Power cuts and electrically operated openings

01 What happens to an electric opening when the mains fails?

Most electric openings are supplied through a control panel that includes a low voltage supply, a battery or a capacitor reserve, and a logic board. On loss of mains, three things happen in sequence. The motor stops because its supply has gone. The control board either holds its last command or runs a programmed fail-safe routine. The release, mechanical or electromechanical, becomes the only way to move the leaf or curtain by hand.

For a domestic roller shutter, the reserve is often a small battery in the switch or a manual override crank. For a gate operator, the panel may hold enough charge to complete one cycle so the gate does not stop half open across a driveway. For an automatic sliding door on a shopfront, the standard behaviour is to open and stay open, because a closed leaf in a dark entrance is a safety and security problem. None of this is universal. The behaviour is set by the panel's configuration and by the risk assessment for the opening, and it must be recorded.

The practical check is a mains-off test, done with the opening in a safe position and someone watching. Note the position the leaf takes, the time it takes to get there, whether the release works under load, and how the opening is restored when the mains returns. A gate that closes on restoration without a warning delay is a hazard, and a shutter that stays locked with no manual release is a fault waiting for the next outage.

Where the opening has to run through an outage, the supply behind it is a small standby system, and the same discipline that applies to a building's backup power applies here: the load list comes first, then the machine. A guide to reading standby generator sizing by machine code, serial number and document version is a useful model for how that dossier should be assembled and kept.

02 Which openings need a standby supply, and which do not?

The question is not whether the opening is electric. It is what happens to people, stock and access while it is unavailable. A domestic garage door that can be released by hand and opened manually needs no standby supply. A trade counter shutter that protects stock and is the only way in needs one, or needs a documented manual procedure that staff can follow in the dark.

Work through the openings one at a time and record four facts for each: the duty, meaning how often and for how long it runs; the failure position; the release method and where its key or handle is kept; and the consequence of a two hour outage. Openings that score badly on the last point are the candidates for a standby supply. Openings that score well are candidates for a written manual procedure and a labelled release.

There is a second category that is often missed: openings that are not safety-critical in themselves but block a safety-critical route. A shutter that closes across the only exit from a stockroom, or a gate that traps a vehicle in a yard, is a problem regardless of what it protects. These belong on the same list, and the fix may be a change to the fail-safe setting rather than a generator.

03 What does a standby supply for an opening actually have to do?

An electric motor draws a high current at the moment it starts, then settles to a much lower running current. A standby supply must cover the start, not the run, and it must cover the number of starts the opening will make during the outage. A gate that cycles twice an hour for two hours is a different load from a shutter that opens once at the start of a shift.

Three figures drive the choice. The first is the locked rotor or starting current of the motor, taken from the motor plate or the operator's technical file. The second is the running current and the duty cycle, meaning how long the motor runs and how long it rests. The third is the supply's own behaviour under a step load, because a small generator asked to start a large motor will drop its voltage and may stall or trip.

For a single small operator, a battery and inverter unit sized for the start current is often enough, and it has the advantage of being silent and needing no fuel. For a site with several openings, a small generator may be the better answer, but it brings its own requirements: a changeover arrangement so the generator and the mains cannot be connected together, a fuel store, ventilation, and a starting procedure that staff can follow. The changeover is the part most often got wrong, and it is the part that matters most for safety.

04 How should the dossier be kept?

A standby supply that cannot be shown to work is not a standby supply. The dossier for each opening should hold the operator's technical file with the machine code and serial number, the panel configuration as commissioned, the mains-off test record with date and result, the release test record, and the maintenance log. Where a generator is involved, add the load list, the assigned ratings, the start checklist and the fuel estimate.

Keep the version of each document. Manufacturers revise panels and controllers, and a setting that was correct on one firmware version may behave differently on the next. A record that names the document version, not just the model, is the difference between a dossier that can be audited and a folder of undated printouts.

05 Are battery backups and generators interchangeable?

No, and treating them as alternatives is the most common mistake. A battery unit is quiet, needs no fuel and starts instantly, but its run time is fixed by its capacity and it degrades with age and cold. A generator runs as long as it has fuel, but it needs starting, ventilation, a changeover and maintenance, and it will not respond instantly unless it is already running.

For a single opening with a short, known duty, a battery unit is usually the simpler and more reliable answer. For a site where the outage may last hours and several openings must keep working, a generator with a changeover is the more honest answer, provided someone is on site to start it. Where neither is practical, the correct answer is a documented manual procedure, a labelled release, and staff who have practised it.

06 What should be checked before the next outage?

Test the release under load, not with the leaf hanging free. Confirm the failure position matches the risk assessment. Check that the release key or handle is where the document says it is, and that the person on shift knows how to use it. Confirm the changeover, if fitted, cannot connect the generator and the mains at the same time. Check the battery date and the fuel level. Record the date of the test and the result.

A power cut is not an emergency for an electric opening. It is a designed condition, and the design is either recorded and tested or it is a guess. The openings that cause trouble in an outage are almost always the ones with no document, no release test and no named person responsible. Fixing that costs an afternoon and a folder. Leaving it costs whatever the opening was protecting.

A bedroom escape window is only useful if it can still be opened when the power is off, and the same question applies to any electric roof vent, blind or shutter elsewhere in the house. The Ledger keeps a separate entry on standby power for electric vents, which sets out what happens to those openings when the mains fails and how a standby set is sized for the loads a modern home actually runs. Anyone checking a bedroom window under Approved Document B should read it alongside the escape route requirements.