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PostPosted: Wed Nov 18, 2015 10:11 am 
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This is done using a system called Carrier Current (sometimes called Current Carrier). This system has been used for other purposes. Some cities used to control their street lights with such a system. There are some collage radio stations that still broadcast this way. IBM and later Simplex used this system for self-regulating clock and bell systems where no direct wiring was needed to the clocks and bells. The signal is picked up from the 120 VAC building power by a small receiver in the clocks and bells.


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PostPosted: Wed Nov 18, 2015 4:06 pm 
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Cost and newer technology. The carrier current signal generating equipment would cost a lot to install and maintain. Years ago we did not have the electronic devices we have now. Many of these can interfere with carrier current systems or absorb the signal making the system nonfunctional.

As the page referenced says, the NEAR system was replaced by the Emergency Broadcast System and outdoor sirens. Now we have 95% coverage by the NOAA radio system. Receivers can be programmed to respond to specific areas. Radios are available with outputs that can operate a strobe light, loud audible device, or a bed shaker.


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PostPosted: Wed Nov 18, 2015 7:45 pm 
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I wonder why they don't have these things in Tornado Alley? It’d be good for those heavy sleepers. If they did re-instate them, the alarms would probably have the 520Hz square tone; not in Code 3, of course.
If memory serves correctly, the carrier current signal can't pass through transformers, so it's only good for a local building or block basically.

we have sirens in most areas, and a lot of major cities have speaker arrays now so instructions can be announced after the sirens have ended. add to that most cell phones receive weather alerts now, even if you don't want them, i think we are pretty well covered.


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PostPosted: Fri Nov 20, 2015 4:49 pm 
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If memory serves correctly, the carrier current signal can't pass through transformers, so it's only good for a local building or block basically.
Chris is correct regarding RF like in a carrier current radio station. The 270Hz used in the proposed NEAR system would pass through transformers.

The IBM/Simplex electronic clock systems I worked on uses frequencies from 2,340 Hz to 9,500 Hz. In single building systems we would inject on the 277/480 volt buss. The signal would go through the step down transformers and be present on the 120 volt circuits. Some buildings have a direct step down to 120/208 and were easy to inject on. I also worked on high line insertion generator systems. In these the signal is injected on the 25,000 volt service that feeds something like a college campus. That would put clock correction signal on every 120 volt circuit in every building on campus. Need a clock added somewhere? Just plug one in on any circuit.


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