Another take on bonding for TNCS

[ianmacd]

No if the neutral fault is external to the premesis. The current (flowwing the wrong way for simplicity ) would go from the line though the appliance back out the neutral and the rcd/rcbo, to the service head where N & E* are linked, back via the cpc to a class I appliance/enclosure, though your now lifeless body and into earth to get back to the source.

Ian.

*Admin we need an Earth symbol too

 

[Apache]

Remember that the E&N are connected. The return path for the Neutral is via the RCD to the E&N connection, and then to earth via the person. Therefore no imbalance between the L&N.

If there is ANY current moving from the live NO current will be moving in the N or E it's going to the mass of earth through a person - so, therefore a difference?

No?

The predecessor to the RCD used to trip if there was current on the earth, but we don't need any fault current to go down the earth to trip the RCD.

 

[ianmacd]

If there is ANY current moving from the live NO current will be moving in the N or E it's going to the mass of earth through a person - so, therefore a difference?

Only if the device in question had an earth fault internally but if its just a stray neutral (Did I really use the word "just" to discribe tat nightmare???) then there would be no fault in the premesis (apart from no power) or any clue what was going on until someone was lit up like a xmas tree.

Ian.

 

[Apache]

Only if the device in question had an earth fault internally but if its just a stray neutral (Did I really use the word "just" to discribe tat nightmare???) then there would be no fault in the premesis (apart from no power) or any clue what was going on until someone was lit up like a xmas tree.Ian.

and then the RCD would trip?

We ineffect have an open circuit?

 

[spinlondon]

and then the RCD would trip?No, because there would not be an imbalance between the L&N.

We ineffect have an open circuit?

Yes, one that you may inadvertantly close.

 

[Andy™]

and then the RCD would trip?We ineffect have an open circuit?

 

[msb2003]

Being new on here I may be missing the point, but if the rcd / rcbo is fitted at the source of the installation, the chance of contact (and creating a fault path through the body) to the neutral conductor on the supply side is remote surely!?

 

[spinlondon]

Being new on here I may be missing the point, but if the rcd / rcbo is fitted at the source of the installation, the chance of contact (and creating a fault path through the body) to the neutral conductor on the supply side is remote surely!?

Very very remote, especially as we are actualy discussing a scenario where the supply Neutral conductor is broken, near to the installation

 

[Doc Hudson]

Being new on here I may be missing the point, but if the rcd / rcbo is fitted at the source of the installation, the chance of contact (and creating a fault path through the body) to the neutral conductor on the supply side is remote surely!?

Welcome to the forum msb2003.

Doc H.

 

[ianmacd]

Andy, you want more flames coming out of the stick man and maybe the grim reaper standing behind him.

Ian.

 

[Apache]

Andy - thanks for the diagram, but I don't see how you could contact current before the RCD? You would have to get a belt at the meter or tails - not the appliance?

I thought we were in a situation where the live was live, but the electric had nowhere to go (no neutral) so when we fiddle with metal kettle the circuit is completed by your body? And therefore it's on the bit that's RCD protected?

I know I'm not a spark, but I have a physics A level (with a good bit of electric in in) and I'm not usually that slow off the mark...........

 

[ianmacd]

Andy - thanks for the diagram, but I don't see how you could contact current before the RCD? You would have to get a belt at the meter or tails - not the appliance?I thought we were in a situation where the live was live, but the electric had nowhere to go (no neutral) so when we fiddle with metal kettle the circuit is completed by your body? And therefore it's on the bit that's RCD protected?

I know I'm not a spark, but I have a physics A level (with a good bit of electric in in) and I'm not usually that slow off the mark...........

The current has already passed though the RCD and then out though the earthed case of an appliance (washing machine etc...) though you (hopefully not you) to earth.

The situation you describe is loosing the neutral the customer side of an RCD so it would trip as you say but this case the neutral is lost by the DNO.

Ian.

 

[Apache]

The current has already passed though the RCD and then out though the earthed case of an appliance (washing machine etc...) though you (hopefully not you) to earth. The situation you describe is loosing the neutral the customer side of an RCD so it would trip as you say but this case the neutral is lost by the DNO.

Ian.

ah!

That's the line I needed.

Thanks Ian - I get it now

 

[batty]

I know steps won't like me saying it but if there is a pme earth I will always use it. I have seen more poor TT installs ie poor earth connection than pme in fact I never have seen a lost neutral on a pme dno side. If this was to happen as long as your side is up to regs ie all bonding it is not your responsibility. Its Dno responsibility to provide a safe supply.

Batty

 

[Andy™]

i normally use the existing earthing arrangement, whether its TT, TNS or TNC-S, unless conditions require it to be changed

but if its TT, i dont go to the hassle to trying to get a PME

 

[Andy™]

Andy, you want more flames coming out of the stick man and maybe the grim reaper standing behind him.Ian.

will this suffice?

 

[ianmacd]

Applaud Smiley

 

[AK-UK]

Firstly nice effort andyc.Applaud Smiley

Secondly whichever way you look at it we all agree, I hope, that loosing neutral in PME (TNC-S) should be prevented. So applying a good and reliable main bonding will save lives...Pray.

 

[Andy™]

Firstly nice effort andyc.Applaud SmileySecondly whichever way you look at it we all agree, I hope, that loosing neutral in PME (TNC-S) should be prevented. So applying a good and reliable main bonding could do the trick...Pray

But when you decide to go water the garden because the power has failed and you touch the tap that is live because PME neutral has died between house and rod, then your the bloke in the diagram making sure everything is fully earthed will not pevent a shock in a neutral fail outside the property.

At least this problem cannot happen with TNS or TT

 

[AK-UK]

But when you decide to go water the garden because the power has failed and you touch the tap that is live because PME neutral has died between house and rod, then your the bloke in the diagram making sure everything is fully earthed will not pevent a shock in a neutral fail outside the property. At least this problem cannot happen with TNS or TT

Yes, you are right but considering the chance that it will happen, and do not forget that the Main bonding is a parallel to the main earth conductor and! it is probably connected to the gas supply. Your body resistance is much higher than the 10mm2 of the main bonding.

At the end of the day it is only an insurance policy, do not want it but hope when you need it it will work for you.