Oil filled Transformers usually have a conservator which maintains the level of oil.  The conservator has a layer of air on top of the oil.  This air communicates with the atmosphere through a breather containing a desiccant,usually, silica gel. 

In some transformers, the tank of the transformer is filled with a layer of nitrogen above the oil in which the windings and the core are immersed.  Nitrogen, being an inert gas, reduces the risk of a fire hazard.  It also protects the transformer oil and the transformer insulation from reacting with oxygen in the air and deteriorating. 

In transformers containing nitrogen, the nitrogen is led into the space above the conservator through a pressure reducing valve.  When the oil level in the transformer rises and the transformer breathes out, the nitrogen inside is released into the atmosphere.  When the oil levels fall due to a reduction in ambient temperature or the load, fresh nitrogen is released into the conservator through the cylinder and the valves. 

Vacuum Circuit breakers are a very popular type of circuit breaker used in the industry.  Vacuum Circuit Breakers or VCBs as they are popularly known use vacuum as the quenching medium.

Vacuum Circuit breakers work by opening the contacts in an evacuated chamber.  The vacuum in the chamber ensures that there are very few ionisable molecules which can sustain the arc.  The arc is, thus, unable to sustain itself and is extinguished.

Vacuum circuit breakers have been used up to 36 kV and can interrupt up to 4000A.  When the arc is initiated during contact separation, metallic vapour is produced from the contacts.  If the contacts are of soft metal, very little vapour is produced.  While this ensure quick arc extinction.  The current waveform can get chopped and lead to high voltage transients.  If too soft metals are used in the contacts, the arc would cause erosion of the material.  Hence, the contacts are made of materials which are neither too soft or hard.  Common materials used are alloys Copper-bismuth or copper-chrome.