04.26.2022

Testing power sources, such as uninterruptable power sources (UPS) and diesel generators, ensure that these devices will provide the required amount of power when needed. A load bank tests these power sources by developing an electrical load. Various load configurations permit testing under different conditions.

A load bank is a self-contained device that has the load elements, controls and cooling systems it needs to operate built-in. Power is delivered to a load bank, which then consumes the electrical power by applying it to a power source and either converting or dissipating the source's power output. Load banks use the energy produced to protect, support or test a power source while a "real" load or actual building load uses energy output productively.

What Is the Purpose of a Load Bank?

Load banks ensure the quality and reliability of your emergency power sources. They verify the accuracy of the generators' output capacities by creating a variety of different loads that replicate real-world use.

If you own, sell or operate generators, especially for backup power, you need to test them regularly. Without testing, you cannot be assured the generator will provide the required power when the lights go out. Many facilities require load tests of all backup power sources. Prime power generators benefit from load bank use, particularly in lightly loaded situations.

What Does a Load Bank Do?

A load bank develops an electrical load. Load banks are absolutely essential for emergency sources of power, as they verify the power source's rated output capabilities before the power source is called into actual use. A load bank should mimic the real load a power source would see during an actual use scenario. The devices usually have different levels of load, which allows critical systems to be tested under a variety of conditions.

Who Uses Load Banks?

You need this type of testing bank if you have generators or uninterruptable power sources in your business. Best practices for testing emergency power sources is regular load bank tests.

If you have diesel generators on-site, you may use a load bank to augment lightly loaded situations and keep your generator running properly. Wet stacking occurs when unburned fuel remains inside the system. The unburned diesel condenses and combines with soot in the exhaust system, leading to wet deposits on exhaust components. Left unchecked, wet stacking reduces engine efficiency until it requires a complete overhaul to restore its former power.

The best way to prevent this from occurring is to test the generator with a load bank, which enables you to complete two maintenance tasks with one action. Monthly testing is a requirement from the National Fire Prevention Association, in its Standard for Emergency and Standby Power Systems. You must test the generator at full load to maintain the minimum exhaust temperature recommended by the manufacturer. An alternative test allows you to use 30% more than the rating on the generator's nameplate for at least half an hour.

A novel use for these banks is for renewable energy sources. The Hawai'i Natural Energy Institute (HNEI) has incorporated a load bank into the power system on the island of Moloka'i. The island could not add more photovoltaic cells because it posed the threat of producing too much electricity and dropping the power produced by the island's diesel generators too low. The addition of a resistive load bank allows the island to add more solar cells because the load bank moderates the amount of power the island gets by absorbing any extra electricity produced.

Difference Between Resistive & Reactive Load Banks
Load banks come in many different forms based on the load they apply to the power sources they test. Resistive and reactive load banks are two primary forms. Each of these has its benefits for specific situations. Understand the differences between resistive and reactive load banks to find the best options for running tests on your power sources.

Resistive vs. Reactive Load Banks

Depending on your generators and how you use them, you may need resistive, reactive or both types of load banks. These devices put different kinds of loads on the generators. Resistive models are the most common, while reactive include inductive and capacitive power loads.

1. Resistive Load Bank Uses

Resistive load banks draw power from a generator in the same way lights or appliances would. These testing devices work well with generators used for general power purposes that involve transforming electricity into heat or light. Light bulbs, lamps and space heaters are examples of tools that convert electrical energy into heat.

These types of load banks draw a specified real power load, in kW, on the generator to ensure the operation of the power supply's cooling system, exhaust and engine operation. They do not test the generator for full apparent power in kVA, which is what a reactive load bank tests.

2. Reactive Load Bank Uses

Reactive load banks can simulate inductive loads, often used commercially in construction or in backup power supplies. Inductive loads create a lagging power factor, whereas capacitive loads generate a leading power factor. Of these two types, inductive appears more commonly in objects that turn power into motor operation through magnetism.

For facilities that rely on emergency generators to run equipment for business-critical operations or life support, such as data centers and hospitals, reactive load bank testing is the most effective.

How Do Resistive Banks Work?
Resistor load banks convert all of the applied electrical energy into heat. Larger kW load banks typically have an integrated blower for cooling. Some load bank designs, such as Duct Mounted, do not have their own cooling system but rather rely on cooling airflow from other sources such as an engine radiator.

