Another source of fire comes from connections, not the heaters themselves.
As the connection and connector ages, the electrical resistance of the connection increases. Up to a point, this is not a problem. A portion of the energy goes to heating the connection, and less energy goes to the heated element. These are in closed loops, so energy is applied longer to compensate.
The connection is using the lost energy to heat itself. This increases the corrosion rate, and also can cause plastic in the connector to flow, which relaxes the connection pressure, which further increases the corrodion, resistance, and heating.
This situation is unstable, and ends in one of three ways:
- the extrusion hot-end or the bed don't get as hot, with implication on the print quality,
- the connection opens, and the heated system simply fails,
- or the worst case, where the temperature increases to the point where the connector housing, wire, heater PCB, or dust and fur on the connector catches fire.
The only real mitigation of this risk is to periodically check power connections for warming. If a connection is warm, it should be watched more carefully. If it is hot, you already have a problem and should fix it.
If you are concerned, focus on preventative maintenance, and on being sure that there are no flammable materials near the printer. Don't put your printer in a simple foam case. Consider the flammability of any enclosure components.
You might also consider adding a smoke and high-temperature detector connected through a relay to disconnect main power. If a connection starts sputtering, you might detect it before the fire can transfer to your house.