In short. ISO 8528 is the international series of standards for reciprocating internal combustion engine driven generating sets. Part 1 defines the power ratings (continuous, prime, limited-time and emergency standby) and the performance classes G1 to G4; other parts cover engines, alternators, controlgear, whole-set performance, test methods and noise. A specification should name the part, edition, rating type and performance class it relies on, not simply quote the series number.
What the ISO 8528 series covers
The series is organised so that each part deals with one aspect of a generating set. Engineers in India most often meet it through manufacturers’ data sheets, consultant specifications and tender documents, where a phrase such as rated to ISO 8528 is common. That phrase is only meaningful when it is clear which part is being invoked.
| Part | Subject | Where it matters in practice |
|---|---|---|
| 8528-1 | Application, ratings and performance | Defining COP, PRP, LTP and ESP ratings and performance classes |
| 8528-2 | Engines | Engine governing and characteristics used in the set |
| 8528-3 | Alternating current generators | Alternator rating, voltage regulation, short-circuit behaviour |
| 8528-4 | Controlgear and switchgear | Set-mounted control and switching equipment |
| 8528-5 | Generating sets | Whole-set transient and steady-state performance limits |
| 8528-6 | Test methods | How performance is measured on test |
| 8528-8 | Low-power sets | Small sets for general purpose use |
| 8528-10 | Airborne noise measurement | Declared noise levels of enclosed sets |
| 8528-12 | Emergency power supply to safety services | Sets supplying life-safety and emergency loads |
| 8528-13 | Safety | Safety requirements for the set |
Parts are revised on different timetables. ISO 8528-1:2018 is the edition commonly cited at the time of writing, but the ISO catalogue should be checked for the current status of every part before an edition is written into a specification. The Bureau of Indian Standards has adopted parts of the series as IS/ISO 8528, and an Indian tender may refer to either designation.
Power ratings: the most misused part
The single most important contribution of ISO 8528-1 is a vocabulary for what a set’s kW figure means. The same set will carry different numbers depending on the rating type, and treating them as interchangeable is a common source of failure.
Continuous operating power (COP)
The power the set can deliver continuously at constant load for an unlimited number of hours a year, with maintenance as prescribed. It suits base-load duty where the set is the main source.
Prime running power (PRP)
The maximum power available during a variable load sequence for an unlimited number of hours a year, with the average load over a 24-hour period not exceeding a stated percentage of the PRP, commonly 70 percent. It suits sites where the generator is the main supply and the load varies, such as a remote site or quarry.
Limited-time running power (LTP)
The power the set can deliver for a limited number of hours a year, typically up to 500, for example in planned peak-lopping.
Emergency standby power (ESP)
The power available during a utility failure for a limited number of hours a year, commonly up to 200, with a variable load whose average is lower than the ESP. Most building backup sets in Hyderabad are bought on this basis. Running such a set as the main supply for months, for example on a project site with no DISCOM connection, exceeds its rating basis even if the kW figure appears adequate. The practical consequences are covered in Standby Power Generators and Prime Power Generators.
The exact hour and average-load limits for a given set are set by the manufacturer within the framework of the standard, so the data sheet governs.
Performance classes G1 to G4
ISO 8528-1 and 8528-5 group generating sets into performance classes according to how tightly voltage and frequency are held in steady state and how far they deviate, and how fast they recover, after a sudden load change.
- G1 suits simple loads such as general lighting and resistive heating.
- G2 suits loads similar to the public supply, such as building services, pumps and fans; it is the usual class for commercial standby sets.
- G3 suits more demanding loads such as telecommunications and some electronic and UPS loads.
- G4 covers exceptionally demanding loads, with limits agreed between manufacturer and customer.
A specification calling for G3 performance with a large block load step may need a larger set, a different governor or a staged load sequence. The interaction between performance class, load step and UPS behaviour is discussed in UPS & Non-Linear Loads.
Turning the standard into a specification
A useful specification clause names the rating type, the site conditions, the performance class and the test evidence required. For example, a clause might ask for a set rated for emergency standby power at 415 V, 50 Hz, 0.8 power factor, at 45 degrees Celsius ambient and site altitude, with G2 performance for a stated maximum load step, demonstrated on a factory or site load test. Ambient conditions matter because the reference conditions in the standard are milder than a Telangana summer; site derating should be confirmed from the manufacturer’s data and is explained in Ambient-Temperature Derating.
The evidence to request is the manufacturer’s data sheet stating the rating type, the transient performance data for the relevant load step, and test records. Datasheets we hold are listed in Datasheets & Brochures.
Common mistakes
- Comparing the standby kVA of one quotation with the prime kVA of another, which makes the smaller set look like better value when it is not.
- Writing rated to ISO 8528 in a tender without naming the rating type or performance class, which allows any bid to claim compliance.
- Using a standby-rated set as the main supply for long periods, which shortens engine life and may void the warranty.
- Ignoring site ambient and altitude when applying a rating, which overstates the power the set can actually deliver on a hot afternoon.
What to send us
Send the specification or tender clause that cites ISO 8528, the load list with the largest single load step, the expected operating hours per year and the site ambient and altitude. If you are comparing quotations, include the data sheets offered. We will reply through the Application Engineering Enquiry route with the rating type and performance class that suit the duty.
Questions engineers ask
Is ISO 8528 mandatory in India?
Not by itself. It becomes binding when a contract, tender or specification invokes it. Statutory requirements such as CPCB emission norms and CEA safety regulations apply independently.
What is the difference between standby and prime rating?
Emergency standby power is for use during utility failure for limited hours each year. Prime running power is for unlimited hours with a variable load whose average stays below a stated fraction of the rating. The prime figure for a given set is lower than its standby figure.
Which performance class should I specify for a commercial building?
G2 is usual for building services. Consider G3 if the set feeds sensitive electronic or UPS loads, and confirm the largest load step the set must accept within the class limits.
Does ISO 8528 cover emissions?
No. Emissions for diesel gensets in India are governed by CPCB norms, currently CPCB IV+ for new sets up to 800 kW. ISO 8528 deals with ratings, performance, testing and safety.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.