Earth current origins

[c1223]

This is just something I'm doing in the garage, having a play with at the moment. It sort of has to do with household electrics.

I've got a 13A plug connected to an RCBO (Pro Elec 16A), which in turn is connected to a 13A socket. I wanted to see what happens when you connect an indicator lamp across the live and neutral of the RCBO (so you can see whether the RCBO is on or off). I've used one of these 230V LED lamps connected across the RCBO outputs.

The whole 2.3mA current appears on the earth. (Measured via a multimeter connected in series on the earth - obviously would never be done anywhere but my garage as it's very much against EAWR).

Is that expected behaviour? Is it something to do with the functional earth, because obviously if this is disconnected, the current doesn't appear on the earth.

 

[Taylortwocities]

You realise that neutral is connected to earth, either at your property or at the substation?

 

[EFLImpudence]

At first guess, I would say that you have the E and N reversed in the socket - but I suppose that's not the case.

Measure the current on the neutral.

 

[davelx]

What is the current in the Functional Earth with no load on the RCBO? The RCBO electronics will be connected between Line in and the FE, so their operating current will always flow in the FE even with no load on the RCBO outputs. Have you measured the current in the Line & Neutral (on the supply side) and on the FE at the same time? I would expect the current in to the RCBO on the Line input to equal the sum of the currents in the Neutral (on the supply side) and FE.

 

[EFLImpudence]

He said it was the same with FE disconnected.

 

[bernardgreen]

The RCBO electronics will be connected between Line in and the FE,

Slighty off topic. The functional Earth is there to ensure the RCBO electronics wil have power if there is a lost Neutral in the supply network and an Earth fault in the installation. At least one manufacturer of RCBOs consider this double fault situation is so unlikely that it can be ignored. Details here click

But if the Neutral is lost then other than in TT earthing the Earth for the installation will be lost which makes the Functional Earth not longer effective.

With a lost Neutral the functional Earth could remain functional if instead of being connected to the Earth derived from Neutral it was connected to an Earth drived from a local Ground rod.

 

[davelx]

He said it was the same with FE disconnected.

No he didn't. He only said he measured the current in the FE, and when that was disconnected there (clearly) wasn't any. He didn't say anything about measuring the input and load currents.

 

[davelx]

Slighty off topic. The functional Earth is there to ensure the RCBO electronics wil have power if there is a lost Neutral in the supply network and an Earth fault in the installation. At least one manufacturer of RCBOs consider this double fault situation is so unlikely that it can be ignored. Details here click

But if the Neutral is lost then other than in TT earthing the Earth for the installation will be lost which makes the Functional Earth not longer effective.

With a lost Neutral the functional Earth could remain functional if instead of being connected to the Earth derived from Neutral it was connected to an Earth drived from a local Ground rod.

Yes I know - I have an Eaton RCBO CU (see my avatar!). I searched around for schematics of those that do have FEs and they all show the FE being used for the return from the electronics.

 

[EFLImpudence]

No he didn't.

Apologies, my mistake.

Not sure now whether the current is in the FE or THE earth.

 

[c1223]

You realise that neutral is connected to earth, either at your property or at the substation?

Yes. It is a TN-C-S setup with PME.

What is the current in the Functional Earth with no load on the RCBO? The RCBO electronics will be connected between Line in and the FE, so their operating current will always flow in the FE even with no load on the RCBO outputs. Have you measured the current in the Line & Neutral (on the supply side) and on the FE at the same time? I would expect the current in to the RCBO on the Line input to equal the sum of the currents in the Neutral (on the supply side) and FE.

Interestingly the current without any load on the earth is 1.12mA. So can the electronics in the RCBO produce that sort of earth leakage? Or is something else causing that current?

I need to measure the other currents and see. Might be easier with a clamp meter.

For clarification, the system is setup as:

13A plug
- L -> RCBO live i/p
- N -> RCBO neutral i/p
- E -> one side of ammeter

RCBO
- L -> 13A socket L
- N -> 13A socket N
- FE -> other side of ammeter

13A socket earth is connected to other side of ammeter as well.

----

I've just done some metering (the load being the LED):

Under load:

L: 3.5mA
N: 1.8mA
E: 1.2mA

No load:

L: 2.3mA
N: 0.8mA
E: 1.2mA

 

[JohnW2]

Interestingly the current without any load on the earth is 1.12mA. So can the electronics in the RCBO produce that sort of earth leakage?

I'm not sure that 'earth leakage' is really the right term, but I could well believe that the resting current taken by the electronics is 1.12mA, and that will flow from the RCBO's FE through your meter to true earth.

KInd Regards, John

 

[davelx]

I'm not sure that 'earth leakage' is really the right term, but I could well believe that the resting current taken by the electronics is 1.12mA, and that will flow from the RCBO's FE through your meter to true earth.

KInd Regards, John

Yes, I agree. Note that getting accurate current readings below 10mA requires a very good quality (£££), calibrated, multimeter. The cheap own-brand ones you can buy at Maplin are not likely to be accurate down there! (They do sell the Fluke range too, which is a different matter))

 

[EFLImpudence]

Where, then, did the 2.3mA - the same as the current drawn by the test lamp - in the FE come from?

Is this, just coincidentally, (roughly) half of 2.3 each in the N and the FE plus the 1.12 resting current in the FE?


If so, what would happen with a larger load?

 

[JohnW2]

Where, then, did the 2.3mA - the same as the current drawn by the test lamp - in the FE come from? Is this, just coincidentally, (roughly) half of 2.3 each in the N and the FE plus the 1.12 resting current in the FE?

As dave1x has said, I think that (unless the OP is using very expensive kit) one probably should take these quantitative very low current measurements with somewhat of a pinch of salt - or, at least, regard them with caution. In what I wrote, I was really just commenting 'qualitatively', about not being surprised that the RCBO electronics was drawing a 'very small current', which was returning via its FE.

Kind Regards, John

 

[ericmark]

I think the word earth leakage is rather confusing, a RCD or RCBO does not measure earth leakage it measures all live wires to see if total is balanced, we assume if not then the unbalance current must be going to earth, but of course it could take another route.

With boats we have to be very careful, we can get an earth current nothing to do with supply, it can be simply due to the earth rod being copper and the boat hull being steel, there are a number of ways to stop this current, one is an isolation transformer the other is a diode, yes we actually put diodes in the earth supply.

These diodes actually go in the earth wire. I would not use one with a house, however very likely nothing wrong with your readings just two earthed items made from different metals.