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How to Balance Safety and Cost when Deploying Intelligent Devices

In the previous post in this series, I gave an overview of how utilities can use the IEC 61508 standard as a framework to maintain high levels of safety while deploying IEDs on electric networks. In that post, I listed three steps toward that goal.

  1. Balance cost and safety
  2. Apply standards
  3. Create a maintenance plan

In this post, I share more detail about the first step—balancing cost and safety.

Instead of piling redundant intelligent devices on the network, the idea is to install just enough IEDs to minimize cost while establishing appropriate safety levels. For utilities to solve the balance equation, they must analyze their network design to understand the acceptable level of risk, and how many customers are affected by particular failures: Areas largely impacted by failures should represent areas of high investment.

Safety functions are added either as stand-alone protection and control devices or in a protection and control system associated with a portion of the electrical network. They reduce the risks associated with the hazards of the electrical network to an acceptable level, as shown in the image below.

Risk reduction (adapted from IEC 61508)
Risk reduction (adapted from IEC 61508)

The standard applies where these safety functions are realized in an electrical system, electronic system, or programmable electronic system. If any safety functions are entirely realized through some other means, then the standard does not apply to those functions or other means.

Scenario analysis for both protection and control functions determine where the risks are largest and greatly inform the design process by which you can maximize safety while minimizing costs.

Protection functions and design

In order to limit the consequences of an incident or fault, protection functions quickly isolate sections of an electrical network. A series of IEDs performs these protection functions. Each IED may be programmed to focus on a particular part of electrical distribution process such as current arrival, current departure, line status, voltage transformation, or motor operation.

For most of the protection functions, non-eliminated faults risk safety, production loss, and equipment damage. On the other hand, unnecessary tripping results in undistributed energy costs and even safety risks for applications where power supply is critical (e.g., tunnel lighting and air circulation). This is why IED protection functions must be properly designed and configured.

Control functions and design

Control functions relieve the burden on operators by automatically executing predefined actions that must take place in a very short time. IEDs are often responsible for these functions, which diminish the risk of human error during circumstances where quick responses are necessary. Consider, for example, the common challenge of how to modify an electrical network’s switching scheme without changing capacity. In order to accommodate such a scenario, IEDs involved in control must be designed and configured according to these rules:

  • Avoid opening or closing a switch where changing its position will establish or cut off a current circuit
  • Avoid opening or closing a circuit breaker where its new position will connect a live circuit to the earth or establish a current circuit through a switch in movement

In this example, if the IEDs are not designed and configured for proper control or automatic sequence, the consequences could include bodily injury and equipment damage.

In the future I’ll post more details about steps two and three in this process—applying standards and creating maintenance plans. In the meantime, you’re welcome to learn more about this topic from a free white paper I recently co-authored with my colleagues: Impact of IEC 61508 Standards on Intelligent Electrical Networks and Safety Improvement.

 Schneider Electric Safety IEC 61508

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