Case study: Selecting electrical systems for water, wastewater plants

During preliminary design, electrical engineers will be responsible for determining the appropriate electrical equipment type based on the application of the project, the anticipated electrical loads, the reliability criteria and the anticipated cost to construct. Two example projects will be examined to identify the electrical equipment requiring power and to determine whether switchboards or switchgear are the recommended equipment type (see Table 2).

The type of project for Example 1 is a new water pumping station. The incoming service is a 480/277 volts, four-wire, three-phase electrical system. The electrical room is located within the pump station. The footprint of the pump station is limited to a certain dimension due to site constraints, so increasing the size of the electrical room is not as feasible.

Figure 6 shows the proposed layout for the electrical equipment in the electrical room. The layout shows that a switchboard, labeled as “SWBD,” will be used in lieu of switchgear. The following are reasons for selecting a switchboard instead of switchgear for this type of design.

• The application for a water pump station does not require the reliability and protection offered by switchgear. Selecting switchgear in this case would provide significant extra costs to that of a switchboard.
• The physical dimension of the switchboard is significantly less than that of switchgear. Because of limited space in the electrical room and overall building, a switchboard will save space compared with switchgear. Consideration to the heating, ventilation and air conditioning equipment can be given as a switchboard will have less heat loss than switchgear, ultimately reducing the size of the required HVAC equipment for proper cooling.

Figure 6: This shows the proposed layout for the electrical equipment in the electrical room. Courtesy: CDM Smith

Example 2 is for an existing wastewater treatment plant where the existing motor control centers were directly fed from the utility transformer. The incoming service is a 480/277 V, four-wire, three-phase electrical system. The design engineers for this project selected new service entrance rated switchgear with a “main-tie-main” configuration.

The new switchgear is housed in a new electrical room, within a larger building, with adequate space and ample working clearances. During design, the dimensions for the electrical room were flexible to allow adequate space for all the recommended electrical equipment. The following are reasons for selecting switchgear in lieu of switchboards for this type of design.

• The application is for a wastewater treatment plant, which typically requires more reliability, flexibility for maintenance and a higher degree of protection for the equipment than a pump station.
• The draw-out circuit breaker design allows for easier maintenance to isolate certain equipment without de-energizing the entire electrical system.

Table 2: Two projects will be compared to identify the electrical equipment requiring power. Courtesy: CDM Smith

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