Help needed with Neutral to Earth short

[howieleem]

If i put my meter across Neutral and earth with the mains switched on the meter fluctuates between short circuit and hi resistance. If i measure between Live and earth i get constant 240Vac.
If i turn off the mains i dont get a short between Neutral and earth.

What could be causing this, and how do i trace the N-E short?

 

[ban-all-sheds]

What sort of meter?

Set to what range?

Where are you measuring <whatever> between N & E?

Why are you measuring it?

Do the terms TN-C-S and TN-S mean anything to you?

 

[holmslaw]

..

 

[howieleem]

Metrix meter, set to auto ranging ohms. The reason i started looking at this problem is because i fitted a new bulb into a ceiling light that has not been used for a while and it blew the bulb. So i measured the wiring to the light and it had 240V between live and earth.
But i have lots of other ceiling lights that work ok.

I am now measuring between live and earth on a wall plug socket and i have the same, 240V between live and earth. I have switched all the breakers off except the one for the socket i am measuring at, and it is the same.

No i do not know what kind of Neutral is used.

 

[howieleem]

Holmslaw

You say i should have 240V between Live and earth, i thought i should only have 240V between neutral and live??

 

[TheOriginalTonkaToy]

240V between live and earth is normal. It's not a case of what type of neutral is used, but rather what type of earthing arrangement.

 

[howieleem]

I measure 0.7Vac between Neutral and earth.

 

[TheOriginalTonkaToy]

Holmslaw

You say i should have 240V between Live and earth, i thought i should only have 240V between neutral and live??

Neutral is earth at the substation, earth is a locally provided earth either by the DNO or, in the case of TT, by an earth rod at your premises.

 

[howieleem]

I'm not sure if it is TN-S, TN-C-S, or TT

 

[ban-all-sheds]

So i measured the wiring to the light and it had 240V between live and earth.

That's as it should be.

I am now measuring between live and earth on a wall plug socket and i have the same, 240V between live and earth.

That's as it should be.

I have switched all the breakers off except the one for the socket i am measuring at, and it is the same.

That's as it should be.

No i do not know what kind of Neutral is used.

Any TN- supply will have N & E connected together - //www.diynot.com/wiki/electrics:earthing-arrangements.

And with a TT supply you'll still see 240V between L & E.

But that's all L - E voltages.


Where were you measuring N - E resistances?

 

[ban-all-sheds]

You say i should have 240V between Live and earth, i thought i should only have 240V between neutral and live??

No - N & E are joined at the substation.

The Wiki article doesn't show the source of the supply - it probably should.

You'll find better explanatory drawings here: http://www.tlc-direct.co.uk/Book/5.2.1.htm

 

[ban-all-sheds]

I measure 0.7Vac between Neutral and earth.

Where, and with what loads in the house switched on?

I'm not sure if it is TN-S, TN-C-S, or TT

It would be good to know.

 

[bernardgreen]

If i put my meter across Neutral and earth with the mains switched on the meter fluctuates between short circuit and hi resistance.

The meter is being confused by voltages and currents induced in the various earth and neutral wires by capacitive coupling from the live wire. The meter ( assuming a "normal" multimeter ) will be testing the resistance with at most a few milliamps of current and looking for voltages less than a volt. The induced currents and voltages are much greater so the meter cannot read the resistance while the live is still energised.

Earth to neutral "short" is normal while the main switch is closed ( ON ). It should not be there when the main switch is open ( OFF ) that is assuming the main switch is a double pole isolating switch that disconnects ALL power to the house.

 

[howieleem]

It looks like TN-S, the earth exits the main feed in cable and goes seperately into the house under the floorboard level, Live and Neutral going to the switch box.

Ok, so 240V between live and earth is ok, so that is not a problem.

I have checked the switched output to the ceiling light bulb and it has 39Vac across L-E when switched off and 240Vac when switched on, where is the 39V coming from?

 

[bernardgreen]

The 39 volts is capacitive coupled from the Live to the Switched Live. Put a filament lamp in and the 39 volts will disappear.