Earth Leakage clamp meters

[Jimmy Boy]

Hi HS,

I find clamping the Earthing conductor misleading, as often you can measure more mA than it would take to trip a 30mA RCD even in a correctly functioning installation.

I wonder if this reading is actually imported ?

A better test is to clamp both tails as this shows how much leakage is present in the installation.
I often do this prior to a CU change, as any significant readings here shows that you will have to test much further before the change, as you are likely to have problems once RCD's are fitted.

Presumably the reading you get by clamping the Earth is the cumulative total of leakage from all circuits not just one circuit, so I guess if you had an upfront RCD then this total could tip it over the edge, depending on the mA, if the leak was <mA setting over several circuits, it shouldn't trip the RCBO, unless of course it was leaking like a sieve lol. Out of interest,could someone tell me if you can clamp a T&E Radial or half leg of a RFC and get a mA reading, or do the cpc and L/N need to be separated ?
ATB J

 

[spark 68]

Presumably the reading you get by clamping the Earth is the cumulative total of leakage from all circuits not just one circuit, so I guess if you had an upfront RCD then this total could tip it over the edge, depending on the mA, if the leak was <mA setting over several circuits, it shouldn't trip the RCBO, unless of course it was leaking like a sieve lol. Out of interest,could someone tell me if you can clamp a T&E Radial or half leg of a RFC and get a mA reading, or do the cpc and L/N need to be separated ?
ATB J

No, Not at all jimmy boy, I have come across installations which are working fine on dual RCD boards yet when clamping the earthing conductor has shown in some cases several hundred mA, I tried this when I first got my earth leakage clampmeter (as you do) on a variety of installs, as I say I think this is external.

When you clamp the tails though they show no appreciable leakage in a correctly working installation, problematic installs always show when clamping the tails together though.


Now I don't even bother checking (clamping) the earthing conductor for this very reason, maybe someone could throw some light on this aspect ?

 

[Jimmy Boy]

As a matter of interest what do the manu's instructions say ?
J

 

[spark 68]

As a matter of interest what do the manu's instructions say ?
J

On my particular meter it just shows clamping one tail to measure current in Amps, or clamping all of the tails to measure leakage in mA.

 

[HandySparks]

No, Not at all jimmy boy, I have come across installations which are working fine on dual RCD boards yet when clamping the earthing conductor has shown in some cases several hundred mA, I tried this when I first got my earth leakage clampmeter (as you do) on a variety of installs, as I say I think this is external.

When you clamp the tails though they show no appreciable leakage in a correctly working installation, problematic installs always show when clamping the tails together though.

Now I don't even bother checking (clamping) the earthing conductor for this very reason, maybe someone could throw some light on this aspect ?

I think it'll depend largely on how well isolated from earth the bonded "extraneous" parts are. In my house, both the gas and water underground pipes are plastic, so the only good connection to "earth" is down the earthing conductor to the TN-C-S head. In a property where the underground services are metal, or there is some other connection to ground (such as structural steelwork in electrical contact with the ground), I guess you may well get all sorts of circulating currents which are nothing to do with "leakage".

 

[spark 68]

That's what I was saying HS,

I was concerned at one TNC-S Install that was showing several hundred mA on the Earthing conductor (just shy of an Amp), (note I did not say leakage), yet the Install itself was fine all tested OK no RCD tripping faults, I clamped the tails this showed about 3 or 4 mA of normal leakage of the installation itself (it was in use at the time).

I spoke to the DNO, and they were not bothered in the slightest and said it is only at mA levels and not to worry.
So clamping the earthing conductor will not necessarily point to a fault within the installation.
So this test is pretty much meaningless.

As I said, the way to test the installation for true leakage is to clamp the tails, this always shows the leakages within the installation.

 

[HandySparks]

Sorry, thought you were querying why this happens on some installations. Hence my theory that it depends on how well "earthed" the extraneous parts are.
I'm guessing that this affects how easy it is for current to flow due to small voltage differences between the means of earthing and the other earthed parts.

 

[spark 68]

Sorry Hs,

I was sort of querying about these readings being imported, as you say it could be due to slight potential differences between the various different extraneous parts.

I suppose clamping the earth conductor on a TT system might give reliable indication of leakage currents in the installation, but I still think other installations nearby could introduce spurious currents via any shared services such as water and gas pipes and the like, Iam not sure.

As I say when I got my meter a couple of years ago I was playing about with it, as you do with a new toy, and it was quite disconcerting to see such currents on the earthing conductor, so much so I queried it with the DNO, but they did not see a problem (I worry too much).
So now I don't bother clamping the Earth at all.

Had it not been for that install already working fine and no RCD tripping, I too would have given a wide birth to doing any work on that install or similar, this is how I found out that clamping the Earthing conductor was a misleading test, and to just clamp the tails, I was just passing on the knowledge I gained to save people the trouble of thinking they had a problem when in fact it seems to be quite normal on some installs.

I would like to know more about this, and what is deemed unsafe limits of current on the Earth, for example does it have to be in the Amps or tens of Amps before it becomes a problem ?

Edit: I also clamped the main bonding conductors, as well as several CPC's and registered nothing of any note (0 mA), this was the other reason I felt it was due to external means beyond my control.

Edit II: I have just clamped the tails here, and registered 7.8 mA, I clamped the Earthing conductor and got 7.1 mA (it is on a TNS system), but as I say on some installs the Earth can measure quite a larger current, which is why I said it is not a reliable test clamping the earthing conductor, I wonder if it depends on the earthing system (TNC-S ?), or if some nearby installs had earth faults, or maybe even electrical noise ?