Specifying, applying load-bank systems

Understanding load-bank specification, installation, and operational issues can help ensure electrical and mechanical system reliability.

03/02/2012


Rack-mount load banks can be used during data center commissioning to simulate the electrical banks can be used during data center commissioning to simulate the electrical and heat load of the computer systems. Courtesy: ComRent InternationalAnywhere there are critical standby or life-safety electrical systems, a load bank may be desirable or even mandated by code to help ensure the reliability of those systems. Although the concept of a load bank as an artificial electrical load is relatively straightforward, consideration must be given to the detailed needs of the site when load banks are specified and installed. 

Why load banks?

The only way to verify that a backup power system will perform during an outage is to periodically test it under load. Both generators and UPSs may appear to run fine when lightly loaded, but may fail to deliver full load power if they are not regularly tested to ensure they are up to the task.

Diesel generators are susceptible to a condition known as wet stacking when operated at light loads for extended periods. Wet stacking occurs when carbon or unburned fuel oil accumulates on the injectors, on the exhaust valves, or in the exhaust system. Readily observable as billowing black smoke or as an oil leak (engine slobber), the generator output will be reduced as combustion gasses leak past the valves. Permanent damage due to cylinder scarring is possible although unlikely. Fortunately, the condition is usually corrected rather easily by running the engine for a few hours under sufficient load—typically 30% to 40% of rated load—to bring the engine to nominal operating temperature and burn off the deposits. Load banks may be required to achieve this load if normal building load is not sufficient.

Because of its durability, this 5-MW low-voltage load bank is being used with a step-up transformer. Courtesy: ComRent InternationalNFPA 110-2009, Standard for Emergency and Standby Power Systems, includes specific requirements for both acceptance testing upon initial installation of an emergency power supply as well as ongoing periodic testing throughout its life. NFPA 110 paragraph 7.13.4.3 requires that a two-hour full-load test be performed on-site as part of the initial commissioning of any standby or emergency system. The building load is permitted to serve as part of the load. Load banks may be required to ensure that 100% of nameplate kW loading is achieved during the test. Unless factory testing was performed at full nameplate kVA rating, reactive load banks may be required during site acceptance testing as well, according to 7.14.3.2.

NFPA 110 paragraph 8.4.1 mandates monthly tests under load for all emergency and standby systems, for a minimum of 30 min. For generators, paragraph 8.4.2 further specifies that a minimum load of either 30% of nameplate kW rating, or as otherwise sufficient to meet manufacturer recommended exhaust temperature, be provided during this test to address wet stacking.

Additional mandates for load testing may be imposed by other standards organizations, such the Joint Commission on Accreditation of Healthcare Organizations, Comprehensive Accreditation Manual for Hospitals, requirement for hospitals to annually test their emergency power supply systems under full load, with additional requirements for monthly and three-year testing under partial load. In other mission-critical facilities, in addition to acceptance testing upon initial installation, monthly or quarterly full-load testing of all generator and UPS systems is a typical operating requirement to ensure that standby systems are capable of providing their full-rated output for the full expected run time of their fuel supply or battery system.

Infrared scanning is quickly becoming the predictive maintenance tool of choice for facilities of all types as planned shutdowns become increasingly difficult to arrange. Per ANSI/NETA ATS-2009: Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems, paragraph 9.3.3, and NFPA 70B-2011: Recommended Practice for Electrical Equipment Maintenance, paragraph 11.17.5.2, “infrared surveys should be performed during periods of maximum possible loading but not less than 40% of rated load of the electrical equipment being inspected.” At many facilities, this can be difficult to achieve with the available building load; an additional artificial load may be required.

Beyond electrical testing, load banks may also be used to test mechanical cooling systems. In particular, it has become commonplace during data center commissioning to use portable load banks on the data center floor to simulate the heat load of the critical electronic systems, simultaneously testing both the electrical and mechanical systems supporting that load. Water-cooled load banks may similarly be used to test chiller plants at their design or nameplate heat-rejection rating.

With the above in mind, the various types of available load banks must be considered prior to specifying a load bank. 

Type of electrical load

By far, the most common type of load bank is purely resistive. However, inductive load banks are also occasionally required, as these are the only way to test to the full kVA rating of electrical equipment with nonunity power-factor rating. NFPA 110 requires the use of a reactive load bank as part of either factory or field acceptance testing for nonunity power factor equipment.

Capacitive load banks are used less often, although they may be needed where generators are required to support leading power-factor loads. Nonlinear load banks, which are often constructed using banks of PC power supplies, are also available for specific applications such as testing K-rated or harmonic-mitigating transformers. 

Voltage and phase considerations

Load banks are commonly available with dual or multiple voltage ratings, which may be attractive if equipment of differing voltage ratings must be tested. Note, however, that the available kW load will be reduced with the square of the voltage. For example, if a 3-phase load bank rated for 1,000 kW at 480 V is used on a 3-phase 240-V system, the available load would be only:

(240/480)2 x 1,000 kW = 250 kW

Similarly, a 3-phase load bank may be used to test a single-phase source by bonding load-bank phases B and C together and connecting the single-phase source from A to BC. However, the available load in this configuration will be reduced to about 66% of the load bank’s nameplate rating. For example, if the 1,000 kW, 480-V 3-phase load bank from the previous example is used to test a 240-V single-phase generator, the available load would be reduced by both the voltage reduction and the single-phase configuration to:

(240/480)2 x 0.66 x 1,000 kW =
                    165 kW

For medium-voltage (1,000-10,000 V) generator systems, one might be tempted to specify a load bank with a medium-voltage rating. However, for a given kW load-bank rating, a higher voltage requires load-bank construction using longer, thinner resistance windings, which implies a more fragile overall design. A low-voltage (less than 1,000 V) load bank in conjunction with a step-up transformer is generally preferable as it will be more durable (see Figure 2). A low-voltage load bank may also offer additional flexibility if low-voltage generators and UPS systems are also to be tested.

Physical configuration

Load banks are available in at least four types of physical mounting configurations: portable, radiator mounted, permanent, and rack mounted.

Portable load-bank sizes range from small suitcase-type load banks to large trailer-mounted units as shown in the photo.Portable: Portable load banks are perhaps the most commonly employed due to their flexibility. They are available for purchase or rental and range in size from small suitcase load banks to large trailer-mounted units (see Figure 3). A single load bank may be used to serve multiple testing requirements at a single site or even shared among various facilities.

Radiator-mounted: Radiator-mounted load banks can be a convenient and economical solution where a site’s sole testing requirement is a single generator, or perhaps a few scattered generators (one for each generator would be required). Radiator-mounted units are typically purchased and delivered as part of a generator package. They use the generator cooling system fan to provide load-bank cooling. Disadvantages include a limitation on maximum available load-bank rating of perhaps 50% to 70% of the generator rating due to the associated increase in cooling air static pressure across the radiator, and the load bank is dedicated to the generator it serves and cannot be used to test other generator or UPS equipment.

Permanent: Permanent load banks are suitable for larger installations with multiple generators or UPS systems. Load-bank busways and breakers with Kirk-key schemes, cam-type connector panels, or hybrid combinations of the two are commonly employed in conjunction with permanent load banks to facilitate the connection of each standby source to the permanent load bank for periodic testing.

Rack-mounted: A rack-mounted load bank is a specific type of temporary portable load bank that is occasionally employed during data center commissioning to more accurately simulate the electrical and heat load of the critical computer systems prior to the computers actually being installed (see Figure 1). In particular, where the data center computer cabinets are direct-ducted to a cooling system exhaust plenum, rack-mounted load banks may be the only practical way available to truly validate that the facility’s cooling systems perform as designed prior to full data center occupancy.

Combinations of these load-bank types and configurations may be used. For example, a resistive load bank may be permanently installed with a tap box to facilitate the occasional connection of a portable inductive load bank. Or perhaps a permanent load bank is used to exercise the generators, but temporary load banks may be brought in to test the UPS systems.

Load-bank options

Other load-bank configuration options include cooling and heating, load-bank controls, and control power. These factors should be considered when specifying a load bank.

Load-bank cooling and space heaters: Even small load banks can generate a substantial amount of heat, which makes cooling a key consideration. Both water- and air-cooled load banks are available. Air-cooled load banks with integral cooling fans are by far the most commonly used for both temporary and permanent installations due to their ease of installation. Self-powered load-bank fans may be specified to be powered from the test source and operate the fans only during testing. Other load banks may need a separate external control power source to allow proper cooling subsequent to testing.

