Encyclopedia of Grounding (CA09040E)

HAZARDS TO ADDRESS

energized line represents the transformer primary and the de-energized represents the secondary. Current will flow in a path consisting of the con ductor, jumpers, Earth or neutral located between the jumpers. The current amplitude depends upon the separa tion of the energized and de-energized lines and the resistances of the path. If the line ends are open, a voltage will be present at the ends. This is a common occurrence when lines share common corridors for long distances. Removal of a grounding jumper may then create a hazard. It would interrupt current flow. Voltage immediately would be induced across the gap created if the jumper is removed (breaking the circuit) resulting in an arc. Successful removal of personal protective grounding equipment depends upon the current and voltage magnitudes present. In some cases, special equipment may be necessary to interrupt the current and quench the arc without causing a flashover to an adjacent grounded point.

The primary hazard to protect against is that of a line being energized from induction or becoming accidentally re-energized after it has been de-en ergized for maintenance. Possible re-energization sources can include incorrect closing of switches or circuit breakers or energized over build lines falling into or contacting the de-energized ones. Other sources that may also re-energize a circuit are back-feed or induced voltage from electric or magnetic fields or both from nearby energized lines. A static charge can be induced from atmo spheric conditions such as wind or lightning. A single, low resistance personal protective jump er placed close to and in parallel with the worker can provide protection for the worker. However, multiple jumpers may be required to satisfy other maintenance or safety aspects. If this is the case, the additional jumpers act to form one or more complete circuits. This allows an induced current to flow in the de-energized line caused by the magnetic field of an adjacent energized line. Think of the parallel energized and de-energized lines as an air core transformer with a 1:1 turn ratio. The Induced Voltages and Currents [21] Magnetic Induction:

Fig. 6-1

6-2

ENCYCLOPEDIA OF GROUNDING