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How 4-Terminal Resistors Work

In a 4-terminal resistor, an ultra-precise resistor (green) is connected to 4 terminals through small, but unknown, resistors (red). These unknown resistors are the combination of lead resistance, screw terminal resistance, connection wire resistance, and other sources of errors. Typical values for these unknown resistors range from 0.01 ohms to 0.2 ohms, and the values …

Other Power Disturbances

The most common disturbances on AC power systems are voltage sags or dips. Other problems, such as transient overvoltages and brief interruptions, occur almost everywhere. Problems with harmonics, voltage regulation, and flicker occur at a wide range of sites. Some other disturbances that occur at specific locations include: Frequency variations: on utility grids, these are …

Voltage Regulation

The term “voltage regulation” is used to discuss long-term variations in voltage. It does not include short term variations, which are generally called sags, dips, or swells.The ability of equipment to handle steady state voltage variations varies from equipment to equipment. The steady state voltage variation limits for equipment is usually part of the equipment …


Flicker is a very specific problem related to human perception and incandescent light bulbs. It is not a general term for voltage variations. Humans can be very sensitive to light flicker that is caused by voltage fluctuations. Human perception of light flicker is almost always the limiting criteria for controlling small voltage fluctuations. The figure …


The electric power distribution system is designed to operate with sinusoidal voltages and currents. But not all waveforms are sine waves. Electronic loads, for example, often draw current only at the peak of the voltage waveform, which always means that the current is distorted, and may distort the voltage as well. One convenient way to …

Transient Overvoltages

Transient overvoltages are brief, high-frequency increases in voltage on AC mains. Broadly speaking, there are two different types of transient overvoltages: low frequency transients with frequency components in the few-hundred-hertz region typically caused by capacitor switching, and high-frequency transients with frequency components in the few-hundred-kilohertz region typically caused by lighting and inductive loads. Low frequency …

Voltage Sags (Dips) and Swells

Voltage sags – or dips which are the same thing – are brief reductions in voltage, typically lasting from a cycle to a second or so, or tens of milliseconds to hundreds of milliseconds. Voltage swells are brief increases in voltage over the same time range. Longer periods of low or high voltage are referred …

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