FAQs - UPS Power Supplies Buyers Guide

An Uninterruptible Power Supply (UPS) is a system to automatically provide battery backup to connected loads when the mains power supplies fails or fluctuates. When mains power is present, a UPS system should provide power protection from typical mains borne pollution including: sags, surges, brownouts, spikes and electrical noise. The UPS charges its battery. When the mains power supply breaks momentarily (power outage) or for a longer duration (blackout) the battery provides power to the inverter section which converts the DC power from the battery into an AC waveform to power the load. When the mains power is restored, the battery is recharged. The load should not see any interruption in output supply from the UPS system when transferring between mains and battery power. This will depend on the type of UPS topology chosen: offline UPS (standby system), line interactive UPS or online UPS (double conversion) system.

There are three types of uninterruptible power supply and they differ in the levels of power protection they provide and cost. The 3 types including Offline (standby) UPS, Line Interactive UPS and double-conversion Online UPS.

• Offline UPS: provide basic power protection and low-cost battery backup when mains power fails or fluctuates (flickers). They are the cheapest and smallest type of UPS system available, typically available up to 1kVA/1kW in size. When switching between mains and battery power there will be a millisecond (ms) break in output power and the output waveform is typically a stepwave or square-wave. The break and waveform are compatible with the type of power supply used in most electronic and computer-type loads (a switch mode power supply – SMPS) and the SMPS uses its electrical capacitance and own electrical characteristics to cover the break in power and waveform output. Typical formats are desktop and tower with internal batteries. Suitable for home PCs, security systems, retail terminals, gaming consoles, network peripherals, routers, switches and storage devices.
• Line Interactive UPS: are next level in power protection. The operating principle is similar to that of a standby UPS. The inverter section is active and in standby mode but switches quicker than a standby UPS and the output waveform may be a stepwave or sinewave. A sinewave is what we expect from the mains power supply itself. Line interactive therefore provide a cleaner output waveform and faster transfer to battery power. Line interactive UPS are the most commonly installed UPS up to 2kVA for IT loads. Typical formats are desktop, wallmount, tower and 19inch rackmount with internal batteries. Some models have battery extension pack options, for longer runtimes. Suitable for more powerful PCs, and file servers
• Online UPS: provide the ultimate power protection. This type of UPS is also referred to as a double (AC-DC-AC) or triple conversion (AC-DC-DC-AC) UPS. In an online UPS the inverter is always powering the load and draws power from a rectified mains power (or generator supply) converting this into the levels of DC power required to charge the battery and power the inverter. This is the AC-DC part of the UPS conversion technology. The inverter converts the DC to AC power; the second stage of the power conversion to give (in a double conversion UPS) the AC-DC-AC power output. Online UPS also feature an automatic bypass which routes the loads connected to the UPS output to the raw mains power supply if there is a UPS fault. Loads connected to Online UPS are fully protected from mains pollution through the double-conversion process and digitally generated sinewave output. As the inverter section is rated for continues running, online UPS can typically be installed with additional battery extension packs for longer runtimes. This type of UPS is the most commonly used in server rooms and data centres and is available in floor standing and 19inch rackmount formats from 500VA to several hundred kVA in single and 3phase configurations. Suitable for file servers and storage devices, complete local are network (LAN) power protection, server cabinets, server rooms, data centres, telecoms, hospital and industrial manufacturing processes

The following is a list of the most commonly seen power problems, both in the UK and worldwide. In the UK we generally have a stable electrical supply who delivery is affected by several factors including demand on the grid a certain times of the day, our national grid electrical infrastructure which can see transformer and substation failures and weather conditions including storms and lightning. Inside the location where you run the loads to be protected, there can also be power problems created by local loads.

