Glass Cartridge Fuse
A glass fuse is the classic protection element made up of a glass tube between two metal caps with a fusible wire inside. Thanks to its transparent body you can see by eye whether the wire has broken. Because it is prone to loosening under vibration in spring-clip holders, it is at a disadvantage against the blade type for on-vehicle use.
Glass fuses are still widespread at device inputs inside cabinets, in measuring instruments and in on-board protection holders. The 5x20 mm and 6.3x32 mm body sizes are the best known; the fast (F) and time-delay (T) characteristics are read from the letter printed on the caps.
- Visual inspection can mislead: the wire may look unbroken while the cap solder has cracked; measure continuity for a definite result.
- Holder contact: the spring clip opens over time and the contact resistance rises. If there is heating or discoloration at the fuse holder, replace the holder as well.
- Vibration: if it is used on a vehicle, the holder must be of the covered and locking type.
If a device does not work at all, the glass fuse is one of the first things to look at, but a visual check alone is not enough. Taking the fuse out and measuring continuity between its terminals, or looking at the voltage drop across it in place, also catches faults invisible to the eye such as a cracked solder joint and a loose holder.
Context in which this term is used: Installation, redundancy and field continuity