Fall arrest stops an unplanned fall over a short distance with a shock load; a descent device lowers deliberately at a controlled rate.

The user of a descent device is conscious, attached and descending on purpose.

There is no shock load, because there is no fall to arrest.

The distances involved are long rather than short.

They are different equipment for different events.

Neither substitutes for the other, and many working positions need both.

Where the escape route is downward and there is no other way out - wind turbines, towers, cranes, offshore structures and tall plant.

In many of those the device is pre-installed at the working position.

That is because descent may need to happen quickly and without preparation.

High-level maintenance positions on plant and structures are the other common case.

Rescue teams also carry them for lowering casualties.

In a building with stairs, they are equipment of last resort rather than routine escape.

Automatic devices descend at a fixed rate and require nothing from the user; controlled devices let the user vary or stop the descent.

Automatic braking suits an injured or panicking person who cannot manage a descent.

Attach and step off is the whole operation.

Manually controlled devices suit rescue, where the descender is managing a casualty.

Some devices offer both modes.

Which is appropriate depends on who is expected to be using it and in what state.

A maximum descent height, and separately a total descent capacity before service is required.

Maximum height is set by the rope or webbing length and by the brake's rating.

Total capacity is the cumulative energy the brake can absorb over its life.

Multiple descents in a rescue can exhaust that capacity.

Both figures are published rather than obvious from the device.

Exceeding either means the device requires service before further use.

Only on devices rated for it - two-person mass changes both the descent rate and the loading.

Rescue variants are specifically designed and rated for two-person descent.

That allows a rescuer to descend with a casualty.

Using a single-person device with two people is outside its rating.

Descent rate rises with mass, which is what the brake design accommodates.

The capability is specified on the device rather than assumed from its size.

Pre-use checks and periodic thorough examination, with additional requirements after any use.

Any device that has been used for a descent needs assessment before reuse.

Rope or webbing is checked for damage, contamination and wear.

Brake condition cannot be assessed visually, which is why the total descent capacity is tracked.

Devices installed permanently at a working position are exposed to weather and need regular attention.

Both the schedule and the competence needed to carry it out come from the applicable regulations.

Yes - and for pre-installed evacuation devices, the training is the whole point.

A device nobody has used is not an escape capability.

Attachment, edge transition and the descent itself all feel very different in practice.

Rehearsal matters more for equipment that will be used once, under stress.

Training requirements are set by the local regime and often by the equipment manufacturer.

Records of training form part of demonstrating the rescue plan exists.