In the Know

Load Bank Testing | Part 2 | Key Reason for Load Testing

The key reason for load testing is that many generator sets seldom run under full load conditions after the manufacturer’s factory testing. Though tested in compliance with the regulatory requirements that permit use of actual loads, over time, this practice can lead to conditions that affect performance and reliability. Modern diesel generator sets designed to meet the stringent EPA Tier Level emission standards are designed to be operated at loads higher than 50% for optimum life and performance. In addition, the use of after-treatment particulate matter filters (DPF) that depend on exhaust temperatures for regeneration to occur can be compromised by low load operation and restrict exhaust gas-flow due to build up, causing higher than recommended exhaust back-pressure and limiting the performance of the reciprocating engine-generator set and/or increased unscheduled maintenance activities.

When multiple units are installed they are often run individually for periodic and annual testing using a test load much less than the manufacturer’s recommended levels. The use of a large capacity resistive reactive load bank can allow testing of multiple units simultaneously thus reducing the time it takes to perform and document the mandatory testing, and additionally, exercise the paralleling controls under the most realistic conditions possible.

Load bank testing is a critical component to meeting regulatory requirements. Today’s diesel engine powered generator sets utilizing electronic engines and emission controls to meet the current and future EPA emission requirements depend on the engines operating at the manufacturer’s recommended load levels and temperatures.

Detailed review of the NFPA testing guidelines refer to minimum load levels of 30%, or as recommended by the manufacturer. Industry associations such as EGSA and the major engine-generator manufacturer’s recommend load testing at higher levels to insure that the full benefits of load testing can be achieved.

Like regular maintenance, periodic testing is required by code in all healthcare applications to maintain compliance with the regulatory agency. It is common for healthcare facilities to perform regular generator set testing during off-peak times when loads are at their lowest. While this practice prevents the possibility of serious interruptions to large and/or critical loads, it does not adequately test the generator set under worst-case conditions.

Periodic weekly or monthly testing is often accomplished using loads much lower than recommended by the manufacturer’s which can lead to equipment not meeting the performance or reliability required for a given facility.

Reactive Load Bank Testing

The ability to simulate more realistic reactive type loads is THE key benefit for a load bank that provides both varying kVA (resistive) and kVAR (reactive) loads. There are critical differences between testing with a resistive only load bank vs. a resistive reactive load bank including:

Resistive Load Bank Testing [KW = KVA @ unity power factor (COS F= 1.0)] 

  • Tests the prime mover (engine) at 100% load 
  • Tests the fuel delivery system operation at maximum rating and fuel consumption 
  • Demonstrates the cooling system operation at the generator set’s full operating capacity 
  • Allows the exhaust and after treatment system to reach normal operating temperatures 
  • Eliminates exhaust wet-stacking by burning off built-up fuel and oil carbon deposits and reseating the rings when part load or low load conditions are encountered during periodic testing 
  • Evaporates moisture from the engine oil that reduces wear causing acid formation 
  • Identifies deficiencies than can be corrected with proper maintenance and repair before failure, avoiding downtime and additional expenses.

Resistive and Reactive Load Bank Testing [Reactive Power Component (COS F = variable )] 

  • Tests the alternator and voltage regulator at its full rated (KVA/KVar) capacity 
  • Simulates the actual load (KW, KVA, KVAR) that the systems are specified and designed for 
  • Simulates transient loads to provide voltage and frequency response characteristics 
  • Tests are used to simulate and verify synchronizing, load sharing and voltage regulation on multiple unit paralleled systems under actual load conditions 
  • Allows thermographic/infrared inspection of the electrical systems and identification of potential hot spots and the condition of cables, terminations and buss work. 

As shown in Table A above, a resistive only load bank can provide adequate testing of the individual prime mover and load sharing (including load add/load shed) controls of a multiple unit facility. A reactive load bank allows testing of the alternator, load sharing and transient responses applying loads more in line with actual loads normally seen in operation. All engine governors respond to loads with a reduction in engine speed.



Figure A compares the transient response for a large diesel standby generator set when applying a block load using a resistive only and a resistive-reactive load bank. The resulting initial synchronous voltage dip (Vdip1) using the 75% load at 0.80 power factor results in an voltage dip approximately 25% greater when compared to the equivalent resistive only load applications. The engine speed related voltage dip (Vdip2) is similar, in both cases, due to the manufacturer’s standard volts per hertz type voltage regulator (data courtesy of Caterpillar Engine Sizer Program).

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