Commissioning best practices: Electrical

Commissioning electrical equipment ideally begins early in the design phase of the project and continues through the completion of functional performance testing.

By John Hebert, PE, LEED AP BD+C, and Catherine Melander, PE, LEED AP BD+C, October 17, 2012

Essential/standby power equipment and systems provide electrical power to a facility during a utility power outage or during a partial outage caused by tripping one or more circuit breakers in a facility. Several types of standby systems can be used. The term “essential power system” refers to the system that provides an alternate source of power if the normal (utility) source fails. To minimize confusion regarding the specific definitions of emergency power, standby systems, legally required standby power, and optional standby power, the term “essential power system” (EPS) has been used throughout this article to represent all of them.

Facilities that are not required to continue operating during a utility outage (office buildings, residential, etc.) often use battery backup lights, which do not require a generator. The battery backup lights are designed to provide 90 min of lighting to facilitate safe egress.

Facilities that support critical equipment/systems (hospitals, data centers, etc.) use generators and supporting electrical equipment to restore power to that critical equipment. Typical supporting equipment includes automatic transfer switches (ATSs), paralleling switchgear, power distribution equipment, and uninterruptible power supplies.

The equipment required to be connected to essential power systems depends on the type of facility. Most general office buildings only require egress lighting; thus the essential system is very simple. The National Electrical Code (NEC) includes specific requirements for equipment that can be connected to each essential system branch in a hospital (life safety, critical, and equipment). Periodic testing is required to verify the operation of these essential systems after occupancy.

Design review

Early in the design phase, the owner should prepare a design intent document (DID). The DID will quantitatively define the performance and operational requirements for the commissioned systems. These performance requirements will be the acceptance criteria against which the systems will be judged.

The commissioning process typically begins in the design phase of the project. The design review focuses on test requirements, sequences of operation, accessibility for maintenance and operations, and essential system equipment design. The intent of the review is to verify that the test requirements are clearly defined and that the system design is clearly documented. It should be noted that the “design review” process often extends into equipment submittal reviews because some critical information is not contained in the design documents. This is particularly true with more complex essential power systems because specific equipment configurations and settings as well as system sequences of operation are often not provided until this stage of the project.

Testing requirements are often specified to comply with the testing requirements included in the Standards for Acceptance Testing Specification published by the InterNational Electrical Testing Assn. (NETA). The design engineer often includes general references to NETA in the electrical specifications. This general reference can result in confusion regarding the intended testing level of rigor. Where NETA is referenced, additional details should be provided to clearly define the scope of testing. For example, testing every circuit breaker on a project site is usually not practical from a project budget and scheduling perspective. It may be more practical in most cases to specify testing for circuit breakers 400 amps or greater. Very critical circuit breakers may be injection tested down to 100 amps or even lower depending upon criticality. 

Specific electrical testing requirements should be communicated in the electrical specifications. Often, equipment suppliers are only given the electrical specifications and, thus, these specifications should either include the testing requirements or refer the suppliers to the specification section in which tests are defined. Factory acceptance testing may also be considered in situations where on-site discovery of major equipment deficiencies may carry unacceptable completion delays.

Design engineers typically define electrical testing requirements either in Part 3 of the individual specification sections or in a separate testing section. A dedicated electrical testing specification section provides a concise listing of all electrical testing requirements in a single location. Electrical testing requirements may be easier to communicate to members of the project team when a dedicated testing section is provided.

Alternatively, the commissioning professional may elect to clearly define the electrical testing requirements in the commissioning specification only. If this approach is taken, the commissioning specification and the electrical specification should be well coordinated and cross-referenced to prevent testing requirements from “slipping through the cracks.”

Functional performance testing

Functional performance testing of emergency power systems includes testing of generators, paralleling switchgear, and ATSs as well as related power distribution equipment including switchboards, panelboards, circuit breakers, metering, conductors, and any other equipment required for power distribution. A large portion of equipment testing is performed prior to energization. Typical pre-energization testing includes insulation resistance (megger), contact resistance (ductor), and overcurrent device current injection testing. Typical energized testing includes verifying the operation of metering and alarming, measuring voltage and harmonic content, confirming proper phasing, and final operational testing by verifying the sequence of operation. A variety of dynamic performance tests may also be performed including 100% block load testing, step load testing, and overload testing.

