Wednesday, 27 March 2013 12:08

Electrical design considerations for mission-critical facilities

Website: The fact that power outages are of major concern in South Africa, which means that certain electrical design considerations need to be put in place to help minimise the risks posed by such outages.

Constant power supply is fundamental to the operation of a mission-critical facility. However, according to Eben Owen, Schneider Electric’s enterprise and solutions manager in South Africa, the fact that power outages are of major concern, which means that certain electrical design considerations need to be put in place to help minimise the risk posed by such outages.

“The first line of defence would be to have a back-up generator when electrical utilities fail. Generator solutions range from a simple, single generator and automatic transfer switch (ATS, to a complex multi-generator plant. However, a generator is only useful if it is able to be online and to power a full equipment load when asked, which means the testing, care and maintenance of your generator is fundamental to the survival of your facility,”  he says.

Testing an emergency generator on a weekly basis with no load will reassure a person that the generator is operational. However, these tests may also lead to “wet stacking”.

Wet stacking

Wet stacking occurs when a generator is operated for extended periods with little or no loads applied. When the generator then needs to power a full load, the “wet” fuel deposits that have built up during no-load tests prevent it from operating at the peak efficiency needed to full power the actual load.

Wet stacking does not usually cause any permanent damage and can be remedied by applying additional load to relieve the condition, effectively blowing out these deposits from the system. Whenever possible, the generator should be tested under loaded conditions in order to prevent this build-up, as it can take several hours to burn off the accumulated unburned fuel.

Engine block heaters

Engine-block heaters allow stand-by generators to start and take up load quickly in an emergency, but the constantly heated water and generator vibration causes wear-and-tear on the hoses and fittings. By installing isolation valves between the engine block and block heaters, the hoses and heaters can be replaced without taking the generator out of service.

An ATS takes care of moving your electric power supply to/from the generator during a power outage. As soon as the ATS senses that the utility power is interrupted, it signals the generator to start. Once the generator is running at the proper load, the ATS safely shuts off the utility power line and simultaneously opens the generator power line from the generator. This switching system also works in reverse once utility power is restored, with most transfer switches waiting a prudent amount of time before automatically switching back. These transfer switches contain parts and connections that can and will fail, therefore continual maintenance is essential.

Uninterrupted power supplies

Uninterruptible power supplies (UPS) are essential equipment at mission-critical facilities. A UPS differs from a stand-by generator in the sense that it will provide almost instantaneous protection from power interruptions by supplying energy stored in these batteries – however, UPS batteries have a limited lifespan and must be tested regularly. With a system-plus-system design or external wrap around the maintenance bypass, maintenance tasks can be performed without affecting the critical load.

Many facilities are designed with state-of-the-art technology. However, all this technology is wasted if the final connection hinges on inadequate cabling, breakers and distribution methods. The 32-amp breaker should be a tested bolt-in breaker rather than a snap-in version, which tends to be used in residential applications. Although the snap-ins are less expensive and easier to use and install, they are also less secure, untested and far less reliable – attracting a failure rate of 20% to 50%.

Lastly, load banks are essential tools in the maintenance of mission-critical facilities. Load banks are devices designed to provide electrical loads for testing power sources such as generators and UPS’s to ensure they can handle the necessary load during an emergency. Load banks are also used to reduce wet stacking problems in generators. Users can install permanent load banks or have their service provider bring temporary units in for maintenance.

Owen concludes: “Either way, be sure you make the provisions in your switchgear to connect the load bank to the UPS system and generators. This extra load bank breaker will save you a great deal of grief later.”

Schneider Electric
Tel: 011 557 6600
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Website: www.schneider-electric.com