Nov 2015

Testing and Inspection of Emergency Generators

Emergency generators are used to maintain electrical power to facilities and equipment considered important and/or critical to continued plant operations. They may be found in occupancies such as data processing, molten materials, communications switching, medical support systems, refrigerated storage and many others. The use of emergency power generators can allow for continuous uninterrupted operations or, if there is insufficient power or no reason to continue full operations, they may allow for an orderly shutdown of most critical systems.

Emergency generators are also commonly required and used to address life safety concerns (hospitals, emergency lighting, fire alarms, exhaust and/or pressurization fans, etc.) associated with a loss of power. NFPA 110 (2016 ed.), Standard for Emergency and Standby Power Systems, defines Level 1 generator installations as those where failure of the equipment powered by the generator could result in loss of human life or injuries. Level 1 installations generally receive a higher frequency of maintenance than Level 2 installations, which is all other generators.

As diesel drive generators are the most commonly found type of generator, this article specifically discusses the diesel engine driven generator.

A new revision of NFPA 110, Standard for Emergency and Standby Power Systems, has been published for 2016. Chapter 8, entitled Routine Maintenance and Operations Testing, provides information on what should be tested as well as the frequency. Maintenance and testing are recommended in accordance with four items: 1) Manufacturer’s Recommendations; 2) Instruction Manuals; 3) Minimum Requirements of NFPA 110; and 4) the Authority Having Jurisdiction (AHJ). Although not expressly stated, one may safely assume that the most stringent of these choices will prevail. Frequencies in Chapter 8 and the Appendix information include Weekly, Monthly, Quarterly, Semi-Annual and Annual, with a few exceptions.

From a practical standpoint, the number of frequencies and different items on each frequency is not truly practical for most users. A better suggestion would be that the few quarterly and semi-annual inspection items be completed on the next high frequency (e.g., Monthly), rather than performing separate inspections for these few items.

Inspection and Testing is broken into three component parts as follows:

DIESEL ENGINE (Or other motive driver, such as propane/natural gas engine, steam turbine, gas turbine, etc.)

NFPA 110 (Figure A.8.3.1[a]) provides a checklist detailing the inspection and testing of engine components into four categories:

  • Fuel system
  • Lubrication system
  • Cooling system
  • Exhaust system

The checklist then includes many items in each category. Rather than re-iterate the items included in the checklist, a synopsis below covers the general inspections necessary to be in compliance:

  • Weekly visual inspection to ensure all components appear satisfactory, that the engine compartment is warm and dry, block heater is on as needed, no evident fuel leaks, etc. These functions can be performed by experienced maintenance personnel.
  • Monthly operation of the diesel driver with a minimum 30% generator load. Check all fluids, gages and components prior to, during and after testing. These functions can be performed by experienced maintenance personnel.
  • Annually check and change engine fluids and service to maintain performance specifications. This servicing would typically be performed by the engine manufacturer’s representative.

ELECTRICAL SYTEM (Electrical wiring and components other than the generator. This includes switches, breakers, panel boards, electrical boxes, UPS systems, etc)

Below is a list of items and frequencies for those items to be checked. A further synopsis covers the general inspections to be in compliance.

  • Monthly General Inspection for Level 2 (Weekly Level 1).
  • Monthly operation under load. Minimum load of 30% nameplate rated kW for 30 minutes at engine temperature; initiate by simulating an outage on Automatic Transfer Switch (ATS).
  • Monthly measurement of storage battery electrolyte specific gravity (generally a storage battery system is used in the UPS system, if provided).
  • Monthly ATS operation.
  • Semi-annual operation of safeties and alarms.
  • Annually tighten control and power wiring connections and clean.
  • Annual functional testing of circuit breakers, switches, and relays.
  • Annual calibration of voltage sensing relays/devices.
  • Five years or 500 hours for Level 2, test for wire insulation breakdown (3/500 Level 1).

Monthly run test under minimum 30% generator load. In most cases, the actual building load which the generator is to support should generally meet this requirement. This would be concurrent with the diesel driver test under load as indicated above. Check operation of the ATS, safeties, and alarms during testing. These functions can be performed by experienced maintenance personnel.

Annually clean, torque, tighten, calibrate, and operationally test electrical connections and operable electrical components. This servicing would typically be performed by a qualified electrician.

Annual infrared thermographic imaging of electrical systems associated with the emergency power systems. This is not an item included in the NFPA 110, but which in our opinion should be included. Ideally, annual infrared imaging should be performed during one of the monthly tests at the minimum 30% load, or under full load during an annual load bank test.

GENERATOR (generally an integrated unit; exciter, stator, windings, brushes, etc.)

Per NFPA 101, an annual electro/mechanical servicing is recommended. The sole exception is that brushes should be checked and cleaned semi-annually. This servicing would typically be performed by a qualified manufacturer’s representative.


An exercise timer is a feature available on most emergency generators. It automatically starts the unit for a timed test run at predetermined intervals, generally weekly or monthly. NFPA 110 is silent on the use of auto-exercising a generator; however, as with any important equipment, Risk Logic Inc. strongly urges that the equipment be well supervised during operation. Considering modern sensor technology, this does not necessarily mean that someone has to be in front of it during testing, but it is possible to monitor operations from a nearby, constantly attended location, provided that appropriate alarms are installed and supervised.

Load Bank Testing is tangentially addressed in NFPA 110, as a possible means to achieve the minimum 30% load during the monthly functional test or as an annual alternative to being able to achieve the 30% load during monthly testing (NFPA 110, C8.4.2.3). While not required by NFPA 110, annual load bank testing is highly recommended. During Load Bank Testing, an artificial load in the form of a portable resistive load bank is applied to the generator in a series of increasing steps up to the nameplate capacity of the generator. Performance results for both the generator and the diesel engine driver are recorded over the 0-100% range of the generator.

Equipment failure most often occurs at or near the manufacturer’s peak ratings (as opposed to under minimal, or partial loads), and load bank testing is able to provide data showing performance over the 0-100% range of the generator and the engine driver, which should be compared to manufacturer’s specifications to determine the relative health of the units. The need for, and importance of, load bank testing data increases proportionally with the need for, and importance of, the generator.

Risk Logic Inc. can help with managing the maintenance and testing of your generator(s).


NFPA 110 (2016 ed.) Standard for Emergency and Standby Power Systems (available online at for viewing only, or purchase as download)

FMDS 5-20 (2007) Electrical Testing (available online for download at

Plant Engineering (Aug 3 2010) close Look at Wet Stacking, by Tom Divine, PE {available online }

Electrical Construction & Maintenance (Apr 3, 2014) Testing Generators with a Portable Load Bank, by Dan Chisholm Sr. (available online Emergency Power Supply Systems)