0
\$\begingroup\$

I'm trying to understand what is going on here - this is in a normal house linked to the grid (240 volts / 3 phase), nothing extra. On two sets of plugs (next to each other), I measure 36 volts between ground and neutral. I see 240V between ground and live (in my minimal knowledge, this seems correct?), and 240V between neutral and live (again, this seems correct?).

In the appliances plugged into these plugs (I've unplugged all of them and get the same readings) you can feel a tingle when touching the metal housing (which I assume is linked to ground). The appliances themselves work just fine.

In all the other plugs in the house, I read zero volts between ground and neutral - this leads me to thing that the plugs showing the voltage are wired incorrectly / differently to the rest of the house.

Can any shed some light on what could be going on here? Should I be calling in my electrician?

\$\endgroup\$
  • 1
    \$\begingroup\$ It's normal for a small voltage to appear between earth and neutral. I'm not sure if 36V counts as small in this case, but if you feel a tingle, that could be cause for concern. 240V between earth and live or neutral and live is perfectly normal as long as you're not in North America or southern Japan or somewhere else with 120V mains. \$\endgroup\$ – Hearth Jan 22 at 15:18
  • \$\begingroup\$ Yea, that is what I figured - thanks. I think I need to get an electrician in. I am still interested to know how this would happen - i.e. how do you wire something so you get that? \$\endgroup\$ – Gareth Jan 22 at 15:20
  • 1
    \$\begingroup\$ Domestic wiring has only a single phase coming in, so the live and neutrals have to be common to all the circuits. I'd guess that there's a break in the ground line, and the voltage you measure is just induced or leakage. You could try measuring the resistance between the ground connection on those sockets and others with the power off. \$\endgroup\$ – Phil G Jan 22 at 15:24
  • \$\begingroup\$ What type of appliances do you feel a tingle when touching? And do you feel a tingle on those same appliances connected to a different socket? A slight buzz is not uncommon for appliances with unearthed switch-mode power supplies under normal circumstances (laptops are good examples of this). I'm not saying you don't have a problem with those sockets, just trying to get better info for diagnostics :) \$\endgroup\$ – marcelm Jan 22 at 17:17
  • 2
    \$\begingroup\$ @PhilG "Domestic wiring has only a single phase coming in ..." - Into the house you mean? That's a blanket generalization you cannot make. Where I live, having three-phase power at home is uncommon but not unheard of (in fact, my mother's house has it; it was once used for an electric range). \$\endgroup\$ – marcelm Jan 22 at 17:20
3
\$\begingroup\$

If you measure 36 volts between neutral and earth and ostensibly 240 volts between live and neutral AND live and earth then clearly this means that 36 volts is close to 90 degrees phase shifted from the 240 volts.

This is because if the two voltages were in phase you would measure: -

  • 240 volts between Live and Neutral AND
  • 240 volts +/- 36 volts between Live and earth

Clearly this isn't happening because you measure 240 volts to neutral or to earth and this largely happens when the earth-neutral voltage is 90 degrees phase shifted to neutral-live.

When this happens, mathematically, instead of simply adding/subtracting 36 volts and 240 volts together to get 276 volts or 204 volts you do a vector/phasor add like this: -

$$\sqrt{240^2+36^2} = 243$$

Another way is looking at it like this: -

enter image description here

In other words you are measuring about 240 volts in either scenario even though there is 36 volts between earth and neutral. This is typically due to a break in the earth line and, what remains of the earth line connected to your socket, is capacitively coupling to live and neutral to the tune of 36 volts. That waveform of voltage will be 90 degrees out of phase hence the imperfect fit with normal additive mathematics.

\$\endgroup\$
0
\$\begingroup\$

Most likely, those sockets are poorly grounded, or not grounded at all.

If you are using a high-impedance digital voltmeter, then the results you get measuring voltages can be a bit misleading, as they can detect tiny capacitive coupling between the different wires in your installation. So something that is not connected can show a voltage.

But get an electrician in to look at it, unless you're competent to investigate for yourself.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.