Electrical reliability is crucial for data center operations performance, and effective acceptance testing and proper maintenance testing of the electrical systems can help achieve this goal. This strategy will help avoid unplanned outages to electrical supply and backup systems. To help facilities develop a plan, the InterNational Electrical Testing Association (NETA) has developed consensus standards for testing devices and systems that include suggested planning tables and describe the specific tests needed. These recommended test protocols are published in the ANSI/NETA Standard for Acceptance Testing Specifications (ATS) and Maintenance Testing Specifications (MTS). These offer key insights into the hows and whys of electrical testing. These two testing guides are similar in the scope of preventative and predictive testing protocols used, but differ in focus, analysis, and trending recommendations.

Acceptance testing is performed prior to commissioning to determine that the system is installed per the engineering design and that both the equipment and the overall system are ready for commissioning and energizing. These testing protocols focus on equipment defects, damage from shipping, and proper installation and can provide baseline information for future testing and analysis.

Maintenance testing is normally done in conjunction with a condition-based maintenance program. The goal is to maximize life expectancy and establish priorities for repair and replacement based on the analysis of the testing results. Knowledge of the equipment, systems, and an understanding of indications for potential failure modes is required to achieve the maximum benefit. These specifications cover the suggested field tests and inspections that are available to assess the suitability for continued service and reliability of electrical distribution equipment, emergency energy sources, and related power systems.

The intent of both the ATS and MTS is to recommend practices that are accomplished with preventative and predictive tests that are appropriate to the type of component under analysis. Those components can include cables, transformers, circuit breakers, automatic transfer switches, UPS system batteries, and backup power generation equipment. Preventative tests are most often executed during a planned electrical outage with the component in a de-energized state, whereas predictive tests are done online with the component in an energized state.



Offline testing is recommended when new to establish operability during acceptance testing, then at three to five year intervals thereafter for maintenance trending and analysis.


Medium Voltage Cables

  • VLF Dielectric Withstand: Cable testing methodologies have advanced from old style direct current high potential testing to utilizing very low frequency alternating current voltage withstand tests. The main advantage of the VLF test is length of cable that can be easily and safely tested. VLF testing is a pass/fail AC stress test but normally performed at 0.1 HZ. This is in addition to insulation resistance (sometimes referred to as a Megger test) that is completed prior to withstand testing to assure that the cable, splices, and terminations are in an acceptable condition for further testing.

  • VLF Tan Delta: VLF Tan Delta diagnostic testing, which can trend the degree of cable insulation degradation, can help determine and prioritize subsequent actions, including, but not limited to, additional testing, repair, or replacement.



In addition to annual visual/mechanical inspections, several periodic electrical tests are recommended to measure and trend transformer performance and condition.

  • Dielectric absorption ratio (DAR)/polarization index (PI): DAR and PI tests are used for determining insulation condition for apparatus with complex insulation systems such as transformers. A polarization index of greater than 1.0 is acceptable, the higher the better.

  • Power factor/dissipation factor: This test measures the insulation’s AC dielectric loss and provides a measure of the overall condition of an apparatus’ insulation system. It is the most effective field test for evaluating the condition of an oil-filled transformer’s solid insulation and its bushings. The test is useful in detecting moisture, contamination, and/or insulation deterioration. Test data is typically evaluated by comparison to prior results, similar equipment, and/or databases. This type of transformer testing is often referred to as Doble testing.

  • Winding resistance: Typically read utilizing a digital low resistance ohmmeter (DLRO) or similar device, these measurements are used to evaluate that the connections are correct and that there are no severe mismatches or open windings within the transformer. Regardless of the transformer configuration, the winding resistance measurements are measured phase-to-phase and are considered acceptable if all readings are within 1% of each other.

  • Turns Ratio Test: The purpose of a Transformer Turns Ratio (TTR) Test is to record the ratio and exiting current of turns of wire in the primary winding of a transformer to the number of turns of wire in the secondary winding. Deviations in turns ratio readings indicate problems in one or both windings or the magnetic core circuit of a transformer.

  • Sweep Frequency Response Analysis (SFRA): This test is completed during acceptance testing once the transformer has been set in place and anytime the transformer is physically moved. It is an electrical test performed on transformers but gives a mechanical picture of the inside of the transformer in graphical form. It is compared with previous test results to determine if there are any changes.


