Categorizing UPS Loads and Load Types

Hardly anyone will argue that the need for uninterruptible power supplies (UPS) is on the rise as ancient power supply infrastructures creek under the weight of increasing demand for energy worldwide. But why do power protection load types need to be categorised?

Firstly, business managers must assess what UPS loads are in terms of their criticality to the continuity of operations in the event of a power cut. Usually, uninterruptible power supply loads are categorised as critical, essential and non-essential.

Another crucial assessment at the design stage of a UPS system is how these loads are synergised, i.e., which loads affect other loads?

Computer loads, in terms of a retail business for example, may affect other systems that are part of facilities management. This may include security cameras, door entry systems, lifts, escalators, PoS terminals, kiosks, cash machines and so forth. In a warehousing business, computer loads may also have a significant bearing on the ability of the business to handle both in-bound and out-bound goods. All of this needs to be taken into account when assessing the criticality of UPS loads.

UPS loads also need to be categorised in terms of their electrical draw and the effect it has on electrical systems; whether they are capacitive, inductive or resistive. This will have a bearing on the size and type of UPS system to be installed.

Load Categories. Critical loads directly affect the ability of an organisation to operate and must either be kept running when the mains power supply fails or be powered down in an orderly manner to prevent systems crashes, data loss or corruption, and life-shortening hardware damage. Their routine operation can also be interrupted when the mains power supply is polluted.

Essential loads provide secondary support services and may be required for health and safety reasons or to maintain ambient temperature. Whilst requiring a form of back-up in case of mains power supply failure, they do not require uninterruptible power and can be allowed to fail or ride through the time it takes for a generator (or alterative back-up system) to start up. Examples include air-conditioning, heating and emergency lighting.

Non-essential loads are those that an organisation can afford to lose when the mains power supply fails. For example, general lighting and non-essential printing services.

Some critical loads, especially sensitive medical and scientific equipment, require tight voltage and frequency regulation and this is only possible from the continuously running inverter of an on-line UPS. Essential loads do not need the quality of supply provided by a UPS and can be powered directly from a generator. This will allow the overall size of UPS to be reduced. Non-essential loads do not require any power protection at all.

The Effect on the Electrical System of Critical UPS Loads. In terms of type, UPS loads are referred to as either linear or non-linear, depending on how they draw their current from the mains power supply waveform. They will be inductive, capacitive or resistive.

An inductive load is one the waveform of which lags the voltage waveform and has a potentially high in-rush current at start-up. Examples of this type of load are SMPS (the most common form of power supply unit in use today and the type of computer loads most often found behind today’s power-hungry data centres), transformer or motor. This may be tempered by a soft-start facility.

Capacitive loads are those that lead the voltage waveform with potentially high in-rush current at start-up. An example of this is the latest high-end server technology such as Blade of Edge Servers.

A resistive load is one that has no inductance or capacitance, an example being a resistive load test bank heater element where the device typically has no initial switch-on surge and the current drawn rises immediately to a steady running state.

Whether a load is inductive, capacitive or resistive will determine its power factor and this in itself greatly influences the overall size of the UPS and generator (or alternative source of back-up power) to be installed. By convention, an inductive load is defined as a positive reactive power and a capacitive load is defined as a negative reactive power. However, power factor is never shown as positive or negative; rather it is displayed as either lagging or leading.

Assessing load types, how they are synergised, and their effect on electrical current is critical to correctly sizing and designing UPS solutions to get maximum power protection and value for money. Expertise from specialists like Riello UPS, whose business is to fully comprehend UPS loads and load types, cannot be overlooked. More detail can be found in a fantastic book on UPS – The Power Protection Guide.