I inherited the maintenance responsibility for an emergency alert siren system in a 10-mile emergency protection zone surrounding a nuclear generation plant. The activation devices were a radio two-tone sequential system, and when tones were received, it closed a contact, which closed a three-phase contactor, which activated an electro-mechanical siren. Shortly after inheriting the system, sirens started activating themselves with no signal. The two-tone sequential system was to prevent bogus signals from triggering them. Yet, they were going off anyway.
This set off a series of events within the utility senior management and the State Emergency Management Services (EMS). They thought the system was unreliable and that something needed to be done right away. There was one siren that seemed to be going off more than the others, so I put the suspect unit on the bench test stand and tried to duplicate the problem. I suspected that it had something to do with power, but what? I got to thinking, maybe there was a power interruption of some sort, but if it was powered down, why would the output relay close causing activation?
I put a dual trace o’scope on the two-tone lines. When the proper tone was received on each decoder, the output would go from 0V to 12V. I started flicking the on/off switch while watching the o’scope. Both lines acted like they were supposed to -- no voltage spikes to give the impression of a received signal. Finally, I got it to pick the output relay up while I was flipping the switch. That recreated the problem. It was definitely power related, but it was not in the decoder lines.
I then put the scope where the two-tone lines combined to produce a single trigger line, and found out that the voltage at this point would go from a 12V to 0V when the proper tones and the correct timing were received. I went through the whole process again. The problem was being able to flip the switch fast enough to make the output relay pick up. It took numerous tries, but it finally happened.
This was what was happening: If the power interruption was short enough, the 12V would almost get to 0V, making it seem as though a proper signal had been received, while not totally powering down the whole unit. The relay would then react to the signal and close it, causing the siren to activate.
We tried to mitigate this by trying different RC constants between the stages, but then it would not operate at all. The whole system was eventually replaced with a dual-tone multi-frequency signaling system.
This entry was submitted by Dennis Buchanan and edited by Rob Spiegel.
Dennis E. Buchanan served six years in the Navy as an electronic technician, and worked for an investor-owned utility for 18 years. While he was working at a nuclear power station, he was part of a three-man design team tapped to improve the radio communications and later to upgrade the emergency notification system. Buchanan is a Motorola-trained trunked radio system technician, and also holds a FCC radiotelephone license with Ships Radar Endorsement.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
No matter where I've lived--in the middle of Los Angeles or a nice suburban town in Northern California--no one pays any attention to car alarms. I would think that's even truer, if that's possible, in NY. I agree about the possible law, and have been amazed such a one has never passed, even here in litigious California.
Your example of a car alarm is a good one, in that the false alarm rate is so high that these alarms are essentially ignored. Indeed, there can and should be a law that that audible alarm portion of car alarms have to be disabled. The siren serves no purpose. The function of a car alarm is to disable the capability of starting the car. That's how it prevents theft, not by warbling so that some samaritan will come by and smite the potential car thief. I live in NY, and if your car alarm goes off, your automobile is on its own, buddy. Nobody's gonna come by and tell anyone to step away from the vehicle..
I haven't been in hotels with false alarms in the middle of the night--that must be terrifying!--but I sure do remember the MGM Grand fire. It's made me nervous about staying in multi-story hotels ever since. I agree, a high rate of false alarms makes us tend to ignore them. Just think about car alarms! The universal signal to ignore an alarm, and a good example of the cry wolf phenomenon (for those who grew up reading fairy tales).
In my grade school, we had fire drills religiously where we marched outside and lined up in our appropriate class lines and waited for the all clear from our school safety officer. That was until we had an actual fire in the school. At this point, the alarm did not go off. Instead the principal called all people on the PA system and told everyone stay in their class rooms until the smoke had cleared. I to this day do not know the rationale of this decision, nor do I know why we did not follow the drill. I guess some plans are only plans until there is an actual problem.
I agree, Tool_maker. I am always amazed at the patience and dogged pursuit of the problems by so many of the contributors in the Sherlock Ohms story. This is a classic example, performed in a very important setting.
This may seem like an insignificant and tangential comment, but apropos of false alarms I just realized that I've been in many hotel situations in the past several years where an earsplitting alarm went off in the middle of the night, you get up worried and don't know what to do, often opening the door a bit in your underwear to find out what's up, and then the alarm shuts off and end of story. No fire, no explanation. This is dangerous because it trains you to ignore hotel fire alarms, so when the real one comes one day, you're gonna be in trouble. This is not trivial. Remember the MGM Grand fire in Las Vegas in 1980? False alarms are a dangerous design flaw. More properly, there will always be false alarms (if you stanch them completely you will miss some real situations). The problem is when you have too high a false alarm rate.
@Ralphy Boy: Your last sentence is true in every line of work. Whether it is a safety system in a power plant, the ignition system in your car or an irregular heartbeat, being able to replicate the problem so one knows what to repair is a gift from God. Either that or dumb luck.
I periodically visited nuclear plants as part of my job in the late '70s and never heard, nor heard of, a siren going off outside. I agree with you, Alex, this is not a small issue. It's scary to think that the problem became evident in retrospect, rather than being prevented in the first place.
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