Case study: Know the power system codes for emergency tunnel lighting
Understand the codes and standards that are necessary for emergency lighting in a tunnel
This project example demonstrates a roadway tunnel application. A roadway tunnel has normally powered lighting as well as emergency lighting. The emergency lighting for a roadway tunnel will be powered by emergency circuits that have source access to the emergency power supply system.
When assessing the building codes before starting design, the electrical engineer has determined that some of the applicable codes and standards include NFPA 70: National Electrical Code (NEC) and NFPA 502: Standard for Road Tunnels, Bridges and Limited Access Highways, NFPA 110: Standard for Emergency and Standby Power Systems and NFPA 111: Standard on Stored Electrical Energy Emergency and Standby Power Systems.
Sorting through the requirements outlined in these codes and standards, the electrical engineer has determined that the emergency lighting systems include emergency lighting for minimum lighting, exit signage and way-finding lighting. The design team also determined that these systems are required to be powered by emergency lighting distribution panels separated from normal power distribution panels.
Figure 5: This shows a partial one-line diagram as an example system configuration of an emergency electrical distribution system powering an emergency lighting panel. Courtesy: CDM Smith
NFPA 502 also requires that there be no interruption to lighting levels greater than 0.5 seconds and that emergency circuits use fire-resistant cables listed for no less than a two-hour fire rating. NFPA 502 also requires these circuits to remain functional for a period of not less than 1 hour.
From this, the engineering team can use NFPA 110 to specify classification requirements for the emergency power supply system. The classification is Level 1, Class 1, Type U. Note that the engineering team decided to use an uninterruptible power supply (UPS) to power the lighting to meet the maximum light level interruption of 0.5 seconds mandated by NFPA 502 in addition to the one-hour functional time period requirement. Alternatively, the UPS may be sized to ride through until power has been transferred to an appropriately sized facility emergency generator.
For feeder and branch circuit wiring the engineering team specified a UL FHIT assembly with a two-hour fire rating.
In locating the EPSS equipment for this project, driven by NFPA 110 Level 1 requirements, our outdoor generator set will feed our UPS located in a dedicated emergency power equipment room along with panelboards for the emergency lighting distribution. The room will be two-hour fire-rated.
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