5.16 Power Line Interference

Power line filters can keep some of the common-mode and differential interference from entering the hardware. In most cases, available filters do not function well below a few megahertz. The filtering reference conductor is usually the equipment grounding conductor that carries filter current back to the service entrance. If the equipment grounding path is inductive (long) then the filter cannot be effective. It is important to note that even with a good filter installation, some of the equipment ground current uses the racks, the signal conductors, and the cables that interconnect hardware to return to the service entrance. These currents represent field energy in the facility.

In a single phase power service, the neutral or grounded power conductor carries power currents for all the loads on a feeder and its branch circuits. This means that this conductor is not at the same potential as the equipment grounding conductor, which does not carry load current. The voltage impressed across a power transformer primary has some common-mode content caused by this neutral voltage drop. The content depends on the attenuation provided by the line filter. Filtering effectivity depends on whether every power conductor is filtered and how the filters are installed.

If there is a power transformer, there is a capacitance from turns on the primary coils to the secondary coils. Common-mode interference on the primary coil will flow through this capacitance to the circuits on the secondaries. The paths involve all conductors that connect to circuit common. This means that there is interference current flowing on signal commons that exit the hardware and any equipment ground connections. These signal commons are often shields or conductors in a ribbon cable.

It is against the code to place any component in series with the equipment grounding conductor. This includes filter inductors. If this component should open circuit then the fault path is broken, creating a hazardous condition. The equipment grounding conductor must connect to the conducting hardware framework. If the connection is made to an inside surface then fields carried on the conductor can directly radiate into the hardware. The right way to terminate the equipment ground on hardware is on an external conducting surface.

It is difficult to build switching circuits without generating some radiated field. This field can couple to power leads and exit the hardware even if there are line filters. In hardware that uses DC/DC converters, the fields that couple out on the power line can be a serious problem. These converters involve semiconductor switches that have short rise times. The currents that flow in a few picofarads of parasitic capacitance can be hundreds of milliamperes. This current can be reduced by careful construction inside the converter transformers and by the proper placement of converter filters. In large systems, there can be hundreds of these converters in operation, all switching at different frequencies. The result can be a very noisy facility.

Large motors often use speed or torque controllers that switch power in mid cycle. This switching can create interference on the power conductors. This can be troublesome, if this same power enters other electronic hardware. The fields resulting from this switching can radiate and leave the conductors. These fields cause surface currents on all nearby conductors and can enter hardware on shields and signal conductors. Fluorescent lamps create fields that can propagate on all nearby conductors. These currents can also enter hardware on signal cables and on power conductors.