Benefits of a TT system?

[Crosswire]

If an installation is supplied by a two wire system (E.G. a TT system) , what are the benefits of using an earth electrode instead of simply joining earth to neutral at the supply head to create a TNCS system?

Surely using the supplier's neutral is a more effective return path for fault current than relying on the earth electrode and the solid mass of earth?

All replies welcome,

Cheers,
Crosswire.

 

[trev]

As soon as I can find a tasty alternative I'm banning Warburtons. Donors to the tories.

 

[Geordie Spark]

As soon as I can find a tasty alternative I'm banning Warburtons. Donors to the tories.

Good lad.

 

[Silly Sausage]

As soon as I can find a tasty alternative I'm banning Warburtons. Donors to the tories.

Bread factories are great.
I used to drive past one every day, watching a 50 foot spiral of loaves going round and round.

 

[Lucien Nunes]

All I can say on the subject of bread rolls is that I've been enjoying a bacon butty in the site canteen of a morning recently. But what I can say about using the neutral as the fault current return path is that one important fact has not been explicitly stated in answer to the OP's question regarding the dangers of a neutral loss in TN-C-S.

If you are relying on the neutral connection all the way back to the transformer as your fault path and it fails, not only is the fault protection lost but all current returning from loads downstream of the fault will be diverted via MEBs and casual earth connections, which might or might not provide a suitably low resistance path to stop everything being lifted to a dangerous voltage WRT true earth. For instance, if the distribution cable N fails in an area with plastic gas and water services, every piece of exposed metalwork in everyone's installation might be raised 230V from earth as soon as anyone switches on a load, even in the absence of a fault within the installations. In other words, the earthing system becomes a source of danger, instead of reducing it.

Hence the need to ensure highest possible integrity of the PEN, and hence PME. Random local N-E links don't guarantee the necessary integrity upstream.

 

[Engineer54]

Far too much scaremongering goes on about DNO PME supplies. If you listen to some on here, death and/or destruction is waiting for you just round the corner if you happen to have a TNC-S/PME supply!!

You are far more likely to have a DNO TNC-S supply, if you're house/flat is older and has been converted from a deteriorating TN-S supply. If you live in a relatively modern house/flat then the chances are that it will be a full DNO PME system where all the relevant cable joints have been N-E grounded. The same will go for DNO overhead supplies, that have been PME'd, it will be a full bona fide PME system. It is also easy to confirm if overhead system has been PME'd by checking the poles for the down lead N-E link to the earth rods/electrodes, (every 3rd or 4th pole)...

Oh and i also take with a good pinch of salt, those that claim they have seen a lost neutral on PME systems on several occasions, ....it's rare, it's very rare to completely lose a neutral connection!!

 

[DPG]

Far too much scaremongering goes on about DNO PME supplies. If you listen to some on here, death and/or destruction is waiting for you just round the corner if you happen to have a TNC-S/PME supply!!

You are far more likely to have a DNO TNC-S supply, if you're house/flat is older and has been converted from a deteriorating TN-S supply. If you live in a relatively modern house/flat then the chances are that it will be a full DNO PME system where all the relevant cable joints have been N-E grounded. The same will go for DNO overhead supplies, that have been PME'd, it will be a full bona fide PME system. It is also easy to confirm if overhead system has been PME'd by checking the poles for the down lead N-E link to the earth rods/electrodes, (every 3rd or 4th pole)...

Oh and i also take with a good pinch of salt, those that claim they have seen a lost neutral on PME systems on several occasions, ....it's rare, it's very rare to completely lose a neutral connection!!

The 2 I have seen were not completely open circuit, but high-impedance. This was on a fairly modern (1970s) street. Keep denying it happens by all means, but next time it happens to me I'll get you to touch the metal clad light switch in my garage and see what you think Daz

 

[Engineer54]

The 2 I have seen were not completely open circuit, but high-impedance. This was on a fairly modern (1970s) street. Keep denying it happens by all means, but next time it happens to me I'll get you to touch the metal clad light switch in my garage and see what you think Daz

Eh, .... A high impedance fault is hardly a lost neutral is it!! lol!!

I am not denying that it can and does happen, but lost neutrals especially on a full PME systems are as almost as rare as hen's teeth!!

Think about it, how can a modern industrialised western nation run an electrical network system that is known to drop neutrals on a regular and ongoing basis, putting both life and property at risk??

Just imagine the costs involved to the electric companies, with the multitude of legal cases!!!

 

[DPG]

It may not be a lost neutral, but they were poor joints on neutrals on a modern PME system. These joints would only get worse and worse over time and become a lost neutral at some point surely.

 

[Lucien Nunes]

If the 'high impedance' multiplied by the load current is above 50V, then for safety purposes it's a lost neutral. A shower running into a 2Ω PEN could cause some nasty shocks even when the EB is up to scratch for PME.

Open or high-resistance connections can occur in any conductor. They are rare in DNO works but in the case of TN-C and TN-C-S PENs they cause a specific hazard that doesn't exist in other supply configurations, namely the protective conductors and ECPs of connected installations can be elevated to hazardous voltages absent any other faults.

I don't think anyone is saying PME is deficient for this reason, only that it's no good to go around making random N-E connections and assuming that a TN-S neutral would automatically be a better place than a rod to connect the earthing conductor.

 

[Engineer54]

I don't think anyone is saying PME is deficient for this reason, only that it's no good to go around making random N-E connections and assuming that a TN-S neutral would automatically be a better place than a rod to connect the earthing conductor.

Now that i CAN agree with!!

High impedance faults don't just magically or instantly just happen by the way, they manifest themselves over a period of time. Again highly unlikely that a high impedance neutral fault wouldn't have made itself known long before it got to the point of being dangerous/hazardous... But again, yes it can and does happen, and yet again, it's going to be a rare, rare, rare occurrence.....

 

[Crosswire]

Now that i CAN agree with!!

High impedance faults don't just magically or instantly just happen by the way, they manifest themselves over a period of time. Again highly unlikely that a high impedance neutral fault wouldn't have made itself known long before it got to the point of being dangerous/hazardous... But again, yes it can and does happen, and yet again, it's going to be a rare, rare, rare occurrence.....

So it seems that the table from wikipedia that ranked the risk of neutral loss on a TNCS earthing system as hgh, is misleading?

This is why I sought the opinion of time served sparks, so as to explode myths and bring clarification on earthing systems. Cheers!

 

[Crosswire]

P.S. Sorry for any earlier sarcasm, just felt threatened so became defensive.

 

[davesparks]

Wikipedia is misleading, end of!

 

[ipf]

Wikipedia is misleading, end of!

.....for the uninitiated.

 

[bossian]

Never seen one occur out in the field in all my years of being a spark, but do understand the potential for a fault.

On another note.......our on site 11kv transformer was changed 3 years ago and the DNO went to a great deal of effort to install an earth nest along with 4x rods buried 3 rods each deep to get a ze of 0.08 ohms.

The DNO does take it's earthing seriously, sometimes