By Mike Holt, Published in EC&M
Contrary to the belief of many in the electrical industry, grounding metal parts of
an electrical system to a ground rod DOES NOT assist in removing dangerous voltage from a line-to-ground
fault by opening the circuit overcurrent protection device for systems that operate at not more than
347 volts to ground! To understand this, we need to review four basic electrical fundamentals:
(1) Electrons leaving the power supply are attempting to return
to the source, they are not trying to go to ground!
(2) The time it takes for an overcurrent protection device to open
is inversely proportional to the magnitude of the fault current (the greater the ground-fault current,
the less time it will take for the overcurrent device to open).
(3) Unsafe voltage can cause the heart to go into ventricular fibrillation
in less than one second, resulting in death in a matter of minutes.
(4) To protect against the dangers of electric shock, unsafe voltage
on metal parts of the electrical system and the building must be removed by opening the circuit’s
overcurrent protection device in less than one second.
To open the circuit protection device in less than one second, the fault current returning
to the source must quickly rise to approximately 6x (fast acting fuses) to 10x (other devices) the
rating of the circuit overcurrent protection device. The following demonstrates the impedance required
to clear a 120 volt fault depending on the overcurrent protection device rating.
Impedance = E/I
||120V/150A = 0.8 ohms
||120V/200A = 0.6 ohms
||120V/500A = 0.24 ohms
||120V/1000A = 0.12 ohms
Note: A GFCI protection device is not an overcurrent protection device and it is designed to
open where the fault is as little as 5 mA.
To insure that dangerous voltage is removed in less than one second, the fault current
path must be permanent and electrically continuous, be capable of safely carrying the maximum fault
likely to be imposed on it, and have a very low impedance to facilitate the operation of overcurrent
devices [110-10, 250-2(d)]. The low impedance path is created when metal parts of the electrical system
are bonded together [250-90] and to the power supply system grounded (neutral) conductor in accordance
with Section 250-24 for service equipment and Section 250-30(b) for separately derived systems, Figure
Naturally, the earth cannot be use as the intended return path to clear a fault because
its high resistance (one billion times that
of copper, IEEE Std. 142 Section 2.2.8) will only allow a few (1 to 10) amperes to flow back to the
source [250-2(d) and 250-54].
Note: Metal parts of the electrical system are grounded (connected to the earth) to prevent the destruction
of electrical components from superimposed voltage from lightning and voltage transients and to help
prevent the build-up of static charges on equipment and material.