CPS or UPS? Better safe than sorry…


    An increasing number of installers and contractors are opting to install uninterruptable power supply (UPS) rather than central power supply (CPS) systems. However, the two technologies do not offer the same high level of protection, warns ABB’s Jenny Paramore 

    Awareness of safety and critical backup systems has never been higher. Public safety has been in the public eye since the Grenfell Tower tragedy in June 2017. Many building owners are now taking the opportunity to look carefully at all types of fire and emergency safety equipment that is installed in their commercial, industrial and residential buildings to ensure it will be up to the job. 

    In addition, many electrical contractors and consultants are re-examining standards and certifications of equipment. This will ensure they are specifying and purchasing the right type of equipment. 

    It is essential that electrical contractors, facility operators and building owners fully understand what electrical equipment is best suited to minimising hazard to people and property in an emergency. If and when it is needed, such equipment will protect people inside the building. 

    However, it will also give peace of mind and professional protection to the electrical designers and contractors responsible for specification, procurement and installation of the equipment. The building owner and operator will also benefit from this peace of mind. 

    Power outages are likely in a fire or other emergency events, and to minimise risk to a building’s occupants, reliable fail-safe power must be made available for safety systems such as emergency lighting, fire pumps, fire lifts and sprinkler systems. 

    Such safety systems literally provide the guiding light that leads people out of the building to safety. In an outage, the backup supply will need to sense the loss of power and switch over to battery power for up to three hours. 

    Outside of emergency operation, operators must test their fire safety and emergency systems regularly to ensure that they will be ready at a moment’s notice if needed. Therefore, many operators specify equipment that is compatible with automatic testing. Some systems are even integrated into building management systems for automatic logging of test results. This helps facility managers minimise red tape and helps them meet safety legislation. 

    However, there is an important additional requirement for emergency power supplies. Grid-connected equipment may be damaged by an explosion or fire, which can lead to abnormal fault currents arising on the network. The emergency power systems must therefore have the capability to clear these faults, as well as providing enough power to ensure that emergency lighting and other equipment work properly for the expected duration.

    CPS or UPS system?

    In practice, either a CPS or UPS system can provide all of this functionality but there are important differences that mean a UPS is not a direct replacement for a CPS. 

    If using a UPS, the electrical designer and installer will need to make additional allowances for fault currents and testing. 

    The main purpose of a UPS is to provide backup power for computer servers, for example in data centres or telecommunications installations. In this job, it will provide a steady and consistent power for server racks, electronic equipment and cooling systems. This will ensure continuity of data services and protect revenues and reputation of the operator and its customers. 

    To achieve this, the UPS will provide backup power over a few minutes as a bridge to ensure power continuity from the time of an outage until a backup generator can come up to speed. 

    It will also provide backup over seconds to ensure high power quality, which will in turn protect high value electronic equipment. In this mode, the UPS will inject and absorb power to overcome power sags and overvoltages that may result from large loads being switched on and off elsewhere on the power grid. 

    In recent years, there has been a noticeable increase in the number of electrical installers and contractors purchasing UPS systems over CPSs. CPSs – or static inverters – are the traditional choice for emergency lighting and safety systems, as they are designed for emergency applications. 

    So, although both systems have their immediate similarities, CPSs are more suited to emergency applications than UPSs. 

    Both contain the same types of components as they integrate battery systems that are built up in modules to meet the voltage, power and duration required. Both also make use of components including converters and inverters to convert power between direct current and alternating current.

    Cost and operational considerations 

    In terms of functionality, both provide backup power in an outage. Furthermore, the required backup duration can met from either type of unit. 

    However, UPS systems are notably less expensive than CPSs, so can seem to be an opportunity to reduce overall project costs. 

    Nevertheless, there are important technological differences that mean a UPS may not operate as expected in an emergency when it is needed the most. 

    CPSs are specifically designed for emergency lighting situations. One of the most notable differences in CPSs compared with UPSs is that they have larger batteries, which gives them a much higher overload protection. They are able to deliver a pulse of high power, which gives them the capability to operate a building’s main switchgear and clear any high-level fault currents. After completing switching operations, the CPS can provide up to three hours of backup power for fail-safe emergency lighting. 

    For example, ABB’s Emergi-Lite EMEX battery system typically has three and a half times the power of its output rating. This gives it the capability to clear substantial fault currents and disruptions before providing power for backup lighting.

    In comparison, a UPS system will generally be limited to provide no more than its power rating. This will give it the capability to provide the same backup duration for emergency systems but it will be unlikely to operate a building’s switchgear as it does not have the required power output. As a result, the UPS may fail to work as required, with the result of cutting power for emergency lighting, fire pumps or lifts and putting the building’s occupants at risk.

    It is possible for knowledgeable consultants and contractors to specify a UPS to perform the same role as a CPS – as long as they take the crucial step of uprating its battery system to supply the high-power impulse to operate switchgear. Only then will the UPS be fit for purpose as a power supply for an emergency lighting system. 

    Rather than performing this additional work, it is much more straightforward to install a CPS in the first instance, as it already meets the required standards. This gives the reassurance that the installation will function as expected.

    Are there any other benefits of a CPS?

    CPSs can also help building owners reduce their facilities management budgets because they have been designed to meet safety standards for buildings. 

    A wide range of standards governs safety and emergency lighting systems in the UK. These include fire safety and workplace safety regulations, as well as standards that govern the design, size, material, clarity and location of light fittings and luminaires, central power supply systems and regular testing. 

    CPSs must meet the BS EN 50171 standard, as they are traditionally used as central power supply systems.

    Under the legislation, building owners and operators must carry out regular testing of fire safety and emergency equipment. Manual testing of emergency lighting systems can prove costly and time-consuming and many operators are keen to save operating budgets. 

    In support of this, CPSs can integrate automatic testing of emergency circuits. These will help the operator meet the requirements of the IEC 62034 standard for automatic testing of emergency lighting systems. 

    This function can be set up to regularly replicate the conditions experienced during a power cut and then test the operation of the emergency lighting circuits. This is an essential check to ensure that they function as expected. 

    It will then generate a report and highlight any circuits that require inspection and maintenance, before logging the results into a building management system. 

    This will ensure that the building managers and owners have a complete centralised record of safety tests. This capability depends on compatibility with building management, such as Lonworks and BACnet. 


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