Pat,
The standard electrical service to homes in the U.S. is a 3-wire system which delivers both 120 and 240 volts. The secondary on the pole transformer provides 240V, but has a center tap which is grounded and becomes the neutral to your house. The two "hot" legs are thus each at
120 volts with respect to ground/neutral, but because these two legs are at opposite ends of the winding they are 180 degrees out of phase and thus you get 240V between them (used to feed your range, dryer, large A/C unit, etc.)The 120V outlets in your house are wired so that some are on one leg of the supply, some on the other. If you plug your intercom stations into receptacles which are on opposite sides of the transformer, the signal will be affected by the degree of coupling which exists between the two sides.
If that same transformer feeds other nearby homes, then you may be able to communicate between houses, but again the signal will be affected by which side of the transformer you are connected to. If you got a poor signal from house to house, you may have found that it would have improved if you'd plugged into a different outlet (on the other leg).
John Hines wrote:
Yes, North America typically has single-phase transformers feeding residential loads, each transformer providing power to a small number of homes.
On this side of the Atlantic, the typical British system uses very large 3-phase transformers, each providing power to a large number of homes by way of a 3-phase 4-wire wye distribution network running at
240/415 volts.Normal residential services are just tapped from the network as 2-wire
240V, houses being distributed between the phases as evenly as possible, while commercial premises can take 3-phase 415V power from the same transformer. You can find smaller, single-phase pole transformers serving a small group of houses in rural areas, but as soon as you get anywhere with more than a couple of dozen homes together you'll find the 3-phase 4-wire system in use.In Continental Europe, it's quite normal to find 3-phase 4-wire
220/380-volt service to even quite small houses, In France they'll even install 3-phase residential power that way with the main breaker set for 15A maximum per phase! Very strange.Going back to Pat's wireless intercoms, you can find a similar situation here in which you might not be able to get a good signal to your next-door neighbors' house (different phase) but can to a house a few doors away (same phase).
I remember a problem of this nature some years ago in which the owner of a general store was trying to use a set of these intercoms to communicate with his apartment above. The signal was very noisy due to the units being on different phases. Fortunately, the store had a
3-phase supply and I was able to solve the problem by installing an outlet for the intercom on the same phase as that used to feed the apartment above.The Victoria line of the London Underground (subway system), built in the 1960s, was designed to run with automatic control right from the start. All the driver has to do is close the doors and push a pair of "start" buttons when ready to leave the station, then the train runs automatically, stopping and restarting at signals if necessary, and coming to a stop at the next station.
The control is provided by safety codes modulated onto a 125Hz carrier injected into the running rails, along with higher frequency signals (up to about 20kHz) which control the actual application of power and braking.
There's a good outline of the system on the "Tube Prune" website (go to "Signalling Pages" then "Victoria Line ATO"):
These days the London Underground uses radio extensively, but for many years it seems the only communication available was by a portable phone kept in the cab. A pair of bare wires a couple of inches apart run on insulators along the tunnel walls. These carry a low-voltage DC circuit which serves a dual purpose. By shorting the wires together a driver can remotely open the breaker which feeds traction current to his section of track. He can then take his portable phone and clip it across the wires to communicate with the control center. Connecting the phone trips the traction current off if that was not done already, so clearly this was used only in emergencies.
The traction supply in the London system is interesting in itself, being 630V DC arranged on TWO separate conductor rails (i.e. the running rails are not used as the return). You can see some pictures of the layout on the link above.
-Paul.
[TELECOM Digest Editor's Note: Thanks for that good explanation. I will tell you another example I found fascinating a few years ago. I worked part time late evenings a couple nights per week for a department store in downtown Chicago. The building was closed at that time of night except for me working in the one small office. When I was ready to leave, I always had to turn the burglar alarm on and put a wooden gate with wires on each end of it across the door to the elevator after I boarded the elevator; i.e. set the alarm, within 15 seconds get on the elevator, insert the gate on the front of the elevator and leave the premises. One night it malfuctioned; the alarm failed to 'set'; I had to call the alarm company to come out and correct it. The alarm company repairman came out with tools and also with a headset. He clamped his headset onto the wires (which otherwise were not connected at the elevator without the piece I put there upon leaving at night), he then diddled up some code on the box I used to set the alarm, and presently was talking over his headset to his office, wherever it was. I told him I thought it was interesting that he was able to 'communicate by voice' over those wires which served as our burglar alarm system; he said it was a routine thing whenever he went out to do repairs to systems like ours around town. This alarm system came from the ADT Company, which I think means 'American District Telegraph' or at least it did in the olden days. PAT]