The oil level in the equipment is used to completely cover components which arc/spark or produce hot surfaces during normal operation, establishing a barrier between these components and any flammable gases which may be present above the oil or outside the enclosure.
In this method of protection, cooling is aided by circulation of the oil (by convection), dispersing the heat produced in hot-spots.
The oil quench arcs occurring at switching contacts. Where minerals oil is used, these arcs will produce hydrogen and acetylene as by-products.
In the UK this condition was considered to be undesirable and, until recently, Ex ‘o’ was limited to Zone 2. Later standards have stricter specifications, and this type of protection is now permitted in Zone 1. They allow for the use of unused mineral oil to IEC 60296 for the protective liquid, as well as other types such as silicone insulating liquids having specified properties.
