Purpose
This instruction outlines the minimum requirements for working on isolated overhead conductors/apparatus that could have voltages induced into them by nearby energised overhead conductors/apparatus. It includes an explanation of, and advice on, how to reduce or eliminate the risk of induced voltages in the isolated conductor/apparatus.
DANGER
Induced voltages may be as little as a few volts and as high as many kV and could be hazardous or life threatening if the correct procedures are not followed.
Note:
• An induced voltage is often referred to as induction.
• All people performing work must comply with the minimum personal protective equipment (PPE) requirements. For more on this, see Section 3 (Personal protective equipment) in this manual.
Instructions
• An 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 apparatus due to it running in close proximity to a live conductor or adjacent apparatus at any point along its length. The danger may not be visible from the worksite.
• Complete the risk assessment process and job briefing to identify all foreseeable risks, including induced voltages.
• Wear PPE and clothing that is appropriate for the risks that have been identified in the risk assessment.
• Obtain the relevant permits for the apparatus to be worked on, from the controlling authority (System Operations Control Centre (SOCC)).
• Apply program earths according to the switching program.
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• Working earths must be placed on 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 the earthing point to create an equipotential work zone.
For more on this, see field instruction 2.10 (Portable earthing / shorting equipment) in this manual.
• Use a proximity tester (e.g. Modiewark) to determine the presence of induced voltages on isolated and earthed apparatus. Start on the highest setting and gradually reduce to the lowest to identify induced voltages.
• 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.
Broken, repaired, separated and joining conductors
• 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.
• If any part of the earthing is broken, dangerous voltages may occur when it is necessary to break a part of the earthing system. Connect a bridging lead across this part of the earthing system before it is broken.
• Do not remove the earth connections between apparatus and the earth system while the apparatus is in service unless this can be done safely. A dangerous voltage may occur if the earth connection is removed from high voltage apparatus while in service.
Conductors on the ground
• There are additional requirements to the normal earthing requirements for electrical access if workers are handling conductors at ground level for any of the following reasons:
o repairing the end of a broken conductor o moving or securing a conductor
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• People working on any conductor on the ground that may have induced voltages must:
o wear gloves rated at a minimum of 500 V over approved wrist length mechanical protective gloves
o apply working earths to the conductor
• If the conductor is disconnected, use a temporary jumper to bridge the gap until the conductor can be pulled together and reconnected. The temporary bridge can then be removed.
Conductors working aloft
• When carrying out conductor jointing, splicing or disconnecting for any other reason:
o both conductors must be earthed as close as possible to the apparatus o a temporary jumper must be placed across the disconnected conductor to
maintain electrical continuity
o the lead must comply with the portable earthing equipment requirements described in field instruction 2.10 (Portable earthing/shorting equipment) in this manual
Extra working earths
If the risk assessment determines that there is a likelihood of induced voltages, use extra working earths at the worksite to reduce the risk of induction. Wear gloves rated at a minimum of 500 V over approved wrist length mechanical protective gloves
Uninsulated elevated work platforms
Uninsulated elevated work platforms working within a substation must have:
• the basket bonded to the electrical apparatus that is being worked on
• the vehicle earthed to an identified earthing point using approved earthing leads (either bolted or clamped)
General information
• People are also affected by induction and may feel this when brushing against earthed structures and apparatus. To avoid discomfort, maintain firm contact with earthed items when working in areas of high induction.
• Test instruments may give false readings when not connected due to induction in the leads.
• Testers in charge must remember that a transfer of earth potential is possible.
Transfer of earth potential is controlled by effective earthing. Accordingly, any contact with a conductor that has the potential to carry a transfer voltage must only occur while that conductor is earthed effectively.
• Where possible, leave all earth switches ON at substation sites unless they need to be opened for testing. Before earth switches are opened, ensure that all personnel on site are aware of the changes to the yard configuration and earthing arrangements within their relevant work area. If operating an earth switch, a “Restricted Use” danger tag must be placed at the switch (see Appendix 1 – Tags and signs). Once the testing is completed, the earths are to be reapplied and all relevant personnel notified.
• Transmission lines are considered one of the major sources of induction. The current in the energised high voltage conductor causes a magnetic field that may cut through de-energised conductors. This induces a current in the isolated lower voltage 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.
Table 1: Guide to induced voltage exposure Distance from energised line
(metres) 1.0 2.0 3.0 4.0 5.0 6.0
Percentage of the phase
value of the energised line 60% 50% 40% 30% 20% 10%
• The distance between the overhead transmission lines and any object in the field may change from time to time according to the electricity loading of the lines as well as the swinging angle of the overhead transmission lines.
Induction can be increased or transferred when a conductive object approaches an energised conductor, i.e. pulling or tensioning equipment, busbars, switchgear and mobile plant.
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• The number of working earths required will be influenced by the value within the electric field, the main earth resistance, the resistivity of the ground and the proximity of the working earths to a person’s working position. In severe cases, induction cannot be totally eliminated and gloves rated at a minimum of 500 V must be worn over approved wrist length mechanical protective gloves.
References
• Work Practice Manual:
o field instruction 2.10 (Portable earthing/shorting equipment) o section 3 (Personal protective equipment)
o Appendix 1 (Tags and signs)
• IEC/TR 61328 Live working – Guidelines for the installation of transmission line conductors and earthwires – Stringing equipment and accessory items
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2.10 Use and management of portable overhead earthing/shorting