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Note: This may be UK-specific, I'm not sure.

I overheard part of a conversation about 3-phase electricity, one guy was saying that mains generators use 3 coils (or something) and as it spins it produces 3 different waves of alternating current that are 120 degrees apart. That's why the power lines often have three (or six) sets of cables, one for each phase. He said that if different phases are electrically connected then bad things happen, and sometimes if only one phase goes down, every third house on a street could lose power.

This raised a few questions for me:

Does that mean all power stations have to be in sync with each other? If so how is this achieved?

Is it true about every third house, I've never seen this?

Does this mean we effectively have 3 grids overlaid on top of each other?

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  • $\begingroup$ A detail: power stations sync from each other across the grid, as do solar inverters, wind farms etc. If a sufficiently large area is turned off, there are special procedures for "black start". $\endgroup$ – pjc50 May 7 '15 at 9:52
  • $\begingroup$ And now imagine how you can plug in photovoltaic and wind power plants, which don't actually use the same system, and don't have a stable phasing and frequency. The electric grid providers really aren't happy about those. $\endgroup$ – Luaan May 7 '15 at 10:48
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Does that mean all power stations have to be in sync with each other? If so how is this achieved?

Yes, they do. Each generating station receives a timing signal from their Energy Management System that essentially tells the generator if it is synchronized to the grid frequency or not.

Grid frequency is primarily driven from a standard and consistent time source, such as from an atomic clock.

Is it true about every third house, I've never seen this?

Kinda true if you really stretch things, but not really. See below.

Does this mean we effectively have 3 grids overlaid on top of each other?

Definitely not. It's one grid using three phases. 1


The basics of what you were told are essentially correct. For a host of reasons, it is more efficient and practical to transfer AC power in three phases.

And yes, if the phases cross and connect with each other, Bad Things really do happen. At a minimum, it will trip the protective relays and shut the area down for a while. The momentary arc that's generated when they cross can be quite spectacular though and can easily destroy large portions of trees that were silly enough to have their branches provide the phase crossing mechanism.

The extent of Bad Things also depends upon the voltage level of the line and amount of current being carried.

You don't see every third house going dark because of the way distribution laterals are setup. There are two main approaches to feeding distribution loads:

  • All three phases transferred along the distribution lateral (aka balanced)
  • A single phase transferred along the distribution lateral.

The single phase per lateral approach is easier to put in place but is less efficient because the loads tend not to balance well across the various laterals coming out of the distribution substation.

It's easier to balance out distribution laterals by transferring all three phases along the length of the line. However, power is delivered to blocks of houses and each block is fed off a single phase from the distribution line. You can verify this if you have overhead lines by looking for the step down transformer (it's a big metal can hanging off of the power pole) and verifying that only one phase feeds into the can. 2, 3

My understanding is that residential service in North America is almost always single phase. The primary driver behind this is that most residential grade motors are single phase, and the demand simply isn't there to justify the additional equipment expense from delivering two phases to a residential drop.

1 I'm explicitly ignoring things like regional interconnections such as what is seen in North America. Then you could argue for there being multiple grids, but that's not really germane to your question.
2 You can identify if you have a 3 phase distribution feed in your neighborhood by looking at the wires at the very top of the distribution power pole. Three wires spread across arms at the top of the pole generally mean a 3 phase service in the area.
3If you see a cluster of metal cans and all three phases feeding into those cans then you've likely spotted a capacitor bank which is used to condition the power on the line. It's not used for stepping down the voltage for residential service.

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  • $\begingroup$ Silly trees growing their branches in the wrong direction $\endgroup$ – user2813274 May 6 '15 at 21:14
  • $\begingroup$ As I understand it, most (but not all) of the houses I've inhabited in the USA have had two phases. Most of the house is on one phase, 110v. Only a couple special outlets have two phases, 220v. Those outlets were for the clothes dryer, the electric stove, the main air conditioner, etc. $\endgroup$ – daveloyall May 6 '15 at 23:08
  • $\begingroup$ The Question mentions that the phases differ by 120 degrees. This does not fit with my understanding. I'd expect two 110v lines out of phase by 180 degrees to combine into a 220v outlet. I also expect that I've misunderstood something. $\endgroup$ – daveloyall May 6 '15 at 23:12
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    $\begingroup$ Houses on so-called 'split phase' power which is common in the US have two legs 180 degrees out of phase with each other. As I understand it, these two legs are generated at the final step-down transformer from a single one of the three phases (120 deg apart.) By contrast, in the US at least industrial buildings are typically fed with all three phases. $\endgroup$ – Ethan48 May 6 '15 at 23:28
  • $\begingroup$ There are areas that are wired up with all three phases though (skipping every third house). It used to be popular among the wealthy to get their house rewired to all three phases so that a blackout will mean only 1/3 of their lightbulbs and wall sockets get affected. $\endgroup$ – slebetman May 7 '15 at 0:03
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I can't speak for the UK, but the "every third house" is certainly not true in Denmark. Here the standard arrangement is that every house gets all three phases at 230 V. Inside the house some of the circuits are fed from each phase, so in some error conditions you can experience that the floor sockets are dead but the ceiling lights work, or vice versa.

Some high-powered appliances such as stoves are often constructed to be connected to all three phases at once -- the difference between two different live phases produces AC with an effective voltage of 400 V instead of the 230 V between a live phase and neutral.

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    $\begingroup$ This is also commonly used in Czech Republic. And if one of the phases "dropped", it's pretty likely the others would as well - it's still the same power plants supplying power for all the three phases after all. $\endgroup$ – Luaan May 7 '15 at 10:45
  • $\begingroup$ I think that's a common arrangement in Europe. Here in Finland we also get all three phases at 230 V. $\endgroup$ – juzzlin May 7 '15 at 14:06
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In North America, you generally don't see 3 phase power being delivered to houses. The transformers (whether they're up in a pole, or in a green box on the ground) take apart the 3-phase 1100V power delivered to them and step it down to 110V. For 220V, two feeds are connected appropriately, and 220V is delivered to each breaker box. In commercial and industrial settings, you can get three phase power delivered. We have big air compressors and hoists that run on 3 phase 220V. A motor has to be designed to run that kind of power. We have had outages on one and on two of the three phases. Bad things happen. Things draw WAY more amperage than they should from the remaining power and smoke and unhappiness results. In our old building, this happened it was because it was old and a circuit-breaker for that purpose wasn't installed. In our new building, the electrician was incompetent and didn't install it. Large installations of fluorescent lights will use 3 phase 367V power. The outage in the new building wrecked the lighting so that afterward, 2 of the 5 bulbs in each fixture wouldn't light. All the ballasts needed to be replaced. We also have transformers in the building that step up the 3 phase 240V delivered to 3 phase 400V and 600V for some specialized equipment.

But in residential neighborhoods, what gets delivered to the house is one phase power 110V on two buses that, when wired correctly can give you 220V power. And if all three phases aren't being delivered correctly to the transformer, a circuit-breaker in that trips and nobody gets any power.

Burning down people's home because of power snafus isn't good karma. The power company ensures that it won't be their fault!

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    $\begingroup$ Three-phase motors are sensitive to failures on one or two phases. They should be used with monitoring relays in order to be shut down in these cases. $\endgroup$ – Rafael Lerm May 7 '15 at 1:47
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    $\begingroup$ @RafaelLerm depends on the topology of the motor. Delta motors would be able to function with only two phases connected. $\endgroup$ – Aron May 7 '15 at 12:43

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