Posted by Mike Agri on 1/26/2012 to
Have you heard the one about...
Today, wireless systems for telephone, alarms, security, and other applications are all the rage. Here is a cautionary tale.
- At one time, IBM, and later Simplex Time Recorder Company, manufactured electronic, self-correcting clock systems that employed a high power frequency generator wired in parallel across the electrical service. The last minute of every hour, the frequency generator would be turned on for a fixed period of time, and insert one of several available frequencies on the entire AC distribution system in the entire building. The secondary clocks, which plugged into any available single phase, 2-wire 120V outlet, each contained a signal amplifier which turned the frequency into a voltage that would energize a solenoid operated correction mechanism in each clock for system-wide synchronization. It was discovered that If two relatively proximate facilities running on the same utility feed just happened to be set to correct using the same frequencies, it would sometimes result in cross talk between building clock systems which might have one or both facilities with a slightly different Master Clock time setting cause an adjacent facility to also go into correction, with the usual result of each or both systems advancing, every hour.
Because you can doesn't mean you should
- An independent care elderly facility installed an inexpensive packaged fire alarm system that included one 24VDC door magnet set to release a stairwell fire door on every alarm. The magnet was controlled through an NO contact of an integrated alarm relay in the FACP, and powered by the panel's [limited] 24 VDC power supply. The first time we tested the fire alarm system on battery standby, the FACP's power supply failed. We replaced the power supply, and it failed as soon as we duplicated the test. The back EMF from the collapsing magnetic door holder's field was apparently severe enough to damage the motherboard voltage regulator. In addition, the door holder load had not been included in the battery calculations of this Auxiliary Fire Alarm System. The manufacturer documentation specified the [limited] available 24VDC output was for powering 4-wire smokes, and other low current devices.
- The contractor who installed fire alarm auxiliary relay-controlled 120 VAC door magnets in elevator lobbies and corridor smoke doors in a local hospital did a very nice, neat job, however, when the alarm system was activated, one circuit of door holders failed to release, even though the 3PDT control relay clearly operated and opened the power to the magnets. If you manually released a door on the circuit, and then tried to re-engage the door magnets, the doors would no longer hold open. Upon further investigation, it was found that the neutral for the circuit in question was used as the switched leg, and through inductive coupling to the EMT containing the wiring, the magnets were able to remain energized enough to hold the doors.
When a ground fault is not a ground fault
- A fire alarm panel in a large, un-sprinklered Vo Tech school went into ground fault [randomly]. A check of all field wiring, and the systems itself divulged no apparent reason. As luck would have it, I happened to be standing in front of a clear panel one morning, when suddenly a ground fault appeared. Coincidently, I noticed the sound of something starting. I walked towards the sound, and around the corner from the FACP sat a brand new VFD (Variable Frequency Drive). Whenever the drive started, the FACP, which was fed out of the same sub-panel that physically sat between the FACP and the VFD, would go into ground fault. Further investigation revealed that the service ground, about 150' from the FACP was badly corroded, and there was no bonding jumper around a water meter that was installed a few feet downstream of the service ground. The sub-panel was part of an old 3 phase 208V, ungrounded system. There was no ground bus in the sub-panel so the installing electrician had “referenced” ground by connecting to the painted surface of the FACP.
- A high rise VA hospital got a brand new FACP that immediately began sending random alarms from one particular floor. No operated devices were found, and no wiring faults could be found, so the zone module was replaced. The problem remained, so all smoke detectors were replaced, with no success.
I was discussing the problem with the hospital's chief mechanic, who also happened to have been assigned as the VA's eyes on site during the building's construction. In passing, he remembered “they had discovered” the building's 7th floor, [the hospital was built on a hill] just happened to be the intersecting point of several local TV and Radio Station transmitter signals, and this had caused some difficulties with other systems when the building was first opened. A sweep of the 7th floor FA wiring disclosed that a number of J-box covers had not been installed by the contractor. A check of the remainder of the building revealed more missing covers. As soon as the system was properly buttoned up, the false alarm incidents promptly ceased.