Self-contained resistive load banks are commonly found up to 3,000 kW. For full kW load testing, make sure to choose a load bank with a capacity equal to or greater than the power source at the rated output voltage.

How Do Reactive Load Banks Work?

Reactive load banks test the full apparent load in kVA of the power source. Though not as commonly used as resistive load banks, reactive models are part of the testing requirements set out by NFPA 110 for nonunity power factor equipment field or factory acceptance testing.

Unlike resistive loads that generate a power factor of 1.0 and a load of 100%, reactive has a 0.8 power factor with a 75% load. This difference in power factors and loads leads to voltage drops from the generator of 25% more than from the resistive loads. In this way, reactive load banks test for systems that have a sensitivity to voltage dips.

Advantages of Resistive Load Testing

Resistive load testing is very common as it is the most cost-effective way to test a power source. For generators, this method checks the ability of the generator to respond at a full load. With the generator producing full power, you can assess the capability of the mechanism's fuel and cooling systems to operate under the effort.

Resistive load banks also prevent diesel generators from losing efficiency through wet stacking, which happens when unburned fuel clogs up the exhaust system. By putting a higher load on the generator than it typically experiences, a load bank encourages the engine to burn more fuel, reducing the chances of wet stacking.

If the condition has already set in, using a resistive load test at full power can reduce the effects by burning excessive moisture in the engine.

Another benefit of using resistive load banks is they allow the generator to exercise its exhaust system to reach high enough temperatures to properly treat the fumes flowing through it and operate at its peak.

Additionally, resistive load banks can be used to supplement the actual load on a generator to optimize its performance. Often you will find generators operating at 50% loads or lower, which can lead to wet stacking. Resistive load banks with Auto Load Leveling Controls can automatically add and subtract kW to help maintain a minimum load.

Reactive load testing offers some additional benefits that resistive testing cannot and can warrant the extra expense for certain applications.

Advantages of Reactive Load Testing

Reactive load testing more closely replicates the type of load a generator will experience during typical use in kVA and kVAR. It helps evaluates not only transient loads, but also load sharing and alternator capacity.

When doing testing with a reactive load on systems in parallel, you can look for hot spots using an infrared examination of the electrical system. You can also use the load conditions to assess the electrical connections, cables, bus work and other components.

Depending on your equipment, you may need both reactive and resistive load testing. If this is the case, we have models that can fulfill both needs. Should you need a unit with the capabilities of both resistive and reactive testing in a single model, we have a solution for you. These load banks have high capacities up to 1,875 kVA and load up to 1,500 kW. Resistive-reactive models do the job of two load banks in one powerful unit.

Inductive Load Banks
An inductive load bank is normally used in conjunction with a resistive unit to provide a lagging power factor load test. This provides the capability to test the generator set fully at 100% nameplate kVA rating. Inductive load banks add inductance to the load and therefore reduce the PF to less than unity (typically 0.8pf on diesel gen-set). Inductive load banks are rated in kilovolt-amperes reactive (kVAr).

Load Bank
A load bank is a device that generates a prescribed amount of electricity draw to test the reliability of electrical switching, generator output, uninterruptable power supply (UPS) systems and cooling in a data center.

Rack-mount server-simulating load banks discharge heat while consuming power to accurately mimic the load of a server processing a workload. Data centers test cooling and power supply operation with load banks rather than software simulations for more real-world accuracy, such as air-flow blockage in the rack caused by the server-sized load bank, or dust in the air from improper filtration. Avtron, ComRent and Mosebach are server simulator load bank manufacturers.

A load bank test simulates real operation by applying the load in situ, with real temperatures and relative humidity, but it is not an exact reproduction. The load bank only draws power (current and voltage) to test the infrastructure, and then dissipates it safely as heat.

Load banks are either portable or fixed. Fixed load banks suit outdoor use, while portable load banks can move around the data center and between sites to test specific power setups.

Load banks cannot mimic the random peaks and lulls of real data center equipment power usage, but they can approximate the equipment's requirements in a controlled setting with high levels of accuracy.

Load testing should be included on UPS maintenance schedules. Capacitive load banks, or capacitor banks, commonly test UPS and other electronic loads by creating a leading power factor. Some power supply vendors, such as Emerson Network Power and Power Systems Specialists, offer load bank testing as a service.

Inductive load banks, by contrast, create a lagging power factor. The third common load bank type is resistive, with a lagging power factor. Resistive load banks, used frequently for industrial purposes, provide equivalent loading on the generator, and are usually portable.

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