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.

 

[Knobhead]

There is far less to go wrong, and if it does, it's easily rectified. Both TN-S and TN-C-S are prone to faliure whereas a well installed TT system will outlast any other means of earthing. TT systems benefit from having noise free earthing. TT systems are far more cost effective from a suppliers point of view. You also have the benefit of having a true earth reference, rather than zero potential introduced by the star point, this is one of the reasons why TN-C-S is not allowed on final connection to caravans and mobile and transportable units.

Define a well installed TT system.

While ever the IET perpetuates the myth of 200Ω being acceptable, TT will remain a 2[SUP]nd[/SUP] rate system.
≤1Ω is possible with a decent interconnected earth nest but you’re going to have to work for it. TT then becomes a viable if not a preferred supply system.

Having installed >20MVA of TN-S systems without problems due to fault. I’ll say it is the best system.

CNE (PME) in its purest form is good. something you’ll not find in this country.

As for the OPs question about a homemade PME, it’s shear folly.

 

[D Skelton]

Define a well installed TT system.

While ever the IET perpetuates the myth of 200Ω being acceptable, TT will remain a 2[SUP]nd[/SUP] rate system.
≤1Ω is possible with a decent interconnected earth nest but you’re going to have to work for it. TT then becomes a viable if not a preferred supply system.

Yes I suppose 'well installed' wasn't the right term to use, but what I was alluding to was a TT system with a sub 1Ω Ze. You can have well installed TT systems that aren't sub 1Ω, but as you say, I wouldn't then consider them superior, just different.

 

[Crosswire]

"As for the OPs question about a homemade PME, it’s shear folly."

I have never done this. The reason I started the thread was due to a colleague of mine conducting a Ze test on an old TNS system, and finding the external earth path not meeting the required values. (0.8 Ohms I believe)

He resolved this by driving an earth rod to convert the system to a TT, thus allowing a Ze of 200 Ohms.

He later spoke to a friend of his who works for a DNO, who ACTUALLY TOLD HIM that he would have been better off converting the TNS system to a TNCS system HIMSELF, and using the suppliers neutral as the earth fault loop path!

This is the background to my initial post.

 

[telectrix]

well then, that DNO employee is a dork. how would you know if the supply was PME before you did that?

 

[Crosswire]

well then, that DNO employee is a dork. how would you know if the supply was PME before you did that?

I guess I wouldn't know. It just got me wondering about the reasons for selecting different types of earthing systems and what the advantages or disadvantages would be of each type might be.

I was hoping a spark with a more in depth understanding than myself would shed a bit of light on the subject so I can improve my own knowledge, and I'm pleased that a couple of forum members have been very informative and helpful.

 

[SPARTYKUS]

I suspect E54 may be along again

 

[Guest77]

lol .

 

[telectrix]

don't think he's digging, mike. just trying to improve his understanding of the 3 earthing systems.

 

[Crosswire]

*sigh*

Thanks to all the helpful members.

To all the trolls, please continue amusing yourselves by mocking me for asking a technical question.

I notice, btw, that none of the trolls have actually put forward even the slightest analysis of the different types of systems. Interesting........

 

[ipf]

*sigh*

Thanks to all the helpful members.

To all the trolls, please continue amusing yourselves by mocking me for asking a technical question.

I notice, btw, that none of the trolls have actually put forward even the slightest analysis of the different types of systems. Interesting........

I think enough has been said, to tell you the truth

 

[Simonslimline]

This is from Wikipedia. We don't use TN-c in the UK though.

	TT	IT	TN-S	TN-C	TN-C-S
Earth fault loop impedance	High	Highest	Low	Low	Low
RCD preferred?	Yes	Yes	No	No	No
Need earth electrode at site?	Yes	Yes	No	No	No
PE conductor cost	Low	Low	Highest	Least	High
Risk of broken neutral	No	No	High	Highest	High
Safety	Safe	Less Safe	Safest	Least Safe	Safe
Electromagnetic interference	Least	Least	Low	High	Low
Safety risks	High loop impedance (step voltages)	Double fault, overvoltage	Broken neutral	Broken neutral	Broken neutral
Advantages	Safe and reliable	Continuity of operation, cost	Safest	Cost	Safety and cost

Maybe worth a read also.

Edit link not working

 

[Crosswire]

This is from Wikipedia. We don't use TN-c in the UK though.

	TT	IT	TN-S	TN-C	TN-C-S
Earth fault loop impedance	High	Highest	Low	Low	Low
RCD preferred?	Yes	Yes	No	No	No
Need earth electrode at site?	Yes	Yes	No	No	No
PE conductor cost	Low	Low	Highest	Least	High
Risk of broken neutral	No	No	High	Highest	High
Safety	Safe	Less Safe	Safest	Least Safe	Safe
Electromagnetic interference	Least	Least	Low	High	Low
Safety risks	High loop impedance (step voltages)	Double fault, overvoltage	Broken neutral	Broken neutral	Broken neutral
Advantages	Safe and reliable	Continuity of operation, cost	Safest	Cost	Safety and cost

Maybe worth a read also.

Edit link not working

Thanks. This is the sort of thing I'm after.

 

[Crosswire]

*Edit* Link didn't work for me unfortunately.

 

[Crosswire]

It seems that risk of broken neutral is one of the primary concerns.

Why is it high for TNCS systems , and low for TT systems?

An earlier poster said that due to TT systems being used for remote locations, there was a high chance of losing the neutral via a storm (and therefore also the fault return path if the neutral was used as such). This is the reason for driving an earth electrode in a TT system, which seemed to make sense.

What am I missing?

 

[trev]

In a TNCS the fault path uses the neutral (yes I know you know that) so if the neutral breaks and there's a fault inside the installation then you could have every metal part inside it live. In a TT you still have the fault path through the general mass of earth.
TTs are not just used in rural/remote locations, we live near a main road into a fairly busy town and we're on a TT (Ra > 1 ohm)