How to Power Emergency Evacuation Lifts with an EN50171 Compliant Backup System

A central power supply system (CPSS) provides battery backup during emergencies but differs in design and performance to an uninterruptible power supply (UPS). A CPSS is used to power safety critical systems within buildings to provide sufficient time for evacuation during a power outage. A UPS system provides conditioned power when the mains power supply is present and battery backup during a power outage to ensure continuos operation of IT systems in server rooms and data centres and for larger sites may be supported by a standby power generator. BS EN 50171:2021 is the European safety standard that defines the requirements for a central power supply system.

Applications for Central Power Supply Systems

A central power supply provides battery backup to several safety circuits when the mains power supply fails. The battery is a 10year design life battery with a runtime for the connected loads of 1 to 3hours, dependent upon the evacuation period required. In comparison, emergency lighting systems have their own battery backup sets, wherever they are installed within a building.

A CPSS should be installed for safety critical backup power in hospitals, multi-floor buildings with firefighter and emergency evacuation lifts, high-rise tower blocks, airports, shopping centres, transportation tunnels (rail and/or vehicles), industrial facilities, schools, colleges and universities. Typical electrical circuits power from EN 50171 compatible CPSS include:

• Evacuation lifts
• Automatic doors for escape routes
• Public address / evacuation systems
• Fire alarm systems
• Emergency lighting
• Exit signs

CPSS Requirements

For a CPSS to meet BS EN 50171:2021 the system must have:

• Backup Power, Long Battery Autonomy and Fast Recharge Time: the CPSS must automatically switch to battery power when mains power supply fails and provide sufficient runtime to sustain operation of the critical load during an emergency, with rapid recharge capabilities to ensure readiness for further events. The runtime will be set at 60minutes or 3hours and will use a suitable and reliable battery set; most commonly valve regulated 10year design life lead-acid or lithium-ion batteries. The system may be suitable for AC and DC backup power and ensure output voltages and frequency are regulated and controlled to within operating limits.
• High Overload Capacity: the CPSS must be designed to handle varying power demands during emergency operations, including dynamic response to load variations from for example evacuation lifts, and provide protection from overloads and short circuit conditions.
• Fire Resistance and Robust Construction: the CPSS must be built to withstand more extreme conditions than a data centre style UPS, including potentially fire and mechanical impacts, and achieve a high operational reliability and MTBF (mean time between failures) value. The overall design of the CPSS system should avoid any single points of failure.
• Centralised Monitoring and Control System: the CPSS must provide intelligent real-time monitoring (SNMP/ModBUS/Dry Contacts), with status alerts (visual and audible) and seamless integration with building management systems (BMS). The CPSS should also continuously monitor the status of its battery set (charge % and temperature), charger status, output circuit voltage and frequency, and fault conditionss.

A CPSS may also be installed with a separate input transformer. BS EN 50171 stipulates that a central battery system (CPSS) must have a constant neutral supply. In case there is 4-pole switching upstream, or in the event that the neutral is interrupted, an input transformer ensures a constant neutral.

Why Use a Central Battery System Instead of Individual Battery Sets?

For life safety applications the use of a CPSS provides several advantages. These are similar to those of a large monobloc or modular UPS system used to power a data centre and include the fact that it can be easier to monitor and maintain a large single system and battery set in one central location rather than multiple systems deployed across a building or site. These advantages do however require a higher upfront capital cost and a larger dedicated space for the system installation. The electrical installation can also be more complex requiring additional works to retrofit into existing electrical circuits. A maintenance bypass facility may also be required to allow preventative or emergency maintenance without the need for downtime.

UPS Systems versus CPSS Applications

Uninterruptible Power Supplies (UPS) are designed for continuous operation, providing a high quality output with zero or negligible interruption to sensitive loads, predominantly IT loads in offices, computer rooms, server rooms and data centres.

Central Power Supply Systems (CPSS) are designed to provide backup power in the case of an emergency power outage to life-safety systems.

The following table summarises the differences.

For more information on BS EN 50171:2021 and CPSS see:
https://knowledge.bsigroup.com/products/central-safety-power-supply-systems-1

Battery Runtime

There are several factors to consider when selecting either a 60minute or 3hour runtime (or any other duration including 90minutes or 2hours for example). These include:

• Evacuation Plan and Time Required: how long will it take to safely evacuate the premises? For most commercial buildings a short duration i.e. 60-90minutes runtime may be sufficient, with longer periods required for larger buildings or building with multiple floors with lifts, and for example elderly occupants.
• Building Type and Occupancy Level: some buildings, including hospitals, care homes, retail stores, and large transportation terminals will require a longer evacuation time due to the number of people in the building, the nature and health of the occupants and/or the complexity of the building. This example could require a full 3hour runtime to ensure safe evacuation.
• Standards and Regulations: in the UK and Europe the CPSS must comply with EN 50171 and this outlines the minimum runtime requirements based on the application with the minimum being 60minutes.
• Critical Systems Supported: CPSS units may have to power AC and DC circuits and a mixture of loads including emergency lighting, fire extinguishing systems, smoke extraction equipment, and evacuation lifts. The runtime for the CPSS must be sufficient to ensure these critical systems operate until the emergency is resolved or the building is clear.

Overall, the final runtime chosen from 60minutes to 3hours will depend on the specific building, systems in use, how long is required for a safe evacuation and what is required to meet regulatory requirements.
Aside from selecting the most suitable battery runtime, care must also be taken with the electrical installation to and from the CPSS itself. Power cables should be routed through low fire risk areas and the installation space for the CPSS should be protected from fire and smoke, with the same principles applied for remote monitoring cables and interfaces.

MCS from Legrand

Legrand is a leading brand within the electrical and data centre industries. Our primary CPSS product is the Legrand TriMOD MCS which is based the Legrand TriMOD HE range. The MCS is a modular system designed to EN 50171 standards and represents the ideal solution for installation in buildings subject to fire safety standards and, specifically, to power emergency lighting systems. The TRIMOD MCS can also be used to power emergency systems including automatic fire extinguishing systems, emergency detection and alarm systems, smoke exhaust and carbon dioxide detection devices and specific safety systems in sensitive areas.

Summary

Server Room Environments supplies EN50171 compliant Central Safety Power Supply Systems to deliver uninterruptible power for safety systems. Our CPPS are custom designed to meet site specific requirements and regulations, and are scalable to adapt to varying loads and operational demands. They are also energy efficient to ensure a long operational working life with minimum maintenance contract requirements.