What Is a Load Bank Test?

Monthly off-load test runs show that a generator runs, but not that it can carry the building load. A set that has been tested off-load for years may, when load is applied during a real outage, produce dark smoke, lose frequency and shut down on a high temperature alarm.

A load bank test is carried out to verify whether the generator can carry real load. A load bank is a device that applies a controlled, adjustable artificial load to the generator. During the test the facility’s real loads are not put at risk; the generator is run and measured at the required load for the required time.

Why is it done?

• Commissioning and acceptance: To show that the set really delivers its nameplate rating on site.
• Periodic verification: For facilities where the building load is too low to apply a meaningful load in the monthly tests.
• Clearing wet stacking: Unburned fuel and soot build up in an engine that runs at low load for long periods. Running at a controlled high load helps burn off this build-up. See the generator maintenance checklist.
• After repairs: To see that the set can take load safely after a major overhaul or parts replacement.
• System tests: Load sharing and loss-of-set scenarios in parallel systems; see synchronized (parallel) operation.

Types of load bank

Type	What does it draw?	What does it test?
Resistive	kW only (power factor 1)	Engine, fuel and cooling system
Resistive-reactive (RL)	kW + kVAr (usually 0.8 power factor)	Engine + alternator, AVR, reactive sharing in parallel operation
Capacitive	Leading reactive power	Special applications, e.g. simulating UPS and filter behaviour

A resistive load bank is more common and more practical. But the alternator is not tested at its rated current: a 500 kVA set on a resistive bank is at full engine load at 400 kW, while the alternator carries only about 80% of its rated current at 0.8 power factor. If the alternator and AVR are also to be fully tested, an RL load bank is used.

Load banks can be portable (trailer-mounted or containerised) or permanently installed at the facility. A permanent load bank is preferred in critical facilities that need frequent testing.

A typical test programme

The programme below is an example; the actual steps and durations are set by the project specification, the manufacturer’s recommendations and the standard applied.

Step	Load	Duration (example)	What is monitored?
1	Off-load warm-up	5–10 min	Oil pressure, temperatures, leaks
2	25%	30 min	Voltage, frequency, current balance
3	50%	30 min	Same values + exhaust temperature
4	75%	30–60 min	Temperatures settling
5	100%	60 min or more	Cooling adequacy, fuel consumption
6	Load steps	Short	Voltage and frequency dip on sudden load, recovery time
7	Cool-down	5 min	Off-load running and shutdown

How far voltage and frequency dip on sudden load steps and how quickly they recover can be assessed against the performance classes (G1–G4) defined in ISO 8528-5. Which class is targeted is set in the project.

Which values are recorded?

• Voltage and current for each phase, frequency
• kW, kVAr and power factor
• Oil pressure, coolant temperature, exhaust temperature
• Fuel consumption (if it can be measured)
• Voltage/frequency dip and recovery time on load steps
• Behaviour of alarm and protection functions

Values are recorded at set intervals throughout the steps. The records are kept so that the next test can be compared against them.

Common mistakes on site

• The test is done off-load. An engine that runs is not necessarily an engine that can carry load.
• Hot air from the load bank comes back into the room. A load bank turns almost all the power it draws into heat. A 400 kW test means 400 kW of heat. The exhaust air must be directed to open space.
• The cables are undersized. Temporary test cables must be selected for the rated current, and connections must be tight and protected.
• Facility load and test load get mixed up. If the building load and the load bank are used together, the total load must be monitored.
• Fuel is not calculated. A full-load test of a few hours can use up a large part of the base tank; see generator fuel tank sizing.

Safety and preparation checklist

• Are the test programme, steps and acceptance criteria written down?
• Are the load bank rating and type (resistive / RL) suitable for the test?
• Have the connection cables and protective devices been selected for the rated current?
• Is the load bank’s hot air outlet facing a safe direction?
• Is there enough fuel?
• How will the facility’s critical loads be supplied during the test?
• Is the test being carried out by authorised, experienced personnel?
• Are the measurement records being kept and stored?

Next step

At Terex Diesel, every set goes through a full-load test before shipment; you can see the production and test steps on our how a generator is made page. For periodic testing and maintenance of your generator on site, see our scheduled maintenance and generator service pages, or contact us.