Document: NRC Regulatory Guide
Document ID: 1731a882-b0c5-418d-b65e-55646c26be5c
Document Type: regulatory_guide
Title: 03/2007
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0703/ML070380553.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.9
CFR Part: 
CFR Title: 

Content:
generator that is able to accept large loads in rapid succession. The majority of these emergency loads are large induction motors. At full voltage, this type of motor draws a starting current of five to eight times its rated full-load current. These sudden large increases in current drawn from the diesel generator as a result of the startup of induction motors can result in substantial voltage reductions. This lower voltage could prevent a motor from starting (i.e., accelerating its load to rated speed in the required time), or could cause a running motor to coast down or stall. Other voltage-sensitive loads might also be lost because of low voltage or if their associated contactors drop out. Recovery from the transient caused by starting large motors, or from the loss of a large load, could cause diesel engine overspeed that, if excessive, might result in a trip of the engine (i.e., loss of the safety-related power source). These same consequences can also result from the cumulative effect of a sequence of more moderate transients if the system is not permitted to recover sufficiently between successive steps in a loading sequence. Rev. 4 of RG 1.9, Page 3 General industry practice is to specify a voltage reduction of 10–15 percent when starting large motors from large-capacity power systems, and a maximum voltage reduction of 25–30 percent when starting these motors from limited-capacity power sources such as diesel generators. Voltage reduction during load sequencing should be evaluated in light of the plant-specific equipment to prevent load interruption. Large induction motors can achieve rated speed in less than 5 seconds when powered from adequately sized emergency diesel generators that are capable of restoring the bus voltage to 90 percent of nominal in about 1–2 second(s). Protection of the emergency diesel generator from excessive overspeed, which can result from an improperly adjusted control system or governor failure, is provided by the