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Filtering Magnetic Fields: A Discussion with Electrical Engineer Al Hislop

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We have electrical wiring in our homes because we want light and energy in many places in our homes. When we turn on a light or use an electrical appliance, currents flow in the wiring, and there is the possibility to create magnetic fields around the wiring. Just having some devices plugged into an electrical outlet (even though not turned "on") can allow the flow of some current in the electrical line.

We'll call anything that is attached to the electrical wiring in your home a "load." Loads can be broadly categorized as resistive (using power), capacitive, or inductive, or a combination of all three. Capacitive and inductive loads (called reactive or non-resistive loads) can allow current to flow without actually using power. Even though these non-resistive loads do not use power, the current that flows through them creates magnetic fields just as well as the current from loads that use power. (Power companies don't like these kinds of loads, because they must supply the current, but you don't pay for any power, because these loads don't use power.)

Most modern homes have wiring that comes in cables with three (and sometimes four) closely spaced wires. A typical three-wire cable has a "hot" (for example 120 volts alternating current [120-VAC] wire), a neutral (or return) wire, and a ground wire. The ground is for safety purposes and in normal circumstances will have no current flowing through it.

In a wiring system with no wiring errors, the current in the hot wire flowing into a load will be the same as the current in the neutral wire flowing back out of the load, and the currents in the two wires of the cable will be flowing in opposite directions. Each wire will produce a magnetic field proportional to the current flowing in the wire. Since the wires are very close to each other, the two magnetic fields created by the opposite currents cancel each other everywhere except very near the wires.

In some cases, one might want to control the lights in a room from any one of several switches. The wiring schemes for these circuits leave open the possibility of un-matched pairs. That is, the hot and neutral wires may not always run in the same cable, and thus the magnetic fields for the two wires do not cancel, because the hot and neutral wires are not very close to each other.

I think we can now get to the topic of filtering. Ideally, we would like to have (in the U.S.) only 60-Cycle Per Second (60-Hz) currents and voltages on our wiring. Voltages and currents other than the desired 60-Hz frequency can come in on the electrical lines from sources outside our homes, or they may be generated by items inside of our homes. For the purposes of this discussion, let's call the undesired, non-60-Hz voltages and currents "noise."

We may want to add filtering to our power lines to reduce the magnetic fields within our homes by blocking noise currents from entering our home at the meter or by keeping noise currents generated by the various items within our homes from propagating into the wiring system of our homes.

A single filter at the meter can block noise coming from outside the home. In an apartment building, a single filter at the electrical input to the building may not be effective, because noise generated by devices in some apartments can get to other apartments without flowing through the input filter. Each apartment would have to have its own filter at the meter to keep out noise generated in other apartments.

Many loads in our homes and businesses create voltages and currents with frequency components other than 60 Hz. Circuits like light dimmers often create harmonics of 60 Hz (120 Hz, 180 Hz, 240 Hz...). Other appliances such as computers, radios and televisions often use Switched-Mode Power Supplies (SMPSs) to provide the operating voltages necessary for the internal circuits. These power supplies convert the alternating current (AC) input voltage to direct current (DC). Then, at high frequency, they turn on and off fast-switching transistors to create high-frequency AC signals to be transformed to the desired operating voltages. SMPSs are used because they are cheaper, lighter and more efficient than older "linear" regulated power supplies. The drawback of these SMPSs is that they can produce noise currents and voltages that "contaminate" the power lines, thus "contaminating" the power lines with their noisy signals.

Let's take a look at filtering out noise generated by devices you may have in your home. Ideally, we would like to keep the noise from getting to the house wiring. A filter is required BETWEEN the device generating the noise and the house wiring. You would plug your device into the filter and plug your filter into an outlet.

Remember that the magnetic fields generated by the noise are proportional to the noise current flowing in the wires. A filter that looks like an open circuit to the house will be best, since it will not allow noise current to flow into it. Filters can also be designed to block noise by appearing as a "short circuit" to the noise. This type of filter usually has a capacitor as its first element. This type of filter, however, will maximize the noise current flowing between the device and the filter. In this case, the power cord from the device that is plugged into the filter will have the maximum possible noise currents, and therefore the maximum potential to produce magnetic fields. The consumer has no way of determining whether the filter is designed to be an open circuit or short circuit unless the manufacturer states so in the specifications. Filters that involve ferrite beads as the first element are more likely to be the desired "open-circuit" type.

At 60 Hz, the filter should be "transparent." That is, it should not block or otherwise affect the 60-Hz power that flows to your device.

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Katie Singer writes about nature and technology in Letters to Greta. She spoke about the Internet's footprint in 2018, at the United Nations' Forum on Science, Technology & Innovation, and, in 2019, on a panel with the climatologist Dr. (more...)

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