Circuit Breakers

Circuit breakers, related relays, and sensors provide for the protection of the electrical system, the computing equipment, and, most critically, the safety of employees during normal operations of the data center. Testing, maintaining, and ensuring a high level of reliability and protection requires a series of electrical and mechanical tests including periodic mechanical operation.

  • Insulation resistance: The purpose of measuring insulation resistance is to determine if the equipment’s insulation system is suitable for operation or high potential test. A megohmmeter is used to record phase-to-phase and phase-to-ground resistance, both with the breaker contacts closed and again across the open contacts. Evaluating and trending insulation resistance is important in identifying insulation deterioration.

  • Contact/connection resistance: Measuring contact and connection resistance helps evaluate, trend, and identify contact deterioration as quickly as possible so the necessary corrective measures can be taken.

  • Vacuum bottle integrity: For medium- and high-voltage vacuum breakers (>1,000 V), overvoltage tests have proven successful in detecting a loss of vacuum bottle integrity. The test should be performed across each vacuum bottle with the contacts in the open position and only after insulation resistance tests have been performed. AC high potential testing is recommended as DC high potential testing may cause damage to the vacuum bottle.

  • Magnetron Atmospheric Condition (MAC) Testing: Magnetron atmospheric condition (MAC) test should be conducted on each vacuum interrupter to measure the life expectancy before failure.

  • Electrical Operability Test: For breakers, switches, starters, etc., which can be operated using remote devices (e.g., are equipped with shunt trips, protective relays, or trip units), the electrically operated circuitry should be tested for proper function. There are two ways of executing the electrical operability test. The most effective option is to test using equipment that primary injects currents and/or voltages to simulate trip conditions, as this tests all of the circuitry and mechanical components including associated instrument transformers, control wiring, and trip unit. Testing can also be done using equipment that secondary injects currents and/or voltages, but this only includes the control wiring and trip unit and provides less comprehensive results. Coordination settings should be verified and adjusted before testing based on the most recent study completed by recording as found/as left.

  • Mechanical Operability Test: For electrical equipment with mechanical components, most mechanical problems are due to the environmental conditions, improper cleaning, and poor lubrication of the switch. Besides removing deposits and contamination, cleaning activities should include complete removal and reapplication of lubricating greases. Additionally, most manufacturers recommend opening and closing these devices at least annually as part of the preventive maintenance activities.


Automatic Transfer Switch (ATS)

Automatic transfer switches (ATSs) are installed in a data center to transfer the electrical loads from the normal power sources to the standby and emergency power sources upon failure of normal power. The acceptance and maintenance procedure should follow those recommended by the manufacturer and/or NETA ATS/MTS. Maintenance programs for transfer switches include checking of connections, inspection or testing for evidence of overheating and excessive contact erosion. Frequency should follow the recommendation from the manufacturer.


Uninterruptible Power Supplies

Batteries are by far the most vulnerable and failure-prone part of a UPS system. Because of this, much time and effort is allocated to maximizing a battery’s reliability and life within the data center. At a minimum, annual testing, verification, and inspection of a battery system should be performed. As with all electrical systems, infrared thermographic surveys should specifically be performed on battery systems on at least an annual basis.

Note: Watering is the single most important factor in maintaining a flooded lead acid battery. The frequency of watering depends on usage, charge method, and operating temperature. A new battery should be checked every month to determine the watering requirement. This prevents the electrolyte from falling below the plates. Care should also be taken to avoid exposed plates at all times, as this will sustain damage, leading to reduced capacity and lower performance.

A properly functioning battery charging system, with healthy batteries, will result in a fully charged and reliable battery system that is available when called upon for critical service.

  • Visual inspection of the battery cells: This includes the electrolyte level, the positive and negative plates, and the amount of sediment. Inspection should also include signs of leakage such as crusty trails or corrosion outside of the jar. Battery post and connections should look new except for a light coating of grease. Note exceptions: Black is lead peroxide, indicating a leak around the positive post. White is lead hydrate, indicating a leak around the negative post. Green is corroded copper and should be remedied with an inspection and replacement of connectors.

