Testing power distribution equipment is important and knowing where to test can be confusing. A good understanding of the one-line helps and as technology has evolved virtualizing the one line is becoming more prevalent. Power monitoring is another initiative that is gaining ground and can help pinpoint where to focus testing efforts based off real-time data.
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In either case understanding the current infrastructure is critical to getting started. Items of importance for electrical distribution testing include Arc Flash Analysis, Load Flow, Short Circuit Study, Harmonics, and Coordination Studies. Once these items are complete in house testing can be incorporated as a second phase of preventative maintenance.
NOTE: Before engaging with any electrical distribution equipment know the correct distances for safety and ensure you completely understand the terminology of power equipment. The world of power can be confusing and getting a solid foundation on these areas are important to overall safety for all personnel.
When starting from scratch gather information about equipment such as fuses, breakers, age, and cables. With new construction require the information up front. With existing infrastructure begin gathering data by reviewing existing one-lines or by an old fashioned site walk. To get started consider the following areas.
Short Circuit and Coordination studies verify that all breakers and fuses work together as designed. They ensure that when a fault occurs, upstream or downstream, the appropriate breaker will trip and open breakers in the correct places. Ideally the breaker closest to the fault will be the one that trips. This avoids shutting down whole sections of the facility unnecessarily. When a Short Circuit study is done, engineers ensure that every device, breaker or fuse, is rated to take the incoming fault and dissipate the fault safely without failing.
Arc Flash (AF) studies are done to help the user understand the available AF energy at different points of their system. It helps to ensure that employees are wearing the appropriate PPE to safely work on the equipment.
Load flow is used primarily when load is added or removed from a system. Review all loads in a system and follow the path of the current. This paints the picture of current flow and will help determine where loads can be added safely. The best way to achieve this study is to physically measure different points of the system.
Harmonic Analysis is used when there are numerous VFD, Motor, and inductive loads. These loads create harmonics or noise on the line. This can create issues downstream to sensitive devices and cause them to not work properly.
What testing can a local team do?
Start with basic visual and mechanical inspections. Test the integrity of the cables, bolted joints, breaker mechanisms. Gear that is not well maintained will have mechanical issues that aren’t shown in electrical testing. Both are needed to make sure that the gear is performing the way it was intended.
Verify torque values on electrical connections and check cabling to make sure it is installed correctly. Use a low resistance meter to check the bussing. This can help determine that all the physical bolted connections of the gear are properly secured by returning a low resistance measured in ohms. Insulation tests ensure there are no cuts in the wiring or the insulation on the copper bus. Breaker testing can be achieved by primary injection or by secondary injection testing. With electronic trip units in breakers, secondary testing can be done by using test sets plugged directly into the trip units and tests are run including long time, short time, instantaneous, and ground fault to make sure that the trip unit is working properly. Primary injection testing is done by using full current thru the primary contacts on the line side of the breaker. This can be done by using more current than the breaker is rated for to check and make sure that breaker trips and performs the way it is intended to.
Don’t skip visual inspections!
What type of breaker is it? Does it have an electronic trip unit? How is the breaker tripping? Instantaneously or over a period? Instant trips indicate a shorted cable or contact. If the breaker is taking time to trip, there might be degradation of something downstream, such as a bad bearing in a motor. Some other conditions that could cause a delayed trip could be temperature or humidity related.
Testing procedures are important because it ensures employee safety and prevents costly downtime and unplanned maintenance.
