Could Someone Please Explain This to Me?

[quote] The devices would be set off in an emergency by altering the regular AC signal being generated and transmitted by the local power companies. This would be done by having a specific alternating current of a specific frequency superimposed upon it; the imposed current would be generated by special generators and would be at 270 Hz at 3 volts RMS for 50,000 cycles. [/quote]

National Emergency Alarm Repeater
Can I have an explanation or a diagram explaining this?

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.

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.

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.

Kind of reminds me of the Functional Devices “Blaster” system I’ve seen in some residential apartment buildings. But they use PLC signals over the electric lines to trigger the blaster units. Very unreliable, and I have no idea how these things ever passed as a legitimate “fire alarm accessory” but they are! One time we unplugged every lamp, computer, charger, etc in an apartment trying to get the system to work and finally got it with practically nothing plugged in. Any computer or electronic device would just absorb the signal and cause the blaster to not operate.

…and that’s why residential fire alarms are 3-wire!

Oh, these aren’t part of 120v smoke alarms. The Functional Devices Blaster system connects to a standard fire alarm control panel. Either triggered off the NAC or dry contact alarm relay. You would use them in an apartment building or condo where the only notification is in the common area hallway. You can plug in a “blaster” into any outlet - like the bedroom inside an apartment - to help make the fire alarm system compliant with sleeping area notification requirements. Saves having to run a wire into every unit and install a horn or sounder. But to me there are several problems with this system. 1.) Unreliable - any interference in the power line makes them inoperative. 2). The blasters and transmitters have no secondary power available. 3.) No supervision and very easy for the layperson to unplug not knowing they are compromising their fire protection.

Link:

http://www.functionaldevices.com/building-automation/blaster.php

I know. I was just saying the reason 120VAC smoke alarms are 3-wire is the same reason those alarms were unreliable.

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.

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.