Checking the accuracy of domestic electricy meter... / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Checking the accuracy of domestic electricy meter...

1 2 next

Move Thread

LAN_403

Nicko

Tue Jan 12 2010, 05:19PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

I'm in the UK. I suspect that our electricity utility provider's main meter is over-reading - its digital, 4 years old, 3-phase, 100A and has two time settings - an over-night rate that it switches to (called Economy 7 here) which is cheaper...

Now E.on (the provider) want to charge me GBP 75 to install a "check meter" in series - the problem is that I don't really want to spend the money as UK regulations say a meter is valid if it over-reads by 2.5% OR UNDER-READS by 3.5%. i.e. I could end up even more out of pocket.

How can I check this meter myself on the QT? Accurately. e.g. to 1 or 2%

PS. Individually checking each phase as well as the overall totals would be great.

Thanks

radiotech

Tue Jan 12 2010, 08:43PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

First, check the creep. Does meter register anything with everything
loadside switched off. To check the meter you need a known load connected at the metersocket. Remember you pay for heating the copper from the meter to the loads you use. With known load and timer, you can check agreement. Why wont this be easy? (1) the kWhr meter may not have the same accuracy at all loads.(2) loads with low power factor, what ever it is, cost you the heating of the copper on your side of the meter to the load. The last suggestion is to try and get hold of the standard test method used to certify that meter for commerce.

Quite a while ago, when updating wiring, I found that a meter had been wired incorrectly in a gutterbox. This meant that one business tennant had been paying for 1/2 of another tennants power, although that tennant had paid full measure. This was subsequently repaired and the electric company refunded a lot of money. The records showed, for years the previous tennant had complained and the file for that address had letters unresolved due to language difficulties some 20 years previous.

Nicko

Tue Jan 12 2010, 08:53PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

We had the house completely rebuilt in 2005 - all the white goods are new with good PF - main heating is a heat pump with a commercial PF-adjusted compressor. There are some low energy bulbs, but not many - most of the lighting is halogen.

Disconnecting the house is probably futile - its all new wiring and shutting everything off would be a pain (there is a LOT of tech in here). What I want is to, over a period of some days, check that the meter is really registering the right number of KWh at the right times.

One bizarre feature is that we seem to be using 75 - 80% of our KWh at night - nothing happens at night! The fridge/freezer is still there. The washing machine & dishwasher might run, but the heat pump just ticks along 24/7 and doesn't care. No lighting is on except blue leds throughout the skirting boards in the common areas for floor lighting (about 30W in total).

I'm puzzled about what is going on...

Electra

Wed Jan 13 2010, 06:26AM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156

Wouldnâ€™t the most straightforward way be to obtain a 3phase meter and wire it as a secondary meter between your companyâ€™s meter and your fuse board?
The total units for all time zones together should add up to roughly the same, to within 2x the meter toleranceâ€™s of course, but at least this will eliminate any major fault with your meter.

Not to state the obvious but I thought economy 7 was only of benefit for large loads at night like water and storage heating. Are you sure youâ€™ve not got a heatstore or something topping up your heating in the cold weather without you knowing.

Finally a bit of a wild one, and someone here will prove me wrong I guess, if you have an unbalenced reactive load connected between 2 differnt phases. Wonâ€™t this cause a positive apparent power in one phase and a negative apparent power in another phase, With respect to neutral ? In a mechanical meter these cancel each other out as they are on the same shaft so the net reading is zero, could an electronic meter have some calculating errors that do not subtract without some small error.
Surely they must test these new meters extensively though.

radiotech

Wed Jan 13 2010, 07:18PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Wattmeters measure E*I*Cos (phaseangle). Pure reactive loads will not register any wattage(in theory) but will have current flowing. The actual reading will be the I^R loss in the wires connecting the load. A wattmater can be made into a varmeter (volt amps reactive) by adding a phase shifting transformer. Here is an interesting paper.

Be wary of measuring reactive loads! It is easy to burn out a wattmeter in the shop, mentioned from bitter experience.

Also, with anything three-phase, take nothing for granted. Elevators always had a phase sequence relay because if somthing happened in the way the feeders were connected later (repairs to transformers or overhead lines) the car would move in the opposite direction, rendering the saftey switches in the shaftway inoperable.

Dinges

Wed Jan 13 2010, 09:31PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

As others have said - disconnect everything in the house (as simple as switching all breakers off) -> electricity meter should now show 0W power use and kWh indication shouldn't increase.

Now plug in a load with known power consumption - I like a 2kW infrared heating lamp. Plug it in with a small kilowatthour meter (energy meter, cost about 25E; known also as 'killawat' in the USA; IMO, every household should have one to monitor the energy consumption of appliances - it will quickly repay for itself in saved energy costs). The value that the plug-in energy meter shows should agree with what the utility meter shows.

If there's a large discrepancy, you may want to repeat with different loads and graph the difference. As the plug-in energy meter doesn't have calibration marks, the readings it yields carry very limited legal weight. But it provides a strong indication on whether you should get the utility company to install a calibrated test meter.

Should your utility meter really be off, I expect you will get your 75 GBP refunded.

Over here, getting your meter verified is without extra charges.

If you can't be bothered to disconnect everything in the house because there's too much high-tech (I don't get that one, but so be it -consider it a man-made power outage) - pay the 75 GBP to have it metered for you. Though they may actually have to (briefly) disconnect your house from the grid entirely anyway to put the meter in series.

It really can't get much simpler than this. You can either do the measuring yourself, or have it done for you (and pay for it, of course)

Peter.

Dave47

Thu Jan 14 2010, 01:26PM

Registered Member #84 Joined: Thu Feb 09 2006, 01:06PM
Location: Dallas, TX
Posts: 47

These guys claim they will have a UK version available this quarter

I've been wanting one, but haven't pulled the trigger, yet.

David

Nicko

Thu Jan 14 2010, 08:01PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

The problem with all these units is that they are a compromise - as they are all driven by a clip-on CT for each phase rather than being in-line, you have to set the line voltage and they all assume a PF of 1 - our line voltage varies between 230VAC and 250VAC (or thereabouts). I have tried the Efergy meters, and they are OK, but unless you have true RMS & PF-aware meter its all rather pointless - we have quite a few low-energy lights, which have a dreadful PF...

Having said that, Google PowerMeter looks cool, and I will almost certainly get a GPM enabled device in due course...

I'm coming round to the idea that the only solution is to get the utility company to install a check meter...

rats

Avalanche

Thu Jan 14 2010, 08:51PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

at work we have a three-phase power meter with rs232 interface for logging. It's a bit like a current clamp meter, with voltage sense. calculates real and reactive power flow, quite a handy piece of kit.

Dave47

Thu Jan 14 2010, 09:38PM

Registered Member #84 Joined: Thu Feb 09 2006, 01:06PM
Location: Dallas, TX
Posts: 47

Nicko wrote ...

The problem with all these units is that they are a compromise - as they are all driven by a clip-on CT for each phase rather than being in-line, you have to set the line voltage and they all assume a PF of 1 - our line voltage varies between 230VAC and 250VAC (or thereabouts).

The TED unit looks like it also measures the voltage on both phases independently. That is, the box that is installed in the breaker panel has the two clip ons, but also has a voltage connection to each phase through a breaker.

Their website is a little light, so I didn't see where they specifically say it is correcting for PF, but it seems so. Anyway, they only have the US version at the moment, so it doesn't help you.

David

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob