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Is Electrical Really Different Than Mechanical?

In 1998 the Getting it Right column concentrated almost exclusively on HVAC system commissioning as a way to inform readers about the process of commissioning. We thought it would be a good idea to start 1999 by stating that commissioning is not, and should not be, limited to HVAC systems. Timothy Coyle’s article in this issue of Engineered Systems introduces electrical commissioning and its importance to proper and safe building operation.

I’d like to discuss the differences between HVAC and electrical system commissioning and want to acknowledge Mr. Coyle for his contributing the lion’s share of this column.

Although the intent and general principles of commissioning electrical systems are no different than for HVAC or other mechanical systems, some inherent differences in the characteristics of electrical and mechanical systems dictate some modifications to the traditional commissioning process when applying it to electrical systems.

Mechanical system operation is often quantitative; an HVAC system intended to provide 50,000 CFM produces only 40,000 CFM, or a chilled water system intended to maintain a relative humidity setpoint of 55 percent is short of capacity on high wet bulb days and achieves only 60 percent. These types of problems manifest themselves over time and as a matter of degree rather than absolute performance.

Much electrical equipment is go/no-go; it functions at 100 percent up to the point of failure and then functions at 0 percent, often providing no warning of impending failure. The mechanisms of electrical insulation failure under the influence of applied voltage, surge voltages, and thermal aging are such that life expectancy is probabilistic and failure does not necessarily coincide with the application of the adverse condition causing the failure. For this reason, electrical system commissioning relies heavily on specialized electrical testing to verify the integrity of insulation, connections, etc.

Similarly, the all-or-nothing nature of electrical system performance can increase the consequences of a deficiency which is overlooked at startup. An error in the setpoint of a temperature control loop will result in an incorrect space temperature which is readily detected through occupant complaints and can be corrected before having a major impact on the building.

Finally, in many electrical systems, and certainly in life safety systems, it is off-normal performance or performance under contingency conditions which we are most interested in. Fuses and circuit breakers, some of the most commonly used electrical distribution system components, serve no purpose under normal operating conditions and are called on to function only when an overload or short circuit occurs. Many control schemes such as smoke evacuation, ground fault protection, or automatic bus transfer are not required and may never be exercised in normal operation of the building. If not fully tested under simulated conditions as a part of start-up, these systems may never betray deficiencies until they are required to operate in a contingency and fail to do so properly.

Unfortunately, the tendency of electrical systems not to betray deficiencies in normal operation can also lead to the perception that an untested system does not have any deficiencies. This may result in electrical systems being overlooked as candidates for commissioning. An analogy to this is the energy-related benefits of HVAC commissioning. An inefficiently operating HVAC system is not necessarily evident to the building operators or occupants. For example, if an un-commissioned HVAC system has outside air dampers whose minimum position is allowing in much more than the required amount of outside air, no one in the building will complain about “too much ventilation,” but the energy costs for conditioning that extra outside air can certainly impact the bottom line of the building owner for the life of the system.

In summary, electrical system commissioning is at least as important to a building as HVAC system commissioning. This is particularly true because electrical systems are so critical to life safety, electrical system failures are often impossible to anticipate, and such failures have the potential to be catastrophic. Therefore, when sitting down to prepare a commissioning plan for any project, be sure to consider inclusion of key electrical systems in that plan.

 

Commissioning: Getting It Right

Engineered Systems, January, 1999

by
Rebecca Ellis, PE, LEED AP, CCP, CxA
President
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318
rteesmag@QSEng.com