Vertical exhaust-air discharge is usually preferred for larger air-cooled load banks as this helps mitigate issues with both heat and noise. Smaller units are more commonly provided with horizontal exhaust. Air-cooled load banks are generally installed outdoors with a sufficient separation from the building to ensure proper cooling airflow. Indoor installations demand careful attention to potential heat issues with building construction, overhead cables, and sprinkler heads. These may require exhaust fans or air conditioning to remove the generated heat. For permanent outdoor installations, space heaters with enthalpy control (combined thermostat and humidistat) should be considered to prevent condensation in the load bank when not in use.

Water-cooled load banks may be preferred if there is a desire to test chiller plants in addition to the electrical sources. They may also be an attractive choice in urban high-rises and other locations where accessible outdoor space is limited.

Load-bank controls: Load banks typically include a control panel to enable load-step switching. For smaller installations and portable load banks, this control panel is usually located at the load bank. For larger installations, it may be preferable to have a remote-control station, perhaps located within a main-building electrical room. For very large installations, a portable controller should be considered. A portable controller can be plugged into any of several jacks located throughout the facility near the generators, UPSs, and transfer equipment to be tested.

In the event of a utility outage during load-bank testing, the load bank must be automatically disconnected to allow the emergency systems to support the critical building loads in accordance with NFPA 110 paragraph 8.4.2.2. Many load banks come with integral controls that facilitate this by connecting a pair of wires for utility-voltage sensing to the transfer controls. In other cases, a shunt-trip circuit breaker can be used.

Control power: Load banks typically require a source of control power to operate their blowers, load-step controls, metering, and possibly space heaters. An integral control-power transformer, connected internally to the test source, may be the easiest way to accomplish this, particularly if the load bank is to be used at a single test voltage. However, such an arrangement will frequently not permit the cooling fans to continue running after testing is completed, which may be an issue depending on the load bank’s cooling rating. In addition, if a load bank is to be used with multiple systems at multiple voltage levels, the control power system must be capable of working with the various voltages.

These problems can be avoided by providing a separate external control-power source. The controls can be arranged to allow the cooling fans to continue operating for a period of time then automatically shut off after testing is completed. Separate control power also affords greater input voltage flexibility. Providing an external control-power source is a frequently overlooked detail during the design phase for both temporary and permanent load banks. 

Installation and operation considerations

Installation and operation issues should also be considered when specifying load banks. Installation and operation focus areas include location, portable load-bank connections, overcurrent protection, impact on total connected load, spare load banks, and predictive maintenance opportunities.

Location: Whether temporary or permanent, the primary issue to be addressed for any load-bank installation is its location. For permanent installations, the manufacturer’s recommendations regarding minimum physical separation must be followed to ensure proper cooling airflow and eliminate the risk of damage to the load bank, building, or other surrounding structures or landscaping. In cold climates, snow may also be a problem, including snow buildup around the load bank and snowdrift from adjacent rooftops into the load-bank ventilation grilles, which could short out the windings. Similar considerations apply when locating a site for the tap box for a portable load-bank connection, a place to park a load-bank trailer, and the proximity of the tap box to that parking spot.

Temporary load banks are often connected indoors, which raises an entirely different set of concerns including the length of temporary cabling to be run between the test source and the load bank, and heat buildup within the space. It may be possible to operate a small load bank in a small enclosed space for a short period of time, but given that even a small load bank is essentially a large electric heater, outdoor locations should be considered wherever possible. If an outdoor location is impractical, a large warehouse or loading dock area with roll-up doors is preferable. Several large fans may be required to avoid overheating the space.

When commissioning data centers or other facilities where the cooling systems are also to be validated, the very point is to operate the load banks within an enclosed indoor space and observe the electrical and mechanical systems do their jobs. Detailed load-bank location plans may be required to help ensure the test installation mimics the actual heat load of the design as closely as possible.

It is critical that temperatures throughout the space be monitored any time air-cooled load banks are operated indoors, as temperatures near the ceiling may be substantially higher than what is experienced at ground level. Potential problems include damaging the insulation on overhead feeders, accidental sprinkler-head discharge, building surface damage or discoloration, or even fire—in addition to the possibility of load-bank overtemperature shutdown or damage to the load-bank windings.

