What an Emergency Lighting Circuit Has To Survive

Why this matters

An emergency luminaire is an ordinary luminaire on an extraordinary supply path, and almost every defect found in the field is in the path rather than in the fixture. The building that fails is rarely the one whose batteries are flat. It is the one where the designated luminaires sit downstream of a dimmer, or a sensor, or a switch, so they come up at whatever level the room happened to be set to, and the monthly test passes every time because the test is run at midday with everything turned to full. What the path has to survive is not one event but three, and only one of the three is the utility going away.

Before any test, and before any tracing

Do not darken an occupied egress path to prove anything. Test out of hours, or clear the route first and station a person with a hand light at each end for the duration of the test. After you open the normal supply, the emergency source can re-energize the same conductors you just opened. An inverter, a generator transfer or a battery unit inside the fixture will back-feed the luminaire circuit on purpose, so isolate the emergency source as well, then prove dead at the luminaire itself before touching a conductor.

The three losses

1. Loss of the normal service. The utility, the main, the building. This is the one everyone tests, and it is the least common of the three in practice.
2. Loss of the normal lighting supply to the area served. A tripped branch circuit or an opened overcurrent device leaves one room dark while the rest of the building is fine. The emergency lighting requirement in NFPA 70 (National Electrical Code) Article 700, in the edition your authority having jurisdiction has adopted, is written around the supply to the area, not around the service, precisely because of this case.
3. Any control device between the source and the luminaire. A dimmer, a relay, an occupancy sensor, a time schedule or a networked controller in the path can reduce or remove the output regardless of where the power came from. The card on what a lighting control system is being asked to do owns the assignment rule; this card owns what the wiring has to look like once that rule is applied.

The performance criterion the surviving light has to meet, which is a floor-level illuminance along the egress path measured at the end of the rated duration, is not restated here. The sibling card on emergency and exit lighting owns it, and a test that proves transfer proves nothing about that criterion.

What may not sit in the path

• A dimming device with the designated luminaires downstream of it. On any of the three losses they arrive at the dimmer's last setting rather than at full.
• An occupancy sensor, a photosensor or a relay whose loss-of-power state is off. The one duty these luminaires exist for is the duty their controlling device fails out of.
• A time schedule or a networked controller with an unrecorded default scene. The default is a value somebody chose, and on the night it matters it is the value that runs.
• A local switch upstream of unit equipment. Battery unit equipment connects to the branch circuit serving the normal lighting in that area ahead of any local switches, so a switch cannot remove its charging supply or make it read a failure.
• A disconnect nobody controls, meaning one a tenant, a cleaner or a kitchen crew can operate, which is a path with a manual off.

The single permitted exception to the first three is a device listed for the duty. A listed emergency lighting control device (UL 924, Emergency Lighting and Power Equipment, which binds through the listing and labeling rule of the adopted National Electrical Code and through the device's own installation instructions) passes normal control through during normal operation and forces its load to full output on loss of the supply it senses. Everything turns on which supply it senses.

 normal panel
     |
     |  area lighting branch circuit
     +----+-------------------+
          |                   |
          |             local switch
   sensing lead and dimmer
   taken here,                |
   ahead of the normal
   local switch luminaires
          |
   listed transfer ------- designated
   device luminaires
          |
   emergency source

The sources, and what each one carries

Battery unit equipment, self-contained per fixture or per group. Its supply arrangement is the whole game: ahead of local switches, on the area's normal lighting circuit, so it sees the loss the occupants see.

Central inverter, one source feeding an emergency panel. It survives loss of the service well and does not by itself survive loss of an area's normal lighting circuit, because the emergency panel stays energized while the room goes dark. This is the arrangement that most often needs a listed device sensing the area supply. A generator with transfer survives an extended service loss, carries a start and transfer interval, and has the same area-sensing blind spot.

Worked example: the path trace of one restaurant

Dining room, kitchen and a rear corridor to the exit. Nine designated luminaires: 5 in dining, 2 in kitchen, 2 in the corridor. Source for the 7 hardwired units is a central inverter feeding an emergency panel; the 2 corridor units are battery unit equipment. The dining room runs at roughly a fifth of full output during evening service and at full during the day.

Each row is checked against three questions, each answered as full output or not.

Four failing cells of the twelve checked, and none of them is a defective product. Three belong to the dining group, which is 5 of the 9 designated luminaires, and one belongs to corridor unit B.

