TNCS - USING NEUTRAL AS EARTH IN BOARD

[telectrix]

No thats not what i mean , forgot war and all that. Why is it that a in one house a tncs system here can be connected to neutral but in another house it cant?

because where PME is provided, the supply has the N (star point) reliably earthed at several points along it's route. therefore, it's maintained at ground potential. best option is to TT it with RCD for fault protection, as wilko stated .

 

[Deleted member 26818]

The OP specifically said the link would be in the bus bar, which on the boards I've worked with would be before any RCD. That way fault current returns through the neutral and through the RCD as normal current would, and the RCD would not operate.

Yes, creating the link at the busbar, or anywhere down stream from the RCD would cause the RCD to trip whenever power was used.

 

[hightower]

Yes, creating the link at the busbar, or anywhere down stream from the RCD would cause the RCD to trip whenever power was used.

Pretty sure it would cause it to never trip even under fault. Gonna draw this out tonight....

 

[hightower]

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).

My question is Whether or not i could create a link between the Neutral and Earth bar to create a tncs system

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

 

[Deleted member 26818]

You won't get to the fault stage, as the RCD will operate as soon as any power is used.
Some of the return current will return to the power supply via the earth instead of via the neutral.
The RCD will see the imbalance between the Line and neutral and trip.

 

[hightower]

You won't get to the fault stage, as the RCD will operate as soon as any power is used.
Some of the return current will return to the power supply via the earth instead of via the neutral.
The RCD will see the imbalance between the Line and neutral and trip.

What earth? There is no earth, it has been linked to neutral to create a bodged TNCS

 

[Rpa07]

You've only got to think of the scenario of wrong neutral leg to wrong neutral bar or when you're working on a circuit switched off at the board but neutral not removed, so easy to trip if neutral and earth touch. The amount of imbalance needed through the RCD coils is minuscule to cause a trip. No way would the install be a happy install if @hightower 's scenario was attempted.

 

[hightower]

You've only got to think of the scenario of wrong neutral leg to wrong neutral bar or when you're working on a circuit switched off at the board but neutral not removed, so easy to trip if neutral and earth touch. The amount of imbalance needed through the RCD coils is minuscule to cause a trip. No way would the install be a happy install if @hightower 's scenario was attempted.

I'd argue quite the contrary - it would be too happy, and would render the RCD completely useless.

 

[Rpa07]

It's electrickery, it knows more than us! We're just wire monkeys! I understand your passion though - we need a "myth buster"

 

[westward10]

Might try this tomorrow, it a controlled environment obviously

 

[Deleted member 26818]

What earth? There is no earth, it has been linked to neutral to create a bodged TNCS

Of course there's an earth.
If there wasn't, then there would never be an earth fault and installing an RCD would be pointless.

 

[hightower]

Of course there's an earth.
If there wasn't, then there would never be an earth fault and installing an RCD would be pointless.

Have the read the original post? You're making no sense to me.....

 

[Wilko]

Hi - not wishing to step into a hole, but would not the luckless chap be standing on the earth and the earth fault current flow through him? This would unbalance the RCD in the usual way, if it had ever been able to stay on line to begin with as per Spin #45 and Rpa #47 thoughts.

 

[Wilko]

Might try this tomorrow, it a controlled environment obviously

Do it, you know you want to

 

[westward10]

Do it, you know you want to

It is my intention but for it to be a scientific thingy you would have to isolate the board from all sources of earth which may be impossible, you have the earthing conductor, bonds and all those circuits linked to extraneous parts like water heaters, boilers etc.. to lose. I see defeat before I start.

 

[Rpa07]

I had to like that twice because of your shaky fingers @westward10!
@hightower, the OP didn't mention any busbars or where it should be linked just that it can be linked.

 

[westward10]

Third time I have done this, yesterday I managed the same post 13 times! I agree I assume the op means before the board not within it.

 

[hightower]

Well he did specifically say bus bars in his first post, which I take to mean in the board.....

 

[westward10]

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.

 

[westward10]

Forgot to say the rcd is as much as a chocolate teapot when connected like this