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matically reduce the lifespan of the device. Electromechanical relays should be tested on a regular basis, per manufacturers' recommendations. In addition to testing the calibration, the unit should be inspected for damage and the contacts should be cleaned. Medium-Voltage Metal-Enclosed Switchgear In most cases, ANSI/IEEE C37.20.3 medium-voltage metal-enclosed switchgear is located at the primary side of unit substations in data centers. The frequency of maintenance depends on the operating conditions. During the shut-down, a hi-pot (dielectric) test should be performed to measure insulation integrity along with general cleaning and tightening of the insulators, bussing, and mechanical parts. The contacts and operating mechanism should be cleaned and lubricated per the manufacturer's specifications. Low-Voltage Switchgear Maintenance on low-voltage switchgear is very similar to maintenance on mediumvoltage switchgear. Construction can vary widely based upon manufacturer and age, so accurate drawings and documentation is essential for use and maintenance. The equipment should be cleaned, lubricated, and exercised on a regular basis. Like with other equipment, an inspection should take place after a short circuit involving the switchgear in the path of current flow. In particular, Insulation and conductors should be inspected for damage. Low-Voltage Circuit Breakers Insulated case circuit breakers (UL 489) can sometimes be maintained similar to power circuit breakers (ANSI C37.13). However, in many cases, their maintenance is similar to a molded case breaker. Manufacturers' literature should be consulted for recommended practices. Molded case circuit breakers (UL 489) require a minimal amount of maintenance. Maintenance is limited to inspection and good operating conditions, and in some cases primary injection testing with the circuit breaker removed from the enclosure. They should be manually operated annually. The "push to trip" button should be 8 | THE DATA CENTER JOURNAL utilized to exercise the tripping mechanism. Switchboards and Panelboards In a data center, switchboards and panelboards are typically found downstream of the switchgear. Maintenance is primarily focused on the electrical and mechanical connections. Regular infrared scanning and cleaning is recommended to insure dependable operation. Low-Voltage Motor Control Centers (MCCs) Maintenance on the enclosure and busbars of MCCs is very similar to switchgear, the major difference being the MCC draw-out buckets. These should be removed from the MCC before maintenance. Once removed, the primary contact stabs should be examined for signs of arcing or overheating. If a stab is badly pitted, it may be necessary to replace the vertical bus within the MCC. In addition, any circuit breakers, switches, starters, control devices, and wiring within the bucket should be examined. Low-Voltage Transformers NEMA low-voltage dry-type transformers are typically used for lighting applications within data centers. During maintenance, connections should be checked for looseness. In addition, any dust that restricts air flow or lies on insulating surfaces should be removed. A visual inspection for overheating, tracking, and overall enclosure condition should also take place. Automatic Transfer Switches (ATS) ATS switch their downstream loads between power sources in the event of a breakdown. The controller/switch combination should be checked often; in fact, one manufacturer recommends testing monthly. Proper electrical operation of the controller and switch should be verified. Another manufacturer recommends annual maintenance on the actual switch which includes cleaning the enclosure, lubrication, and inspection of all contacts and cable connections. Uninterruptible Power Supply (UPS) Systems UPS systems provide the final safety net for a data center power system. They also can remove the momentary power interruption during open transition switching between utility and generator sources. Due to the unique and technical nature of a UPS system, most manufacturers recommend that their service personnel perform any major maintenance and system tests. The room should be clean and cool, air filters replaced, and the battery system inspected on a regular basis. Busway Busway is designed to be low-maintenance equipment. However, a regular visual inspection and IR scan are recommended to look for exterior damage and loose connections. An inspection should be made if a short-circuit or ground fault has taken place downstream of the busway. Grounding System Grounding and bonding within the system should be inspected on a regular basis. The electrical connection to earth should be measured. In addition, voltage should be measured between the equipment grounding conductor and the grounded conductor at multiple locations. Also, current should be measured on the grounded conductor. If any of these readings are outside of standard levels, their cause should be immediately found and corrected. CONCLUSION All studies of electrical equipment maintenance programs show a strong correlation between maintenance levels and the reliability of electrical equipment within the power distribution system. No critical power system should be without an effective electrical preventive maintenance and testing program to help minimize downtime and prevent potential breakdowns. The program should be tailored to each individual system. Details such as types of equipment, availability of downtimes, operating conditions, and the nature of the loads must be considered. n About the Author: Ken Agent, P.E. Schneider Electric Engineering Services References: NFPA 70B®, Recommended Practice for Electrical Equipment Maintenance, Quincy, MA: NFPA www.datacenterjournal.com