5.5 High voltage isolation and access
5.5.1 General
Relevant sources of hazards listed in section 4.4.1 also apply to high voltage. Additional sources of hazards for high voltage isolation and access include:
- Induction from other circuits and communications equipment such as radio transmitters;
- Build up of static charges due to weather conditions;
- Ferro-resonance13;
- Transferred earth potentials14;
- Feedback from secondary or tertiary systems;
- Stored energy in high voltage capacitor banks; and
- Working under or over other live conductors.
From an electrical safety perspective, the risks and consequences of an incident involving high voltage are significantly higher. Under fault conditions, the higher potentials (voltages) and fault current levels release massive quantities of energy. Control measures should be adopted to reduce exposure to these risks.
5.5.2 High voltage isolation and access system
People near exposed live high voltage parts must maintain the safe approach limits detailed in section 3 of this Code.
Where people intrude within the safe approach limits for exposed live high voltage parts, the parts must be de-energised and earthed.
The principles in section 5.4 of this Code also apply to high voltage.
Elements of a safe system of work for high voltage isolation and access should include:
- A mechanism to instruct people on what to isolate to permit access to the high voltage parts as well as to surrender access and reverse isolation – and how to do it;
- Effective isolation to prevent lightning or switching surges from compromising the isolation point or points. Electrical safety in the area for access should not be compromised;
- Work practices and devices used for earthing and short-circuiting should be capable of withstanding the various prospective fault current levels and clearing times or fault duration15;
- Except in emergency situations, authority to access high voltage parts that have been earthed and short circuited should be provided by way of a written permit or authority. The permit or authority may be issued by telephone or radio etc;
- In emergency situations the actions taken should be recorded.
- Testing to prove de-energised to be in line with section 6.5 of this Code.
- Where possible, work and access should occur with visible earths and short circuit devices;
- Placement of "other precautions" should be a control measure for directing people to electrically safe work areas. Other precautions usually include:
- Taping or roping off;
- Safety signs; and
- Barriers.
- A mechanism or process implemented that demonstrates that the isolation and access process is being complied with at all levels;
- Having mechanisms in place to deal with protection operations that minimise exposure to electrical hazards and risks. If a feeder trips or ‘locks out’, whether or not after reclosing, the actions to energise the feeder should be outlined;
- Effective isolation to include control circuits and secondary systems. Unless fit for purpose, an open high voltage circuit breaker should not be considered an effective isolation point; and
- Procedures or mechanisms to be in place to address worker and community safety in emergencies eg damage from cyclones, mains on the ground.
Earthing installed in a power system is one method used to prevent or minimise electric shocks. When operating a power system (eg to switch, de-energise, isolate, prove de-energised and so forth) additional risks could be present. For example, when a power system is operated, voltage transients of short duration can create lethal step and touch potentials. Transients can occur when network operations cause huge quantities of electrical energy to stop flowing suddenly.
As it is foreseeable that lethal levels of step and touch potential will exist, appropriate control measures should be used. Depending on the situation, methods, or a combination of methods, that should be considered include:
- Design and installation measures:
- Physically separate or isolate articles eg mount the equipment in a safe location such as above 2.4 metres; and
- Use alternative insulation, better earthing, or both.
- Operational measures:
- Use of insulating gloves or mats, or both, when performing system operations; or
- Use of equipotential zones where conductive materials create a zone around the worker, so that the effects of step and touch potentials are negligible.
If insulating gloves or mats or both are chosen, they should be used in at least the following situations:
- Operating high voltage isolators, switchgear and earth switches via handles, operating rods and similar equipment;
- Applying and removing portable earths and short circuits; and
- Using high voltage test devices, operating rods, measuring sticks etc on, or near, high voltage parts.
13 The term "ferro-resonance" is defined in Appendix A.
14 The term "transferred earth potential" is defined in Appendix A.
15 The term "fault duration" is defined in Appendix A.
Last updated July 16, 2009
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