Working with electricity is dangerous. Hermetic seals help lower the danger of electrocution. Some places even make hermetic seals mandatory. Optical Finer Packaging lists what you need to know about a hermetic terminal seal.
There are many types of hermetic seals for electrical components.
Glass-to-metal hermetic seals safely fuse glass parts to metal parts. This not only makes it safe for electricity to move about, but the seal also prevents contaminants like dust from coming in to block the electrical flow. You see this kind of hermetic seal every day in lightbulbs. This kind of hermetic seal is reliable because it can withstand high temperatures of 100 degrees C and low temperatures of -55 degrees C.
Ceramic-to-glass hermetic seals connect ceramic parts to glass parts. Although both ceramic and glass may seem fragile, they are very strong. This kind of hermetic seal is reliable for machines using high voltages of 3,000 volts. They work great in places of extreme heat or cold. This makes them ideal for motors. This also makes them the most expensive kind of hermetic seal.
Another kind of hermetic seal is called epoxy-to-glass since it connects epoxy parts to glass parts. It is used in electronics but does not withstand extreme temperature changes. They are most used for coaxial cables.