High voltage switching is a high-risk task, and as such the use of switching programs is critical. A switching program is a written list of sequential logical operations to permit isolation and re-energisation of electrical plant and/or equipment.
The prime considerations of a switching program are safety of the operator; safety of the persons working with the system; welfare of the plant and/or equipment; and continuity of supply.
The workflow illustrated below is intended to provide you with an overview of the processes involved in developing switching programs, including the isolation and access of high voltage apparatus and/or equipment.
Switching program development
Before the development of any HV switching program, there are several points to consider:
- What is the task to be done?
- How long will the switching take?
- What and where are the isolation points?
- How many and where will the earths be applied?
- Is there any testing to carry out?
Consider the task of changing the HV fuses in a high voltage cabinet. The risk to the high voltage switching operator is exposure to an arc blast and electric shock; therefore, any HV fuse changes inside the cabinet would require a switching program and earths to be applied.
When writing a high voltage switching program you will also need to consider:
- Reasons for isolation
- Equipment characteristics
- Isolation points
- Earthing points
- Phasing out
- Load levels
- Voltage levels
- Fault levels
- Access to equipment
- Other parties
Possible effects of switching operations on the system can include:
- Loading: Examine whether equipment still in service can cope with any additional load it is required to handle during the course of the switching program, specifically:
- Is there enough capacity on the low voltage conductors?
- Is there enough capacity on the HV feeder?
- Is there enough capacity from the substation supplying the HV feeder?
- Has allowance been made for increased loads that may occur during the course of the program?
- What is the rating of underground cable in the circuit?
- Is any part of the aerial systems conductors too small?
- Could distribution transformers become overloaded with additional load?
- Voltage: Check that statutory supply voltage can be maintained during the course of the switching program. Voltage levels may be affected by:
- The additional load?
- Increased distances from the customers to the supply points?
- Conductor size and lengths.
- At least the minimum statutory voltage levels must be maintained, therefore more interconnection may be required, or the program delayed until loads have reduced.
- Proving de-energised: Where and how can the equipment be proved de-energised? Plan this in the program.
- Earthing: Earths should be placed at all entrance points to the isolated area. Where practical earths are to be placed on both sides of the work area and within sight of the work party.
- Phasing out: If this is necessary, where is the best place for phasing out? Plan how this is to be done before starting to write the program. It may require additional items to be included in the program.
- Special circuit arrangements: Are there any special problems involved with this program? There may be a need for an alternative supply.
Here are some other handy tips to remember when writing a switching program:
- Allow enough time to write the program. Do not try and rush it.
- Avoid interruptions when writing the program. Turn your mobile phone off and work in a phone free environment.
- Non-emergency programs should be submitted for approval at least twenty-four hours before the proposed start time.
- Make sure the equipment is called by its right name. If uncertain, personally verify on site yourself. Do not take the word of a third party.
- Try and write the program in an efficient way to minimise the operator travelling. Be aware of traffic hazards; try to keep the vehicle close to the operator at all times.
- If possible, keep operations on voltage levels together (i.e. complete the low voltage switching first before starting the high voltage switching).
- Do not take drawings or diagrams for granted, get out on site to confirm the equipment is as shown and what its current position is. It may be damaged; it could be shown as OFF on a drawing, but it is actually in the ON position.