As noted throughout this discussion, current large PWRs require a safety-related AC power bus backed by an emergency diesel generator (EDG) to provide assurance that active safety-related equipment will function when required. There are usually two EDGs per large LWR onsite. The EDGs are governed by plant technical specifications and must be tested at least once a month. Every six months, each EDG must be shown to come up to speed and voltage, and begin the loading sequence within 10 seconds of receiving a start signal (NRC, 2012b). Although necessary to prove the safety basis of the plant, this kind of testing can be problematical for the EDGs. In the event that one EDG is declared inoperable, the remaining EDG must be tested within 24 hours. As a result, it is not unusual for EDGs at large LWRs to be tested quite frequently, leading to higher maintenance requirements.

By removing the need for safety-related diesel generators in the various iPWR designs, the planned diesel generators become ancillary power supplies that enhance iPWR defense-in-depth by maintaining the availability of normal operating equipment that is not safety-related. Testing of these ancillary diesels can be less frequent and fast load testing can be relaxed such that the ancillary diesels have an opportunity to warm up for a few minutes before loading. Therefore, less maintenance can be anticipated for the iPWR ancillary diesel generators.

Current large PWRs maintain EDG-backed safety-related AC buses, nominally at 480 Volts and 4160 V AC to operate active safety equipment. In addition, current LWRs require safety-related power to operate instrumentation during and following an accident. This is provided by battery-backed 120 V DC and low voltage AC through an inverter. The iPWR designs will not require high-voltage safety-related buses, because there is no active safety-related equipment component with this need. Power to reposition any iPWR safety-related valves will be required initially. Only battery-backed 120 V DC and low voltage AC through an inverter will be required to be safety-related, which will meet the needs for repositioning valves and powering instrumentation. So, the electrical distribution systems of current large LWRs and iPWR designs will be similar, but the approach and amount of testing and maintenance will differ.