Meters - inaccurate or consuming power themselves?

zagmund

We had mini-meters installed on each circuit in the house to help monitor usage patterns. They are similar in appearance to these ones from ABB - http://www.abb.ie/product/us/9AAC100486.aspx.

The weird thing is that the sum of the meter readings is less than the actual main meter reading. All things being equal the sum of the mini-meters should equal the main meter reading.

Say the main meter records 100 KwH, the mini-meters might add up to 97 KwH. They're in the right ball park, but certainly out by more than a minor rounding amount. From observation over a period of months the variance is about 3%.

One of two things is happening here -
1) The mini-meters aren't sufficiently accurate - I can live with this as I know the total consumption and that's what's ultimately important.
2) The mini-meters consume power in order to measure power - if they consume 3% of my total power usage then this is not great.

I'm inclined to think that it's an accuracy thing rather than a consumption thing. Does anyone know for sure whether a din rail meter would be expected to consume power just to measure the power flowing through it?

z

[edit] there's a third option - that there's power being consumed without passing through one of the meters. The house was rewired from scratch and I'm pretty sure that the electrician who did the wiring would have run everything through the mini meters, so I reckon we can pretty much rule out #3.

coylemj

I'd be inclined to give the technology the benefit of the doubt and say that the mini meters themselves are the cause of the discrepancy - they are only reporting the kWh consumed by your appliances and lights and ignoring the tiny amount of juice they're using themselves which is of course seen as power consumption by the main meter.

frankmul

I think the main meter is accurate to 0.2%, class 0.2. The abb meter can be up to 2% accuracy. This might be the cause of the difference. The meter I would say is using only a few watt.

tomdempsey200

3% sounds ok

ted1

Your looking a meter tolerances, with multiple meters it's being exaggerated.
The meters themselves will only use a minimal amount.

tomdempsey200

ted1 wrote: »

with multiple meters it's being exaggerated.

if each meter is 2% off doesn't that mean the overall discrepancy is max 2%

zagmund

The figures above were based on a month or so, but I just went back and checked the difference over the last 6 months and it's actually lower than I thought.

The main meter recorded 2281KwH, the mini-meters recorded 2296KwH. This gives a difference of only 12KwH which is actually only 0.52%. The original 3% above may have been a function of the sample period being too small.

I can live with 0.5% as a variance

z

Bruthal

tomdempsey200 wrote: »

if each meter is 2% off doesn't that mean the overall discrepancy is max 2%

It does I'd say

antoinolachtnai

Was it expensive to get the mini-meters? Do they have a flashing LED?

zagmund

I don't know the cost as it was bundled in as part of the "rewire house - €x,000" figure we had as part of a bigger job.

I did get the impression that they weren't unduly easy to source locally, but I've seen them on Amazon and equivalent since then. Have a look for "DIN rail meter" and you'll see the options.

There is a red flashing light when a certain amount of power is consumed. When the washing machine is on it goes like billy-o. However since I don't have the documentation for the meter and since I don't want to stand there trying to count (without blinking) flashes for a couple of minutes I haven't worked out what the consumption interval is.

If you are thinking of having them fitted, consider getting ones that can be monitored remotely. I trundle out to the garage once a week and write the readings down and then enter them into a spreadsheet for tracking purposes. It would be generally easier to have them networked and let the computer do the reading & recording for me. It doesn't have to be expensive wifi (which probably wouldn't like being right *on* the mains anyway) - there are other home networking systems for this sort of setup. Modbus is one, but there are others. Google is your friend.

z

ted1

antoinolachtnai wrote: »

Was it expensive to get the mini-meters? Do they have a flashing LED?

The pro1T is a nice meter http://www.dhmeters.com/Din-Rail-Electricity-Meter.php

It has a display and a pulse output. Not to expensive and the same size as a single phase MCB.

zagmund

I had a look at the front of the meter this morning. From what I can tell, there are 2,000 flashes of the light per kWh.

z

Heroditas

0.5% variance is pretty good, particularly for typically small loads at a domestic level.

zagmund

Here's the type I have in place - "Smart IMT80". The lower number (556.9) is the total kWh since the meter was installed. The upper one is the usage since it was last reset.

I wasn't able to find them online anywhere and I've no idea where the electrician sourced them. I've just seen them on this site though - http://www.camax.co.uk/electricity-meters/single-phase-meters/imt80a-kwh-meter

z

zagmund

Excellent news. Now that I've found the documentation for the meter on the camax.co.uk site I can work on remote monitoring. I've heard of, but never done, pulse monitoring and the documentation gives the details of the connections for pulse monitoring.

All I need to do now is to wire up an Arduino and some bits and bobs and we're laughing. I was especially happy to read the bit where they say the pulse connections are fully separated from the inside circuit. *Obviously* I wasn't going to start sticking wires into mains meters without knowing the specifics. Now that I have the manual I can fire on ahead.

That's a good result for today.

z

antoinolachtnai

You could just use the LED pulse to be sure you are fully isolated. Be interested to hear how you get on.

ted1

Pulse relays are genreally a relay, reed switch or transistor output.

I have never seen one that is different than the above and I specialise in metering

zagmund

ted1 wrote: »

Pulse relays are genreally a relay, reed switch or transistor output.

I have never seen one that is different than the above and I specialise in metering

I'm afraid you lost me there Ted (as a certain inhabitant of Craggy Island might say).

I think you're saying something like "of course you couldn't do anything dangerous with a pulse connection."

If so, the important bit is where you are the expert and I'm the novice and I'm erring on the side of not electrocuting myself by first of all waiting until I have the instructions and then once I have the documentation, proceeding according to the instructions. I think.

z

ted1

Your quite right, Always best to proceed on the side if caution.