Generator step-load acceptance — performance classes ISO 8528-5

The guide on load-bank testing covers why a diesel generator set must periodically run under sufficient load to avoid "wet stacking". This guide covers a different, equally practical aspect of the same generator set: how much load it can accept in a single step without the engine stalling or the voltage/frequency temporarily falling outside permissible limits.

Why a generator set cannot simply accept the full load in one step

A diesel generator switched from no load (or light load) to a large load step experiences a sudden demand for mechanical power from the engine. The governor and fuel delivery have a finite response time to restore engine speed — and thereby mains frequency. During that transition period the frequency temporarily dips (frequency dip), and voltage often dips as well, because the generator's excitation also needs time to re-regulate. With too large a load step at once, the dip can become deep enough for the engine to stall, or for connected equipment (particularly sensitive electronics or motors with under-speed protection) to fall outside tolerance or even trip.

The performance classes G1-G4 of ISO 8528-5

ISO 8528-5 defines four performance classes for generating sets, which among other things determine how large an allowable frequency and voltage deviation may be during a load step, and how quickly the set must return within tolerance:

G1 — basic quality, intended for applications where voltage/frequency may deviate substantially during a load step (for example simple resistive loads such as lighting or heating, without sensitive electronics).
G2 — the deviation during a load step is more limited and recovery faster than G1; suitable for most general applications where normal mains quality is expected.
G3 — even stricter frequency and voltage deviation limits; applied where equipment is more sensitive to transient deviations (for example certain data centres or medical environments without separate UPS buffering at every level).
G4 — the strictest class, with the smallest permissible deviation and the fastest recovery; intended for the most critical, power- electronics-sensitive loads where even a brief, small frequency or voltage deviation can cause problems.

The higher the class, the more heavily dimensioned (and generally more expensive) the engine, governor, and excitation control must be to absorb the same load step with less deviation.

Practical consequence: sequential (staged) load restoration

Because a generator set has a limited capacity to absorb a single large load step at once, larger installations are often designed with sequential (staged) load restoration: after transfer by the automatic transfer switch, not all loads are reconnected to the generator simultaneously, but in a predetermined order with a short time delay between each step (for example critical IT load first, then cooling, then remaining load), so that each individual step stays within the generator's load-acceptance limit. This is particularly relevant for large motor loads (such as cooling compressors), whose starting current can be a multiple of the rated current and therefore constitutes a disproportionately large load step for the generator set.

Relationship to generator set sizing

The rated (continuous) power capacity of a generator set is not the same criterion as its load-acceptance capacity: a set may be more than sufficient in continuous power terms for the total load, yet still be unable to absorb the single largest load step without excessive frequency/voltage dip if the step size is relatively large compared to the set's rating. When selecting a generator set, the largest single load step that can occur (particularly with motor loads) and the associated required performance class must therefore be considered explicitly, alongside the total power.

Typical mistakes

Sizing a generator set solely on total kVA rating without accounting for the largest single load step — the set may have sufficient continuous power yet still stall or fall outside tolerance when a large motor load is switched back on in a single step.
Restoring all load in a single step after ATS transfer, instead of applying a staged sequence — especially risky with multiple large motors starting simultaneously.
Not matching the required performance class to load sensitivity — connecting critical electronics to a G1-rated set can cause repeated trips during every load step, even though the set otherwise functions correctly.
Not verifying manufacturer load-acceptance data when replacing a generator set with a different (smaller or different-brand) model — an apparently equivalent kVA rating does not guarantee the same step-load capacity.