Why the monthly test never found it. The test is run at 11 a.m. by pressing the test button and confirming the units illuminate. At 11 a.m. the dining dimmer is at full, so the 5 dining luminaires illuminate at full and the test is recorded as a pass. At 8 p.m., with the dimmer at a fifth, the same test would show the same 5 luminaires holding at a fifth. The test condition and the operating condition were never the same condition, and the record shows twelve consecutive passes.

Correction for the dining group, and it is a re-basing of the sensing point rather than an added device in series. A listed emergency lighting control device goes in ahead of the dimmer, taking its sensing lead from the dining area's normal lighting branch circuit ahead of the local switch. What it senses moves from "the emergency panel is energized", which was never in question, to "the dining area's normal supply is present". During normal service the 5 luminaires dim exactly as before, because the listed device passes normal control through.

Correction for corridor unit B. Reconnect it ahead of the corridor switch, on the branch circuit serving the corridor's normal lighting. Until that is done it false-operates every time the corridor lights are switched off, running its battery down during normal hours, so it would begin any real event already partly discharged.

Re-trace after both corrections. All 4 rows now answer full output on loss of service and on loss of the area circuit, and no row answers yes to a control being able to reduce or remove output: 0 failing cells of 12.

Test discipline that follows. Run the transfer test with the room in the condition it normally occupies, dimmer at the evening setting, and record that setting on the test sheet beside the result. Transfer is what this test proves. Duration against the performance criterion is a separate test on a separate schedule, and the sibling card owns it.

Checking your own figures

• Nine designated luminaires, 4 path rows, 3 checks per row, 12 cells. 5 + 2 + 2 = 9 luminaires. Failing cells before correction: dining 3, corridor B 1, total 4 of 12. After: 0 of 12.
• The failing luminaire count is printed, not implied. The dining defect covers 5 of the 9 designated luminaires, which is 56 percent of them, on the three losses this card defines.
• The dining failure is stated at the operating condition, with the figure. Roughly a fifth of full output during evening service, against a test performed at full output at 11 a.m.
• No performance criterion is stated here. No duration, no floor-level illuminance figure and no measurement plane appears in this card; each is cited to the sibling that owns it.
• The correction is named as a re-basing and the two bases are printed. Sensed condition before: emergency panel energized. Sensed condition after: dining area normal supply present.
• Every instrument that is not a federal regulation carries its adoption path. NFPA 70 Article 700 and the listing and labeling rule, in the edition the authority having jurisdiction has adopted; UL 924 through that listing rule and through the device's own instructions.

Hazards this work creates

• Never test, trace or de-energize an emergency lighting path with the egress route occupied. Clear the route and station a person with a hand light at each end, or do the work out of hours.
• Isolate both supplies before touching a conductor. Opening the normal supply does not de-energize a luminaire fed by an inverter, a transferred generator or an internal battery. De-energize, lock and tag under 29 CFR 1910.333(b)(2) for electrical work on utilization equipment, isolate the emergency source at its own means, and prove dead at the luminaire with a meter checked on a known source before and after (NFPA 70E-2021, 120.5, binding through your employer's electrical safety program or your contract).
• Battery unit equipment is stored energy that no upstream lockout reaches. Disconnect the battery lead the way the instructions state before working inside the housing, keep metal tools off both terminals at once, and wear eye protection, because a shorted cell vents.
• Reaching an exit-path luminaire is work at height, on a properly set ladder or lift under 29 CFR 1910 Subpart D for general industry or 29 CFR 1926 Subpart X for construction, with the route below closed to traffic while the ladder is set.
• On a construction site the general illumination minimums still apply while you have the lighting apart, under 29 CFR 1926.56, and those minimums are a floor for the work area, not a design level.

References

• NFPA 70 (National Electrical Code), Article 700 and the listing and labeling rule of Article 110, in the edition your authority having jurisdiction has adopted
• UL 924, Emergency Lighting and Power Equipment, binding through that listing rule and through the device's own installation instructions
• 29 CFR 1910.333(b)(2), electrical work on utilization equipment conductors; 29 CFR 1926.417 is the construction counterpart for lockout and tagging of circuits; 29 CFR 1926.56 for construction illumination minimums
• NFPA 70E-2021, 120.5, as adopted through an employer electrical safety program or by contract
• See related: What a Lighting Control System Is Being Asked To Do; the sibling card on emergency and exit lighting, which owns the floor-level performance criterion and the duration test