Overview of IEEE Standard 493
What is IEEE 493?
IEEE Standard 493 is called the IEEE Gold Book due to its golden cover. The gold covering was its unique identifier as part of the IEEE "Color Books" containing power system standards. IEEE 493 contains recommended design practices for improving the reliability of industrial and commercial power systems while balancing the benefits of the project against the cost.
Overview of IEEE 493
IEEE 493, the IEEE Gold Book, is one of IEEE’s sets of recommendations regarding power system design. IEEE recommends that electrical system analysis and project to improve electrical power system reliability should include a cost-benefit analysis.
IEEE 493 offers electrical engineers standardized formulas to perform a cost benefit analysis for power system improvements and projects intended to improve power quality, such as improving the power system’s reliability by reducing the odds or duration of outages or improving the quality of the power supply by eliminating voltage and current fluctuations.
IEEE 493 requires companies to perform a trade-off study comparing the cost of a power outage relative to the cost of improving system reliability. The cost of additional fail safes may not be worth the expense when the power outages are rare or have little impact on the bottom line.
Calculating the Cost of Your Current Power System
The cost-benefit analysis in IEEE 493 looks at the potential losses when there are electrical shorts or power surges. IEEE 493 recommends that businesses consider the losses from damaged equipment and its repair costs. IEEE Standard 493 also suggests that companies consider the likely cost of product that is ruined when the power goes out or equipment malfunctions due to power fluctuations. Additional maintenance costs are also a consideration, since a less reliable power system requires more frequent checks and repairs.
The cost of an interruption in service can be broken down into an hourly cost and set cost. The set cost will include costs such as the replacement cost for damaged machinery, new fuses and replacement parts. It can also include retooling costs to reset and re-position equipment thrown off by the power interruption Hourly costs include lost production and labor costs as employees sit idle. When determining the cost of an interruption, the set cost is the minimum cost. The hourly cost should be multiplied by the average downtime of a production line when there is a power interruption. This information should be
available from your plant's maintenance logs. IEEE 493 contains failure and repair rates for major electrical components so that engineers that do not have actual data to work from have mean time between failure or MTBF values to use the in the cost-benefit calculation. The hourly cost multiplied by the average duration of an outage is added to the set cost to get the total cost of an interruption. This cost per outage is multiplied by the average number of outages per year to get the total cost of the current power system’s reliability.
Identifying Possible Improvement Projects
Identify several possible projects for evaluation. Should you install a backup power generator to kick in when the main power goes out? Do you need to upgrade your fuses or power transformers? Would the addition of backup circuits prevent production stoppages when the main circuit overloads or burns out?
Determine the cost of each project in terms of equipment costs and any infrastructure costs such as new electrical wiring. Estimate how long will the production line be down during the implementation. Multiply this by the cost of lost production per hour. Add the set up or tear down costs, installation costs of the new equipment and any money spent disposing of old electronics. Total up the material costs, labor costs for the project and lost production time to determine the costs of implementing each of the possible power system improvements.
Quantifying the Benefits of Power System Improvements
The benefit of the power system improvements will be the money you do not have to spend in the future. How many outages would be prevented? Would the severity of each incident be reduced? Estimate the new number of failures that would be seen if the new improvements were put in place. Determine the costs each incident would result in. Would set costs be reduced because the new failures would only require repairs to a circuit breaker? Would the hourly losses of the failure be reduced because the production line is kept running by secondary circuits while the primary ones are out?
Multiply the new cost per the average outage in the planned design by the number of outages the more reliable system would experience. This will give you the estimated cost of power losses under the new design.
Subtract the estimated losses from the selected power system design from the annual cost the current state of affairs. This is the cost-benefit of implementing the power system improvements. Any power system improvements must cost less than the cost savings from the power system improvement. If your power system improvement saves $100,000 a year, the total cost of any new power system improvements must cost less than this in order for the project to be worth the money.
For example, the cost of additional fuses or transformers may be cheaper than the cost of items that would have to be replaced if they were ruined by a power surge. Conversely, the project’s cost is not worth the benefit if the power system is highly reliable and brief outages have little impact on the bottom line. The cost of installing private power generators have been found worthwhile in areas with an unreliable power grid, given the high costs of repeated power outages and plant shut downs.
IEEE 493 Working Group
The IEEE 493 standard is maintained by the IEEE 493 Working Group, also called the IEEE 493 WG and Gold Book Working Group. The 493 WG is itself part of the industrial and commercial power systems committee.
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