How does a three phase electricity meter work?

[netorius77]

Someone recently told me that if you have a three phase electricity supply at your house, the meter reads the highest current of any phase and then calculates the kWh based on that current on all phases. He said it is therefore critical that the loads on each phase are as even as possible, otherwise you are paying for energy you are not actually using. This is obviously assuming you have multiple single phase loads across each phase.

Is this correct?

 

[Jola]

I have three different meters, one for each phase, so don't think mine works like that.

My phases are very uneven, one consumes a lot more than the others.

 

[LazyLion]

I have three different meters, one for each phase, so don't think mine works like that.

My phases are very uneven, one consumes a lot more than the others.

Ours at our business also has three separate meters.
One for each phase, you can see them moving at different rates.

Three of these thingies... they haven't switched us over yet to the digital, but it is on the cards soon.

 

[Freaksta]

We have 3 meters but I can't remember how you read it. The guy that reads it did tell me once but I don't recall it being straight forward.

 

[sgs]

At a previous property I had 3 phase installed for a tankless water heater (which needed the extra juice to enable it to heat water to the required temp instantly).

Everything else in the apartment was on single phase, with the stove on 1 phase, plugs on another and lights on the 3rd. (They didn't even bother to connect the stove to 3 phase, which would have been the more balanced approach to take.)

The meter was a 3 phase prepaid meter.

When I switched on the kettle, the meter would run at rate A. If I switched on the stove, the rate would be B. Switching on both kettle and stove would make the meter run down at rate A + B. (I specifically tested this as I wasn't sure about how the metering worked either.)

In other words, it did not matter if one phase used more power than another. It simply measured each phase and added it all up.

So while it is preferable to be as close to balanced as possible, any imbalances do not appear to affect residential metering at all. (What happens with industrial users may be an entirely different story. The meters used there are much more complex and take things like e.g. the power factor into account as well.)

 

[Sonic2k]

The usage across all three phases is summed.
In years gone by, they made meters with three discs on a common spindle.

 

[Arthur]

Indeed. Each phase is metered individually. For billing purposes the readings are summed and you are invoiced for energy used.

Power factor correction is an interesting topic. We are typically billed in kilowatts (per time period, typically kWh) but when power factor is less than unity this ain't the same as KVA.

 

[netorius77]

Thanks for the replies - seems that my information was actually "misinformation". Always good to check if something doesn't sound quite right. Appreciate the comments.

 

[Gnome]

Indeed. Each phase is metered individually. For billing purposes the readings are summed and you are invoiced for energy used.

Power factor correction is an interesting topic. We are typically billed in kilowatts (per time period, typically kWh) but when power factor is less than unity this ain't the same as KVA.

SA consumers aren't billed for power factor, or rather we are billed as if your power factor is 1.0

I would assume that you could therefore push power back into the grid and incur R0 (not sure what it would do with negative values...)

 

[richjdavies]

Yes - if anyone is charging you on a tariff that is kWh -- then they have to charge you based on the ACTUAL kilowatthours used. They can't just decide to bill you based on some random other figure ==> e.g. like you said the maximum across any phases.

I believe it is important to 'balance' the phases if you are charged based on kVA -- e.g. your on a larger commercial tariff.... but I'm not 100% sure why, as surely having 10A on all phases or 30A on one and nothing on the others, would seem to be the same thing to me.

 

[Arthur]

SA consumers aren't billed for power factor, or rather we are billed as if your power factor is 1.0

I would assume that you could therefore push power back into the grid and incur R0 (not sure what it would do with negative values...)

If you have relatively high inductive loads (eg electric pumps, motors, aircons) in relation to resistive loads, then power factor can drop to say 0.95, 0.9 or even 0.85. If so, correcting the power factor can make a difference to your electrical bill. This isn't significant for the vast majority of domestic consumers, where almost all loads are resistive and hence PF=1.0 most of the time. Most municipalities contractually stipulate a PF of 1.0. For commercial and industrial users, Power Factor minimums are specified. Some years back City Power in Joburg even had a clause saying they were not obliged to deliver power if PF was less than 0.85.

 

[richjdavies]

Hate to raise this zombie thread. But stuck on a very related issue and so wanted some wisdom from the crowd!

I'm starting to notice a lot of Solar installations where they add up all the phases before working out what is 'imported' and 'exported' from the grid; whereas some meters seen don't sum them before adding to the total e.g.:

Red Phase = Using 1kW
White Phase = Using 2KW
Blue Phase = Feeding back 1.5kW
Assume the above happens constantly for 1 hour

Solar monitoring system shows: 1+2-1.5 = 1.5kWh used from the grid, 0 exported...
Advanced meter shows: 1+2kW = 3kWh used from grid AND 1.5kWh exported to the grid.

Which is really right? What do you guys think? Anyone know what is REALLY right?

My in going belief (can be swayed...!) is that the Advanced meter is right - as you are feeding back and using at the same time, the 3 phases don't care... Same would be same for the 3 x single phases meters (as per CBI's old meters).

But is there any technical standard for this and if so - how come I have a prepaid meter on a site that does it like the solar monitoring system!!?

 

[Gordon_R]

I'm not sure of the exact details, but I do know for a fact that some models of prepaid meter cannot tell the direction the current is flowing. My cousin has a grid-tied solar installation, with exactly that problem. He has to manually read the old 3-phase meter, and claim a periodic refund on the difference between the two meters.

 

[Sinbad]

Depends what rate the municipality credits you at. Pick the measurement that makes most financial sense.

 

[Xtm]

Strange... I'h three phase connected to single meter

 

[thehuman]

Strange... I'h three phase connected to single meter

You get meters that can measure all 3 in one unit, most probably smart meter , he would need to figure out to add some extra load onto the phase grid tied connected to