A power quality investigation of a flickering CRT screen demonstrates the importance of a smart electrical design in the information age.
Our firm recently received a call from an electrical contractor who was performing work in a hospital that was having problems with a flickering CRT monitor. What seemed like a fairly simple problem at first, this disturbance wouldn't respond to the most obvious corrective efforts. After arriving on-site, we first inquired how frequently the monitor flickered. If it only flickered occasionally, we could attribute the problem to other office equipment cycling on and off on the circuit, which may have been close to its capacity rating. Such culprits might have included a laser printer, fax machine, or copier that would cycle when copies were made or when the heater cycled on and off. However, the problem proved to be more elusive.
Once we had ruled out the effects of other office equipment on the monitor, the quickest way to identify the problem was with the “black-box” approach. We started by moving the monitor to other parts of the building and powering it from different sources. This proved that the disturbance lay in the monitor's original power source, so we plugged in a different monitor at the offending location. Predictably, the problem resumed. Therefore, we concluded the power quality issue stemmed from this particular area of the building, not the monitor itself.
Next, we took power readings on the branch circuit supplying this area, measuring volts, amps, power factor, and total harmonic distortion for both current and voltage. When all readings fell within acceptable ranges, we suspected an outside source was behind the flicker. We constructed a wire mesh shield around the monitor to determine if radio frequency interference (RFI) was the cause of our problem, but the flicker remained.
At this point, we asked the local electric utility to visit the site. Their investigation revealed that the flickering monitor was located near a wall adjacent to utility-owned transformers, and directly above the main switchgear for the facility. The utility's power quality specialists measured the magnetic flux densities in the room with a Gauss meter. After the room was mapped, the problem stood out immediately.
Any time electric current flows through an appliance or wire, it creates a magnetic field. This field is measured in units of milligauss (mG). The highest magnetic field magnitudes are typically found at a transformer or high-current conductors. Field strength at one side of the room in this location — the side the jittery monitor was on — exceeded 10 mG. Industry research has shown that video display problems from magnetic field interaction can begin at flux densities in the range of 10 mG to 14 mG.
Avoiding power quality problems in new buildings requires more vigilance and pre-planning than ever before. Addressing these difficulties in existing facilities can lead to extensive and expensive rework to the electrical system. In many cases, problems are the result of remodeling over time, during which new panels are installed to carry added load requirements. Invariably, one or more of these panels will feed different types of electrical loads, including lighting, receptacles and motor loads. Installers will often do this because the panel in question was the only one with any available room when a certain phase of construction was completed. Troubleshooting problems often are complicated by the fact that one must first identify what equipment a particular circuit serves and then determine from which panel the circuit is fed.
The solution? Take your pick.
Once we pinpointed the true nature of the problem, we had a range of solutions to consider, including the following:
Change the scan rate, or refresh rate, of the monitor. You can reset this rate to a frequency that closely matches the source frequency. In general, the closer you get the scan rate to 60 Hz the less the monitor will flicker.
Relocate the monitor to another part of the room where the magnetic field strength is lower. Since field magnitudes drop off fairly quickly as distance increases from the current source, you could relocate the monitor to a part of the room where the magnetic strength is lower.
Install a magnetic shield around the monitor or around the field source. Magnetic field lines will cut right through concrete walls and floors. When shielding the monitor with a special 60 Hz shielding material, you must take into account all aspects of the room, including mapped flux densities, monitor dimensions, and monitor location. A more expensive option would be to shield the entire room. However, this requires much more material, and you have to make sure all penetrations in the wall are addressed in the shielding design, including data and voice out-lets, power receptacles, lighting switches, TV outlets, and wall-mounted devices.
Replace the standard CRT monitor with a flat-screen monitor. Flat-screen monitors are not affected by magnetic fields because they don't include a typical cathode ray tube refresh scan rate, and are not affected by magnetic fields at these low levels.
Avoiding problems in the first place.
Magnetic fields are a direct function of current and an inverse function of conductor separation. Fields cancel as conductor separation decreases, and increase as conductor separation increases. In addition, magnetic field magnitude increases as current increases. But the field intensity drops quickly as distance from the source increases.
You should remember these concepts when selecting routes for power conductors in a new facility. Where possible, carefully analyze the distances between the main switchboard and working areas in the facility where CRT monitors will be present. For example, this may be as simple as locating the main switchboard on the opposite side of the electrical room adjacent to the staircase. You should also avoid routing electrical conduits tight to the first-floor ceiling deck when CRTs will be located on a desk on the floor above. Likewise, you should avoid routing large feeder conductors in walls adjacent to rooms that will house computer equipment and monitors.
By applying good detective work and the process of elimination, we eventually got to the bottom of the situation. But this episode reaffirms the growing need to incorporate electrical engineering requirements into the earliest design stages of today's smart buildings to maintain high levels of power quality. With their more intricate electrical systems that support a maze of electronic data-management hardware, today's modern buildings require us to be more careful in the design and placement of the electrical infrastructure.
Koch is a project electrical engineer for HDR in Omaha, Neb.