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 oíscope 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 oíscope. 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.
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