TNCS - USING NEUTRAL AS EARTH IN BOARD

[hightower]

The experiment has taken place at a board with the earthing conductor disconnected and none of the circuits in contact with extraneous parts. With a link across the earth and neutral bars the board functions, as soon as you reintroduce the earth the rcd trips.

Yes, so providing the OP installed as such (using the neutral link as the earth, not a rod) it would sit happily all day long even under most fault conditions. The only time it might trip is when a user introduces a true earth, but you could have an oven with a line-earth fault and it would sit happily energised all day long.

 

[Des 56]

Yes, so providing the OP installed as such (using the neutral link as the earth, not a rod) it would sit happily all day long even under most fault conditions. The only time it might trip is when a user introduces a true earth, but you could have an oven with a line-earth fault and it would sit happily energised all day long.

And just to the side you have a washing machine with a neutral earth fault
Then there is the nasty "double fault" monster ready to pounce

Systems could have most nuisance tripping and blowing of fuses vastly reduced if that pesky earth wasn't around
Its when the double fault occurs things get interesting

 

[westward10]

Yes, so providing the OP installed as such (using the neutral link as the earth, not a rod) it would sit happily all day long even under most fault conditions. The only time it might trip is when a user introduces a true earth, but you could have an oven with a line-earth fault and it would sit happily energised all day long.

I did wonder what would happen but yes you are quite correct, tried rcd tester and did high current two lead loop test and rcd stayed on. Didn't try the test button just in case something went amiss. This only works if there is absolutely no connection to earth. A sufficient line/earth fault though should trip the mcb.

 

[KennyKen]

Australia uses an MEN system. Main earth neutral or PEN conductor. In domestic boards there is a visible Earth link between earth and unprotected neutral

 

[mache]

Okay, so the scenario in the OP was that a link was made between the earth and neutral bus bars. This is my interpretation of that, as depicted by the purple line. (Apologies about the Paint job, not quite Richard Burns spec).



So current flows through the line through the RCD, and out on the first circuit. There's a fault, and it returns via the earth path. The earth path is linked to neutral before it passes through the RCD, and the current then passes through the neutral via the RCD. The RCD wouldn't see a fault and doesn't operate.

View attachment 35580

But if you connected the earth to the neutral on the incomer to the rcd then at a fault condition of line to earth that should show the imbalance on the RCD output side to trip it. I think ??

 

[Pete999]

But if you connected the earth to the neutral on the incomer to the rcd then at a fault condition of line to earth that should show the imbalance on the RCD output side to trip it. I think ??

Only if the N is connected to the N at the supply,

 

[Lucien Nunes]

There is an awful lot of waffle and nonsense in this thread. To dispose of Hightower's comment first, when the OP mentioned linking N & E at the 'bar' he presumably did not mean the N terminal bar DOWNSTREAM of the RCD. Of course that won't work! As people keep saying, If there is any kind of external earth present any load on the system will tend to split between N (passing through the RCD) and E (missing the RCD) and cause a trip - just like an N-E fault in a final circuit that causes a trip when a heavy load elsewhere in the house is switched on. And an L-E fault wouldn't trip the RCD, as Hightower says.

But this is beside the point for the OP - he hasn't got an E to work with. He wants to make his E bar 'E' solely by connecting it to N UPSTREAM of the RCD, i.e create a TN-C system, which is a no-go. The reason it's OK here is that it is done as part of the DNO works if, and only if, the N conductor meets the necessary criteria for TN-C-S. If it is not earthed at sufficient points, or is resistance is too high, or other connected installations are not compatible, then it cannot be used as TN-C-S, not least because of the risk of ALL THE EXPOSED METALWORK BECOMING LIVE IF THE N CONDUCTOR FAILS! How could you possibly be confident that the supplier's cable meets all the necessary criteria? Have you tested it all the way back to the substation and inspected all their earth rods?

Nope, TT earthing might never achieve stellar Ze figures, but at least it's better than seemingly low Ze 'earthing' that suddenly turns live on you when a badly made splice somewhere up a pole gives out.

 

[assegayer]

There is an awful lot of waffle and nonsense in this thread. To dispose of Hightower's comment first, when the OP mentioned linking N & E at the 'bar' he presumably did not mean the N terminal bar DOWNSTREAM of the RCD. Of course that won't work! As people keep saying, If there is any kind of external earth present any load on the system will tend to split between N (passing through the RCD) and E (missing the RCD) and cause a trip - just like an N-E fault in a final circuit that causes a trip when a heavy load elsewhere in the house is switched on. And an L-E fault wouldn't trip the RCD, as Hightower says.

But this is beside the point for the OP - he hasn't got an E to work with. He wants to make his E bar 'E' solely by connecting it to N UPSTREAM of the RCD, i.e create a TN-C system, which is a no-go. The reason it's OK here is that it is done as part of the DNO works if, and only if, the N conductor meets the necessary criteria for TN-C-S. If it is not earthed at sufficient points, or is resistance is too high, or other connected installations are not compatible, then it cannot be used as TN-C-S, not least because of the risk of ALL THE EXPOSED METALWORK BECOMING LIVE IF THE N CONDUCTOR FAILS! How could you possibly be confident that the supplier's cable meets all the necessary criteria? Have you tested it all the way back to the substation and inspected all their earth rods?

Nope, TT earthing might never achieve stellar Ze figures, but at least it's better than seemingly low Ze 'earthing' that suddenly turns live on you when a badly made splice somewhere up a pole gives out.

Thank you for the clear in depth explanation

 

[Marvo]

I'm not sure about Libya as such but what size ie how many amps is the supply?

I often work in central African countries and if I'm there for any length of time and need to set up a clean safe supply we use an isolating transformer. It's not something that would be practical in countries like the UK with their 80Amp residential supplies but if the incoming supply is only single phase 10 or 20Amps like it is in some countries it's easily doable.

You install an isolating transformer at the point of supply entry, whack in a decent rod (12-16 meters in length) and create your own TN-C-S arrangement on the secondary side, that way you won't end up with your earthing arrangement grounding large faults from some badly wired factory up the road.

 

[lincoln]

Don't link from neutral to make your own earth . Sink a rod and use that.

Just for info for the rest of the respondents you could find this useful I use it when working abroad .
http://www.legrand.com/files/fck/File/pdf/Guide-International.pdf

 

[The Claw]

I recommend reading wikipedia "Earthing System"
Earthing system - Wikipedia - https://en.wikipedia.org/wiki/Earthing_system