The HSE classifies working at height as when you:
Ladders can be used for working at height tasks when a risk assessment of the task shows that using equipment with higher level of fall protection is not justified e.g. task is low risk, duration of task is short or there are existing workplace features which cannot be altered. The assessment must be undertaken by someone that is competent and a safe system of work is in place.
A working platform includes:
Collective protection is equipment that does not require the person working at height to act for it to be effective and protects all those that might be working there. This includes designed walkways, guardrails, scissor lifts and tower scaffolds.
Personal protection on the other hand requires the individual to act for it to be effective. An example would be correctly wearing a safety harness correctly and connecting it, with an energy-absorbing lanyard, to a suitable anchor point.
There is no clear answer as every fall is different and best not fall at all. The depth and severity of injuries depend on factors such as the distance, the person’s age and weight, the way you land and what you might land on e.g. debris or sharp objects.
Working at height requires the right skills, knowledge and experience to perform the task. Undertaking training gives the trainee the necessary tools to understand the risks associated with working at height. Ultimately it is an employers responsibility to define what the competence levels are based on knowledge, training, experience and knowing when they have reached their level of competence and stop to gain further help.