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rebekahlin-blog · 7 years ago

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Emergency Lighting: UPS or CBU? Credit: Voltimum

Voltimum interviews ABB Emergi-Lite’s Jenny Paramore about a growing trend for contractors to install a UPS (Uninterruptible Power Supply) instead of a static inverter, also known as a Central Battery Unit (CBU). While the two types of unit are similar, there are important differences that mean a UPS may not deliver power in an emergency.

Why do electrical contractors need to know about powering emergency lighting?

Health and safety systems are in the public eye following the Grenfell Tower disaster in June 2017. Therefore, now is a good time to check that safety and emergency systems are effective and fit for purpose.

Installing the right equipment will not just protect a building’s occupants should the worst happen – it will also protect the electrical consultants and contractors who are responsible for specifying, purchasing and installing such systems.

Why are contractors installing UPS systems instead of CBUs?

Simply put, UPS and CBU systems are quite similar at first glance, but the purchase price for a UPS is often significantly lower than a CBU – so a UPS can seem to offer an opportunity to find a saving on the overall cost of a project.

Once in operation, both systems provide backup power in the case of an outage. In addition, both use battery systems built up in modules to meet the required power rating, as well as components such as converters and inverters to convert power between alternating current and direct current.

Because they have a lot in common, it’s easy to think they’re interchangeable. However, there are important differences that mean that CBUs are more resilient in unusual fault conditions that can arise in a building’s electrical system, during an emergency situation.

So why are CBUs more expensive?

Emergency lighting systems need fail-safe power for lighting to guide people to safety should the worst happen.

Fire or explosion damage to grid-connected equipment can lead to unusual fault currents building up. Therefore, the backup power supply for emergency lighting must be able to clear these faults as well as providing up to three hours of power for emergency lighting.

Being designed specifically for emergency lighting installations, CBUs have more energy storage capacity than comparable UPSs. This gives a higher level of overload protection that enables the CBU to operate a facility’s main switchgear to clear high-level fault currents and disruptions. For example, ABB’s Emergi-Lite EMEX battery system can typically deliver three and a half times the power of its output rating.

UPSs do not have this capability built in, as their main purpose is to provide backup power in the case of a straightforward outage of mains power, where high fault currents are unlikely. As a result, UPSs are not rated to deliver power to operate main circuit breakers. The result is that a UPS may not work as expected when used for emergency lighting – and therefore it may not operate when it is needed most.

Therefore, the high level of overload protection and fail-safe performance accounts for the price difference between a CBU and a relatively simple UPS.

Where should contractors install UPS systems?

Although they do not always meet the requirements for emergency lighting, UPS systems are ideal to provide backup power in telecoms installations and data centres. They usually provide power over a period of minutes, giving enough time for backup generators to start up.

They also have a secondary use in absorbing and injecting power to ensure a consistent high- quality power supply. This protects sensitive and high-value electronic equipment from switching transients that arise when large loads are switched on and off elsewhere on the power grid.

What standards and regulations are important for central power supplies for emergency lighting?

Emergency lighting systems in the UK are governed by a range of standards. These include fire safety and workplace safety regulations, as well as standards that govern the design, material, size, clarity and location of light fittings and luminaires, central power supply systems and regular testing.

Central inverters, or CBUs, have traditionally been used as the central power supplies. They must meet the BS EN 50171 standard and have the ability to clear faults and then provide backup power to illuminate escape routes in commercial, industrial and residential facilities.

Whereas a UPS system can also achieve this, it must be sized to provide the required power – only then will it be fit for purpose for emergency lighting.

Rather than dedicating additional engineering work to uprating a UPS system, it is simpler for a contractor or consultant to select a CBU certified to BS EN 50171. This provides reassurance that the installation is fit for purpose.

Are there any other benefits of a CBU?

The components inside a CBU include a set of batteries, battery charger, control circuitry, alarms and instrumentation. These are engineered to provide up to three hours for all emergency lighting lamps and luminaires with no more than a five-second response time.

CBUs are also designed to meet the legal requirements for periodic testing to meet fire safety regulations. Testing can be expensive, time consuming and disruptive, therefore some CBUs have in-built capability to meet the IEC 62034 standard for automatic testing of emergency lighting systems.

This function will replicate a power cut and then check emergency lighting circuits to ensure they are working as expected. It will then upload test results to building management systems to ensure a complete centralised record. This relies on compatibility with building management including BACnet and LONWORKS to help building owners and managers keep reliable and consistent records.

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