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RES - Technical Corner
Technical Corner by Joshua Doores, Electrical Engineer, Colliers Engineering & Design For the article this month we will consider X/R ratios and their relationship to 3 phase fault current and line to ground fault current. This article will try to answer the following questions: What is X/R?, Why does it matter?, and How is it used by electrical engineers and designers?, particularly as it relates to fault current. We remember from ohms law, that current is a function of voltage divided by impedance, I=V/Z. Impedance is the combination of resistance “R” and reactance “X”. This ratio of an electrical systems reactance over its resistance is known as the X/R ratio. Note, that the electrical system in question could be as small as a branch circuit or as large as the entire electrical distribution for a building, including the utility. Reactance depends on the inductance and frequency of the system. In real-life applications in the USA, we deal primarily with 60Hz voltage and current; it is a constant, therefore reactance varies based on the inductance of the electrical system. Inductance reflects how difficult it is to change the current. Coils of wire are a common electrical component that generates inductance. Voltage leads current in inductive electrical circuits. Resistance is the measure of how hard it is for current to flow, having to do with the physical properties and circular area of the conductor being used. You may be familiar with the concept of the impedance triangle which is used to visually demonstrate the components that make up impedance and the phase angle between them. See figure below. As you can see, the larger the inductive reactance the larger the phase angle and the more voltage leads current, and the two waveforms become more out of phase.
Figure 1: Impedance Triangle, source: website: https://www.electrical4u.net/ electrical-basic/impedance-reactance-impedance-triangle/
6 | The ROCHESTER ENGINEER AUGUST 2022
res - technical corner
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