The load test almost nobody runs
A generator started off-load every month proves that it starts. It does not prove that it will carry the site's load, nor that the transfer will happen without anything stopping.
In most installations the monthly test of a standby generator consists of starting it and letting it run for a quarter of an hour. The engine fires, the lamp turns green, the maintenance sheet is signed. That procedure verifies starting. It verifies nothing else.
Three faults an off-load test does not reveal
Load acceptance. A generator starts without difficulty off-load and may still fail to absorb the step load at the moment the transfer switch throws the site across. Behaviour in the transient regime depends on the nature of the loads — motors, compressors, switched-mode supplies — and does not follow from the rated output.
Wet stacking. A diesel engine run for long periods below thirty per cent of its rated load fouls. Incomplete combustion deposits unburnt fuel in the cylinders and the exhaust. The monthly practice meant to maintain the set therefore degrades it, slowly, and the first real call comes on a machine in worse condition than the day it was accepted.
The complete chain. What protects the site is not the generator: it is the whole formed by the mains failure detector, the transfer switch, the set, the fuel circuit and the UPS units that cover the starting interval. A test bearing on one of those elements leaves the others unverified.
What a load test establishes
A load test consists of cutting the public supply upstream, letting the installation transfer on its own, and holding the site on the generator for a meaningful period. It produces four findings.
The transfer happened within the expected time, and the equipment covered by UPS crossed the interval without restarting. The set carried the real load, measured rather than estimated. The observed consumption allows the stated autonomy to be checked against the fuel actually present. And nothing stopped that had not been identified as due to stop.
That last point is the one that surprises. A load overlooked during the original wiring, a service outlet fed from the wrong distribution board, a server added two years after commissioning onto an unprotected circuit: these gaps exist in most installations, and only a load test reveals them before the incident does.
The load bank
Where the site cannot be interrupted, the test is run with a resistive load bank connected to the generator. It simulates the demand without touching operations. It is a routine service, and its cost bears no comparison with that of a failure discovered on the day the mains goes.
The test also brings the engine up to temperature and load, which reverses part of the fouling accumulated by monthly off-load runs.
What to require
A load test at acceptance, with the transfer time and the demand recorded. Its annual repetition, on real load or on a load bank. A consumption reading allowing autonomy to be recalculated. And, after every test, an update of the schedule of protected circuits: it is the document that ages fastest and that nobody keeps.
