Testing an emergency luminaire means simulating a failure of its normal supply. Doing that by pulling a breaker interrupts the lighting for a whole circuit, so installations provide a way of doing it luminaire by luminaire. The key switch is the traditional mechanism and it exists purely so that testing does not become an event.
What the key switch does
A key operated switch interrupts the mains supply to the luminaire's charger while leaving the rest of the lighting circuit intact, which makes the fitting change over to its battery. Turning the key back restores the supply and the charger resumes. The key is removable so that the switch cannot be left in the test position, which would flatten the battery and leave the fitting dead when it was needed.
Test facilities in practice
Arrangements vary: an individual key switch at each fitting, a key switch serving a group of fittings on one circuit, or a central test switch for a whole area. Grouped arrangements are faster to test and give less granular information, because a group that fails does not say which fitting failed. The design should record what the test facility is, and the testing procedure should match it.
The risk of leaving it switched
A key switch left in the test position discharges the battery and then holds it discharged, which damages it and leaves the luminaire unable to operate. This is a common finding during annual testing, and it is why the key is removable and why the test procedure ends with a check that the charging indicator is lit on every fitting.
Self testing as the alternative
Self testing luminaires run the monthly function test and the annual duration test themselves on a schedule, report failures by an indicator or to a central panel, and log the results. In buildings with many fittings they remove a large recurring labour cost and produce a more complete record than manual testing. They cost more per fitting and the saving arrives in the testing budget rather than the capital one.
Questions people ask about emergency light key switch
Where should the test key be kept?
Somewhere it is available for testing and not left in a switch. Losing it is the usual reason a manual testing regime lapses.
Can a breaker be used instead?
It works and it takes out the normal lighting for the whole circuit while testing, which is why dedicated test facilities exist.
Do self testing fittings still need a record?
Yes. They generate it, and somebody still has to read it and act on the failures it reports.
What emergency light testing frequency does the standard expect?
A short function test monthly and a full rated duration test annually, both recorded in the logbook. The monthly test operates the key switch or test facility so the fittings run on their own batteries for a few minutes while somebody walks the route and notes any that failed. The annual test runs the system for its rated duration, three hours for most designs, and needs the building to be left with the lights discharging and a recharge period afterwards, which is why it is usually done out of hours.
Is the emergency lighting test frequency different for a small building?
No. The monthly and annual pattern is the same whether there are four luminaires or four hundred, because it is the battery in each fitting that is being proven and batteries do not care about the size of the premises. What changes with size is how the test is organised: in a small building one person with a key does both tests in an evening, while in a large one the annual test is usually done in sections so the whole building is never unlit at once, with each section's result logged separately.
Is there an emergency light test certificate, or only a logbook?
The logbook is the record and there is no statutory certificate. What a contractor issues after a commissioning or an annual test is a completion or test report, which is worth having and is not a substitute for the site's own log: the monthly function tests are yours to do and to record, and an inspector reads those entries to see whether the system was being maintained between visits. So keep the contractor's reports in the front of the logbook, keep the monthly entries behind them, and treat a certificate offered instead of a log as half the evidence.