When it comes to establishing an electrical maintenance program for your facility, there are many resources at your disposal. For inspection of energized electrical equipment, an infrared camera is probably the first tool that comes to mind. However, a growing number of maintenance and reliability professionals have come to realize that airborne ultrasound is a viable tool that can be used to inspect and diagnose conditions such as corona, tracking, arcing, and partial discharge in energized electrical equipment.
The standards that maintenance and reliability professionals access for guidance in establishing procedures and best practices for inspecting and maintaining electrical equipment in facilities are National Fire Protection Association (NFPA) 70B, and Canadian Standards Association (CSA) Z463. Both of these documents have been established to create recommended practices for inspecting and maintaining electrical assets in order to prevent accidents and reduce downtime.
According to NFPA 70B, the standard for electrical maintenance, an electrical preventive maintenance program is “a managed program of inspecting, testing, analyzing, and servicing electrical systems and equipment with the purpose of maintaining safe operations and production by reducing or eliminating system interruptions and equipment breakdowns.”
When it comes to the inspection tools that NFPA 70B recommends for inspecting energized electrical equipment, the document only mentions ultrasound in two sections. The first mention is in Section 11.21.3.3.2.2 when discussing partial discharge. The other mention is in Section 15.1.2.2, which discusses corona in substations and switchgear.
Canada’s NFPA 70B equivalent, CSA Z463, goes into more detail when discussing ultrasound technology. CSA Z463 reads in Section 7.10.2 that “ultrasound inspections use collectors that detect the high frequency produced by the emissions caused by electrical arcing, tracking, and corona.” This section also goes on to mention the fact that the sound heard by the inspector can be recorded and then further analyzed in spectrum analysis software for an accurate diagnosis of what was detected by the ultrasound instrument.
Users of airborne ultrasound, a technology that was once considered just a leak detector, have come to realize the instruments could be used for other applications, such as condition monitoring of rotating equipment and electrical inspection. To many, airborne ultrasound has become a necessity when inspecting energized electrical equipment. The primary driver of this application is safety. In most cases, airborne ultrasound inspection can be done without opening energized electrical cabinets. Performing an inspection without opening energized electrical equipment helps to reduce the risk the inspector faces from potential exposure to arc flash hazards.
The ultrasound inspection can be done by scanning any openings on the cabinet itself, such as vent openings, seals around doors, or louvers. Therefore, before any energized electrical cabinet is opened for maintenance or further inspection, a pre-inspection can be done with ultrasound to see if any anomaly is heard. If there is a condition, such as corona, tracking, or arcing inside the electrical equipment, the ultrasound produced by those anomalies will be heard by the inspector via those openings. The source of the ultrasound produced is ionization, and in some cases, vibration from mechanical looseness.
In all fairness, NFPA 70B is in the midst of a revision period. The next revision is scheduled to be released in 2016 and could include additional information regarding ultrasound and its usefulness for electrical inspections. Both documents go into great detail regarding the inspection and maintenance of any electrical asset in a facility. There are even recommended procedures discussed regarding asset criticality and establishing an electrical maintenance program.
