> agents. (b) I personally have tried so-called 'wireless intercoms'
> between different locations nearby; sometimes they worked (although in
> a rather piss-poor way; other times not at all. I have no personal
> experience with (a) but have been told the connections are very
> 'noisy' many times, and (b) when they worked, they seemed to have a
> lot of 'hum' in the background. When they did not work (all I got
> was hum with no audible voice at all) I am told this was because the
> two intercom stations involved were on opposite 'legs' of the
> transfomer. Can anyone explain this better to me?
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:
> My understanding is there are two different electrical systems in the
> world, the 220v version the rest of the world uses, and the 110v the
> US uses. In the rest of the world, a fairly large number of residences
> are run off a single transformer, where in the USofA, a much smaller
> number are run.
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.
> An electric wire can carry currents of different frequencies, so AC
> power current, DC power current as well audio frequency and radio
> frequency can be all carried on the same line. For example, a RR line
> powered by AC has both the AC power (25 or 60 Hz) as well as the
> control signal (100 Hz and others) sharing the medium. Some lines
> even have multiple power sources from a separate wire, such as DC via
> third rail. There are "filters" (IIRC, "impedence bonds") that
> separate out the stuff.
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"):
http://www.trainweb.org/tubeprune
> I know that the third-rail seems like an awful way to transmit voice
> communications. On the one occassion I had to see the CTA system in
> action, I called into the CTA main headquarters phone number (MOHawk
> 4-7200) and the operator switched me to a supervisor in one of the
> control towers several miles away for whom I had a question. The
> connection, frankly, was not all that good. Once I also called Grand
> Central Station in downtown Chicago to the Lost and Found; she
> switched me to the Lost and Found in Baltimore, OH, also via the
> trackside phone lines. That connection sounded pretty bad also. PAT]
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]
Paul Coxwell (paulcoxwell@tiscali.co.uk)