On the fiddle ______________

[mainline]

I know what you are saying, but an IT system with no Isolating Transformer isn't really.....

What sort of earthing system is it ?

 

[timhoward]

What sort of earthing system is it ?

It's Friday night and I started early.... but I'd say still TN-C-S really, Just the S is happening at the CPC bar not the cut-out.

The neutral is disconnected from the meter, so when the switch is open there is only one neutral conductor and that's the same one as the cpc,

completey agree

where is the separation of the cpc and neutral ?

The CPC bar. The main earth arrives there which is being misused as a PEN conductor. Imagine the G/Y conductor going to the main switch is blue. We don't have to agree! It's a crazy setup after all. But I think the RCDs would work.

 

[mainline]

I wonder why it was left like that and not removed when they left

 

[timhoward]

Do you think L to Cpc would trip the rcd ? or mcb ?

Yes. Forget the RCD a minute. If current can flow from the MCB through a load and back via the N bar, via the RCD, and the load works, then we've proved it's possible for current to flow. Lower the resistance, all the way to negligable if you like, and lots of current will flow, the and MCB will trip. Or has the 1664 completely broken my brain?

For the RCD, anything that goes through it on the L side and doesn't come back on the 'other pole' will cause an imbalance and it will trip.

If you take the meter completely out of the installation and forget it, this is electrically equivalent to every TN-C-S install. Just the point where PEN becomes N and CPC has shifted but it's still upstream of the RCDs.

 

[mainline]

Yes. Forget the RCD a minute. If current can flow from the MCB through a load and back via the N bar, via the RCD, and the load works, then we've proved it's possible for current to flow. Lower the resistance, all the way to negligable if you like, and lots of current will flow, the and MCB will trip. Or has the 1664 completely broken my brain?

For the RCD, anything that goes through it on the L side and doesn't come back on the 'other pole' will cause an imbalance and it will trip.

If you take the meter completely out of the installation and forget it, this is electrically equivalent to every TN-C-S install. Just the point where PEN becomes N and CPC has shifted but it's still upstream of the RCDs.

I need a holiday ....

 

[timhoward]

I need a holiday ....

No worries, it was a most welcome distraction from trying to reassemble an over-locker sewing machine....I'll explain in Arms in a minute...

 

[Other_Tucker]

How would you stop the neutrals from acting as proper neutrals though since they're all physically connected correctly in the board?

I'm trying to visualise a set up whereby the board can look normal but can be switched to use earth as neutral.

There are two large diameter cables going from the CU to the same point in the head, a blue and a green yellow. The blue passes through the meter and the other doesn't, swapping them won't affect the installation because they're joined anyway, essentially it's just a cable colour changing crossover switch.

The CSA is different 16mm-25mm if you're fussy , but if the deteriorating rubber DNO tails to the meter are 10mm anyway I can't see it being an issue

 

[davesparks]

What sort of earthing system is it ?

TN-C

 

[mainline]

There are two large diameter cables going from the CU to the same point in the head, a blue and a green yellow.

No there isn’t the blue conductor isn’t connected to the meters neutral as the incoming to the meter has been disconnected, leaving a switched shared cpu/neutral connected to the meters output.

 

[mainline]

TN-C

Thats what I thought but was more concerned with using a single conductor to combine N and Cpc I would have thought could upset an rcd.

 

[mainline]

Thats what I thought but was more concerned with using a single conductor to combine N and Cpc I would have thought could upset an rcd.

Residual current devices (RCD) rely on detecting currents flowing in the earthing conductor by measuring the out of balance currents between phase and neutral conductors. In a TN-C installation, the neutral and earthing conductor are combined, thus making the detection of residual current impossible.

You cannot tell what is a legitimate neutral current, and what is an earth fault current using the PEN, as the earth and neutral combined.

 

[timhoward]

In a TN-C installation, the neutral and earthing conductor are combined, thus making the detection of residual current impossible.

I don't think it's helpful to label this installation as a TN-C. I understand that in some ways that is correct, but it is causing confusion.
In a true TN-C system there wouldn't be an N or a CPC for any of the loads, just a PEN. In a true TN-C setup earth faults are indeed impossible to detect using an RCD.


In this installation, the neutral and earthing of loads are separated. The loads have N connected to the RCD and the CPC connected to the CPC bar. In turn, the CPC bar is connected to the N incomer on the main switch.
Therefore any current flowing through the CPC will not pass back through the RCD and the RCD will trip due to the imbalance.


Apart from the colour of the wire, I maintain that it is electrically exactly the same as this:


Discuss!

 

[Other_Tucker]

I cant find a ~100a version of the crossover switch in the picture, one that will link straight through L-R and then when turned join L green to R blue as well as L blue to R green. It's easy enough to do in electronics but I guess no-one makes an electric sized version. Sorry for the floppy CAD, turns out windows paint 3D is ----

 

[newfutile]

In a meter the neutral plays no part in the measurement unless its used to power (with the line) any electronics present in the meter .
i have taken single phase meters apart and there is always a solid neutral link with the live sent though a current shunt , the volt drop is then measured over the shunt to calculate amps

 

[mainline]

I cant find a ~100a version of the crossover switch in the picture, one that will link straight through L-R and then when turned join L green to R blue as well as L blue to R green. It's easy enough to do in electronics but I guess no-one makes an electric sized version. Sorry for the floppy CAD, turns out windows paint 3D is ----

A generator cross over switch or a contactor would do what you want.

I don't know how you would switch a 100amp mains using electronics cheaply.

I don't know what you are trying to achieve in your pic, The OPs had his N supply to the meter disconnected, so the meter electronics wouldn't work, as already said a number of times.

Apart from switching colours and Csa it won't do anything to thwart the meter reading.

 

[littlespark]

If it was as easy as disconnecting the neutral from the supply of the mainswitch…. And using earth as the return… basically not having anything connected to neutral out of the meter… to stop the meter running…. Everyone would be doing it.

The meters don’t need the neutral to operate… even the old mechanical ones.


Someone comes to read my meter 2 or 3 times a year, even though I fill it in online when asked to.
I’m sure these guys are trained to notice bypasses, strange boxes, smoking nails hammered into the tails….