Encyclopedia of Grounding (CA09040E)

Capacitive Induction:

Electric field induction (capacitive coupling) from adjacent energized lines can induce high voltages on isolated, de-energized lines. Asinglegrounding jumper on the conductor is sufficient to bleed this charge off to the Earth. The jumper may carry a continuous current as highas 100milliamperes per mile of parallel line. However, the higher current amplitude resulting from the magnetic induction into a closed loopwill not bepresent, becausewith a single ground jumper there is no loop. A step potential hazard is defined as the voltage across a ground support worker who steps across or otherwise bridges an energized path of Earth. The transfer of the rise in line voltage during a fault to Earth is by way of a jumper or other direct connection. This raises theEarth’s point of contact to approximately the same voltage as the line itself during the fault. The Earth itself has resistance [20] . Remember, cur rent flowing through a resistive element creates a voltage drop. Aswith any voltage drop, it is spread over the resistance itself. Consider the Earth as a string of resistors all connected in series. Each resistor in the serieswill developa voltagebecause of the current flowing through it. This is the voltage drop bridged by the worker who steps across it. As thedistance fromthepoint of contact increases, voltageat that remoteEarthpoint decreases. Tests indicate that the voltage drops to approximately half of the Earth’s point of contact voltage in the first 3 feet, at least at distribution voltages levels. It drops to half of that voltage again in the next 3 feet until it can (for all practical purposes) be considered zero. This is a hazard for ground personnel. It is a real danger for workers leaving a truck that may have becomeenergizedthroughaccidental contactwith an energized conductor and maintenance work ers around underground distribution equipment. Step Potential [1,4,12]

NOTE: The distance from the fault to points A and B de pend on fault magnitude and soil resistivity.

Fig. 6-2

Protectionmethods include insulation, isolationor development of an equipotential zone.

Touch Potential [1,4,12]

Theworker has still another hazardtocontendwith: Touch Potential. This is the voltage resulting from touching a conductive element that is connected to a remote energized component. The voltage is called transferred potential and it rises to the samevalueas thecontact that becomesenergized. It could be thought of as standing on a remote Earth spot while holding a long wire that becomes energized on its far end. Touch voltage between the remote site and the voltage where the worker stands can be quite different. Refer to Fig. 6.2. The voltage is developed across the ground worker’s body. Methods of protection remain the same: Isolate, insulate or develop an equipotential zone.

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