The most commonly seen power problems include the following:

• Spikes and Electrical Noise: are sudden surges in the electrical supply over a short-term period. They are caused by local lightning strikes and the switching of local loads. Fast transient spikes and long-term exposure to electrical noise can disrupt the operation of the power supplies within your connected loads, leading to processing errors, erratic operation and over term, component degradation and potential failure. Offline and Line Interactive UPS have built-in circuits to clamp or surpass spikes and electrical noise. Online UPS have similar but more sophisticated protection circuits, and digitally generated their output waveform which is not disrupted by the incoming mains power supply levels.
• Brownouts and Sags: brownouts are long duration low voltage levels (typically 10-15% below the nominal voltage) which can be just outside the operating input window of your connected loads. In the UK we expect the nominal mains power supply voltage we plug-into to be 230Vac 50Hz, single phase and 400Vac, 50Hz 3phase. Sags can be any momentary voltage lower than these levels whereas Brownouts will last for several minutes, hours or even days. When within the input voltage range of the power supply within your loads, these conditions simply cause the power supply to work harder to deliver the power required. When outside the input voltage window this will be seen as a power failure and your load can crash. Offline UPS will switch to battery power during brownouts and sags that fall outside their input voltage and frequency windows. Line Interactive UPS will typically have a built-in automatic voltage stabiliser which will boost the incoming mains supply level up before resorting to battery power for voltage and frequency levels outside their input window. Online UPS will be unaffected as their inverter section is constantly running, powered either from the rectified mains power supply or battery.
• Surges: surge conditions in electrical supply are the opposite to Sags. They are increases in the electrical supply voltage (typically above 10-15%) and are referred to as ‘high mains’ and can again be caused by sudden changes in the load on the local electrical grid or building where you operate. Surges will cause the power supply in your connected loads to work harder, and over time can lead to wear and tear and eventual component failure. For Offline, Line Interactive and Online UPS the operation is the same as for brownouts and sags. If the operating conditions for the local mains power supply are within the UPS operating parameters, the UPS will continue to operate without resorting to battery power. Outside of these they will use their connected batteries and discharge them until either exhausted or the mains power supply stabilises.
• Power Outages and Blackouts: power outages are momentary fluctuations in electrical supply voltage levels which are seen by connected loads a power breaks. The level of mains voltage available falls outside the input voltage window of your connected loads. Typical examples include local storms which can make the lights flicker and alarms to sound, and substation switches as the electricity utilities make automatic changes within the grid to power local supplies. Blackouts are longer duration power failures lasting minutes to several hours. Whilst less common than momentary power interruptions, their effect can be as severe for IT operations that cannot be interrupted and must continue to operate. Protection for these types of power problem are provided by a combination of Online UPS with battery extension packs and generators.

Frequency problems and harmonic distortion are not commonly seen within the UK and where they occur are local in nature, typically caused by local site electrical infrastructure and the systems being operated. See our guide to UK mains power supply voltages for more information on the voltage and frequency levels to be expected.

UPS systems are sized according to the amount of power the connected loads will draw. This is measured in VA (Voltage x Amps) or Watts. Voltage represents the local mains power supply voltage which in the UK is 230Vac, 50Hz (single phase) or 400Vac, 50Hz (3phase).

VA is referred to as the ‘Apparent Power’ drawn by an electrical load.

Apparent Power (VA) = Volts (V) x Amps (A)

For a computer type load drawing 4 Amps the calculation would be:

VA = 230 × 4 = 920VA

The nearest UPS size to this would be 1kVA. It is always recommended to size your UPS to run at 80% or less. For more applications we add a further 20% for headroom.

VA = 230 × 4 = 920VA x 1.2 = 1104VA or 1.1kVA

The nearest UPS size to this would now be 1.5kVA.

The Amps used in the formula can be obtained from rating plates, brochures, datasheets and manuals, or with the correct electronic/electrical devices, measured. It is important to note that when taken from rating plates and published data, the figures will be topside and actual running may be at 50% less.

For a three phase application the VA or kVA calculation is made per phase i.e.

Three phase Apparent Power = kVA Phase 1 + kVA Phase 2 + kVA Phase 3

It can be harder to calculate the Watts (W) required. Watts is referred to as the ‘Real Power’ required by the load. The Watt rating of a load is measured in Watts and this number is used to size batteries for a load size and runtime. Watts is a typical measurement for linear resistive loads and can be calculated as:
Real Power (W) = Amps (A) x Volts (V).

