Difference in amperage though household meter question

[alasdairp]

If it's a TT system then the current returning via earth should be almost zero as there should be no connection between earth and the live conductors (other than filtering caps etc that can cause a small leakage.)

You are correct if there are no wiring problems. For filter caps (etc) it should be a max of less than 1 mA/device (cookers and IT equipment can be a few mA). So nowhere near the 120 mA apparently missing.

However, even a TT supply Neutral is connected to Earth at the distribution transformer. If there is a short or low resistance connection between the house Neutral and the local ground Earth, then stray current can flow (immersion heaters are prone to cause this type of N-E fault). If his clamp-meter readings are correct, then I would take a bet that the "lost" current was probably going via the Earth connection. Should be easy enough to determine using a suitably sensitive and accurate clamp meter.

Another thought is... 'an old 50mA RCD'?? Is that what the protective device is? Or is it an old 50 volt voltage-tripping ELCB that sits between the Earth spike and the CU Earth busbar? That would explain why the '50 mA RCD' is not tripping. That was what used to be fitted in ages past. I have seen plenty in old houses in rural locations.

Another concerning thought - Earth is to an old lead water pipe - only house on the Estate like that now... are you sure - do all the other houses have proper Earth Rods or were they upgraded to supplied PME Earths by the network company sometime in the past? Your water pipe should be formally tested.

Although 120 mA is not a large stray current, it does represent about 30 watts of power - so the cause really should be tracked down and remedial action taken.

So, sorry, it sounds as if you should get a full EICR test carried out.

 

[Lucien Nunes]

I think there's some crossed wires, would be helpful for the OP to clarify. He does not seem to be talking about earth leakage at all, but about the supply current. I.e. he has switched off (nearly) all of his appliances and the supply current from the service cable is 0.85A, as measured by putting the clamp meter around either the line or neutral meter tail alone (not both) on the supply side of the meter.

But the current between the meter load side and upfront RCD is 0.97A, measured likewise. So his query is not about earth leakage but the difference in the line conductor current upstream and downstream of the meter, and far from going missing, it appears that more current is coming out of the meter into the RCD than it is drawing from the service head. If I am understanding this question correctly, it cannot relate to earth leakage because the meter cannot create any, not being earthed.

There is a possible explanation. The small load measured is likely a few timeswitches, PIRs etc that are still on when all normal appliances are turned off. These often have capacitive power supply droppers, which have a strong leading power factor. There is also usually some capacitive leakage current from suppressors in other 'live-but-off' appliances and the wiring itself. The clamp ammeter is correctly measuring the total of these at 0.97A. The kWh meter is also drawing current through the supply side tails for its voltage coil, but as R-fur points out, this is inductive with a strongly lagging power factor, so this current is more than 90 degrees out of phase with the house load. Hence, the input current to the meter is lower because it is compensating for the leading power factor of the house load.

If rather than current, the OP measured the power before and after the meter, he would likely find that although the current before is lower, the real power is higher, due to the dissipation in the voltage coil. In any case, measuring the currents without also comparing the phase, when the power factors are extreme, can give odd-looking results.

Also as mentioned above, there is a risk of measurement error with a clamp at low currents, including from stray fields from the magnetic circuit of the kWh meter. A significant part of the difference in readings could be due to measurement error.

 

[alasdairp]

Thanks, Lucien. A thoughtful and interesting contribution.

I strongly agree that local magnetic fields (espercially from the meter if an old rotating disc type) would be likely to affect the clamp-meter reading - though the OP says he has measured multiple times and the values are very consistent. It does depend on the quality and sensitivity of clamp meter and the harmonic content of the current waveform.

I do not understand how the PF can affect a clamp-meter current reading. The clamp meter is just measuring the magnetic field from the current flow. It is not calculating power. So it does not matter what the phase relationship of the current waveform to the voltage waveform is. A really bad PF can lead to higher harmonic content and therefore erroneous readings (especially on cheap clamp meters).

An investigation by a competent and knowledgeable qualified electrician is really needed if the OP wants to resolve this. Especially as his main Earth is an old lead water pipe and most other houses nearby have utility supplied main Earth connections.

 

[static zap]

Are the cables being measured a similar distance apart ,any curves nearby --
(forming a partial turn rather than a long straight run) ... Expecting something like this as a source of error .
Neutral - earth pd ,with house in OFF condition
may have some clues !

 

[Lucien Nunes]

I do not understand how the PF can affect a clamp-meter current reading. The clamp meter is just measuring the magnetic field from the current flow. It is not calculating power. So it does not matter what the phase relationship of the current waveform to the voltage waveform is.

I'm not saying power factor is affecting the readings, rather that it is responsible for the apparent anomaly that the input current to the meter is lower than the output current. My theory is that each reading is correct in itself, and the input current is indeed lower, but since the voltage coil magnetising current is heavily lagging, if the house 'standby' current is leading then subtracting the meter output current from the meter input current can yield a negative result even while both the house and the meter coil apparent power is positive. If the OP replies and confirms that we've understood correctly, I'll draw a phasor diagram.

 

[alasdairp]

I'm not saying power factor is affecting the readings, rather that it is responsible for the apparent anomaly that the input current to the meter is lower than the output current. My theory is that each reading is correct in itself, and the input current is indeed lower, but since the voltage coil magnetising current is heavily lagging, if the house 'standby' current is leading then subtracting the meter output current from the meter input current can yield a negative result even while both the house and the meter coil apparent power is positive. If the OP replies and confirms that we've understood correctly, I'll draw a phasor diagram.

Ah, I now follow. IF (quite a big if in my opinion) the inductance of the meter coil is enough to improve a bad capacitive PF and move it closer to unity, then the current on the supply side will be lower for the same total power. Yes, that is true. It would be very interesting to measure and calculate the PF in both places.

I may have a play with an old meter and some low current capacitive loads over the w/e.

 

[David Prosser]

Hi,

Its a TT system but it does not have a dedicated earth rod. Its earthed via the old copper going into lead mains water pipe. I am the only person on my estate still on lead and I have it all to myself and I have a long drive so there is plenty underground.

I don't think you should be using the water supply pipe as your earth electrode. 542.2.6

 

[davesparks]

I don't think you should be using the water supply pipe as your earth electrode. 542-02-04.

In a new installation this is unlikely to be allowed, but at the likely age of the installation it was a requirement of the regulations that an earth wire be connected to the metallic water supply pipe as a means of earthing.

 

[David Prosser]

In a new installation this is unlikely to be allowed, but at the likely age of the installation it was a requirement of the regulations that an earth wire be connected to the metallic water supply pipe as a means of earthing.

Could be it was installed before 1966 ?

 

[ruston]

And if my memory serves me , a private water supply pipe could, and I think can still be used.