Transmission lines are considered one of the major sources of induction. The current in the energised HV conductor causes a magnetic field that can induce dangerous voltages and currents in conductive materials that are in proximity to the energised line. The induced voltage in the de-energised circuit is a function of:
o the current in the higher voltage line
o the distance separating the two lines
o the nature of the dielectric, ambient conditions, moisture, heat, wind, etc.
There is no point in trying to calculate the value of the induced voltage due to the variable nature of all of the above factors. The common approach is to establish whether a risk exists and then introduce additional precautions to mitigate against it. For example, an initial inspection of the line route or examination of a map indicating all other powerlines adjacent to the line to be isolated must be conducted. This is to determine the likelihood of the existence of any induced voltages. Hazardous induced voltages may be present on the lower voltage line due to it running in close proximity to a live conductor or adjacent transmission line at any point along its length. The danger may not be visible from the worksite.
There is a distinct possibility that a voltage detector used to prove that the line is dead prior to the application of earths may give a false live indication.
If there is an earth switch available on the feeder at a ground mounted substation, then that should be the first earth applied and it should remain closed throughout the work, unless it is required to be opened for testing.
Working earths must be placed both sides, and as close as possible, to the workers who are working on the apparatus.
Ensure that the earth grounding leads are bonded at a single earthing point, or rod, if possible to create an equipotential work zone.
Apply and remove portable earths using an approved and rated insulated stick.
When operating on the structure, connect mobile equipment (e.g. cranes) to the portable earthing point on the structure to create an equipotential zone.
Wear a minimum of 500 V rated gloves with approved wrist length mechanical protective gloves to avoid touch potential which may occur when working around mobile plant and other conductive apparatus.
If any part of the conductor being worked on is disconnected in any way, a temporary jumper must be installed to bridge out the break in the conductor. If this is not done, the worker touching both ends of the disconnected conductor will act as a bridge and may receive a shock if an induced voltage is present in the isolated conductor.
Never allow a situation where a person could become part of the ‘circuit’, a bond must always exist across a break in a conductor where access to both ends is possible.
Regards
Marmite
