How would a "clip-on current transformer" for tracking domestic electric

June 5, 2013 11:12 PM

An Australian start-up, wattcost.com, is developing a "clip-on current transformer" device for analysing energy usage via domestic electricity meters. Is this real technology, or vaporware?

They're asking for $99 each to fund a device with sticks or clips onto domestic electricity meters, transmits electricity usage data via a wifi device back to them for analysis (via my home router, I assume), in order to send usage details and alerts back to users' smartphones. They're using a kickstarter-style crowd funding business model.

I live in an apartment block and, as is common in Australia, all the apartments' electricity meters are in a locked room in the basement, to which no residents have access. But they've told me via Twitter that the device will even work when stuck onto the cable leading from the fuse box in my kitchen out to the meter.

I'm interested in the concept, and I'm not paranoid enough to worry about sending them my electricity usage details, or letting them connect via my wifi router to do it. But I'd like to know more about how the "clip-on cable transformer" device would actually work. They're not particularly forthcoming with the technical details on their website or via Twitter (maybe to protect IP).

Is this all plausible? Is anyone aware of its usage elsewhere - not necessarily at the domestic level, maybe for industrial applications?

posted by infinitejones to Technology (10 answers total)

Clip-on current measuring devices are A Thing. They work well if the rest of the circuitry is designed correctly, and they're safer than the traditional way of measuring current (cut/break the wire and insert your meter between the two ends).

Some nice clamp-on current meters
A hobbyist version
posted by spacewrench at 11:23 PM on June 5, 2013 [2 favorites]

I first saw conceptual designs of devices such as this whilst on a design competition jury.

There are also prepaid electricity meters that have been popularized by South Africa initially, and now available in other African and Asian countries, primarily for those on a budget.

I can see this being a viable concept.
posted by infini at 11:25 PM on June 5, 2013 [1 favorite]

I run one of these to monitor electrical consumption in my home. They are cheap and readily available - I'd check one of the existing inexpensive consumer devices first before investing in a science project. In my case, a small computer logs current and transfers it to my website, where I can monitor my power usage. It's very accurate and replicates my formal electricity bill within a couple of percent.

Here is a description of how current transformers work, and some resources.

Mine also drives a small monitor attached to my wall which shows power consumption, and lights LEDs when power exceeds a certain threshold.

I'd recommend it. It really is remarkable how much energy you waste by standby devices, leaving lights on, running your heating inefficiently (if it is electricity based) etc. and how quickly your behaviour changes when you can see what's going on.
posted by falcon at 11:34 PM on June 5, 2013 [2 favorites]

Look, it even has a Wikipedia page.
posted by themel at 12:16 AM on June 6, 2013

Electrical currents create a magnetic field around the conductor, with AC the magnetic field changes, which in turn will create an electrical current in a conductor around that field. So, you just measure that current and you can find out how much electricity if flowing through the wires.

The technology is already available, but I don't know if you can get one today with wifi, etc for $99.
posted by delmoi at 12:29 AM on June 6, 2013

They totally exist (they're a common way to measure AC current in a line for test purposes) but they only work if you can clip over a single leg of the line— they essentially measure the total current through the hole in the clip-on core, and if both the 'hot' and 'return' are going through the core their sum is roughly zero. $99 doesn't seem insane for a version with a small attached wifi transmitter and without strict calibration requirements ... a little optimistic but probably achievable in volume.

Anyway, from the website I wonder if they're not doing a conventional clip-on current sensor but are instead doing something that watches the rotating disk on your electric meter and counts the number of revolutions it sees. Which seems maybe a little harder to get working reliably (and it won't work in places that have digital meters; it sounds like they've considered this but they're awfully short on details) but there's nothing fundamentally hinky about the idea.
posted by hattifattener at 2:35 AM on June 6, 2013

You can read disc meters with an optical sensor, hattifattener — the Blue Line/Black & Decker one does. Most digital meters have an optical pulse port that flashes every few watt-hours, and these are easily readable.

There are a bunch of these products available, and a couple of open-source projects to do it to, like OpenEnergyMonitor.
posted by scruss at 3:34 AM on June 6, 2013

Here is an expensive ($500) industrial/lab grade clip-on amp meter from Fluke. they are one of the respected names in electrical measuring stuff.
posted by ArgentCorvid at 8:12 AM on June 6, 2013

My lab has one the meters ArgentCorvid links, and we use it regularly to confirm the voltages that ought to be running. That part, at least, is real, and the price seems in the right general range for something that's likely to be measuring a standardized, narrow range of voltages without pinpoint accuracy.
posted by tchemgrrl at 1:08 PM on June 6, 2013

infinitejones: "But they've told me via Twitter that the device will even work when stuck onto the cable leading from the fuse box in my kitchen out to the meter."

All of the devices (except the ones watching the dial/flash on the Service Meter) that have been linked in this ask depend on being able to clip a meter onto a single conductor of your service. They can't work if you clamp the loop around both conductors of a circuit (this fact is the basic theory that lets GFCIs function).

Current Transformers can be dangerous, especially as the current being measured starts climbing into 200A range common for household service. Because of that and because of the requirement to isolate a single conductor a permanent CT like this generally will be installed within the service equipment (IE: inside your fuse panel). And, in Canada/USA, you actually need two CTs; one for each hot leg of your service.

A CT install to monitor whole house house power draw isn't a job for even a moderately handy homeowner because the available fault currents are very high. To give you an idea: where I work even 4th year apprentice electricians aren't allowed to do that sort of thing. It is reserved to journeymen and then only when circumstances prevent a shutdown of the the equipment.

Obviously if you just have to stick a sensor onto the power meter that watches the physical state of your meter the install will be straightforward and safe. But it doesn't sound like that is what you've been promised (IE: you have no access to your meter) so either they are lying to you, or they are making promises not understanding the situation you are in, or they aren't using a user installable device.
posted by Mitheral at 8:26 PM on June 6, 2013