Electric meter question

If there is a brownout are you still paying as if the normal voltage was being supplied? Are the meters amperage with assumed voltage based or actual wattage based which will factor in voltage sags?

AFAIK, electricity meters actually measure the current. So you’d paying for the normal voltage even during a brownout.

From Wiki (which jibes with what I remember):

So no, you should not be paying for 110 volts x amperage, you should be paying for the actual power (kWH) being drawn.

The meter is always measuring real power (in watts) on a continuous basis, and then integrates it over time to calculate total energy used over time (in kWh).

So, in essence, the meter is counting the actual electrons that pass through?

There is more than one type of electric meter. If you’ve got a really old one (say from the 1930s or 1940s) then all they measured was the current and they won’t accurately measure power during a brownout. I’m not going to say there aren’t any of these still in operation, but if there are any still out there they are exceedingly rare. If you have one, I’d like to know about it just because I’d like to know if there really are any still in use.

Most folks have the spinning disk type of meter. This has both a voltage coil and a current coil which are used in such a manner that they make the disk spin proportionally to the power. Therefore it does accurately measure power and not just current. In a brownout situation it will accurately measure the usable watts that you use and you will be billed accurately.

There are also newer fancy digital meters, which are getting more and more popular. Newer homes will usually have these, and older homes are also slowly getting retrofitted with them. These also measure both the voltage and current, and not only do they provide an accurate measurement (like the older spinning disk meters) but they also measure things like power factor and noise and all sorts of things that they can then report back to the utility company so that they can make things more efficient. The meters also can talk to each other so the power company can automatically read them and they don’t need to send someone out to read the dials and write down the numbers. While most of the benefit of digital meters is to the power company and not to you, digital meters do allow the power company to measure stuff accurately which helps them to make things more efficient. If they keep the difference as extra profits then it doesn’t help you, but if they then lower their price per kilowatt hour then it does benefit you indirectly. So having digital meters is actually a good thing (there’s a bit of paranoia against them, for some reason).

So the short answer is unless you have a freaking antique meter, it is accurately measuring power and not just current. You don’t need to worry about being billed incorrectly for brownout periods and such.

The newer units simultaneously sample (i.e. convert from analog to digital) the voltage and current waveforms. I have no idea what the sampling rate is, but I am guessing it is a few thousands samples per second. Each simultaneous “voltage & current sample” (e.g. V = -145.09 V and I = -28.32 A) is multiplied together to get an instantaneous power value. The amount of energy used between the *current *power value and the *previous *power value is calculated and then thrown into a long-term additive accumulator. The value of the accumulator represents total energy used.

At least that’s the way I think they work.

Just how does the remote reading process work? Can the power company address each individual meter thru the power lines and get a response? If so, how does that not screw up the pure sine wave that most powered equipment is expecting?

If you’ve ever measured AC line voltage with an oscilloscope, or better, a harmonic distortion analyzer, it is anything but a pure sine wave; well, pretty close but passing it through a simple low-pass RC filter is enough to noticeably smooth it out, or if high-pass, pass the noise through (most of the distortion is noticed at the peaks, where flattening occurs due to all of the rectified loads in modern homes). Here is a particularly bad example of AC line distortion (the current shown is typical of rectified loads, as mentioned above).

As for power line communication, typically a very high frequency is used (100s of kHz or more), which has negligible effect on most loads, although capacitive loads would preferentially shunt the high frequency, including the line filter capacitors used in most electronics and PFC capacitors used by the power company to balance inductive loads; thus, I don’t really see how long-distance power line communications can be reliable enough (I have experimented with this myself; in order to make it reliable enough, I had to use an isolated wire run, with an inductive filter on both ends with the transmitter on one end and the receiver on the other). As it is, most smart meters use wireless to communicate.

That article isn’t very informative…

…and Wiki’s isn’t much better, at least to answer my question. Is the wireless signal sent for many miles to a central office? Is it one-way, read-only? Or is it piggybacked onto the wire line by modulation? Is it detected by a driveby capture unit? What frequency is used? Same for all meters? What data encoding is used?