Portable load-bank connections: Whenever periodic load tests using portable load banks are required, provisions should be made to facilitate the load-bank connections. This can be as simple as a spare breaker in a switchboard with temporary wiring routed to the load bank. For larger installations, it may be advantageous to provide permanent power, control, and control-power wiring to an outdoor tap box to facilitate the load-bank connection.

Tap boxes can be constructed with either bus bars for termination of wiring with lugs, or cam-type connector panels for quick connection. Small hinged panels can be provided in the tap box to permit covers to be closed even when temporary wiring is routed to the box. Special attention should be given to the choice of materials used to construct cam-type connector panels. As noted in NFPA 70: National Electrical Code (NEC) paragraph 300.20, nonmagnetic materials such as aluminum or fiberglass should be used to avoid inductive heating. In some situations, cam connector panels fabricated from plain carbon steel occasionally can make their way into the field—even when shop drawings indicate aluminum. These can reach temperatures exceeding 220 F from inductive heating when full load is applied, necessitating a retrofit with an appropriate nonmagnetic material.

Overcurrent protection: Proper overcurrent protection must be provided for all load-bank feeders, whether temporary or permanent. This can be an issue at times, especially with temporary load banks sized for only partial loading (such as might be used to eliminate wet stacking, or to heat-load a data center floor during commissioning). There is a disturbingly common misconception that temporary installations such as these are not subject to the overcurrent protection requirements of NEC. However, according to NEC Article 590, Temporary Installations, this is not the case. Paragraph 590.3(C) makes it clear that installations for testing fall under the scope of Article 590, and paragraph 590.4 specifically requires that wiring and overcurrent protection for temporary installations be provided in accordance with the applicable requirements of Article 230: Services, and Article 240: Overcurrent Protection. The permanent building electrical distribution systems should be designed to accommodate these requirements—cost and schedule can otherwise be powerful temptations to perform load-bank tests without the required overcurrent protection in place.

Conversely, if a large switchboard or substation is to be tested under full load, ensure that provisions are made to connect a large load bank. A main switchboard circuit breaker might be suitable on its own to provide the required overcurrent protection at full load. But in this case, consider providing an additional bussed switchboard section to land the load-bank cables.

Impact on total connected load: Many standby systems are designed with various bypass pathways for redundancy and to facilitate maintenance operations. These bypasses are likely to be tested during load-bank testing. However, keep in mind that the load bank presents an additional load beyond the normal building load, and possibly beyond the building design load. Ensure that a planned manual bypass or unexpected automatic-bypass operation during load-bank testing will not overload an upstream circuit breaker, resulting in a critical system power outage.

Spare temporary load banks: When multiple temporary load banks are to be rented for large-scale testing such as the commissioning of a new data center, consider ordering a few spares in case one fails. Such tests often imply big resource-intensive days, with representatives from several parties—owner, contractor, subcontractors, equipment and controls vendors, electrical and mechanical design engineers, third-party testing firm, and commissioning agent—on hand to witness the testing. One failed load bank can derail the entire test and require it to be rescheduled; having a spare load bank or two on hand can help ensure the test goes smoothly.

Infrared scanning: It’s worth mentioning that a load-bank test is also an excellent opportunity to perform infrared scanning. The more the system is loaded, the greater the likelihood that an impending failure will be detected during an infrared scan. In particular, when temporary load banks are brought to a site to test generator or UPS systems, consider loading other portions of the electrical distribution system for infrared scanning purposes.

Utility demand charges:  Most electrical utility companies include a demand charge as part of their overall monthly power bill, which may account for roughly half of the overall bill each month. It is not uncommon for the demand charge to be calculated by looking at the peak 15-min demand usage, then applying a charge based on that 15-min peak to the entire month’s bill. Full-load testing can result in high spikes in the facility demand load. Although the demand spikes may be of short duration, possibly lasting only a few hours, the impact on that month’s power bill can be substantial. Many utility companies understand this issue and are open to requests to reduce or eliminate demand charges during testing—it can be worthwhile to discuss this with your utility company a month or two before performing testing.

Summary

As the demand for reliable electrical systems increases, the need for load-bank testing will increase as well. Load-bank testing can go a long way to ensure system availability and conformance to code requirements. Careful consideration when specifying, installing, and operating load banks can help ensure reliable electrical and mechanical system operation while avoiding unpleasant surprises or unnecessary costs. 

Bearn is an associate electrical services engineer at KlingStubbins. He is a member of the Consulting-Specifying Engineer editorial advisory board.



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