Residual Current on Mains Tails - Installation Disconnected.

acvc · Nov 21, 2018

I bought a new MFT1710 a couple of weeks ago and got a free clamp meter with it. (UNI-T UT210B, 200A Mini Clamp Meter).

I finally got it out the box today and thought I'd try it on my mains tails to see if/how it's working. Surprisingly, with the consumer unit main-switch off, and the main earthing conductor plus all bonding disconnected from the consumer unit, the clamp meter read 0.36A.

I checked with my Fluke T600 and it read 1A (doesn't go to decimal points).

The only thing connected is the actual meter, a single phase watt hour digital meter rated at 2W/230V.

If I'm paying 20p a unit (not sure about that) that means I'm paying £139.20 a year for this residual current.

The only thing I can think of is that this is normal and is deducted from everybody's bill as a matter of procedure, so isn't a problem.

I've got a three-phase mains head with two supply cables rising into two different phases, but only one is in use. I don't know if there's a fuse in the other two carriers.

TNS earthing.
Ze of 0.44ohms.
Voltage is 241V - I'm twenty odd meters from the substation.

Any thoughts?

Thanks.

snowhead · Nov 21, 2018

Here are the specs for that clamp meter;

UNI-T UT210B Mini AC Clamp Meter Key Features

Measure AC current from 2A to 200A
17mm small size jaws
Compact body for measurements in tight applications
2000 count backlit LCD screen
Autoranging
True RMS
Non-contact voltage detector built-in
Data hold function temporarily stores the last value taken on the screen
Relative mode
Auto power off when left idle for 15 minutes
Low battery indicator

TJ Anderson · Nov 21, 2018

If your double pole main switch contacts are open then it is impossible to have current flow on the tails directly feeding them.

MFS Electrical · Nov 21, 2018

Me thinks there might be a bit of induction going on here.... not on the tails but the clamp meter itself

Wilko · Nov 22, 2018

Hi - if you fiddle about with getting the tails in the middle etc it might zero out. But anyway, as Sir Snow has pointed out any reading below 2A would be unreliable. How's the weather in Mongolia just now?