Path of Least Resistance? (2-8-2K)

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By Mike Holt, for EC&M magazine, Back-to-Basics.

No, electricity does not "take the path of least resistance." It takes all paths available-in inverse proportion to the impedance of the paths. Current flows through all available paths. The magnitude of the current flowing in each path depends on the voltage and impedance of each path. The lower the impedance of the path (assuming voltage remains constant), the greater the current. Conversely, the higher the impedance of the path (assuming voltage remains constant), the lower the current.

Picture two unequally-sized resistors in parallel. The current flowing through one resistor depends on the size of that resistor-not the one next to it. Assuming an infinite power supply, you could add 1,000 resistors in parallel and the current in that one resistor wouldn't change. Because of this principle, such things as the integrated circuit are possible.

IEEE Std. 80 uses a value of 1000 ohms for the human body for touch voltage calculations. A 25-ohm ground rod in parallel with a 1000-ohm human will not make an installation any safer from electric shock. For example, if you touch a metal pole energized by a 120V line-to-case fault and there's no effective fault current path, the touch voltage will be enough to kill you even if you bond the metal pole to a ground rod with a measured ground resistance of 25 ohms. The Figure helps illustrate the following:

  1. A ground rod with a resistance of 25 ohms does not provide an effective fault current path. The pole will remain energized with dangerous touch voltage because the fault current will be only 4.8A (I = 120V/25 ohms). This is not enough to trip a 15A breaker, click here.

  2. Electrons take all available paths, and one of those paths is your 1000-ohm body.

  3. OSHA and NFPA 70E say dangerous touch voltage is any value over 30V, and death from electric shock can occur from as little as 50 mA in a few seconds.

  4. Touch voltage from an energized object is about 75% of the line-to-case voltage. So a 120V line-to-case fault results in a touch voltage of 90V. This can result in 90mA flowing though the human body indefinitely(I = 90V/1000 ohms), click here.

For many years, the street lighting and traffic signaling industries used ground rods without an effective fault current path, to ground metal parts of the electrical system. The thinking was that these installations were safe because "electricity takes the least resistive path and it bypasses high resistive paths." People also thought the ground rod (earth) provided an effective fault current path to quickly remove dangerous touch voltage. Unfortunately such thinking resulted in practices that causes many deaths, click here.

This thinking still exists. Worse yet, some equipment manufacturers state in their installation instructions that a 25-ohm ground rod without an equipment grounding conductor is an acceptable means of providing a safe installation, click here.

Make sure poor grounding practices don't lay you to rest. Yes, electricity does take low resistance paths, including the one of least resistance. But, it takes every other path available to it also. You can't suspend Ohm's Law and Kirchhoff's Laws by driving 10 feet of copper-clad steel into dirt. To make an installation safe, ensure the touch voltage on metal parts never exceeds 30V for more than a few seconds. You can do this by bonding all metal parts to an effective fault current path in accordance with Article 250, click here.