Prior to the start of functional testing, it is critical to clearly define equipment settings. Complicated essential systems use multiple pieces of equipment which require coordinated settings.

Electrical testing is typically defined at the equipment level and the systems level. The electrical commissioning professional is involved in the testing of the individual pieces of equipment (generator, ATS, etc.) and then the overall system. The complexity of essential system testing depends on the needs of the facility. The following illustrates the testing and acceptance criteria for three levels of complexity.

Level 1: Office building: egress only

The Level 1 scenario applies to office buildings and other similar occupancies that can be evacuated when a utility power outage occurs. The Level 1 essential system includes battery backup lighting to facilitate safe evacuation.

The functional testing for battery backup lighting is relatively simple. System testing includes simulating a power outage to verify that egress lighting is provided. The Level 1 testing scenario listed below assumes the system consists of battery backup emergency lighting.

  • Simulate a utility power outage by opening the circuit breaker serving the facility.
  • The battery backup emergency lights turn on and remain on for a minimum of 90 min.
  • The battery backup lights provide a light level of at least 1 foot candle along the egress route. Note: This test may need to be performed late in the evening, outside normal working hours, if the egress path includes any windows or other sources of daylight.
  • Restore power to the facility by closing the main circuit breaker feeding the facility.
  • Normal lighting is restored and the battery backup lights start recharging. 

Level 2: Fire stations: generator and ATS

The Level 2 scenario applies to small- and medium-sized facilities that will not be evacuated when a power outage occurs. Healthcare clinics, fire stations, municipal maintenance facilities, and ambulance garages are examples of facilities that would use Level 2 essential systems. Some hospitals use Level 2 systems.

The Level 2 essential system includes a generator and two ATSs. One transfer switch serves life safety equipment/systems including egress lighting and the fire alarm system. The other transfer switch serves non-life safety equipment/systems including computer equipment, overhead doors, HVAC systems, and other user defined loads.

Essential power system testing includes equipment testing and system testing. Equipment testing includes testing the generator, ATSs, and power distribution equipment individually. Typical generator equipment testing includes a 4-h full-load test, alarm verification, shutdown verification, remote annunciator verification, and overall system integrity. Typical ATS equipment testing includes verifying equipment settings, phase rotation, manual and automatic operation, and maintenance bypass operation (if applicable). Typical power distribution equipment testing includes insulation resistance, contact resistance, and overcurrent device current injection testing.

System testing includes simulating a power outage to verify the essential power system restores power to the essential loads. The Level 2 “live” testing scenario listed below assumes the system consists of one generator and two transfer switches.

  • Simulate a utility power outage by opening the circuit breaker serving the facility.
  • The ATSs will signal the generator to start.
  • When ATSs determine the power is “acceptable,” the ATSs will transfer to the alternate source (generator) and restore power to the essential loads.
  • Restore utility power to the facility by closing the main circuit breaker feeding the facility.
  • The ATS will start a retransfer delay timer.
  • Following the retransfer delay the ATS will transfer to the normal source (utility).
  • The generator will cool down and turn off.

Code requires power to be restored to the loads connected to the life safety system within 10 sec. of a utility outage. The power is not required to be restored to the non-life safety loads within a particular time period. For life safety loads it is important to verify that the loads see power restored within the 10-sec life safety requirement.

Level 3: Hospitals and data centers: generators, paralleling switchgear

The Level 3 scenario applies to medium/large mission critical facilities that will not be evacuated when a power outage occurs. Hospitals and data centers typically use Level 3 essential systems.

The Level 3 essential systems can include multiple generators, paralleling switchgear, multiple ATSs, and a fuel oil system. The design of the essential power system for healthcare is code-dependent, and the design for data centers is user defined. Level 3 systems can be designed using a wide range of strategies.

The Level 3 testing scenario listed below assumes the system consists of three generators, four transfer switches, paralleling switchgear, and one uninterruptible power supply.

