A Mechanical Engineer's Perspective on Electrical System Commissioning

By David Sellers, P.E., Senior Engineer, Facility Dynamics Engineering, Portland, Ore. -- Consulting-Specifying Engineer, 5/1/2007

An unexpected power failure and the ensuing recovery can wreak havoc on a building and its systems, generating complaints, loss of environmental control, quality control problems, and occasionally, debris.

But one often overlooked aspect of power systems failure is the effect on HVAC and building automation systems, and the problems that can ensue from loss of power.

Here, I want to discuss the important HVAC and building system considerations that merit attention when assessing and testing the ability of a building and its system to recover from a loss of electrical power, as well as the loss of other types of utilities.

The bottom line is that power failures happen—they will happen irrespective of whether the design of a building and its systems anticipated them or not. Similarly, the building will recover when power is re-applied, be it via emergency generators or the restoration of normal power.

The only real question is, as Dr. Phil would say, “So tell me, how did that work for you?”

There are a number of important characteristics associated with a power outage that need to be considered. One is simply, what constitutes a power outage?

The answer, of course, depends somewhat on personal perspective. Let's say, for example, an individual is in a windowless office located in the core of a building. The outfall of the circuit that serves those occupants' lights and receptacles tripping off is indistinguishable from the outfall of the circuit breaker serving a panel feeding all of the lights and receptacles on the floor tripping off.

In contrast, nobody else on the floor may know of the localized outage until a co-worker stumbles out of his or her darkened office. If the panel serving the floor looses power, then everyone is impacted.

The implications of the outage also can vary. Our worker in the windowless office with the circuit breaker trip will experience a significant drop in productivity until the problem is resolved.

But, the productivity of other workers on the surrounding floor remains high unless the outage impacts the entire floor. If our worker gets promoted, and the windowless office becomes a broom closet, a power outage affecting only the broom closet may go undetected for weeks or months and have no impact.

If, however, the windowless space was an active operating room in a surgical suite, the loss of light and power could be life threatening, not just an impact on productivity.

From the perspective of the load or occupants in the area served by an HVAC system, the ripple effects of the outage can be as significant as or more significant than the actual loss of power. Consider a 100% outdoor air surgical air-handling system serving a hospital in a hot and humid environment. The surgery may be operating at 65°F, 50% RH (Figure 1) due to the nature of the procedure being performed. Typically, this would require a discharge temperature in the low 50ºF to 52°F range to ensure adequate dehumidification of the 98°F dry bulb/76°F wetbulb ambient outdoor air one might encounter in a hot and humid location (relative humidity 37%; dew point 67°F). If the power fails, emergency generators will typically start and restore lighting airflow in a matter of seconds; in most instances, code will require this.

However, if chilled water service is not restored at the same time, then the hot, humid air drawn in by the system will cause condensation on any of the surfaces that are at or below its dew point. In the case of our example, this would include the duct system and all of the surface in the operating room. This will lead to some serious consequences including:

• Saturation of the HEPA filters in the diffusers serving the surgery
• Water dripping from the diffusers into the sterile field and open wound
• Condensation in the sterile supplies stored in the suite
• Condensation inside tools and equipment in use in the surgery, including inactive electrical and electronic equipment
• A slip hazard due to water accumulation on the floor.

In light of the preceding, it may be desirable not to restart the air-handling system when emergency power is restored, if chilled water is not available and the outdoor air dew point is above the temperatures typically found in the surgical suite and the systems serving it. Such an approach may be counter to common wisdom and, in some instances, counter to code or infection control requirements targeted at re-establishing air change rates designed to control infection and protect the patient.

It is interesting to note that in our example, the events described could be triggered by a power outage that was:

• Area wide: a thunderstorm knocks out the local utility serving the entire district in which the hospital is located
• Building wide: a fault in the transformer serving the hospital's central plant building could take the chilled water plant off line but not the hospital building
• Localized: a controller fault could shut down the chilled water distribution pumps serving the surgery AHU with out impacting the hospital buildingor chillers.

Most HVAC systems have energy place into them from an electrical source via a motor. At first blush, the concept of a power outage and the ensuing recovery from a power outage is associated with the loss and reapplication of electricity to the motor. But, the fact is that the energy input from the motor is conserved in the system as mass in motion; typically a fluid flowing in a duct or pipe (chilled water, air, refrigerant, steam, etc.). It is this mass in motion that actually provides the thermal phenomenon (heating, cooling, dehumidification, etc.) required to serve the load. Thus, anything that disrupts this mass in motion will disrupt or corrupt the thermal processes required to meet the load. Loss of electrical power is only one possibility. Others include:

• Drive system failures: the failure of a coupling, belt or VFD eliminates the energy input to the system required to keep the mass in motion just as effectively as removal of electrical power to the motor.
• Motor failures: a motor can fail for reasons other than loss of electrical power. Windings can burn up, bearings can seize, shafts can shear.
• Controller failures: controllers can fail or cold-start, shutting down the equipment they serve, even though power is available at the starter or drive serving the machinery.
• Operator error: it is alarmingly easy to throw the wrong selector switch or command the wrong point off; I know because I've done it. The result is indistinguishable from an electrical outage.

The preceding are but a few examples of how the integrity of a building's electrical system and the power it supplies are crucial to the integrity of HVAC systems. Power failures are a reality for virtually all operating buildings. By taking the time to consider the implications on all fronts, when inevitable occurs, you'll be able to look the Dr. Phil's of the world in the eye when they ask “So tell me, how did that work for you?” and respond “Pretty well, actually; we'd thought it through and we were ready.”

Buildings are becoming more technologically advanced. For this reason, commissioning of power systems must consider all types of systems. And systems are becoming much more complicated. New requirements in fire protection, building safety, energy performance and information technology are driving engineers to constantly learn complex systems.

There are a number of resources one can make use of when dealing with electrical systems and the related engineering issues. Of critical importance is a working relationship with knowledgeable electrical engineers. If you're as lucky, you'll meet a number of them as you work on various projects. Take the time to develop relationships. They understand the mysteries behind the “magic” in the wires that power the motors that make our systems work. They will become an invaluable resource, be it via a quick phone consult or as a co-consultant assisting you with a project. Other useful resources include:

InterNational Electrical Testing Association (NETA): NETA is an excellent resource for guidelines targeting the testing of electrical equipment, including emergency generators, uninterruptible power systems, and automatic transfer switches. Visit their website for additional information and to subscribe to their bi-monthly magazine at www.netaworld.org.

Single Phasing and Motor Protection Issues: Bussman's Overcurrent Protection and the 2002 National Electric Code provides valuable insights into motor protection topics, including single phasing and is available at www.bussmann.com.

The Functional Testing Guide: This free, downloadable resource, developed by STAC (State Technologies Advancement Collaborative) includes a chapter on Integrated Operation and Control. Contained with-in that chapter, under topic 2.2 – Test Guidance and Sample Test Forms is a test guidance document on the topic of System Recovery from Power Failure available at www.peci.org/ftguide.