Shall I convert from TT to TNCS ?

low Re

ITM Ze

True, but if currently TT then it will be fully RCDed anyway, so the Ze will matter less.

Depends a bit on your starting point. PME/TN-C-S will give you a good, consistent earth with a low external loop impedance. So it is easy to clear faults without needing RCD protection on all circuits. Arguably, in this day and age with RCD protection on pretty much everything anyway, that is less of an advantage than it used to be.

PME makes it harder to export an earth to outbuildings etc since you will need to export the whole equipotential zone with it. (since you need to mitigate the unlikely situation where you loose your protective earth and neutral to the property - that could leave all your earthed metalwork floating at mains voltage).

So for a self contained domestic install, TN-C-S is the default route since it works and does not depend so heavily on RCD protection.

If you have a big sprawling installation with lots of sub-mains running to outbuilding etc, and each of those configured as smaller TT setups, then TT is often easier. However making sure all the RCDs function properly is important and so periodic testing is more important.

(my house is wired as TT, but later the supply was upgraded to PME, So I have the same option. I have left it as TT)

Well, we hope. There are still installs about that don't comply with the basics. Last one of those I saw was TT with no RCD anywhere, an all electric house running off a 30A neutral fused incomer. First not ok install I ever saw had the earth rod disconnected, and of course no RCD anywhere. The voltage controlled ELCB was never going to trip.

means what?

Ze is Earth Impedance :-)

Is it *really* necessary to invoke impedence in this situation? I know it's referred to as "Earth Impedance" but we're only talking about

I think we can safely rule out earth inductance. Is there a significant capacitive effect? I assumed not. Unless I'm mistaken Re = Ze.

Because that is how it is specified in the wiring regulations...

Dave

Quite. It's kind of hard to imagine any kind of phase shift arising between the fault and earth over the loop lengths one typically encounters.

and . . .

Because Impedance incorporates Resistance - and not vice-versa.

PA

The only thing you (the OP) might not anticipate is if he has otherwise-grounded stuff, like an unbalanced radio aerial. Keep its earth and the PME'd power system well apart!

Otherwise, go for it! I'm too stingy to make the changeover, but since our local supply was PME'd the dynamic impedance of the PME is less than an ohm; the conventional house earth is over 100 ohms!

PA

Someone asked about RCDs, I have 30ma RCDs RCBOs on every circuit, some shared in split consumer units. Before them all is a 100ma 100ms RCD.

There are no ring circuits.

There are no radio ham arials

I put in an Building Control notice for new circuits about a decade ago, and have ripped out all the old perishing rubbery wires and replaced them ...

I havent seen anyone say what is the advantage of converting from TT to TNCS, ( adding a 16mm greeen wire ) is it safer in some way?

George

PS I am us> Shall I convert from TT to TNCS ?

Several of us have told you that the impedance of the TNCS earth is much lower that of a local earth.

And, as the old history book "1066 and all that" says:

"This is a GOOD thing" ;-}

That's all very good, but what about the case of the "Broken PEN"? It could be that with the advent of RCDs that TT might now be safer.

TN-C-S will offer a very low fault impedance and so can clear faults quickly, using just just the MCB in many cases - so you have slightly fewer points of failure. Historically it was also slightly cheaper to install if starting fresh. (these days with lots of RCDs being the norm, it does not make much difference)

In your case with a properly setup TT install with time delayed master RCD and cascaded 30mA trip devices for direct and indirect contact protection, there is no clear performance of functionality improvement that would result. TT is also easier if you want to run external circuits and you don't need to worry about exporting an equipotential zone should you export an earth etc.

There are tiny differential risks with both options. I'd be more inclined to trust PME than RCBOs/RCDs, and if that assessment is correct then TNCS is safer. It's minor though, you face way bigger risks in life.

Is it *really* any easier to run external circuits from a TT install than a TN-C-S one? Surely, in both cases it's easiest to not worry about exporting the equipotential zone and just to make the external circuit TT by providing it with its own earth rod.

Its easier in the sense that with a TT install you can export the earth, and don't need to worry about its proximity to another "local" earth, since the earth you are exporting *is* the local earth. So a bit simpler an cheaper since you don't necessarily need an extra earth rod and RCD setup for each.