  • Simulate a utility power outage by opening the circuit breaker serving the facility.
  • The ATSs or paralleling switchgear will signal the generator to start.
  • The paralleling switchgear will connect the first generator to the bus.
  • The remaining generators will synchronize and connect to the bus, thus operating in parallel.
  • When each individual ATS determines that the power is “acceptable” and after the appropriate time delay, each ATS will transfer to the alternate source (generators) and restore power to the essential loads. The ATS transfers are typically staggered so the entire load is not transferred onto the generator supply at the same time.
  • Restore utility power to the facility by closing the main circuit breaker feeding the facility.
  • The ATSs or paralleling switchgear will start a retransfer delay timer.
  • Following the retransfer delay, the ATSs will transfer to the normal source (utility).
  • Once all loads are transferred to the normal source, the generators will run unloaded (cool down) for a predetermined time delay and will turn off.
  • Power to the equipment connected to the uninterruptible power supply has not been interrupted during any of the transfers.

Integrated systems test

An integrated systems test, sometimes called a “blackout” or “loss of power response” test, is often conducted near the end of the construction process. The test should be conducted after all other functional testing has been completed, and ideally after the occupant’s systems are installed and operational. This electrically initiated test is a facility-wide test and an excellent way to help confirm that all facility systems will operate correctly during a utility or partial power outage.

Throughout the project, the commissioning professional needs to engage with the project team at key times to facilitate the successful execution of the commissioning process. A brainstorming session with key operations, design, and commissioning team members is often worthwhile to help confirm that no reasonable failure modes are left unanticipated and uncommissioned. The functional testing for EPS is critical to ensure the systems are functioning as designed. Following the successful completion of functional performance testing, the commissioning professional’s involvement in the project is nearly complete. Often the commissioning professional is also asked to oversee and confirm that the owner is adequately trained to coordinate ongoing routine maintenance and testing.

Maintenance testing

Power distribution maintenance testing is generally conducted annually. Testing typically includes visual inspections and thermal imaging. Subjecting the equipment to the highest anticipated load during thermal imaging is ideal, but at minimum, thermal imaging should not be conducted until the power distribution equipment is subjected to the typical facility load.

The equipment associated with the EPS should be tested in accordance with the manufacturer’s instructions, and the operation should be verified regularly. Many essential power systems are tested monthly by individually operating each ATS using the test switch. The frequency of testing depends upon the type of facility. In critical facilities, this needs to be carefully scripted. Some facilities test the generator(s) under full load, using load banks, every year or two.

John Hebert is senior electrical engineer and has 15 years of engineering experience, including project management and technical experience commissioning electrical systems including medium- and low-voltage main power, emergency power, mission critical power, voice/data, fire alarm, and building automation and control systems. Catherine Melander is senior electrical engineer and has 30 years of engineering and project management experience. She has spent the past 12 years commissioning electrical systems, including incoming medium- and low-voltage power, emergency power, mission critical power, low-power, and building-wide “loss of power response” testing.


Several of the technical terms included in the body of the article are described below.

Automatic transfer switch (ATS): An ATS automatically starts the generator when utility power is lost, transfers the load to generator, and returns the load to utility power when stable utility power has returned.

Load bank: A load bank is used to simulate building load on a generator. During functional testing, adequate building load is not available to load the generator to 100%. A resistive (versus reactive) load bank is generally acceptable.

Paralleling switchgear: Power distribution equipment that connects two or more generators. Connecting multiple generators provides increased flexibility, redundancy, and reliability.

Insulation resistance test: A dc voltage is applied phase-to-phase and phase-to-ground to verify the integrity of the insulation or separation between conductors and bus.

Contact resistance test: A low current is applied through switches and circuit breaker contacts to verify the resistance is consistent through the contacts.

Primary current injection test: Current is applied through circuit breakers to verify that the circuit breakers trip according to the published trip curves. Primary injection testing verifies that the entire circuit is operational, not just the electronics.

Re-commissioning: After initial commissioning, a schedule for re-commissioning should be considered and adopted. These procedures may, and often are, somewhat smaller in scope than initial commissioning.