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Standard for Emergency and Standby Power Systems

The NFPA 110 standard covers performance requirements for emergency and standby power systems providing an alternate source of electrical power in buildings and facilities in the event that the normal electrical power source fails.

Systems include power sources, transfer equipment, controls, supervisory equipment, and accessory equipment needed to supply electrical power to the selected circuits.

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Chapter 8 of NFPA 110 contains requirements for performing maintenance on Emergency Power Supply Systems (EPSSs). Planned maintenance is a critical part of any backup power system. Regular verification that an installed system is functional is necessary to ensure that standby power will be available when called upon at a loss of primary power.

All components of the system, from fuel quality up to the operation of the transfer switch, are addressed in this chapter because each relies on the other. These operational testing requirements are the minimum benchmarks for proper operation of the EPSS.

NFPA 110 requires that the manufacturer’s maintenance recommendations be followed along with the instruction manuals, the minimum requirements of this chapter, and any additional requirements of the authority having jurisdiction (AHJ).  (8.1.1) Random inspection and testing are not a basis for maintaining the dependability of an EPSS. The continued reliability of the EPSS is dependent on an established program of routine maintenance and operational testing.

It also requires that consideration be given to temporarily providing a portable generator or alternate source of emergency power whenever the emergency generator is out of service or during routine testing. (8.1.2).

Generator maintenance
Technician setting up remote generator monitoring


At least two sets of instruction manuals for all major components of the EPSS shall be supplied by the manufacturer. These manuals should contain detailed explanations of the operations and maintenance tasks, an illustrated parts list with part numbers, and schematic diagrams of electrical wiring systems (one-line drawings), including operating and safety devices, control panels, instrumentation, and annunciators. (8.2.1)

For Level 1 systems, instruction manuals shall be kept in a secure, convenient location, one set near the equipment, and the other set in a separate location. (8.2.2)

Special tools and testing devices necessary for routine maintenance shall be available when needed. (8.2.3)

Spare parts as recommended by the manufacturer or by experience should be stocked and maintained in a secure location on the premises. (8.2.4)

It is recommended that the instruction manuals, special tools, and spare parts be stored in a metal cabinet in the generator room. The instruction manual should be stored on the inside of the cabinet door.


The EPSS shall be maintained to ensure to a reasonable degree that the system is capable of supplying service within the time specified for the type and for the time duration specified for the class. (8.3.1). It is suggested that the manufacturer’s maintenance recommendations, procedures, and frequency be followed.  However, NFPA 110 outlines an alternate maintenance program if no manufacturer guidelines are available. Figure A.8.3.1(a) and Figure A8.3.1(b) provide alternate suggested procedures.

A routine maintenance and operational testing program shall be initiated immediately after the EPSS has passed acceptance tests or after completion of repairs that impact the operational reliability of the system. (8.3.2). A written schedule for routine maintenance and operational testing of the EPSS shall be established. (8.3.3)

When components of an EPSS are repaired or replaced, an operational test is necessary to verify the proper operation of the system. 

The operational test shall be initiated at an ATS and shall include testing of each EPSS component on which maintenance or repair has been performed, including the transfer of each automatic and manual transfer switch to the alternate power source, for a period of not less than 30 minutes under operating temperature. (

8.3.4 – Transfer switches shall be subjected to a maintenance and testing program that includes all of the following operations:

  1. Checking of connections
  2. Inspection or testing for evidence of overheating and excessive contact erosion
  3. Removal of dust and dirt
  4. Replacement of contacts when required

8.3.5 – Paralleling gear shall be subject to an inspection, testing, and maintenance program that includes all of the following operations:

  1. Checking connections
  2. Inspecting or testing for evidence of overheating and excessive contact erosion
  3. Removing dust and dirt
  4. Replacing contacts when required
  5. Verifying that the system controls will operate as intended

8.3.6 – Storage batteries, including electrolyte levels or battery voltage, used in connection with systems shall be inspected weekly and maintained in full compliance with manufacturer’s specifications. Note: A major cause of problems when starting generator sets is the lack of battery maintenance.

  • Maintenance of lead-acid batteries shall include the monthly testing and recording of electrolyte specific gravity.
  • Battery conductance testing shall be permitted in lieu of the testing of specific gravity when applicable or warranted.
  • Defective batteries shall be replaced immediately upon discovery of defects.

8.3.7 –  A fuel quality test shall be performed at least annually using appropriate ASTM standards. Note: Fuel quality and fuel management are concerns since many generator failures are attributed to poor fuel quality, contamination, and other fuel system problems. While NFPA 110 does not specify a specific ASTM standard for the fuel quality test, ASTM D 975 – Standard Specification for Diesel Fuel Oils, contains test methods for existing diesel fuel.  Your fuel supplier or generator service provider can also provide test kits or fuel sampling to comply with this requirement.


NFPA 110 requires that the EPSS, including all appurtenant components (i.e. generator sets, transfer switches, circuit breakers, fuel systems, and other equipment) be inspected weekly and exercised under load at least monthly. (8.4.1)


A weekly inspection procedure might be provided for a specific generator. In lieu of that, there are weekly inspection items for EPSS installations identified in Figure A.8.3.1(a). There are also inspection items for monthly, quarterly, semi-annual, or annual intervals.