  • Battery charging performance verification: Test each cell voltage and total battery voltage against manufacturer’s data. Verify appropriate charger float and equalizing voltage levels. Test each cell for specific gravity and temperature.

  • Acceptance/periodic electrical testing: Along with a load test for verification when new and comparison purposes when service-aged, there are additional, more sophisticated tests that can be performed to accurately determine battery health. These methods measure the internal ohmic values of the battery or associated cells and include both AC and DC measurement methodologies. Both types of tests are reliable and can be trended from a base reading taken when the batteries were new, measuring conductance and resistance.


Backup generators

Backup generators are usually engine-driven stationary units that are interconnected into the data center’s electrical infrastructure. ATSs provide a signal for the generator to start and transfer the load from the normal supply to the generator. As with the UPS batteries, backup generator maintenance should include occasional load testing.

  • Exercise the engine generator: The engine-generator should be run under load on a periodic basis. During dynamic testing, engine parts become lubricated, oxidation is prevented, old fuel is consumed, and overall functionality is ensured. Vibration analysis, normally a predictive test, is also often performed during the run test.

  • Visual inspection: The area around the engine-generator should always be kept free of debris to ensure sufficient ventilation during operation. Additionally, components of the engine and generator should be checked, including radiator leaks and contamination, leaking fluids (such as lubricating oil and the fuel system), and the exhaust system, which includes the manifold, muffler, and pipes. Also, the engine jacket water heater should be checked for correct operation.

  • Electrical system testing: Electrical connections should be tight and free from corrosion, batteries should be properly maintained, and the generator head tested for insulation resistance, polarization index, and proper operation.



Online testing can often be used to develop predictive trend points for a viable predictive maintenance program. These are recommended to be performed on an annual basis for regular monitoring.

  • Physical inspections: Physical inspections should be performed on both high- and low-voltage equipment. Subassembly failures, poor equipment condition, overheating, and corona should be recorded. For some equipment, a periodic “maintenance route” should be developed to ensure that timely physical inspections are performed. For example, most liquid-filled transformers have liquid level, temperature, and pressure indicators. These should be monitored periodically to ensure that the transformer is operating within acceptable parameters. Similarly, protective settings for breakers should be periodically reviewed to ensure that the appropriate settings are in place.

  • Oil sample analysis: Oil sample analysis, including a dissolved gas analysis, should be performed on all liquid-filled transformers and oil circuit breakers. To obtain maximum benefit from this analysis, a qualified person should pull the samples and the samples should be sent to a qualified laboratory for analysis. At a minimum, the analysis should include testing for dielectric breakdown, acid neutralization number, interfacial tension, color, moisture, and dissipation or power factor. External oil sample ports can be installed on pad-mount transformer cabinets to ensure safety in pulling of the sample while the transformer is energized.

  • Infrared inspection (IR): Infrared inspections, or thermographic surveys, should be performed on all high- and low-voltage equipment. This includes surveying most electrical equipment for thermal differences or high limits, which are indicative of problems that could result in equipment failures. IR Windows can be installed on equipment panels for safer online surveying. It is good practice to complete this just before preventative testing to identify potential problems.

  • Ultrasonic emission (UE): UE surveys should be performed on high-voltage equipment only. This includes surveying electrical equipment for ultrasonic variations, which are indicative of problems that could result in equipment failures. This is non-invasive and again can identify problems prior to preventative testing. UE detects noise, whereas IR detects heat.

  • Partial discharge (PD): The integrity of high-voltage insulation systems can be assessed through utilizing on-line partial discharge (PD) testing and analysis and can provide the foundation for a viable predictive maintenance program for high-voltage equipment. Periodic use of the technology allows for the identification of problem areas with higher voltage terminations and splices prior to failure, which attribute to the majority of all cable failures.



The key electrical element for most data centers is the combination of the UPS and backup generators with a properly functioning ATS that provides seamless transfer to the alternate power sources. Secondly, the electrical distribution system, consisting of cables, transformers, and circuit breakers must be designed, installed, and properly maintained to protect both the equipment and the personnel. Reliable operation of this distribution system will depend upon baseline quality assurance during acceptance and commissioning and planned predictive and preventive maintenance activities. Utilizing the effective strategies of task planning, appropriate testing, and professional maintenance are all critical to keep the level of reliability needed for effective data center uptime management.

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