# earth fault loop path TNS to TN-C-S



## petesrepairs

Hi, I'm trying to get some info as I appear to be going round in circles with this. On a TN-S system, when there is an earth fault on a load the live current travels along the CPC to the consumer unit, before leaving to go down the earthing cable to the electricity supply box. once here it travels down the steel armour (it being in the earth from the supply) to the transformer. 

(now here is were things go a bit screwy on my understanding due to people chattering in my ear)

The current then travels back down the neutral supply to the consumers house, before travelling through to the consumer unit and due to this causing the MCB to blow. now is this right? or does the current travel down the live cable back to the consumers house?

I am assuming that in a TN-C-S system, the flow will be the same, so if its down the live from the transformer, then the current passes down the neutral cable to the transformer and then back along the live. but if it travels down the neutral from the transformer, i am assuming that in a TN-C-S it avoids going down the neutral to the transformer and goes directly back up to the consumer unit.

if anyone can clarify or anything, that would be great, if anyone has a picture with a arrow flowing with the routes, that would be even more impressive. cheers in advance.

Pete


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## brian john

Peter a few issues here, we are primarily an American site and the terms you use are British terminology so I am having some problems following you.



petesrepairs said:


> Hi, I'm trying to get some info as I appear to be going round in circles with this. On a TN-S system,



What is a TN-S system?



> when there is an earth fault on a load the live current travels along the CPC to the consumer unit, before leaving to go down the earthing cable to the electricity supply box. once here it travels down the steel armour (it being in the earth from the supply) to the transformer.


What is a CPC are you saying there is a fault in the in the residence downstream from the main panel? 

(now here is were things go a bit screwy on my understanding due to people chattering in my ear)



> The current then travels back down the neutral supply to the consumers house, before travelling through to the consumer unit and due to this causing the MCB to blow. now is this right? or does the current travel down the live cable back to the consumers house?


 Here's were I get lost are you saying what happens if there is a fault in the residence or on the utility side of the panel?

If there is a fault in the residence the current travels down the ground path (armor) back to the utility transformer the low resistance of the fault connection results in a high current which exceeds the current carrying capacity of the OCP (over current protection fuse or circuit breaker)



> I am assuming that in a TN-C-S system, the flow will be the same, so if its down the live from the transformer, then the current passes down the neutral cable to the transformer and then back along the live. but if it travels down the neutral from the transformer, i am assuming that in a TN-C-S it avoids going down the neutral to the transformer and goes directly back up to the consumer unit.


What is a TN-C-S system?



> if anyone can clarify or anything, that would be great, if anyone has a picture with a arrow flowing with the routes, that would be even more impressive. cheers in advance.
> 
> Pete


This is basic electricity and it is not magic, if you can define some terms we can help.


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## frenchelectrican

Here Brian John this will help ya

http://en.wikipedia.org/wiki/Earthing_system


BTW our eathing sytem in France is simauir to the USA 

We run TN-C-S or TI in USA I know you run TN-C-S or TT

But The TN-C-S is the most common one we deal with it.

Merci,Marc


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## frank

Pete

With a TN-S the earth fault is carried by the supply company eartning facility to the star point of the supply transformer ( or the joint neutral/earh point if you wish). The fault current then flows unrestricted via the supply maindphase conductor ( live) through the protective device ie mcb - and being unrestricted opperates the mcb at a high fault current ratio. Mains now off. Circuit safe.

Using a TNC (pme) system the parammeters are exactly the same except that the supply authority earth is both neutral and earth at the consumers location. To prevent neutral currents during normal opperating conditions being carried by the grounding portion of the circuit - and subseqently making metalwork live to the touch - multiple earths are added to internal systems ( equipotential bonding etc) and the combined earth/neutral core of the supply cable which is usually the concentic outer cable is bonded via earth electrodes all the way down your street.

Hope this helps.If I missed someting or said it incorrecty - then someone will add to this post for sure.


Frank


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## chrisb271

Adding to Franks excellent teach in there.

With such a large current being able to flow in the Live ( or phase ) conductor you need to pay special attention to Mcb ratings ie the type of Mcb B , C or 3, D , older type 2's when designing layouts.
This is where your enquiry or measurment of Ze comes in , so if your looking at a Ze of 0.09 ohms then you could potentially expect 2.55 kA ( 2,500 amps )of fault current ( bit much for the old type 2's to handle )
Hence we have ratings on them of 6 , 10 or 24 kA ( 40 -80 kA on Bs 88 fuses ) 
Its important to consider such as it would be possible to blow an Mcb or main switch to bits if incorrectly sized.

