Diesel and Gas Generator Sets: ISO 8528 Rating and Selection Guide

How to distinguish standby, prime and continuous generator duty, size the alternator and fuel system, coordinate transfer equipment and specify hospital or data

1. ISO 8528 Application Classes

ISO 8528-1 defines the application, rating and performance language for reciprocating internal-combustion engine generating sets, including the engine, alternator, controlgear, switchgear and auxiliaries. A project must state whether the set is emergency standby power (ESP), prime power (PRP), continuous operating power (COP) or limited-time power; a single kVA figure without a duty class is incomplete.

2. Rating and Load-Step Checks

Selection itemEngineering question
DutyIs the set a standby source, a variable-load prime source or a continuous baseload source?
AlternatorCan the winding, excitation and AVR accept the largest motor start or step load?
Fuel / gas trainDoes the tank, pressure regulation and filtration support the required autonomy and transient response?
ParallelingAre synchronising, load sharing, reverse power and island-mode controls specified?

The three-phase apparent-power check is:

S = √(3) · ULL · I

Real-power demand then depends on power factor, efficiency, non-linear loads and the start sequence. UPS rectifiers, elevators, pumps and VFDs must be represented in the transient load study rather than added as steady-state kVA only.

3. Standby, Prime and Safety-Service Boundaries

Standby sets are selected for utility-outage operation with the site transfer system; prime sets are selected for variable load over an operating period; continuous sets support essentially constant load and long running hours. For hospitals, high-rise buildings and public-safety loads, ISO 8528-12 adds requirements for emergency power supply to safety services. ISO 8528-13:2026 adds safety requirements for the complete RIC generating set and associated equipment.

4. Specification Deliverables

  • Publish altitude, ambient temperature, humidity, acoustic limit, emissions regime and fuel quality.
  • Define ATS/AMF transfer time, black-start behaviour, battery charger, jacket-water heater and remote monitoring.
  • Require load-bank test, step-load data, short-circuit capability, protection settings and maintenance access.
  • For gas sets, add gas pressure range, methane-number assumption, ventilation and gas-detection interfaces.

*Reference: ISO 8528-1:2018; ISO 8528-6:2023; ISO 8528-12:2022; ISO 8528-13:2026.*