Grounding in Facilities to Reduce Electrical Interference

Circuit Theory—A Valuable Tool

Electrical engineers use circuit theory to predict the behavior of circuits that are used to distribute current and voltage to various loads. The current levels, the wire sizes and the voltage drops can all be calculated using this theory. Circuit theory is not the right tool to describe the role of grounding, or how interference is propagated on the power conductors. To better understand these issues the underlying physics must be considered.

How is Power Actually Moved?

When a power switch is turned "on" the power that is needed must eventually come from the power generator. If the generator is 10 miles away it takes about 0.1 milliseconds for the request to get to the generator and back. The generator may take seconds to adjust to the request for more power. In real life it seems that the power is available immediately. How is this possible?

Power is not carried in conductors. It is carried in the electric and magnetic fields between the power conductors. The role of these conductors is to direct where the power or signal can travel. When a demand is made for more energy it is taken immediately from the fields in the nearby conductors. As time progresses the energy is supplied from the fields that thread through the entire facility. An examination of the power line voltages will show a drop in voltage near the load that is almost not visible at the building entrance. This drop in voltage (often called a spike) will affect the voltage supplied to nearby hardware. When power is turned off, the power that is flowing produces an excess of voltage which is also a spike. The sudden changes in power demand can occur in each power cycle or it can occur infrequently. The nature of these power demands depends on the hardware using the power. The demands of a motor start are different than that of a switching power regulator.

The Nature of Interference

Changes in the demand for power propagate through the facility at near the speed of light. The fields associated with these fields are most intense near the conductors that supply the power. The intensity of these fields at a distance from the power conductors depends on the geometry of the power conductors. It is important to have conductor layouts that limit the extent of these fields. Any uncontrolled field will cause currents to flow in all the other conductors of the facility. These currents are the interference that must be considered. In large facilities, hundreds of devices are pulling power from the utility at one time. In most electronic hardware the power is not taken on a steady basis but in small gulps. If the interference fields that result are not limited, the facility will be electrically noisy. These interference patterns may be changed by facility grounding but the interference is rarely reduced.

Trays and Conduit

The most important factor in field generation is loop area. When power conductors are spaced apart the associated fields are larger. When power conductors are placed in trays, the space between conductors can allow significant circuit loops to exist. Twisting or braiding the conductors in each circuit can control these loop areas. The best approach is for each braided circuit to contain a neutral and an equipment grounding conductor. The National Electric Code requires that the tray itself be grounded as a part of grounding the electrode system. This grounding is a safety precaution and it is not a part of noise reduction.

The twisting or braiding of power conductors has the advantage of reducing external magnetic fields. One or two twists per foot is usually sufficient at power frequencies. Twisting is a far better solution than attempting to contain fields through the use of conduit. Electrical metal tubing (EMT) is not an effective shield against magnetic fields at power frequencies. Twisting conductors inside EMT is obviously difficult. The advantage of EMT is that it does keep the current loop areas small.

Here is an example of how power wiring can generate interference. Consider a large power disconnect switch mounted on a wall. The power conductors are spaced apart for connections to the switch. The resulting loop area allows field generation that is most intense near the switch. A power switch should not be mounted on a wall that might later house sensitive equipment. Remember that most building walls are transparent to electrical interference.

Power Line Filters and Isolated Grounds

Electric filters are provided in almost all pieces of electronic hardware. The purpose of a power line filter is to limit the flow of high frequency current into and out of the hardware. If the hardware demands small bursts of energy this energy can be supplied locally from the added filter rather than from the power grid. Line filters can also limit the entry of high frequency current generated by other hardware.

Line filters function by using series inductors and shunting capacitors. The inductors are used to restrict the flow of high frequency current. The capacitors can be used to return unwanted pulses of current back to the source or they can be used to supply energy over a small part of each cycle. Filter capacitors are usually connected to equipment ground (the rack or housing of the hardware). Unwanted current flows in a loop that involves the equipment ground and the power conductors. Attempts to return this current to a special "earth" will generally result in a noisier facility. The reason rests in the fact that the loop areas for the return path are not controlled. One version of this approach involves the use of "isolated grounds." Here each equipment ground conductor is returned and grounded separately at a panel or service entrance rather than to the immediate grounding electrode system. This "isolated ground" technique is not recommended as it increases the loop area for the currents in the filter. The separate grounding conductor is inductive and this limits the performance of the line filter. The end result is a noisier facility. There are excellent methods that can be used to limit these fields that will be discussed in Part III of this series.

Neutral Current in the Earth

Outside of facilities, neutrals in general power distribution are often connected to earth at several points. If the loads are not exactly balanced then there will be some neutral earth current. The field associated with this loop extends between the power conductors and the earth. These loop areas can be very large. This means that a field can exist throughout a facility that is not generated in the facility. Neutral earth current is apt to follow buried conductors as these conductors provide a lower impedance path than the earth. These fields can not be reduced by making changes to the facility. This means that a facility by its very nature invites noise currents to flow in the earth under the facility. This field can be eliminated if each nearby facility is provided with a separate distribution transformer.