The difference between Apparent Power and Real Power is known as the Power Factor. This is the degree to which the current and voltage waveforms are out of phase with one another.

Real Power (W) = Apparent Power (VA) x Power Factor (pF)

or

Apparent Power (VA) = Real Power (W) / Power Factor (pF)

See our VA to Watts and Power Factor conversion table for more information.

The amount of battery runtime you need from your UPS depends on whether you want to protect from just momentary power outages or longer duration blackouts. UPS manufacturers will publish runtimes on their datasheets, and these are a guide only. The amount of battery runtime will also depend on the size of UPS system chosen, the connected load (VA/Watts), the size of the battery pack installed, how charged the batteries are and even the local temperature and age/condition of the battery set.

Offline UPS have a small battery (1 or 2 blocks) built-in. They are designed to provide power backup for short duration runtimes at full load of around 3-5 minutes. If you need longer, choose a larger model than the calculated load. For example, a 1kVA offline or standby UPS with a 3minute battery will provide 6-10 minutes at 50% load (500VA).

Line Interactive UPS again have built-in battery sets and may have the facility for connection to external battery extension packs. The oversizing principle for battery runtime sizing applies. Typical runtimes with battery extension packs can run from several minutes up to 2-3hours at lower rated loads.

Online UPS have inverters rated for continuous running and plug-in battery extension packs are generally available for most models up to 10kVA. Their higher DC voltage specifications (from 36-60Vdc for single phase UPS and larger for 3phase UPS) means that more battery blocks are required for any given Ampere-hour (Ah) rating. A battery block is either 12Vdc or 6Vdc rated. A 36Vdc rail in a UPS will require 3×12Vdc blocks etc. The batteries may be housed inside the UPS cabinet (up to 20kVA) or in separate battery cabinets with installation by certified engineers. Typical runtimes are 5-10 minutes as standard with runtimes available up to 4-8 hours or longer. For larger kVA rated UPS or long runtimes of 10-12 hours or more, a local standby power generator is recommended.

How long you need to run for depends on your load an application. Most UK power outages only last a few seconds to minutes. You should select a battery runtime that either allows you to ride through power outages or close down your systems in an orderly manner (manually shutdown or using software) should the blackout be of a longer duration.

Blue Light services, hospitals, industrial manufacturing, server room and data centre facilities may require uninterrupted service provision, and this leads to installing a UPS system with sufficient battery or generator power for several hours.

Note that additional battery packs increase the overall space required for the UPS system and weight (kg) loadings. This will mean allocating more floor space, or space within a server cabinet. For large UPS systems a separate UPS room may be required, typical in mid to mega-sized datacentres.

The larger the UPS battery, the higher the total cost of ownership (TCO) as the batteries will require replacement during the life of the UPS system. For lead acid batteries the recommended working life if 3-4 years (5-year design life) or 7-8 years (10-year design life). For lithium batteries, the working life is longer, calculated at 10 years. Lead acid battery performance and working life is also dependent on room temperature. The recommended ambient is 20-25°C with the design life halving for every 1 degree rise above 30°C.

UPS are available in several formats including desktop, wallmount, rackmount, tower and floor standing. The topology of the UPS and its size, limits the form factors available.

UPS Topology	Form Factors
Offline (standby)	Desktop, Wallmount, Tower
Line Interactive	Desktop, Wallmount, Tower, Rackmount
Online	Desktop, Wallmount, Tower, Rackmount, Floor Standing

If you want to use an Offline UPS for rackmount applications, then the UPS can sit on a suitable rack shelf.

When selecting the UPS form factor consideration should be given to the physical space available for and required by the UPS and the total weight loading. It is good practice to leave sufficient space around the UPS for airflow and cooling. A UPS is not 100% efficient and so there will be some heat generated from its operation. Batteries are also endothermic (absorbing heat) when charging and exothermic (generating heat) when discharging. A reasonable working space is also required with good access for UPS service and maintenance, and future battery replacement.

