Neutral to PE link in TNC-S system

Papa_Bear · 06-03-2015 4:50am #1

Hello Sparks,

I was curoius to know where in the installation shown where the meter and consumer unit are co-located would the connection between neutral and earth typically be? Is it usually in the consumer unit (a conductor between the N and E bars) or somewhere else?

Cheers.

06-03-2015 6:39am

Normally done at the cut out.
Not in that case though.

bountyhunter87 · 06-03-2015 8:51pm

There is no main over current protection that cu. That is a 80amp isolator on the left. Not a 63a mcb

Papa_Bear · 07-03-2015 2:02am

2011 wrote: »

Normally done at the cut out.
Not in that case though.

Should this be changed then, by connecting a PE conductor between the earth bar in the CU to the N terminal of the cut-out (instead of one between the E and N bars of the CU)?

Cheers.

Papa_Bear · 07-03-2015 2:06am

bountyhunter87 wrote: »

There is no main over current protection that cu. That is a 80amp isolator on the left. Not a 63a mcb

The left-most mcb on the CU is a 63A main breaker. The cut-out is a 60/80A fuse.

07-03-2015 7:15am

Papa_Bear wrote: »

Should this be changed then, by connecting a PE conductor between the earth bar in the CU to the N terminal of the cut-out (instead of one between the E and N bars of the CU)?

Cheers.

Assuming that this installation should be neutralised, yes.

07-03-2015 7:16am

Papa_Bear wrote: »

The left-most mcb on the CU is a 63A main breaker. The cut-out is a 60/80A fuse.

That would be against the regulations.
It should be a main switch fuse.

bountyhunter87 · 07-03-2015 8:28am

It's a isolator not a mcb. Hence it's red.

07-03-2015 9:03am

bountyhunter87 wrote: »

It's a isolator not a mcb. Hence it's red.

I would think you are correct.

There may be a rating written on the isolating switch but this would refer to the maximum current that it can switch, not the current that it should trip at.
Either way to comply with the National Rules for Electrical Installations, it should be a switch fuse unit like this:

[URL="javascript:;"][/URL][URL="javascript:;"][/URL] ge-63amp-s-p-neozed-switch-fuse_l.jpg?osCsid=0c4ae0479e346ce6d5f3fdfdb72ef462

ge-63amp-s-p-neozed-switch-fuse_l.jpg?osCsid=0c4ae0479e346ce6d5f3fdfdb72ef462

bountyhunter87 · 07-03-2015 9:14am

Correct op.
But also a 63amp mcb complies with regulations. Once it's a mcb and not an isolator.. the hager boards usually come with an isolator now. . Isolater with a red switch like the pic in post 1.And a 63amp mcb is 0ut in the meter box in a ip housing . Obviously not the case above. As the meter is beside the cu.
op should change the red isolator to a suitably rated mcb or switch fuse.

17larsson · 07-03-2015 10:02am

2011 wrote: »

I would think you are correct.

There may be a rating written on the isolating switch but this would refer to the maximum current that it can switch, not the current that it should trip at.
Either way to comply with the National Rules for Electrical Installations, it should be a switch fuse

A 63amp mcb is suitable

07-03-2015 10:35am

17larsson wrote: »

A 63amp mcb is suitable

You could be right.

I have not wired a domestic installation in a long time, so this may have changed.

My preference would be for a fuse because the characteristics are different to an MCB especially under short circuit conditions. This means that if something goes terribly wrong the chances of blowing the ESB fuse are reduced.

Papa_Bear · 07-03-2015 6:40pm

According to documentation that came with the CU the switch on the left is an NBR163A/NBN163R which is an MCB by their description and guidelines.

Are you saying there should be an isolator also intalled in the CU beside it.

Regarding my original query. Should there be a 2.5sq grenn/yellow conductor running between the cut-out and the E bar of the CU?

07-03-2015 9:04pm

Papa_Bear wrote: »

Regarding my original query. Should there be a 2.5sq grenn/yellow conductor running between the cut-out and the E bar of the CU?

No, the neutralised link should be at least a 10 sq. cable.

07-03-2015 9:15pm

I should point out that an electrical installation should only be neutralised at one point.

Tuco88 · 09-03-2015 10:58pm

keeping this topic going, Is there a danger in having the neutralized link in the cu? its not correct or fair from good practice is understood, but is it effectively the same thing? what would be the risk of having more than one neutralised point? is that not what the PME is?