Diesel generator sets in service shall be exercised at least once monthly, for a minimum of 30 minutes, using one of the following methods:

  1. Loading that maintains the minimum exhaust gas temperatures as recommended by the manufacturer Note: there is no load percentage required for this test. Rather, the load needs to be sufficient to achieve the exhaust temperature recommended by the manufacturer.
  2. Under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating. Note: The minimum load is intended to limit the possibility of wet-stacking (incomplete combustion), which may occur when a diesel generator is lightly loaded.

Diesel-powered EPS installations that do not meet the minimum requirements listed above shall be exercised monthly with the available EPSS load and shall be exercised annually with supplemental loads (load bank testing) at not less than 50 percent of the EPS nameplate kW rating for 30 continuous minutes, and at not less than 75 percent of the EPS nameplate kW rating for 1 continuous hour for a total test duration of not less than 1.5 continuous hours.

Spark-ignited (gaseous fueled) generator sets shall be exercised at least once a month with the available EPSS load for 30 minutes or until the water temperature and the oil pressure have stabilized. Note: Spark-ignited (e.g. natural gas or propane) generators are not subject to the minimum load requirements of diesel generators and can be tested with available loads for 30 minutes or until the water temperature and oil pressure have stabilized.

The EPS test shall be initiated by simulating a power outage using the test switch(es) on the ATSs or by opening a normal breaker. Opening a normal breaker shall not be required. (8.4.3) Where multiple ATSs are used as part of an EPSS, the monthly test initiating ATSs shall be rotated to verify the starting function on each ATS. Note: Rotating the ATS that initiates the cold engine start of the monthly test will verify the operation of each transfer switch. Consideration should be given to ATS criticality.

Load tests of generator sets shall include complete cold starts. (8.4.4) Time delays shall be set as follows: (8.4.5)

  1. Time delay on start:
    1. 1 second minimum
    2. 0.5 second minimum for gas turbine units
  2. Time delay on transfer to emergency: no minimum required
  3. Time delay on restoration to normal: 5 minutes minimum
  4. Time delay on shutdown: 5 minutes minimum

Transfer switches shall be operated monthly. (8.4.6) The monthly test of a transfer switch shall consist of electrically operating the transfer switch from the primary position to the alternate position and then a return to the primary position. Note: Although it is not required to verify the type rating of the system during the monthly test, it is required to annually confirm that the system responds within the time dictated by its rating.

EPSS circuit breakers for Level 1 system usage, including main and feed breakers between the EPS and the transfer switch load terminals, shall be exercised annually with the EPS in the “off” position. (8.4.7) Circuit breakers rated in excess of 600 volts for Level 1 system usage shall be exercised every 6 months and shall be tested under simulated overload conditions every 2 years. Note: It is not intended that the circuit breakers be exercised under load.

EPSS components shall be maintained and tested by qualified person(s). (8.4.8)


Level 1 EPSS shall be tested at least once within every 36 months. (8.4.9) This requirement applies to all generator types if they supply a Level 1 EPSS. The intent is to ensure that the EPSS with all its auxiliary systems is capable of running with load for the duration of its assigned class. It is not intended that a full facility power outage be conducted as part of these tests. This 36-month test is not a requirement for a Level 2 EPSS.

Level 1 EPSSs are tested for the duration of their assigned Class if less than 4 hours. Where the assigned Class is greater than 4hours, it shall be permitted to terminate the test after 4 continuous hours. (

The test shall be initiated by operating at least one transfer switch test function and then by operating the test function of all remaining ATSs, or initiated by opening all switches or breakers supplying normal power to all ATSs that are part of the EPSS being tested. ( A power interruption to non-EPSS loads shall not be required. (

The minimum load for this test shall be as follows: (

  1. For a diesel-powered EPS, loading shall be not less than 30 percent of the nameplate kW rating of the EPS or that which maintains the minimum exhaust gas temperatures as recommended by the manufacturer. A supplemental load bank shall be permitted to be used to meet or exceed the 30 percent requirement.
  2. For spark-ignited EPSs, loading shall be the available EPSS load.

The 36 Month Test shall be permitted to be combined with one of the Monthly Tests and one of the Annual Tests as a single test. ( Where the test is combined with the annual load bank test, the first 3 hours shall be at not less than the minimum loading required by (30 percent for diesel EPSs) and the remaining hour shall be at not less than 75 percent of the nameplate kW rating of the EPS.


Records shall be created and maintained for all EPSS inspections, operational tests, exercising, repairs, and modifications. (8.5.1)

The record shall include the following: (8.5.3)

  1. The date of the maintenance report
  2. Identification of the servicing personnel
  3. Notation of any unsatisfactory condition and the corrective action taken, including parts replaced
  4. Testing of any repair in the time recommended by the manufacturer

This new section was created in the 2016 edition to consolidate record. No retention time is specified to reflect actual operating conditions. However, the retention period may be defined by the facility management or the authority having jurisdiction (AHJ) and must be made available to the AHJ on request.

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