Case study: Hospital electrical system design allows for expansion
When it comes to electrical system design, the electrical engineer must consider various strategies to provide a safe operating system, particularly in a hospital where human life is at stake. A recent example is that of a hospital facility located in northern Texas. The electrical system was designed to support approximately 350,000 square feet of full hospital services including diagnostic and treatment, women and children’s services and general and intensive care patient spaces. The project requirements also included electrical infrastructure to support future horizontal and vertical expansion.
At the onset of the project, the teams met with the owner’s group to establish the owner’s project requirements, which identified desired redundancies, flexibility, future expansion requirements, energy targets and overall program of the facility. Once the project parameters were established, the design engineers worked to prepare the basis of design documents and began planning the electrical distribution systems.
As the architect commenced the programming and conceptual design phase, the engineer worked alongside to identify initial space constraints and optimum layout of electrical equipment rooms. Access to equipment, ease of maintenance and overall safe operating systems were all top priorities. During this process, several studies were conducted, including options for a remote central utility plant versus an attached plant; medium–voltage versus low–voltage service; and indoor versus outdoor generators.
With the space program and initial conceptual layouts in hand, the electrical load estimates were prepared based on the anticipated usage of the space. Load calculations included heavy power use elements such as elevators, medical imaging and surgical loads, kitchen cooking equipment and sterilization equipment as well as HVAC, plumbing and medical gas equipment. The OPR included a strong desire for a reliable system that would result in minimum downtime for any service, maintenance or outside factors affecting the system.
The normal power service was designed with redundant utility feeds connected to two main service entrance switchboards. The switchboards were connected in a main-tie-main arrangement, which allowed for one service to support the entire hospital, providing a backup should one of the services fail. Similarly, the emergency power system was designed with two paralleled generators and provisions for two additional units to ultimately provide N+1 redundancy for the emergency power supply system. Temporary provisions for an external roll–up generator also were included to protect in the event of a sustained power outage or extensive repair necessitated on the installed EPSS.
Safe system design strategies were integrated throughout the entire electrical distribution system, including:
- Draw-out type circuit breakers were installed on the main normal and emergency power service switchboards. This type of arrangement allows for breakers to be safely disconnected and de-energized for maintenance with minimum impact or downtime on the rest of the system. In addition, the larger main and tie breakers were electrically operated so that they could be remotely disconnected before racking them out to their draw-out position, providing an added layer of protection to the worker.
- Infrared viewing windows were installed on all main normal and emergency power switchboards to allow for annual infrared scanning of connections without opening the equipment, thereby preventing exposure of dangerous arc hazards.
- Bypass isolation automatic transfer switches were used to allow for service and maintenance of critical and life safety components without de-energizing an entire branch of emergency power.
- Zone selective interlocking was incorporated into the larger feeder breakers to aid in arc hazard reduction.
- The normal power switchboard main and feeder breakers were provided with ground fault equipment protection.
- An extensive power monitoring system was incorporated to aid the facility engineer with tools for predictive maintenance upon review of load trends. The level of detail is provided down to the departmental level.
- Main equipment rooms were planned with multiple egress points and adequate working clearances. Branch equipment rooms were planned in key locations throughout the facility for maximum flexibility with appropriate maintenance access and working clearances.
- An arc flash analysis and selective coordination study were performed to identify key arc hazards and the appropriate protections required for servicing of the electrical equipment.
- The equipment was appropriately labeled when installed detailing the potential hazards and available arcing faults.
This list highlights some of the key components and features that were incorporated into the electrical distribution system design. The hospital also was fully commissioned by a third-party commissioning agent, which provided an added level of security for the owner, proving that the systems were installed and operating as intended.
The entire team (owner, design and construction) worked successfully together to fully equip the owner and facility engineering director with the appropriate components to allow for safe day–to–day operations and safe handling of the equipment. Provided the systems are maintained properly, the building will remain a safe working environment for the building occupants for the longevity of the facility anticipated to be more than 50 years.