There can be a number of different definitions of what a major incident is, but in general, a major incident can be an event or situation which may have the potential to involve or cause a significant number of casualties, may have actually involved or caused a significant number of casualties, may have significant potential or actual risks or may have special circumstances.
Generally, major incidents are not frequently declared, and the chances of being involved in declaring one is low. However, having a basic understanding and idea could allow you to act quicker in managing one and thus potentially help save lives. There isn’t a general rule book as to what is and what isn’t a major incident, and it will often come down to your experience and instinct. There are obvious examples where there is a mass casualty situation, such as an explosion in a high occupancy building with significant casualties and people trapped, or a train derailment with mass casualties involved. Less obvious may be where there is a potential issue at a chemical factory or plant which could threaten a large proportion of the population in the general area or a reservoir damn that may break causing a threat to residents in a village further downstream.
As mentioned, a major incident doesn’t necessarily have to involve mass casualties, only the potential risk. An example of this is Novichok and the Salisbury incident, where police declared a major incident.
In all cases where you may not be sure, advice can always be sought. Using control, they can discuss the situation with other specialists and management. A majority of ambulances often carry a major incident booklet or folder you can refer to. On-call NILO’s (National Interagency Liaison Officer) and commanders can also be contacted.