Earth/Neutral resistance ?

[lonspwi]

Hello,
My undertanding of earth and neutral in a domestic premises, is that earth is earthed at the premises and the neutral is earthed at the sub-station.

But should'nt there be open circuit between them ? I recently measured the resistance between earth and neutral in a lighting circuit and it was almost short circuit (a few ohms). Is this correct ? The resistance was measured using a DVM and with the power off.

Regards

 

[sm1thson]

there are different ways of earthing this explains them:
//wiki.diynot.com/electrics:earthing_supply_types_and_bonding:earthing_arrangements
so your situation looks pretty normal to me, allthough it doesnt tell you anything as (as you have discovered) you cant distinguish a correct set up from a neutral earth fault like this!

 

[plugwash]

Hello,
My undertanding of earth and neutral in a domestic premises, is that earth is earthed at the premises and the neutral is earthed at the sub-station.

the transformer neutral is tied to real earth at the substation.

depending on the particular install your domestic earth may be either seperately tied to real earth (TT), connected via a seperate route independent of the neutral (TN-S) or connected to the neutral (TN-C-S)

But should'nt there be open circuit between them ?

if the mainswitch is off then they should be isolated from each other (and if they aren't you have a fault somewhere) otherwise no (individual MCBs don't normally isolate the neutral)

on a TT you'd expect between about 20 ohms and 200 ohms depending on local soil conditions. On a TN-S you'd expect it to be fairly low (less than an ohm) and on a TN-C-S you'd expect it to be *really* low (less than a hundredth of an ohm if measured at the int)

 

[securespark]

Max Zs on TT = 200 Ohm

Max Ze on TN-S = 0.8 Ohm

Max Ze on TN-C-S = 0.35 Ohm

 

[lonspwi]

Thanks guys for all the excellent advice.
I only measured the resistance with the lighting CB off. I'll try it again this weekend with the main CB off, and ascertain which earthing system I have. The house is a 1920's build, and the Electricity board meters look pretty old, although the CU looks new.

 

[Steve]

The house is a 1920's build, and the Electricity board meters look pretty old, although the CU looks new.

Go on, lets have a look, show us a picture we like pictures of dangerous things!