Understanding standby, prime, and continuous gensets

Electrical Power & Protection

March 2024 Electrical Power & Protection

The increasing reliance of South African businesses on generator sets (gensets) to mitigate power disruptions highlights a crucial need for proper selection based on specific operational demands. Despite their growing usage, there’s still widespread confusion about how to choose the appropriate genset, often leading to inefficient and costly decisions.

Understanding the differences between standby, prime, and continuous applications is essential to optimise genset performance and longevity. This is according to Craig Bouwer, senior manager at WEG Africa, who explains that many customers mistakenly select gensets based solely on nameplate rating.

“Understanding the specific application of the genset is crucial for the right selection, and the first step is knowing that genset applications are broadly categorised into standby, prime, and continuous, each with distinct operational requirements,” he says.

Standby gensets are seldom used, and are typically kept for emergency situations. These units have a limit on operational hours per year and a specific load factor. In South Africa, due to frequent loadshedding, few gensets are used solely for standby purposes. Prime and continuous applications are more common in the country. Prime gensets can run unlimited hours annually with variable loads, but maintaining an average load factor below their maximum rating. Continuous gensets also operate unlimited hours, but with a constant and predetermined load.

Damian Schutte, engineering manager at WEG Africa, explains that understanding the difference between prime and continuous ratings is also critical. The load factor is a key differentiator and not the unlimited time requirement, with prime applications having variable loads, and continuous ones having fixed loads.

Schutte uses a vehicle analogy to illustrate the differences: a continuous genset is like a car on cruise control operating at a steady speed within its capacity on a long-distance trip, while a prime genset is akin to a vehicle driving in the city. Standby can be perceived as racing between traffic lights.

The choice of genset rating impacts its expected lifespan and maintenance needs. For example, continuous power may be required in mines during loadshedding to supplement limited grid power, while industrial applications like workshops, with variable loads, would need a prime-rated genset.

Bouwer notes that standby power remains vital in essential service sectors for health and safety reasons, especially in environments like mines, hospitals, and data centres.

WEG Africa, as an original equipment manufacturer, uses these categories to guide customers in their genset choices, aiming to match their specific needs and expected service life. The company cautions against oversimplifying the selection process by just matching the total load with a genset’s nameplate rating, as this can lead to premature failure and additional costs.

“To ensure the correct choice, we work closely with customers to assess their load requirements, usage frequency, and operational conditions, and through this process ensure optimal genset selection,” Bouwer concludes.

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Understanding standby, prime, and continuous gensets

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