An electrical connector is a conductive device for joining electrical circuits together. The connection may be temporary, as for portable equipment, or may require a tool for assembly and removal, or may be a permanent electrical joint between two wires or devices. There are hundreds of types of electrical connectors. In computing, an electrical connector can also be known as a physical interface (compare Physical Layer in OSI model of networking). Connectors may join two lengths of flexible wire or cable, or may connect a wire or cable to an electrical terminal. Although cable glands are often called "connectors", a technical distinction can be made in the terminology, which differentiates them from quick-disconnect, conducting electrical connectors. The distinction is often not made. Higher voltage applications bring a diverse set of challenges in design and wear of electrical contacts. Some of these high voltage contacts are operated in oil, others in air, others in a vacuum and still others in varying gases to deal with the unique requirements of high voltage applications. Some of the problems encountered will be high wear, arcing and sticking (or welding).