I know that we (typically) get 120 V AC in USA, but what does the waveform look like? Can anyone post a pic? How many phases are there, and what's the peak voltage?
Electronic – What does the US power supply waveform look like
mains
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Impedance depends on frequency. Assume 50 or 60 Hz unless otherwise noted.
Impedance is typically in the 0.1 - 0.5 Ohm range at the home switchboard with the latter being unusually high and under 0.1 Ohm not uncommon in well provided systems with short feeders.
Note that a resistance of 0.1 Ohm will drop 10 V at 100 A
As a percentage of Voltage on a 230 VAC circuit that's 10/230 = 4.3%.
Main impedance meter slide show.
- They note in slide 27 that NFPA-70 suggests 5% drop at farthest outlet. 5% at 110 VAC = 5.5V.
At say 20A load that's a maximum R of 5.5 V/20 A = 0.275 Ohm at the furthest outlet!
ZM-100 mains impedance meter manual note typical result on page 1-8 of 0.l24 Ohms at 60 Hz (rising to 0.52 Ohms at 1 kHz.) . This diagram is simply meant to show effects of frequency but also shows what they consider a useful value for an example. The metter considers > 0.99 and < 0.01 Ohms as faults.
Mains quality perspective here - around 0.1 Ohm mainly resistive for practical purposes in many cases. YMMV.
Notes:
Thevenin Voltage = Voc = nominal mains Voltage (110 VAC, 230 VAC, ...)
Thevenin current = Isc.
You do not usually run a mains outlet at anything like Isc!.
For practical purposes Zth (Thevenin impedance) will be nearly pure resistive. At the power entry point to a home switchboard short circuit current is liable to be of the order of hundreds of amps.
For eg 460 A at 230 V, R = V/I = 0.5 Ohms.
Lower is better and usual.
Maximum power transfer occurs when Vout = 0.5 x Voc and you don't even run a mains outlet anywhere near that heavily loaded ! - and Isc is MUCH greater again.
Comment: Possibly the device that comes closest in formal use to maximum power transfer conditions is an arc welder and similar - a very special case.
To specify Thevenin impedance for a mains power supply you need to specify point of measurement, as the result varies substantially. Example ponts may be
Just on consumer size of pole-fuse or feed point to consumer's individual circuit.
At home switchboard mains supply point
In switchboard on load side of fuse of circuit breaker.
At wall outlet or light socket
At end of an extension cord.
Depending on aim of exercise - assume 0.1 Ohm at switch board input at 50 Hz will probably be an OK starting point in many cases.
This post is mainly about ground/earth/soil as conductor and importance of grounding for safety. Maybe it's not accurrate answer but it may be useful too. For other meanings of ground - see other answers.
I think answer for your question is here:
http://en.wikipedia.org/wiki/Earthing_system
See TN, TN-C and TN-C-S systems.
Cables you are asking about terminate at closest transformer.
More about grounding and why ground/soil is not used as conductor for power lines.
Ground in medium and high voltage power grids is not carrying significant power in normal conditions. High voltage power lines are 3-phase, current flows mainly between phases and ground is just reference "zero" for them. Ground works more as reference and discharges static electricity from high voltage equipment housings and other conductive parts that should be at earth potential. High voltage equipment is well insulated and it may accumulate large electrostatic charges.
Carrying power by ground (soil) would probably end up with very fast electrodes corrosion and maybe some environmental changes in soil, because soil contains water, salts, acids. All of this becomes mix of electrolytes.
Grounding also works as lightning protection. Thats the way to route/control lightning power into ground with low "power losses". When there is no intentional grounding - lightning will find 50 other ways anyway. In that case - grounding can be considered as high impedance/resistance grounding. Very high power can be emitted on high resistance and that may cause uncontrolled fire or explosion anywhere, in many places etc. So it's better to make a "highway" for the lightning by grounding big metal things.
In some networks "functional grounding" is used and in this case earth is indeed used to carry power.
In low voltage networks (110V or 230V in europe) grounding is used as "protective grounding", to allow RCD protection. Older method of protection is connecting conductive devices housings to ground. If device is damaged (burned insulation, mechanical damage etc) and voltage "coming out" to grounded housing - fuses will blow up because there is a short circuit.
Your black wire probably goes to transformer at power station/transformer station. It's grounded there. In some countries ground wire has to be connected to earthing system near house (metal tapes burried under house), but in that case - that wire is not black, but yellow with green stripes. That depends on earthing system used in your country. You can read about diffrent earthing systems on wikipedia (link below).
Grounding/earthing is a thing with "many faces"...
And there is no such thing like "negative" or "positive" line in alternate current. There is phase wire, zero and/or ground wire. Phase wire becomes positive (voltage above zero) or negative (below zero voltage) over time. Zero stays at zero relatively to... earth/ground :)
Can someone correct/check for my language mistakes? My english is bad, I don't want to mislead anyone in such important matter (grounding/earthing).
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Best Answer
I took some oscilloscope captures of the mains waveform in my house in Houston, Texas. The house is over 50 years old and the wiring is definitely not recent (no grounding). We have LED bulbs on dimmers that weren't designed for them as well as the usual collection of electronics with switching power supplies. This is definitely not ideal!
First, here's a large-scale view of the sine wave with measurements. Normal household outlets have only one phase. I see 110 Vrms with a peak of about 150 V, which is on the low end of the nominal range. Distortions are visible near the high and low peaks.
Here's an FFT, which shows third harmonic (180 Hz) distortion at roughly -30 dB relative to the fundamental. The next-largest harmonic is the seventh (420 Hz) at about -45 dB relative to the fundamental. The other harmonics are in the -60 to -70 dB range.
Here's a zoomed-in capture of the distortion, showing variations of perhaps 5V from an ideal sine wave.
UPDATE: I took some scope captures of the mains voltage in the lab in Sugar Land, Texas, where I work. The building was built a few years ago in a rapidly-developing commercial area, so I'd assume the infrastructure is up to date. Nearby loads include lots of test equipment and LED lights. There's an ATE test floor on the other side of the building, but I doubt it's a huge factor.
Here's the full waveform. It's monotonic and the distortions are much less obvious. Note that the voltage here is 122 Vrms with a peak of about 165 V, slightly above the nominal range. Both this and the household mains show a peak voltage that's lower than what we'd expect for a pure sine wave with the measured RMS value.
Here's the FFT. The harmonics look very different from the household mains. Surprisingly, the next-largest peaks after the fundamental have about the same magnitudes as before. The other harmonics are worse than the household mains.
Finally, here's a zoomed-in view of the positive peak. The flattened top is much more obvious here.