the cut-out will be at the same potential, and effectively be exactly the same as a service head PME connection. The earth bonded to steel frame is still effectively a neutral connection....


Not quite, Yes neutral and earth will be at the same potential under normal conditions but earth will be separate back to the main. For example take a neutral fault on the service on one supply in the steel framed building, (service and main PME) neutral becomes Open circuit so being PME the current flows through the neutral earth terminal, though the steel frame (bonded) and down the neutral of the other supplies. Not a good situation as overloading of the neutral and bonding conductors can result.
Now take the same idea and the services are TN-S back to the main which is PME'd in the service joint, open circuit fault on one service neutral will mean no current can flow in the steel frame as the only neutral earth connection is in the service joint on the main. Therefore only one service is affected.


(quote)
In a new PME cable installation, (for simplicity let's talk about a single phase main cable) the neutral is regularly rodded/grounded along it's length, from the sub-station. There is no common earth in the cable construction. So to give an installation an earth, the earth is derived from the neutral conductor at the service head. Any neutral fault will/should then only affect service heads beyond the fault. That is Not So with a converted TN-S cable, where it's sheath has been included in the N-E connections at joints and/or at the service head cut-outs. Acting as a common conductor, any neutral fault will be transmitted to installations that shouldn't be affected by the fault, the length of the cable in fact. Which is why TN-S sheaths should NOT be connected to a PME cut-out, and underground joints should not include the lead sheath in N-E rodded conversion connections...

TN-S cables have a connection between the neutral core and lead sheath at the substation and at joints along it's length. A neutral fault ie open circuit neutral core (very rare) will result in all neutral current flowing through the lead sheath which is of adequate CSA to carry this current. You would have to lose the lead sheath and neutral core for there to be major problems.
Neutral faults are thankfully rare and are usually caused by alu CNE cable when moisture gets in to the alu neutrals and corrodes them away.