If a situation like the OP had came up, ie no main Earth, would it be a good idea to place a temporary link in the cu until the ESB can sort it out?

Risteard81 · 10-03-2015 2:46am

No, because then it ceases to be a TN-C-S installation and would in fact be a TN-C installation as you would have a PEN conductor within the installation. Neutral and Earth must be separate within the installation (Terra/Neutral Combined and Separate (TN-C-S)).

Bruthal · 10-03-2015 10:42am

Combined at the cu makes no operational difference really.

However, it adds extra connections for neutral failure upstream or at the neutralizing point, which itself is a good reason why the earth should be connected to neutral at the very first connection point the neutral makes at the installation.

10-03-2015 9:01pm

Tuco88 wrote: »

what would be the risk of having more than one neutralised point?

In a properly functioning circuit current flows down the phase conductor and returns on the neutral. If an installation is neutralised at more than one point the returning neutral current now has a parallel path. Instead of all of the current flowing back on the neutral a portion of the current will flow through the sections of the earth cable that is connected between the neutralised points. Current flowing through the earth conductors when an electrical installation is functioning properly is not desirable.

I saw this causing issues on a large electrical installation. In addition to the "normal" neutralising point within the main LV board the standby generator was also neutralised. This caused a number of issues including the potential of earth bars local to the generator to be raised under certain load conditions. It also made it difficult to predict what would happen under fault conditions.

In reality in a small domestic installation such as this it will make little difference although but many would consider it bad practice.

is that not what the PME is?

No.
PME (Protective Multiple Earthing) is when a continuous earth conductor is run from the star point of the supply transformer to the MET of an installation, independly of the neutral (also connected to the star point).

If a situation like the OP had came up, ie no main Earth, would it be a good idea to place a temporary link in the cu until the ESB can sort it out?

Best to do it properly IMHO.
Temporary solutions have a habit becoming permanent.

Risteard81 · 11-03-2015 3:00am

2011 wrote: »

PME (Protective Multiple Earthing) is when a continuous earth conductor is run from the star point of the supply transformer to the MET of an installation, independly of the neutral (also connected to the star point).

What you are describing sounds like a TN-S earthing system.

PME on the other hand is a form of TN-C-S system whereby the PEN conductor is earthed at multiple points, typically in Britain or the north under the Electricity Safety, Quality and Continuity Regulations. There is not usually an earth electrode connected at the MET of a TN-C-S installation in these places.

gctest50 · 11-03-2015 3:23am

Tuco88 wrote: »

...........
If a situation like the OP had came up, ie no main Earth, would it be a good idea to place a temporary link in the cu until the ESB can sort it out?

no - it'll end up being left there - do something proper or phone someone who can

and you can get a weird and potentially fatal fault with that sorta craic :

if the neutral breaks upstream of your board ( think of the stormy weather now)

the metal-cases ( of things that are earthed ) will become live

(since most earth-pole thingys are a bit dismal )

if you touch one of these the RCD may not pop (since you are touching something upstream of it)

Bruthal · 11-03-2015 8:12am

gctest50 wrote: »

if the neutral breaks upstream of your board ( think of the stormy weather now)

the metal-cases ( of things that are earthed ) will become live

That will happen when the neutralizing point is at the correct position too. Its just that doing it at the board means there is at least 1 more connection to fail.

It is very bad practice to do what tuco is asking about. Everyone agrees there I'd say.

if you touch one of these the RCD may not pop (since you are touching something upstream of it)

The RCD won't trip because you are outside the circuit alright. Even if it did trip, the hazard may still be there, unless all loading is on the RCD circuits.

The above mentioned metal cases are already directly connected to earth anyway, so if a person touching one had any chance of tripping an RCD, it would trip before any contact anyway. But as said, the path is outside the RCD circuit(s).

aido79 · 12-03-2015 11:55am

Bruthal wrote: »

Combined at the cu makes no operational difference really.

However, it adds extra connections for neutral failure upstream or at the neutralizing point, which itself is a good reason why the earth should be connected to neutral at the very first connection point the neutral makes at the installation.

I live in Australia and that's the way its done here. Its called an MEN link. As far as I know it is illegal in Ireland and the UK.

Neutral to PE link in TNC-S system

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