A fire alarm panel restricts what can be done at it by access level, and the key is the physical part of that. Most panels also use a code or an enable switch. The point of the levels is that silencing an alarm and disabling a detector are very different acts and should not be available to the same people.
The access levels
The first level is available to anybody and shows the panel's state without allowing any action. The second level, reached with a key or a code, allows an authorised person to silence and reset and to see more detail. The third and fourth levels are for maintenance and for the system engineer, allowing devices to be disabled and the configuration to be changed. The key most buildings hold is the second level one.
Why disabling is a controlled act
Disabling a zone or a device removes detection from part of the building, and it is done legitimately during hot work, building work or to manage a faulty device. It is also done illegitimately to stop a nuisance alarm, and a disabled zone that nobody re-enabled is a serious and completely invisible defect. The panel logs disablements and the log is worth reading.
Key control in practice
Keys migrate. They end up in drawers, on contractors' rings and in the pockets of people who left. A building should know who holds a panel key, and the key should not be so freely available that any member of staff can reset the panel after an alarm without the incident being recorded. Equally, the key must be available at all times the building is occupied, because a panel nobody can operate is its own problem.
Panel keys are not the only ones
Buildings accumulate fire related keys: call point test keys, detector removal tools, smoke vent override keys, riser keys, fire service access keys and key safes. A schedule of which keys exist, what they operate and who holds them is a small piece of work that saves a great deal during an incident.
Questions people ask about fire panel keys
Are fire panel keys standardised?
Many panels use a common pattern within a manufacturer's range, which is convenient and means a key is not itself meaningful security. The access control is the discipline around who uses it.
Should a panel key be kept in a key safe?
It has to be available to authorised people whenever the building is occupied. A key safe by the panel is common, and the number of people with the code is the control.
Who may disable a device?
Somebody trained and authorised, with the disablement recorded and a plan to re-enable. Disablement without a record is how a building loses detection permanently.