Safe manual handling is also an important aspect. In general, the maximum recommended weight that one person should lift is 25kg for men and 16kg for women. These limits are guidelines, and should be observed but the shape of an object such as a UPS can also play a part in how safely they can be managed. For UPS systems, a 2-person team and lifting or trolley arrangements may be required to place the UPS and its battery packs inside a room or server cabinet.

The physical size of the UPS system and its battery packs will also dictate location. For any rack mount UPS, the available U-height within a server cabinet will an important consideration. UPS depth should also be considered to ensure the server cabinet maximum internal depth is sufficient.

In a server rack, the UPS should be installed at the bottom of the rack due to its weight and batteries. In terms of temperature rise, the higher the location within a server rack, the higher the temperature and for lead acid UPS batteries, a 20-25°C (or less) ambient inside the rack is recommended to ensure battery life and performance. If in doubt the UPS should be installed with its own external temperature sensor (if available) or a suitable environmental monitoring system installed.

Location distance to the UPS load to be protected should also be considered. Longer cable runs can lead to voltage drop (and potentially interference) and it is good practice to install the UPS as close to the loads to be protected as possible.

In Centralised UPS installations, where a single UPS power a complete room or facility, such as a server room or data centre, additional power distribution should be considered alongside installation of a UPS maintenance bypass. For reference a decentralised UPS installation is one where each location has their own UPS i.e. one UPS per server rack or data cabinet.

UPS mains power and load connections is reasonably straight forward. In the UK, UPS power supplies up to 3kVA are supplied as standard with power cords to plug into a BS 1363 wall sockets. Hardwired versions are available for some models as a special order and are connected to a fused spur. Above 3kVA, UPS systems are hardwired with some UPS models up to 10kVA offering a mixture of hardwired and local output socket connections.

UPS power supplies under 10kVA will offer either C13 or C19 outlets or a mixture of the two. These are the IEC sockets, which in the UK we sometimes refer to as kettle-lead types. C13 outlets are rated at 10A, 250Vac and C19 at 16A 250Vac. Some small UPS below 1kVA may also offer BS1363 UK or Schuko outlets, making it easier to connect plug-in loads.

A UPS system may be installed with a power distribution unit (PDU) which provides additional outlets. The PDU will have a C13 or C20 plug or be hardwired for connection to the UPS. It is important to ensure that the total load on the PDU does not exceed the total capacity of the UPS outlet it is connected to or the UPS itself.

So, to connect the loads to be protected to a UPS system, you ned to list the number of loads and their plug requirements and select a suitable UPS power supply (and potentially PDU) arrangement.

Note, it can be useful to label each load connection, whether direct to the UPS or PDU for later reference, especially in server racks. This can help with load planning and also business continuity testing to ensure every critical IT load is UPS supported.

The most commonly purchase accessories for a UPS include the following:

• UPS Software and Monitoring Apps: this is the most commonly selected for clients who want to monitor their UPS remotely and/or configure an automatic shutdown of their connected loads. The software is generally available for download from the relevant UPS manufacturer’s website.
• Interface Card: plug-in cards are available for UPS systems with an appropriate communications slot. Cards include SNMP, signal contacts and Modbus.
• UPS Maintenance Bypass: this is an external device, either wall mounted or rack mounted that allows the UPS system to be bypassed for maintenance, service or removal. The bypass will use either plug-in (3kVA UPS and under) or hardwired (above 3kVA) connections.
• Power Distribution Units: PDUs provide additional outlets and way to distributed protected power paths within a server rack. For installations with A and B power supplies, the PDUs will be connected to one or more UPS systems.
• Automatic Transfer Switches (ATS): also referred to as an STS (static transfer switches), powers connected loads from one of two power sources. These can be two UPS systems or a combination of one UPS and one mains power supply. The ATS/STS increase the resilience of the UPS installation by providing a redundant power path.

Please refer to each UPS product page and the related products tab, brochures or data sheets for a list of the available UPS accessories available.