Potential Difference Between Neutral And Earth

Karin · Aug 22, 2016

evening guys, i think i'm gonna need this one spelled out to me a little if anyone can help. everytime i think i've worked it out i just start to doubt myself and go in circles!

double pole isolation is required for TT but not TN systems where neutral can be said to be reliably connected to earth through a suitably low impedence.

so i'm thinking this means the reason we isolate the neutral when working on TT systems is in case of a pd between neutral and installation earth (as opposed to general earth, as both TT and TN-S have the exact same connection to general earth?).

..how does this happen? something is telling me voltage drop is involved but i'm just not sure!

happysteve · Aug 22, 2016

On a TN system the reference to Earth is at the supply transformer, and this is also the reference to the neutral point of the installation.

On an TN-C-S system, N and E are linked at the PME block, so they will be at the same potential, near enough.

On a TN-S system, N and E are linked at the supply transformer. The impedance will be roughly the same as Ze I guess, so a maximum of 0.8Ω. "

On a TT system, although the neutral is referenced to Earth locally via your earth electrode, this is (a) some distance from the supply transformer, and (b) will have a resistance Ra. Think of Ra as the resistance between your MET and "true earth", whatever that may be. Perhaps you could think of "true earth" as the voltage at the neutral point of the substation transformer, which is pretty much the potential of your neutral. Because there is a resistance between this and your MET (and exposed conductive parts) on a TT system, wherever there is resistance, there is the potential for there to be a potential.

I've been on the vino, so that might not make sense. Makes sense to me, though...

Karin · Aug 23, 2016

thanks for the response. i've drawn out some diagrams and am slowly getting this. athough i think i'm still a little hazy with the science as to how it happens, i think i'm understanding that the reason is to avoid a pd across the MET/installation earth and the neutral that could be an issue through indirect contact when working on the installation?

so you've essentially got a neutral that is quite poorly referenced to the MET, in general service as well, healthy installation, regardless whether the line is isolated or not.
the installation earth and neutral kinda float to different potentials in relation to each other based on the Ra?

i'm likely of course to be getting two (or more!) concepts mixed but i appreciate the help in clarifying
i've again typed out and deleted my thoughts multiple times but just getting into a headspin!