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The Path of Least Resistance

Jul 1, 2001 12:00 PM, By Mike Holt, Mike Holt Enterprises, Inc.

Contrary to popular belief, electricity takes all paths available — in inverse proportion to the impedance of the paths. The magnitude of the current flowing in a path depends on the path's voltage and impedance. The lower the impedance (assuming voltage remains constant), the greater the current. Conversely, the higher the impedance (assuming voltage remains constant), the lower the current.

Imagine 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 1000 resistors in parallel and the current in that one resistor wouldn't change.

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.

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 anything over 30V. Death from electrocution can occur from as little as 50mA in a few seconds. 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 through the human body indefinitely.

For many years, the street lighting and traffic signaling industries used ground rods, without an effective fault current path, to ground metal parts of an electrical system. Electricians thought these installations were safe because “electricity takes the least resistive path, and it bypasses high resistive paths.” Unfortunately, such thinking resulted in several deaths. This thinking still exists. Some equipment installation instructions require a ground rod without an equipment grounding conductor, claiming it's a safe installation. Electricity does take low-resistance paths, including the one of least resistance. But it also takes every other path available to it. You can't suspend Ohm's Law and Kirchhoff's Law by driving 10 ft 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 Art. 250.

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