Normal mode noise has to pass through the system's power supply where at least some of it will be filtered out before it reaches the circuit boards.
In addition, the AC electrical system was designed to protect itself and its loads against normal mode noise. How? There's good news: It does this by shorting itself out at the high voltages and high frequencies of power line noise, and dumping excess energy to ground. But there's the bad news, too: Because of this, most of the normal mode noise is converted into common mode noise.
Common mode noise is harmless to most of the loads that the power system was designed to operate, because they only use the ground wire as an alternate path back to ground for safety reasons. Unfortunately, modern computer systems and their peripherals use ground as a reference point and common mode noise is a serious threat to them. In a typical system, the safety ground is either directly or through a capacitive manner coupled to the computer's logic reference ground. If more than a half a volt of noise gets into the ground bus, it will disrupt system operations. This is why a very quiet ground is required for any computerized system. The vulnerability of computerized systems to power problems is well documented. Several studies have shown that up to 80 percent of all computer failures are power related. Removing noise from the electrical environment is one of the most important things that you can do to improve a system's reliability.
How do we effectively remove noise from electrical power? We apply the same principles to electricity that we apply to gasoline, oil, air, or water -- we filter it and we condition it. There is a lot of confusion about the meaning of the words "power conditioning." The public at large and we security professionals confuse power conditioning with other devices, such as surge suppressors, voltage regulators, spike arrestors, EMI/RFI filters, and so on. Most of these devices are designed to treat one or two symptoms of noisy power. A true power conditioner should protect a system completely from all the effects of power line disturbances. There are four leading technologies for industrial power conditioning. Those would be the following: isolation transformers, ferroresonant (constant-voltage) regulators, electronic tap-changing regulators, and low-impedance power conditioners.
The next step should be a comprehensive evaluation of the electrical system ' and I do mean comprehensive!
The best thing to do is to have a full range power monitor plugged into the wall receptacle and monitor conditions for a full week. I strongly recommend plugging it in on a Sunday morning and unplugging it on the following Saturday morning. Instructions are implicit -- the monitor it is to be left undisturbed. The tape should be kept rolled up.
Someone can do that once in the morning and just prior to locking the doors at night. When the test is complete, the machine is turned off, the tape collected, and the analysis made. The written report will pinpoint the problems as they occurred and recommend corrective action. If there were short-term power outages or very low voltage conditions, which amounts to the same thing, then an uninterruptible power supply may be required. The report may determine that grounding must be improved. Certain tests may be recommended. We strongly recommend they be completed before any improvements are undertaken. Power conditioning will accomplish wonderful things, but it will not work to its maximum potential without good low resistive grounding.
The completed survey is to facilitate site preparation or site restoration for installing or restoring of a reliable and effective alarm system, free of false and nuisance alarms. There is proof that thorough site preparation, proper sensor selection, installation methods and techniques, along with owner/operator education and training, incidences of false and nuisance alarms are significantly reduced. Scarce security and law enforcement resources can be effectively utilized in either a proactive manner, or squandered by responding to spurious alarms. With a proper plan of action that takes into account potential alarm system disruptions, those calls will more likely be for real incidents rather than nuisance alarms.