Emergency generator test runs — load-bank testing and "wet stacking"

The emergency generators guide covers the mutual exclusion of mains and generator and star-point earthing during parallel operation in accordance with §551. A separate, practical point of attention is how a generator must be tested periodically to remain genuinely deployable during an actual mains failure.

The problem: "wet stacking"

A diesel engine that regularly runs unloaded or lightly loaded (for example, a short weekly test run without any connected load) does not reach sufficient cylinder temperature for complete combustion. Unburned fuel then ends up in the exhaust system — a phenomenon known as "wet stacking", recognisable by a black, oily deposit around the exhaust manifold, the piping and the turbo. Over time this damages the engine and exhaust system and reduces the reliability of the generator precisely at the moment it is needed.

Rule of thumb: a diesel engine should not run for extended periods below around 30% of nominal power (the lower bound most often cited by engine manufacturers to prevent wet stacking); for genuinely complete combustion and warm-up to operating temperature, around 60% is often used as a practical target. The exact percentage varies by engine manufacturer.

Load-bank testing as a solution

A load-bank test artificially loads the generator using an adjustable, resistance-based load bank up to (well) above this threshold, independent of the actual operational load at that moment. This:

• Prevents wet stacking by actually running the engine at operating temperature.
• Verifies the actual nominal power of the generator under realistic load.
• Exposes hidden defects that do not come to light during an unloaded test run.

Note: the required test frequency and duration are usually laid down in the maintenance contract or the organisation's internal procedure — this article provides the technical principle, not a universal test schedule.

Practical relevance

For a horticultural business or commercial building with an emergency generator as backup for climate installations (see also the UPS guide for the distinction between short-term UPS bridging and longer-term generator deployment), a regular, correctly loaded test run is essential: a generator that is only tested unloaded can still fail during an actual outage due to a dormant wet-stacking problem that the regular test never revealed.

Common mistakes

1. Only test-running the generator unloaded or lightly loaded — this tests whether the engine starts, but not whether it functions reliably under actual operational load.
2. Ignoring wet-stacking symptoms (black deposits at the exhaust manifold) until a failure occurs — this is an early, visible warning sign.
3. Not including a periodic load-bank test in the maintenance contract, meaning the generator's actual load capacity is never verified under realistic conditions.

Related

• Emergency generators (§551)
• UPS & uninterruptible power supply