This is the second in a 2-part blog series discussing the safety and reliability of medium voltage (MV) and low voltage (LV) electrical distribution systems. In my first post, I noted how both specifiers and facility teams need to go beyond the equipment itself, by addressing safety-related considerations during the design and installation phase, as well as during operation and maintenance. In that post, I looked first at considerations regarding arc faults, shocks, accidents as well as changes of conditions over time.
In this post, we’ll talk about additional design considerations specifically related to an MV electrical distribution system, including criteria related to installation, location, and operation. Once again, please note this is only intended as a brief overview. More detail and tips can be found in the resources at the end of this post.
Consider the installation and location
Though choosing the type of connection from neutral to earth is not always possible at the design stage, it is nonetheless influential on system performance. Deciding on the best method should take into account requirements for the protection scheme, potential step/touch voltages, transient voltages on equipment, and service continuity.
We think of single line diagrams primarily defining the electrical installation at the design stage, in terms of loads, supplies, redundancies, protection, etc. But they should also be depended on to support the safety of personnel during the system’s entire lifetime. For example, they can help determine ease of grounding operations, capability for maintenance on dead switchboards, and the opportunity to separate equipment into several rooms.
When civil work is done to accommodate the MV portion of an installation, a number of actions can help improve safety levels. First, to reduce risk of faults, it’s recommended that there be climatic control around equipment, protection against ingress of elements (water, dust) and animals, and access control to restrict unskilled people. To reduce risk of personnel being in contact with live equipment, the MV system can be split between multiple rooms and remote switchgear control capability can be added. Installing auxiliary equipment (e.g. communications) in an MV equipment room should also be avoided.
Other location-specific factors such as the impacts of a gas explosion, dust in mining operations, and possible seismic events should all be input for design criteria for switching rooms. As noted above, the proper amount of space for safe and efficient work should also be factored in. Adequate lighting is also an important safety consideration.
In terms of switchgear, controlling access to high-voltage compartments, provisions for good grounding, proper identification of functions and circuits, and clarity of operational instructions should all be addressed to ensure safety. In general, all equipment needs to meet the designated performance criteria for the electrical installation. Beyond the determination of service levels and fault characteristics, network studies can provide data for specification of other characteristics; though, dependent on available input data, some simulations may require conservative assumptions.
Regarding MV protection systems, if a malfunction occurs it’s important to have knowledge of the goal, principle, and thresholds of the system. This will help in the assessment of the situation and restoring service. In addition, a monitoring system will deliver the information needed to diagnose the cause of the protection operation, which is essential in reducing any safety risk before attempting a corrective action.
Designing with operation in mind
How a site is operated has a big influence on risk and safety levels, so it’s important that this is taken into account when defining design constraints. An analysis of normal and fault conditions should be done, including the variables of incoming power loss, need for maintenance, and changes in the installation. These will impact the single-line diagram, organization of switching rooms and layout, choice of equipment, and the required skill level of maintenance personnel. During operation, if a fault event does occur, there should be clear procedures in place. This includes specifying the acceptable loss of continuity for the sake of ensuring the safety of those personnel tasked with restoring power.
Maintenance procedures need to define when and by whom they are performed, as well as prerequisites and procedures. A maintenance program should balance work safety and performance of the installation. For example, special protection settings could be used during maintenance work to limit the consequences if a fault occurs, even if this temporarily impairs protection performance. Maintenance policy should clearly define task limits for each individual, and a backup plan if a person in unavailable. Equipment maintenance should follow manufacturer guidelines, but should also take into account local conditions, changes in strategy (e.g. from periodic to predictive), and changes to the installation. Finally, attention should be paid to cleaning, plumbing, and any other factors affecting the electrical installation.
Work procedures should be viewed as a means for better defining operations, tools, and methodology surrounding safety aspects. They can also be used to keep track of lessons learned and to ensure workers and management share a common understanding. Working around high voltage equipment also requires specialized equipment, some of which can be considered burdensome. Making sure workers are comfortable and reinforcing that such equipment is mandatory will help avoid neglect and compromised safety. Any specialized tools required for MV work should also be clearly listed within procedures.
Consistent training helps maintenance teams become more knowledgeable and safety aware. A training refresh should be triggered anytime there is a change in the workforce, an introduction of new equipment, or an evolution of process or installation operating conditions. Regular reviews of working procedures will help increase situational awareness and, in turn, increase confidence during emergency situations.
Ultimately, electrical system safety should begin at the initial design stage to help ensure operation efficiency, optimized costs, service continuity, and personal safety. To learn more about this subject, refer to the white paper ‘How Medium Voltage Equipment Performance and Reliability Depend on Early Safety Considerations’. To discover how an advanced power management solution from Schneider Electric supports MV system reliability, visit the EcoStruxure™ Power web page.
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