The powerline communications systems that work in the 100’s of kHz region don’t work too well through US electric distribution system- signals that high in frequency are difficult to get through a typical residential distribution transformer, and are mostly filtered out by power factor correction capacitors. That band is much more popular for whole-building systems (where the transformer blocking effect is a feature), or in Europe, where hundreds of houses can be fed from one transformer.

There are several meter reading systems that operate below a few kHz - the closer you are to the 60Hz powerline frequency, the better the signal travels (since by definition the AC distribution system passes 60Hz well). The problem here is that normal powerline noise starts to become a problem for reception. Despite that, there are several successful meter-reading systems, including some below 1kHz. Generally the signal levels are much smaller than the 60Hz sine wave, and aren’t a problem for regular equipment (in fact so small that it’s a challenge to design a receiver to intentionally detect it). Some of these systems are very slow (e.g. 20 hours to read one meter), and/or one way.

There are pros and cons to RF vs. powerline meter reading schemes. Powerline systems sometimes have an advantage in rural areas, where cellular coverage isn’t good, and meters are too far apart for fancy mesh networks or efficient dedicated radio coverage.

For RF meter reading, there are several types:
[li]cellular modem, using a standard carrier such as Verizon or AT&T[/li][li]licensed spectrum with dedicated transceivers blanketing the area (often mounted on utility poles), and small radios in the meters. This is often near the 900MHz band. The pole-mounted transceivers use fiber or cell modem connections to back-haul to a regular network[/li][li]RF Mesh networking, where each meter has a low-power transmitter that can only reach neighboring meters. Messages are passed through the mesh until it reaches a concentrator, which would use some other means to get back to a central network.[/li][li]Drive-by RF, where each meter has a short-range transceiver that can only reach the street, and a person in a truck slowly drives by, picking up all meters in range[/li][/ul].

These are all generally two-way communications systems, with varying amounts of real-time ability (obviously the drive-by systems are only real-time if someone’s actually nearby).

Many utilities have a variety of systems, including a mix of RF and powerline communications systems.

Where I grew up we had the spinning disk meter. It was right around the corner from our front porch so we often looked at it wondering how it worked. I surmised that the spinning disk ran an ‘analog-style’ odometer, that is instead of a row of digits it had a row of dials with each digit represented by were its needle was (sort of like how very old, early tabulator displays worked).

Something that struck me as a conundrum way back then: Were we being charged for the electricity to run the mechanical meter itself? I realized it had to be a miniscule amount, but still I figured somebody somewhere might have complained about the concept… :smiley:

entertainment for you is worth charging for.

There are two coils in this type of meter. The voltage coil connected before the metered part of the circuit, so you don’t pay for the power going through it.

The current coil has current going through it proportional to how much current you are drawing. In order for it to work it has to get its energy from the metered side of the circuit. So yes, even though it’s a small amount, technically you paid for the power flowing through this coil.

So the power company paid part of the electricity required to operate the meter and you paid part of it. On the plus side, as those meters aged the mechanical bits tended to run a bit slow due to increased friction, so you probably made out ahead in the long run.

The fancy new digital meters have the same inherent measuring problem. It’s just that because they use a low power A/D converter instead of a magnetic spinning disk the amount of energy you pay for is much lower. It’s still not worth worrying about either way, but technically you do pay for a portion of the metering. There’s also no mechanical bits to wear out, so the meters won’t read low as they age either. Oh well.

Another, ah, ‘advantage’ with the mechanical meters was if you had a friend at the utility company who could get you the security tamper-tags that sealed the ring around the glass meter housing you could pull the meter off (it essentially ‘plugged’ into the base with four prongs), and plug it back in upside-down. This caused the meter to literally run backwards like an older car’s odometer in reverse. So if you switched it in the middle of each billing period (usually two months) you’d get almost free electricity. If the utility guys ever came by and saw your meter upside-down you’d be in big trouble though!

We never did this at our house, in fact I only ever saw it done by one friend of ours, but it did work (this was 25+ years ago!)