low earth loop

[andy]

was doin a job 2day and the external earth loop was 0.15 ohm. from a TT supply. isnt this a bit low?

 

[pdcelec]

Are you sure it is TT all the way and not tapped off from somewhere else which is TN? Does sound low for a TT.

 

[andy]

Are you sure it is TT all the way and not tapped off from somewhere else which is TN? Does sound low for a TT.

at no point does an earth come from the service fuse. inside the CU i couldnt see anything directly connecting neutral and earth. there was 3 10mm earths leaving CU. presumably earth rod, gas and water.

 

[Big_Spark]

Whilst this is low, even by TN-C-S standards, I would not worry about it unduly as there is obviously a very good earth path. What sort of building is it?

 

[plugwash]

it could be a TN-S with the connection point outside your property. it could also be one of the water/gas bonds giving a low impedance path via someone elses system.

another possibility is a N-E fault in the installation (possiblly cross cuircuit or from a cuircuit to some kind of pipework)?

what happens if you disconnect the 3 10mm earths from the CU and loop test them seperately.

you DON'T get those kind of readings from a rod alone! it could be something totally innocent like pipework to other houses or it could be something worse like a NE fault you really need to find out.

 

[andy]

Whilst this is low, even by TN-C-S standards, I would not worry about it unduly as there is obviously a very good earth path. What sort of building is it?

i know this is not a problem, and it could make it safer. but i was wondering why it was so low.

its a 2 story house, next to loads of houses/flats

 

[andy]

it could be a TN-S with the connection point outside your property. it could also be one of the water/gas bonds giving a low impedance path via someone elses system.

another possibility is a N-E fault in the installation (possiblly cross cuircuit or from a cuircuit to some kind of pipework)?

what happens if you disconnect the 3 10mm earths from the CU and loop test them seperately.

you DON'T get those kind of readings from a rod alone! it could be something totally innocent like pipework to other houses or it could be something worse like a NE fault you really need to find out.

1: i agree, its possibly fed by a seperate earth

2: i was thinkin earlier that it could be a N-E fault, however, all apart from lights are on the RCD side, so it could only be the lights at fault.

3: didnt go that far. was just doin a quick test on the new circuit and was a bit surprised by the reading

 

[Big_Spark]

N-E fault is unlikely to give you a low Earth Loop reading. More likely there is an Earth plate in the ground that does several properties or there is a metallic foundation in the ground from previous constructioon of the site.

I have also known a very low reading for a stake in ground on the site of a former Aluminium processing factory, the ground was heavily contaminated by aluminium oxide and other metal rich compounds, these metals included iron, tin, antimony, tungsten and numerous others. It produced the lowest readings I have ever personally seen (0.08 Ohms), however I have heard of others lower but never seen this in writing.

 

[mapj1]

N-E fault is unlikely to give you a low Earth Loop reading. More likely there is an Earth plate in the ground that does several properties or there is a metallic foundation in the ground from previous constructioon of the site.

An NE fault on say a short cable to a cooker circuit might cause this, effectively giving you an earth reading rather like PME - but, without the proper PME precautions being in place. Mind you, round our way 0.15 ohms is quite low for an L-N loop reading too. - Usually see a kilamp or so as the highest PS-CC for an LN fault.

If I could not see an obvious earth route, I'd assume PME on neighbouring properties and a shared water main, if there is no N-E fault on a final circuit, but I'd not rule it out as meaning it must be safe without looking a bit further. A high fault current isn't on its own a good thing automatically.

so

Is the L-N loop the same reading?
Is the incomer some sort of RCD? - if its TT it normally is.
How far are you from the transformer ? - more than 100m and I'm tempeted to say 'I dont beleive it' (apologies to one foot in the grave fans)

PS on simple TT systems the earth rod at the transformer end isn't always up to much either - for a lone pole pig in field it could be as high as 20 ohms -though it sounds like that is certainly not the case here.

 

[kai]

What is the megger reading between neutral and earth on each circuit, with the Main Switch in OFF position?
Do you get something like 165 Meg Ohms N-E at the end of the circuit?