I know of two cases where this has actually happened when a small factory unit placed close to the supply transformer happened when using the wrong ratings,quite an eye opener !

Paying a little attention to your results of Ze , Zs & Earth loops etc will tell you if your circuits are within the safe margins.

It always makes me laugh when people say " its easy being an electrician , just bung a few wires in here and there ,it's easy money "

Yeah,figure !

Regards

Chris B


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## RePhase277

If the TN-S system is as show in the diagram in the link posted above by Marc, the French Electrician, then the fault current path is from the live wire, through the fault, and onto the CPC/PE, and back to the neutral point of the transformer. It's circuit is complete, there is no need for it to go back to the consumer unit.

In a TN-C-S system, the fault current travels from the live through the fault, on the CPC/PE to the CU, where the CPC/PE is bonded to the supply neutral. Another complete circuit for the current. Remember the the CPC and the neutral are at the same potential because at some point they are electrically connected. In the TN-S system they are connected at the transformer, and in the TN-C-S they are connected at the consumer unit. What this means is, the live wire loves to complete a circuit with the CPC just as much as it does with the neutral, because the CPC and neutral are really just parallel wires joined at one end.


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## chrisb271

InPhase277 said:


> In the TN-S system they are connected at the transformer, and in the TN-C-S they are connected at the consumer unit.


TN-S and TN-C-S are always connected at the transformer and TT circuits fed from a generator set are connected at the supply end.

The consumer unit NEVER has a neutral/earth link as our Rcd's/ Rcbo's (GFCI)would never allow current to flow.All Neutral/earth connection are made on supply lines externally,overhead lines are earthed at mutliple points hence the term PME (Protective Mutlitple Earth)

There is NO connection to neutral/earth anywhere in the supplied location both being totally seperate.
There is a connection made PRE consumer unit/fuse board at the point of entry on the supply company cut out


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## frenchelectrican

Chris thanks for your explation on your system however In France we do little diffrent than what you do and we do run the netural thur our RCD's

We don't have very many place that run PME unless you are in Metro area.

Merci,Marc


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## nobber

fed up just spent 25 mins doing a post but had to nip out all was explained the best i could got back been loged out lost the lot, 

but if your still unsure of this let me know and ill write it all again


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## septiclecky

Brian John 

CPC stands for Circuit Protective Conductor

When I started my apprenticeship in 1973 it was known as earth to us in the UK or round to you in the US.

It is all to do with terminology used by CENELEC in Europe.


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## Docara

frenchelectrician - whats the point of answering about what you do in france???
I have noticed over the months the answer you give by and large are incorrect for the UK but they sound close enough to be believable. You clearly know what you are talking about but IN FRANCE please make sure you KNOW the answer or state that you are a little unsure so that the OP knows to do some of their own investigation based on what you have posted. British regs are British regs not French 

As an example

"BTW our eathing sytem in France is simauir to the USA 

We run TN-C-S or TI in USA I know you run TN-C-S or TT

But The TN-C-S is the most common one we deal with it."

or

"Chris thanks for your explation on your system however In France we do little diffrent than what you do and we do run the netural thur our RCD's

We don't have very many place that run PME unless you are in Metro area."

We have three types of earthing systems 

TT - TERRA TERRA - LIVE + NEUTRAL supplied by a 2 core cable from (usually) an overhead transformer next to property but no earth consumer MUST provide their own Earth Rod

TN-S - TERRA NEUTRAL + SEPARATE - the bit after the hyphen is the earhing method. So this mains the suppliers cable would carry the Live, Neutral and earth ( separately) as say the armoured of a SWA cable.

TN-C-S - TERRA NEUTREAL +COMBINED+SEPARATE also known as PME (Protective Multiple Earth) In which the neutal and earth is combined together but is fed separatly like the above but at the consumer end gets split to form both neutral and Erth connections 

In fairness you Wiki link is excellant

Now Chriss

Have a look in the regs book and their are diagrams in there concerning the earth fault path thus answering your question. If you ever do the 2391 exam they might ask exactly this question and you have to be able to draw these diagrams.

Docara
Docara


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## frenchelectrican

Docara .,

If you don't mind if I can PM ya I have few question I need to ask ya direct.

Merci,Marc


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