When should a CBS be installed instead of a UPS?

Power Control
23 Dec 2020

CBS (Central Battery Systems) or as they may also be known as a CPS (Central Power Systems) and UPS (Uninterruptible Power Supplies) share many of the same design and operational similarities as one another. Both systems provide backup power in the event of a mains failure or fault. Both utilise modules within the battery system to meet the required power rating and both contain similar components. However, it is important to realise that they are not interchangeable and recognise the reasons why.
Since the Grenfell tower tragedy in 2017, public safety has been at the forefront building design and construction. Landlords of existing infrastructure have been re-assessing their existing fire and safety equipment, including critical backup systems.

Earlier this year, the government set out proposals for a new building safety regime, the new regulations will initially apply throughout the lifecycle of new builds and will later apply to the occupation stage of existing buildings.

Contractors and consultants have taken the opportunity to review standards and certifications of equipment that they specify for projects. Ensuring that suitable equipment is being installed. Suppliers of such equipment are also reviewing and updating their advice and product documentation to ensure compliance with the latest review.

In addition, greater pressures have been put on mitigating safety risks within any building including commercial, industrial or residential. Ensuring critical backup systems are deployed, installed and maintained to meet the relevant regulations have been no exception.

Power outages are a common occurrence during a fire, which is why installing a secondary power supply to support life safety systems is crucial. Such systems include emergency lighting which illuminate the fire escape route, firefighter lifts and fire suppression systems including sprinklers.

With the knowledge that the buildings life safety equipment is up to standard, and a source of failsafe power can be relied upon, building owners, electrical contractors and consultants will have peace of mind that in the event of a fire or other emergency event, members of the public are safe.

While both a CBS and a UPS contain the same types of components, Central Power Systems are designed to adhere to a set of government standards that have been put in place to ensure a minimum level of safety provided by a CBS to the life safety equipment that they are designed to support. The most familiar is BS EN 50171.

This standard sets out a requirement for a minimum of 120% permanent overload capacity. CBSs are specifically designed to meet this requirement as they have larger batteries, which gives them a much higher overload capacity. After delivering a pulse of high power, operating the main switchgear and clearing any high-level fault currents, the CBS must provide up to three hours of backup power for emergency lighting.

In comparison, a UPS is typically limited to provide up to its power rating meaning that without over sizing the UPS, which can lead to other issues and add unnecessary costs, it would be unable to tolerate an overload. As a result, the UPS may fail to work when required and put public safety at risk.

Also set out in the EN50171 is the requirement for the batteries to have a 10 year design life and the charging current must have a 10 hour capacity (must be at least a C10 rated battery).

Specialist UPS systems can be installed with 10-year design life batteries, but these are often specified for specialist applications like within healthcare facilities to meet healthcare safety regulations. CPS are supplied with 10 year design life batteries as standard.

Polarity reversal protection where reversed battery connections can be done without damage or impact is also referred to in EN 50171, a feature of all CPS.

Another regulation is the IEC 62034 which supports integration of automatic testing of emergency circuits. A CBS can integrate these tests making the task of carrying out regular testing of fire and safety equipment less time consuming and save on operating budgets.

BS 5266-1 covers the speed in which power must be restored to any life safety system. A CBS must be able to provide and recover 80% autonomy within 12 hours in the event of a mains fail.

The final directive to note is EN 60598-1 which stipulates that the CBS must be housed within an IP20 metal enclosure. Guidance is also given with regards to operating modes. An ‘Uninterrupted mode’ must have the load continuously fed by the inverter. A ‘changeover mode’ must have the load fed continuously though the bypass and a ‘changeover mode with control switching’ needs to include a switching device for emergency only loads.

CBS Topologies
CPS are available as either static or modular units. Both systems include batteries, a battery charger, control circuitry, alarms and instruments and are designed to support the same applications; life safety systems. How to choose the most suitable topology depends on the existing infrastructure, budget and the likelihood of future expansion.

Static CPS are a cheaper alternative for those not expecting a significant infrastructure growth in the near future. Modular CPB architecture are more suitable for those who require additional flexibility. They are ideal for facilities that are expecting future expansion. Modular CBS also provides the flexibility to have N+X redundancy within the cabinet whereas to achieve the same redundancy in the static alternative, more than one unit is required.

Power Control has partnered with leading UPS and CBS manufacturers, Borri Spa and Legrand to provide a comprehensive CBS product portfolio that includes both static and modular architecture. From supply and installation to commissioning and maintenance, Power Control offers a complete service for consultants, contractors and end users.