Emergency Lighting vs Normal Illuminance

Frequently asked questions

Why does a minimum change the design so much?

Because averages forgive dark spots and minimums do not. In a normal lighting design, a bright area compensates for a dim one and the mean across the room can meet the target while individual points vary — uniformity limits how far that goes, but the criterion is still an average. In an emergency scheme the requirement is a minimum along the escape route's centre line, so the point midway between two luminaires, where the light from both is weakest, is what the scheme is judged on. Adding brightness elsewhere does nothing for it. That makes spacing the governing variable rather than total output, and it is why emergency layouts have fittings at closer, more regular intervals along corridors than an efficiency-driven normal scheme would place them.

Where are luminaires required regardless of the calculation?

At the points where someone evacuating needs to see something specific, and the lists in the standards are explicit: at each exit door intended for escape; at every change of DIRECTION along a route; at every change of LEVEL, with each flight of stairs lit so the treads are visible; at corridor intersections; outside and near each final exit; at every fire alarm call point and item of firefighting equipment; and at first aid points. Those are required whether or not the illuminance from surrounding fittings would satisfy the minimum, because they are the decision points and the hazards. A scheme designed purely from a spacing calculation and lacking a luminaire at a stair or a direction change is non-compliant even if the numbers pass, which is the most common way a scheme fails review.

How long must emergency lighting run?

For a duration the regulations set for that building type and evacuation strategy, and it is commonly one or three hours depending on the regime and the use. The duration matters more than it looks: it determines the battery capacity, and a battery's capacity degrades over its life, so a system that achieved its duration when commissioned may not several years later. That is precisely what the annual full-duration test exists to establish — the system is run on battery for its full rated period and the illuminance is checked at the end of it, which is the condition that matters rather than the first minute. A system that passes a monthly flick test and fails at fifty minutes of a three-hour requirement is a system that will not do its job.

What does the testing regime involve?

Short functional tests at frequent intervals and a full-duration test annually, with the results recorded — and the record is as much a part of the requirement as the test. The short test confirms each luminaire switches to its battery and illuminates; the annual test runs the system for its full rated duration to confirm the batteries still hold, which is the one that finds degraded cells. Both have to cover every luminaire, which in a large building is a substantial exercise — hence automatic test systems, which perform and log the tests themselves and report failures, and which are increasingly specified for that reason. In most jurisdictions the regime is a legal duty on the building's responsible person rather than a best practice, and the log is what demonstrates it.

What is high-risk task area lighting?

A higher category of emergency lighting for places where a sudden loss of light is itself dangerous — a process that must be made safe before anyone leaves, machinery that has to be stopped, a laboratory procedure, a control room. The requirements are stricter in two ways: a higher illuminance, so the task can actually be completed, and a much shorter response time, because a person operating machinery cannot wait even a second in darkness. It is distinct from escape route lighting and from open-area (anti-panic) lighting, which is the third category and covers larger spaces where the objective is to prevent panic and let people find their way to a route. A building commonly needs all three in different parts, which is why the design is by space rather than uniform.

Does a layout change affect the emergency scheme?

Almost always, and it is the change most often missed. Emergency lighting is designed around escape ROUTES and hazard points, so moving partitions changes where the routes run, creates new changes of direction, and can put a new corner or a new door where no luminaire exists. A fit-out that leaves the ceiling grid and the light fittings largely alone can still invalidate the emergency scheme entirely by rearranging what is beneath them. The same applies to a change of use, which can change the evacuation strategy and therefore the duration and the categories required. Reviewing the emergency lighting is therefore part of any alteration that moves walls or doors, and it is cheap to do at design stage and expensive to discover at inspection.

Can normal fittings double as emergency ones?

Yes, and it is the usual arrangement: a maintained luminaire operates normally and switches to its integral battery when the supply fails, and a non-maintained one is dark until it is needed. Which is used depends on the space — maintained fittings are required where people may be in darkness when the supply fails, such as in a place of entertainment. What that arrangement does not do is remove the need for a separate emergency DESIGN: a fitting can be convertible and still be in the wrong place for an escape route, and a scheme that simply converts a proportion of the normal fittings without checking the route's minimum illuminance and the hazard points is a scheme that has not been designed. The conversion is a procurement decision; the layout is a design one.

How is the escape route illuminance measured?

On the FLOOR, along the centre line of the route, with the normal lighting off and the emergency system on battery — which is a different measurement from the one a normal lighting design is verified by. Normal illuminance is measured on the working plane, typically at desk height; escape lighting is about seeing where to put your feet and where the route goes, so the reference plane is the floor. The measurement is taken at the points where the illuminance is lowest, which is between fittings, and along the route's centre line rather than at its edges. Verification also covers the duration, so the meaningful measurement is at the end of the rated period rather than at the start, when the batteries are fresh and the output is at its highest.